essay about earthquake and volcanoes

Earthquake Essay for Students and Children

500+ Words Essay on Earthquake

Simply speaking, Earthquake means the shaking of the Earth’s surface. It is a sudden trembling of the surface of the Earth. Earthquakes certainly are a terrible natural disaster. Furthermore, Earthquakes can cause huge damage to life and property. Some Earthquakes are weak in nature and probably go unnoticed. In contrast, some Earthquakes are major and violent. The major Earthquakes are almost always devastating in nature. Most noteworthy, the occurrence of an Earthquake is quite unpredictable. This is what makes them so dangerous.

Types of Earthquake

Tectonic Earthquake: The Earth’s crust comprises of the slab of rocks of uneven shapes. These slab of rocks are tectonic plates. Furthermore, there is energy stored here. This energy causes tectonic plates to push away from each other or towards each other. As time passes, the energy and movement build up pressure between two plates.

Therefore, this enormous pressure causes the fault line to form. Also, the center point of this disturbance is the focus of the Earthquake. Consequently, waves of energy travel from focus to the surface. This results in shaking of the surface.

Volcanic Earthquake: This Earthquake is related to volcanic activity. Above all, the magnitude of such Earthquakes is weak. These Earthquakes are of two types. The first type is Volcano-tectonic earthquake. Here tremors occur due to injection or withdrawal of Magma. In contrast, the second type is Long-period earthquake. Here Earthquake occurs due to the pressure changes among the Earth’s layers.

Collapse Earthquake: These Earthquakes occur in the caverns and mines. Furthermore, these Earthquakes are of weak magnitude. Undergrounds blasts are probably the cause of collapsing of mines. Above all, this collapsing of mines causes seismic waves. Consequently, these seismic waves cause an Earthquake.

Explosive Earthquake: These Earthquakes almost always occur due to the testing of nuclear weapons. When a nuclear weapon detonates, a big blast occurs. This results in the release of a huge amount of energy. This probably results in Earthquakes.

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Effects of Earthquakes

First of all, the shaking of the ground is the most notable effect of the Earthquake. Furthermore, ground rupture also occurs along with shaking. This results in severe damage to infrastructure facilities. The severity of the Earthquake depends upon the magnitude and distance from the epicenter. Also, the local geographical conditions play a role in determining the severity. Ground rupture refers to the visible breaking of the Earth’s surface.

Another significant effect of Earthquake is landslides. Landslides occur due to slope instability. This slope instability happens because of Earthquake.

Earthquakes can cause soil liquefaction. This happens when water-saturated granular material loses its strength. Therefore, it transforms from solid to a liquid. Consequently, rigid structures sink into the liquefied deposits.

Earthquakes can result in fires. This happens because Earthquake damages the electric power and gas lines. Above all, it becomes extremely difficult to stop a fire once it begins.

Earthquakes can also create the infamous Tsunamis. Tsunamis are long-wavelength sea waves. These sea waves are caused by the sudden or abrupt movement of large volumes of water. This is because of an Earthquake in the ocean. Above all, Tsunamis can travel at a speed of 600-800 kilometers per hour. These tsunamis can cause massive destruction when they hit the sea coast.

In conclusion, an Earthquake is a great and terrifying phenomenon of Earth. It shows the frailty of humans against nature. It is a tremendous occurrence that certainly shocks everyone. Above all, Earthquake lasts only for a few seconds but can cause unimaginable damage.

FAQs on Earthquake

Q1 Why does an explosive Earthquake occurs?

A1 An explosive Earthquake occurs due to the testing of nuclear weapons.

Q2 Why do landslides occur because of Earthquake?

A2 Landslides happen due to slope instability. Most noteworthy, this slope instability is caused by an Earthquake.

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How are volcanoes and earthquakes interrelated?

• Updated 17/11/22
• Read time 5 minutes

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Both volcanoes and earthquakes occur due to movement of the Earth’s tectonic plates. They are both caused by the heat and energy releasing from the Earth’s core. Earthquakes can trigger volcanic eruptions through severe movement of tectonic plates. Similarly, volcanoes can trigger earthquakes through the movement of magma within a volcano. Therefore, one aspect of how are volcanoes and earthquakes interrelated is the cyclical relationship where earthquakes cause volcanic eruptions and magma movement causes earthquakes.

Earthquakes and volcanic eruptions in itself are dangerous natural phenomena which poses risk to humans. Furthermore, tsunamis are an equally deadly secondary effect caused by underwater disturbances such as earthquakes, volcanic activity, landslides, underwater explosions or meteorite impacts which pose significant danger to human lives.

What are tectonic plates and how do they explain volcanoes and earthquakes?

The outer layer of the Earth is made up of solid rock, called lithosphere. The lithosphere is broken up into 17 major separate pieces that fit together like a bad jigsaw puzzle. A piece of the puzzle is called a tectonic plate . The plates are horribly placed, with overlapping pieces, gaps, and are forced to fit with each other even when they don’t. Because the tectonic plates don’t go well together, it creates earthquakes and volcanic activity when two plates collide, diverge or slide past each other.

There are three types of boundaries caused by tectonic plates on Earth: first, transform boundaries when two plates slide or grind past each other. Transform boundaries are horizontal movements of plates and do not create or destroy plates. Second, divergent boundaries when two plates create a gap in between each other. Divergent boundaries create ocean basins when plates move apart from one another. Third, convergent boundaries when two plates crash towards each other and overlap to form a subduction zone.

Tectonic plates are extremely large and can encompass both land and ocean. These tectonic plates interact with one another because of the Earth’s internal heat. This heat causes movement of material beneath the Earth’s crust and releases energy in the form of earthquakes and volcanic eruptions. Interactions between the tectonic plates occurs in three main places: first, oceanic-continental convergence, where the continent meets the ocean. Second, continental-continental convergence, where two continents meet. Last, oceanic-oceanic convergence, where two oceans meet.

So, how are volcanoes and earthquakes interrelated? Both volcanoes and earthquakes are caused along the boundaries of tectonic plates due to their movements and interactions.

How dangerous are volcanoes and earthquakes to humans?

How many deaths there are and how severe a natural disaster is depends on the interactions we have as humans with our environment. A strong earthquake can be completely overlooked if it happens in the middle of land where hardly any people live in, for example in December 2003, when a 6.5 magnitude earthquake in Central California killed two people. Consequently, a similar earthquake in a densely populated metropolitan area can cause many deaths, injuries and destruction of property, for example, a 6.6 magnitude earthquake in Bam, Iran killing 30,000 people, injuring 30,000 more and destroying 85% of property.

The same goes for volcanic eruptions, if an underwater volcano erupted in the middle of the ocean, it would have little impact to humans living far inland. However, if the volcano had been dormant for a significant amount of time and humans started populating the land close to the base of a volcano it would be devastating should it ever erupt. Further, indirect consequences of a volcanic eruption can have a greater impact than the eruption itself. For example, volcanic ash can induce climate change that have serious agricultural, economic, and sociological impacts on people’s lives.

Read these articles to learn more about the structure of the volcano and the internal structure of the volcano .

To find out more about where the next volcano will erupt in Australia, read this article.

Tsunamis as a result of earthquakes or volcanic activity

Tsunamis are giant waves that occur when a large volume of water is displaced. The most common causes of underwater disturbances that lead to tsunamis are earthquakes and volcanic eruptions, especially because most volcanoes are underwater. Although tsunamis can also be caused by landslides, meteorite impacts and underwater explosions. Tsunami waves travel extremely fast, with wave speeds reaching up to 900 km/h in deep water. The waves get taller the closer they reach the shoreline because of rapidly decreasing depth of the sea floor, and slow down to 20 – 50 km/h.

We can deduce that volcanoes and earthquakes are caused by tectonic plate movements, they both pose a significant danger to humans, and result in deadly secondary effects like tsunamis.

Additional Reading

https://web.archive.org/web/20120913041924/http://www.enviroliteracy.org/nsfmod/NaturesFury.pdf

https://australian.museum/blog-archive/science/earthquakes-and-tsunamis/

https://australian.museum/learn/minerals/shaping-earth/plate-tectonic-processes/

https://media.australian.museum/media/dd/Uploads/Documents/4645/volcano_structure.134d97b.pdf

https://australian.museum/learn/minerals/shaping-earth/structure-of-volcanoes/

https://australian.museum/blog-archive/science/amri-where-next-volcano-erupt-in-australia/

The Australian Museum respects and acknowledges the Gadigal people as the First Peoples and Traditional Custodians of the land and waterways on which the Museum stands.

Image credit: gadigal yilimung (shield) made by Uncle Charles  Chicka  Madden

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Earth’s Dynamic Forces: Understanding Volcanoes, Earthquakes, and Plate Tectonics

The Earth, our home, is a dynamic and ever-changing planet shaped by powerful geological forces. Volcanoes, earthquakes, and plate tectonics are key players in this geological symphony, shaping the landscape and influencing the course of life on our planet.

In this article, we delve into the fascinating world of Earth’s dynamic forces, exploring the intricacies of plate tectonics, the awe-inspiring nature of volcanoes, and the seismic events that shake our world.

Plate Tectonics

At the heart of Earth’s dynamic forces lies the theory of plate tectonics, a groundbreaking concept that revolutionized our understanding of the Earth’s lithosphere and asthenosphere. The lithosphere, comprising the rigid outer shell of the Earth, is divided into several tectonic plates that float atop the semi-fluid asthenosphere beneath. These plates are in constant motion, albeit slowly, driving the geological processes that shape our planet[1].

Types of Plate Boundaries

Plate tectonics manifests itself primarily at plate boundaries, where the interactions between these massive pieces of the Earth’s crust give rise to various geological phenomena. There are three main types of plate boundaries, each characterized by distinct geological activities.

Divergent Boundaries

At divergent boundaries, tectonic plates move away from each other. This movement creates space for magma to rise from the mantle, solidifying at the surface and forming new crust. The Mid-Atlantic Ridge is a prime example of a divergent boundary, where new oceanic crust is continuously being generated.

Convergent Boundaries

Convergent boundaries involve tectonic plates colliding with one another. When two plates converge, one may be forced beneath the other in a process known as subduction. This collision leads to the formation of deep ocean trenches, volcanic arcs, and mountain ranges. The iconic Himalayas, formed by the collision of the Indian and Eurasian plates, exemplify convergent boundary effects.

Transform Boundaries

Transform boundaries occur when two plates slide past each other horizontally. The movement along transform boundaries can cause earthquakes as stress builds up and is released along fault lines. The San Andreas Fault in California is a well-known example of a transform boundary.

Understanding these plate boundaries is crucial for comprehending the geological processes that shape our planet’s surface. The dynamic interactions at these boundaries drive the creation of mountains, ocean basins, and volcanic activity.

Volcanoes, among the most visually striking geological features, are a direct result of the Earth’s internal heat and the movement of tectonic plates. These majestic structures can take various forms, each with unique characteristics.

Volcanoes are openings in the Earth’s crust through which molten rock, ash, and gases are expelled. The formation of a volcano is closely linked to the movement of tectonic plates. When plates diverge or converge, magma from the mantle can rise to the surface, creating a volcanic vent[2].

Types of Volcanoes

There are three main types of volcanoes: shield volcanoes, stratovolcanoes (composite volcanoes), and cinder cone volcanoes. Shield volcanoes have broad, gently sloping profiles and are primarily composed of low-viscosity lava. Stratovolcanoes, on the other hand, are characterized by steep slopes and alternating layers of lava, ash, and volcanic rocks. Cinder cone volcanoes are smaller, conical structures formed from the accumulation of ejected volcanic materials.

Volcanic Eruptions

Volcanic eruptions can be explosive or effusive, depending on the viscosity of the magma. Explosive eruptions involve high-viscosity magma that traps gases, leading to violent explosions. Effusive eruptions, on the other hand, involve low-viscosity magma that flows more easily, resulting in gentler eruptions.

Volcanic Hazards and Their Impact

While volcanoes captivate us with their beauty, they also pose significant hazards to nearby communities. Eruptions can release ash clouds, pyroclastic flows, and lava flows, all of which can have devastating consequences. Understanding these hazards is essential for mitigating the risks associated with volcanic activity.

Earthquakes

Beyond the awe-inspiring spectacle of volcanoes, Earth’s dynamic forces also manifest in the form of earthquakes, powerful seismic events that can reshape landscapes and impact communities. Understanding the causes, characteristics, and consequences of earthquakes is crucial for mitigating their potential harm.

Earthquakes are the result of the Earth’s crustal plates interacting at plate boundaries. The release of stress built up along faults causes the ground to shake. These faults are fractures in the Earth’s crust where movement has occurred, allowing for seismic energy to be released.

Seismic Waves and Their Characteristics

When an earthquake occurs, it generates seismic waves that radiate outward from the epicenter, the point on the Earth’s surface directly above the earthquake’s point of origin. There are three main types of seismic waves: Primary waves (P-waves), Secondary waves (S-waves), and Surface waves. P-waves are the fastest, while surface waves cause the most significant ground movement.

Measurement of Earthquakes

The magnitude of earthquakes is measured using various scales, with the Richter scale and the moment magnitude scale being the most commonly employed. These scales quantify the energy released during an earthquake, providing a standardized way to assess seismic events[3].

Seismology and Earthquake Monitoring

Seismologists use a network of seismometers to monitor and analyze seismic activity worldwide. By studying the patterns and characteristics of earthquakes, scientists can gain valuable insights into the Earth’s interior structure and the dynamics of plate tectonics.

Earthquake Hazards and Preparedness

Earthquakes can result in various hazards, including ground shaking, surface rupture, and secondary effects like tsunamis and landslides. Preparedness measures, such as earthquake-resistant building designs, early warning systems, and community education, are essential for minimizing the impact of seismic events on human populations.

Interactions between Volcanoes, Earthquakes, and Plate Tectonics

The dynamic forces of plate tectonics, volcanoes, and earthquakes are interconnected, creating a complex and ever-changing geological landscape[4].

The Connection between Plate Boundaries and Volcanic Activity

Volcanic activity is closely linked to plate boundaries. At divergent boundaries, magma rises from the mantle to create new crust, leading to the formation of volcanic features. Convergent boundaries, where plates collide, can also trigger volcanic eruptions as one plate is forced beneath another, creating intense heat and pressure.

Earthquakes as Indicators of Tectonic Activity

The occurrence of earthquakes often serves as a key indicator of tectonic activity. Most earthquakes happen along plate boundaries, reflecting the stress and strain associated with the movement of these massive tectonic plates. The correlation between earthquakes and plate boundaries underscores the interconnected nature of Earth’s dynamic forces.

Case Studies Highlighting the Interplay

Examining specific regions provides insights into the intricate interplay of these dynamic forces. For instance, the Pacific Ring of Fire, characterized by intense seismic and volcanic activity, exemplifies the complex interactions at convergent and divergent plate boundaries. Understanding these case studies helps scientists and communities prepare for and mitigate the potential impacts of geological events.

Human Impacts and Mitigation Strategies

The captivating dance of Earth’s dynamic forces, involving plate tectonics, volcanoes, and earthquakes, is not merely a geological spectacle but also a force that shapes human societies and ecosystems. Understanding the impact of these dynamic forces on human communities is crucial for developing effective mitigation strategies[5].

Impact of Volcanic Eruptions and Earthquakes

The human impact of volcanic eruptions and earthquakes can be profound. Eruptions can result in the destruction of infrastructure, displacement of communities, and even loss of life. Earthquakes, with their potential to cause ground shaking, landslides, and tsunamis, pose similar threats. The consequences of these events can reverberate through communities for years, necessitating thoughtful preparedness and response measures.

Strategies for Monitoring and Predicting Events

Advances in technology and scientific understanding have enabled the development of strategies for monitoring and predicting volcanic eruptions and earthquakes. Seismic monitoring networks, satellite imagery, and ground deformation measurements contribute to early warning systems, giving communities valuable time to evacuate and prepare for potential disasters.

Emergency Preparedness and Response Measure

Effective emergency preparedness and response are critical components of mitigating the human impact of geological events. Evacuation plans, community education, and the development of resilient infrastructure can significantly reduce the risks associated with volcanic eruptions and earthquakes. Collaborative efforts between scientists, government agencies, and local communities are essential for creating comprehensive and effective strategies.

Ongoing Research and Future Prospects

As our understanding of Earth’s dynamic forces continues to evolve, ongoing research endeavors offer a glimpse into the future of geological studies and the potential applications of this knowledge.

Current Scientific Advancements

Contemporary research in seismology, volcanology, and plate tectonics is unveiling new insights into the Earth’s interior structure and the mechanisms driving dynamic forces. Advanced imaging techniques, such as tomography and satellite-based observations, provide unprecedented views of subsurface features and volcanic activity. These advancements contribute to a more nuanced understanding of geological processes.

Areas of Ongoing Research and Exploration

Scientists are actively exploring various areas to deepen our understanding of Earth’s dynamic forces. Subduction zones, where one tectonic plate is forced beneath another, are a focus of research due to their association with powerful earthquakes and volcanic arcs. Deep-sea exploration and the study of mid-ocean ridges contribute to our knowledge of the processes occurring beneath the Earth’s crust[6].

Implications for Earth’s Geology

Ongoing research has broad implications for our understanding of Earth’s geology. Insights into the behavior of magma, the dynamics of plate boundaries, and the factors influencing seismic activity contribute not only to scientific knowledge but also to the development of strategies for mitigating the impact of geological events on human societies.

In conclusion, Earth’s dynamic forces, encompassing plate tectonics, volcanoes, and earthquakes, weave a narrative of constant change and evolution. The interconnected nature of these geological phenomena shapes the planet’s surface, influences ecosystems, and impacts human communities. While the raw power of these forces can be destructive, our growing understanding, coupled with advancements in technology, equips us with the tools to navigate and mitigate their impact.

The ongoing research into Earth’s dynamic forces opens doors to new frontiers of knowledge and exploration. As we delve deeper into the complexities of plate tectonics, volcanoes, and earthquakes, we gain not only a better understanding of our planet’s past but also insights that can guide us in building a resilient future. By combining scientific advancements with effective mitigation strategies, we can coexist with Earth’s dynamic forces, appreciating their beauty while minimizing their potential harm. The journey into the heart of Earth’s geology continues, promising a future where our evolving understanding allows us to navigate and thrive in a world shaped by dynamic forces beyond our control.

• T. H. Jordan, “Structure and Dynamics of Earth’s Lower Mantle,” Annual Review of Earth and Planetary Sciences, vol. 23, 1995, pp. 215–238.
• R. S. J. Sparks, “Volcanic Processes in Ore Genesis: The Relationship Between Volcanism and Volcanogenic Ore Deposits,” Economic Geology, vol. 85, no. 8, 1990, pp. 1927–1944.
• S. Stein and M. Wysession, “An Introduction to Seismology, Earthquakes, and Earth Structure,” Blackwell Publishing, 2003.
• P. J. Tackley, “Mantle Convection and Plate Tectonics: Toward an Integrated Physical and Chemical Theory,” Science, vol. 288, no. 5473, 2000, pp. 2002–2007.
• R. R. M. Rawlinson et al., “Earthquake Preparedness and Response: Lessons Learned from Recent Earthquakes,” Current Psychiatry Reports, vol. 21, no. 9, 2019.
• National Research Council, “Living on an Active Earth: Perspectives on Earthquake Science,” National Academies Press, 2003.

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Volcanic Eruptions and Their Repose, Unrest, Precursors, and Timing (2017)

Chapter: 1 introduction, 1 introduction.

Volcanoes are a key part of the Earth system. Most of Earth’s atmosphere, water, and crust were delivered by volcanoes, and volcanoes continue to recycle earth materials. Volcanic eruptions are common. More than a dozen are usually erupting at any time somewhere on Earth, and close to 100 erupt in any year ( Loughlin et al., 2015 ).

Volcano landforms and eruptive behavior are diverse, reflecting the large number and complexity of interacting processes that govern the generation, storage, ascent, and eruption of magmas. Eruptions are influenced by the tectonic setting, the properties of Earth’s crust, and the history of the volcano. Yet, despite the great variability in the ways volcanoes erupt, eruptions are all governed by a common set of physical and chemical processes. Understanding how volcanoes form, how they erupt, and their consequences requires an understanding of the processes that cause rocks to melt and change composition, how magma is stored in the crust and then rises to the surface, and the interaction of magma with its surroundings. Our understanding of how volcanoes work and their consequences is also shared with the millions of people who visit U.S. volcano national parks each year.

Volcanoes have enormous destructive power. Eruptions can change weather patterns, disrupt climate, and cause widespread human suffering and, in the past, mass extinctions. Globally, volcanic eruptions caused about 80,000 deaths during the 20th century ( Sigurdsson et al., 2015 ). Even modest eruptions, such as the 2010 Eyjafjallajökull eruption in Iceland, have multibillion-dollar global impacts through disruption of air traffic. The 2014 steam explosion at Mount Ontake, Japan, killed 57 people without any magma reaching the surface. Many volcanoes in the United States have the potential for much larger eruptions, such as the 1912 eruption of Katmai, Alaska, the largest volcanic eruption of the 20th century ( Hildreth and Fierstein, 2012 ). The 2008 eruption of the unmonitored Kasatochi volcano, Alaska, distributed volcanic gases over most of the continental United States within a week ( Figure 1.1 ).

Finally, volcanoes are important economically. Volcanic heat provides low-carbon geothermal energy. U.S. generation of geothermal energy accounts for nearly one-quarter of the global capacity ( Bertani, 2015 ). In addition, volcanoes act as magmatic and hydrothermal distilleries that create ore deposits, including gold and copper ores.

Moderate to large volcanic eruptions are infrequent yet high-consequence events. The impact of the largest possible eruption, similar to the super-eruptions at Yellowstone, Wyoming; Long Valley, California; or Valles Caldera, New Mexico, would exceed that of any other terrestrial natural event. Volcanoes pose the greatest natural hazard over time scales of several decades and longer, and at longer time scales they have the potential for global catastrophe ( Figure 1.2 ). While

the continental United States has not suffered a fatal eruption since 1980 at Mount St. Helens, the threat has only increased as more people move into volcanic areas.

Volcanic eruptions evolve over very different temporal and spatial scales than most other natural hazards ( Figure 1.3 ). In particular, many eruptions are preceded by signs of unrest that can serve as warnings, and an eruption itself often persists for an extended period of time. For example, the eruption of Kilauea Volcano in Hawaii has continued since 1983. We also know the locations of many volcanoes and, hence, where most eruptions will occur. For these reasons, the impacts of at least some types of volcanic eruptions should be easier to mitigate than other natural hazards.

Anticipating the largest volcanic eruptions is possible. Magma must rise to Earth’s surface and this movement is usually accompanied by precursors—changes in seismic, deformation, and geochemical signals that can be recorded by ground-based and space-borne instruments. However, depending on the monitoring infrastructure, precursors may present themselves over time scales that range from a few hours (e.g., 2002 Reventador, Ecuador, and 2015 Calbuco, Chile) to decades before eruption (e.g., 1994 Rabaul, Papua New Guinea). Moreover, not all signals of volcanic unrest are immediate precursors to surface eruptions (e.g., currently Long Valley, California, and Campi Flegrei, Italy).

Probabilistic forecasts account for this uncertainty using all potential eruption scenarios and all relevant data. An important consideration is that the historical record is short and biased. The instrumented record is even shorter and, for most volcanoes, spans only the last few decades—a miniscule fraction of their lifetime. Knowledge can be extended qualitatively using field studies of volcanic deposits, historical accounts, and proxy data, such as ice and marine sediment cores and speleothem (cave) records. Yet, these too are biased because they commonly do not record small to moderate eruptions.

Understanding volcanic eruptions requires contributions from a wide range of disciplines and approaches. Geologic studies play a critical role in reconstructing the past eruption history of volcanoes,

especially of the largest events, and in regions with no historical or directly observed eruptions. Geochemical and geophysical techniques are used to study volcano processes at scales ranging from crystals to plumes of volcanic ash. Models reveal essential processes that control volcanic eruptions, and guide data collection. Monitoring provides a wealth of information about the life cycle of volcanoes and vital clues about what kind of eruption is likely and when it may occur.

1.1 OVERVIEW OF THIS REPORT

At the request of managers at the National Aeronautics and Space Administration (NASA), the National Science Foundation, and the U.S. Geological Survey (USGS), the National Academies of Sciences, Engineering, and Medicine established a committee to undertake the following tasks:

• Summarize current understanding of how magma is stored, ascends, and erupts.
• Discuss new disciplinary and interdisciplinary research on volcanic processes and precursors that could lead to forecasts of the type, size, and timing of volcanic eruptions.
• Describe new observations or instrument deployment strategies that could improve quantification of volcanic eruption processes and precursors.
• Identify priority research and observations needed to improve understanding of volcanic eruptions and to inform monitoring and early warning efforts.

The roles of the three agencies in advancing volcano science are summarized in Box 1.1 .

The committee held four meetings, including an international workshop, to gather information, deliberate, and prepare its report. The report is not intended to be a comprehensive review, but rather to provide a broad overview of the topics listed above. Chapter 2 addresses the opportunities for better understanding the storage, ascent, and eruption of magmas. Chapter 3 summarizes the challenges and prospects for forecasting eruptions and their consequences. Chapter 4 highlights repercussions of volcanic eruptions on a host of other Earth systems. Although not explicitly called out in the four tasks, the interactions between volcanoes and other Earth systems affect the consequences of eruptions, and offer opportunities to improve forecasting and obtain new insights into volcanic processes. Chapter 5 summarizes opportunities to strengthen

research in volcano science. Chapter 6 provides overarching conclusions. Supporting material appears in appendixes, including a list of volcano databases (see Appendix A ), a list of workshop participants (see Appendix B ), biographical sketches of the committee members (see Appendix C ), and a list of acronyms and abbreviations (see Appendix D ).

Background information on these topics is summarized in the rest of this chapter.

1.2 VOLCANOES IN THE UNITED STATES

The USGS has identified 169 potentially active volcanoes in the United States and its territories (e.g., Marianas), 55 of which pose a high threat or very high threat ( Ewert et al., 2005 ). Of the total, 84 are monitored by at least one seismometer, and only 3 have gas sensors (as of November 2016). 1 Volcanoes are found in the Cascade mountains, Aleutian arc, Hawaii, and the western interior of the continental United States ( Figure 1.4 ). The geographical extent and eruption hazards of these volcanoes are summarized below.

The Cascade volcanoes extend from Lassen Peak in northern California to Mount Meager in British Columbia. The historical record contains only small- to moderate-sized eruptions, but the geologic record reveals much larger eruptions ( Carey et al., 1995 ; Hildreth, 2007 ). Activity tends to be sporadic ( Figure 1.5 ). For example, nine Cascade eruptions occurred in the 1850s, but none occurred between 1915 and 1980, when Mount St. Helens erupted. Consequently, forecasting eruptions in the Cascades is subject to considerable uncertainty. Over the coming decades, there may be multiple eruptions from several volcanoes or no eruptions at all.

The Aleutian arc extends 2,500 km across the North Pacific and comprises more than 130 active and potentially active volcanoes. Although remote, these volcanoes pose a high risk to overflying aircraft that carry more than 30,000 passengers a day, and are monitored by a combination of ground- and space-based sensors. One or two small to moderate explosive eruptions occur in the Aleutians every year, and very large eruptions occur less frequently. For example, the world’s largest eruption of the 20th century occurred approximately 300 miles from Anchorage, in 1912.

In Hawaii, Kilauea has been erupting largely effusively since 1983, but the location and nature of eruptions can vary dramatically, presenting challenges for disaster preparation. The population at risk from large-volume, rapidly moving lava flows on the flanks of the Mauna Loa volcano has grown tremendously in the past few decades ( Dietterich and Cashman, 2014 ), and few island residents are prepared for the even larger magnitude explosive eruptions that are documented in the last 500 years ( Swanson et al., 2014 ).

All western states have potentially active volcanoes, from New Mexico, where lava flows have reached within a few kilometers of the Texas and Oklahoma borders ( Fitton et al., 1991 ), to Montana, which borders the Yellowstone caldera ( Christiansen, 1984 ). These volcanoes range from immense calderas that formed from super-eruptions ( Mastin et al., 2014 ) to small-volume basaltic volcanic fields that erupt lava flows and tephra for a few months to a few decades. Some of these eruptions are monogenic (erupt just once) and pose a special challenge for forecasting. Rates of activity in these distributed volcanic fields are low, with many eruptions during the past few thousand years (e.g., Dunbar, 1999 ; Fenton, 2012 ; Laughlin et al., 1994 ), but none during the past hundred years.

1.3 THE STRUCTURE OF A VOLCANO

Volcanoes often form prominent landforms, with imposing peaks that tower above the surrounding landscape, large depressions (calderas), or volcanic fields with numerous dispersed cinder cones, shield volcanoes, domes, and lava flows. These various landforms reflect the plate tectonic setting, the ways in which those volcanoes erupt, and the number of eruptions. Volcanic landforms change continuously through the interplay between constructive processes such as eruption and intrusion, and modification by tectonics, climate, and erosion. The stratigraphic and structural architecture of volcanoes yields critical information on eruption history and processes that operate within the volcano.

Beneath the volcano lies a magmatic system that in most cases extends through the crust, except during eruption. Depending on the setting, magmas may rise

___________________

1 Personal communication from Charles Mandeville, Program Coordinator, Volcano Hazards Program, U.S. Geological Survey, on November 26, 2016.

directly from the mantle or be staged in one or more storage regions within the crust before erupting. The uppermost part (within 2–3 km of Earth’s surface) often hosts an active hydrothermal system where meteoric groundwater mingles with magmatic volatiles and is heated by deeper magma. Identifying the extent and vigor of hydrothermal activity is important for three reasons: (1) much of the unrest at volcanoes occurs in hydrothermal systems, and understanding the interaction of hydrothermal and magmatic systems is important for forecasting; (2) pressure buildup can cause sudden and potentially deadly phreatic explosions from the hydrothermal system itself (such as on Ontake, Japan, in 2014), which, in turn, can influence the deeper magmatic system; and (3) hydrothermal systems are energy resources and create ore deposits.

Below the hydrothermal system lies a magma reservoir where magma accumulates and evolves prior to eruption. Although traditionally modeled as a fluid-filled cavity, there is growing evidence that magma reservoirs may comprise an interconnected complex of vertical and/or horizontal magma-filled cracks, or a partially molten mush zone, or interleaved lenses of magma and solid material ( Cashman and Giordano, 2014 ). In arc volcanoes, magma chambers are typically located 3–6 km below the surface. The magma chamber is usually connected to the surface via a fluid-filled conduit only during eruptions. In some settings, magma may ascend directly from the mantle without being stored in the crust.

In the broadest sense, long-lived magma reservoirs comprise both eruptible magma (often assumed to contain less than about 50 percent crystals) and an accumulation of crystals that grow along the margins or settle to the bottom of the magma chamber. Physical segregation of dense crystals and metals can cause the floor of the magma chamber to sag, a process balanced by upward migration of more buoyant melt. A long-lived magma chamber can thus become increasingly stratified in composition and density.

The deepest structure beneath volcanoes is less well constrained. Swarms of low-frequency earthquakes at mid- to lower-crustal depths (10–40 km) beneath volcanoes suggest that fluid is periodically transferred into the base of the crust ( Power et al., 2004 ). Tomographic studies reveal that active volcanic systems have deep crustal roots that contain, on average, a small fraction of melt, typically less than 10 percent. The spatial distribution of that melt fraction, particularly how much is concentrated in lenses or in larger magma bodies, is unknown. Erupted samples preserve petrologic and geochemical evidence of deep crystallization, which requires some degree of melt accumulation. Seismic imaging and sparse outcrops suggest that the proportion of unerupted solidified magma relative to the surrounding country rock increases with depth and that the deep roots of volcanoes are much more extensive than their surface expression.

1.4 MONITORING VOLCANOES

Volcano monitoring is critical for hazard forecasts, eruption forecasts, and risk mitigation. However, many volcanoes are not monitored at all, and others are monitored using only a few types of instruments. Some parameters, such as the mass, extent, and trajectory of a volcanic ash cloud, are more effectively measured by satellites. Other parameters, notably low-magnitude earthquakes and volcanic gas emissions that may signal an impending eruption, require ground-based monitoring on or close to the volcanic edifice. This section summarizes existing and emerging technologies for monitoring volcanoes from the ground and from space.

Monitoring Volcanoes on or Near the Ground

Ground-based monitoring provides data on the location and movement of magma. To adequately capture what is happening inside a volcano, it is necessary to obtain a long-term and continuous record, with periods spanning both volcanic quiescence and periods of unrest. High-frequency data sampling and efficient near-real-time relay of information are important, especially when processes within the volcano–magmatic–hydrothermal system are changing rapidly. Many ground-based field campaigns are time intensive and can be hazardous when volcanoes are active. In these situations, telemetry systems permit the safe and continuous collection of data, although the conditions can be harsh and the lifetime of instruments can be limited in these conditions.

Ground-based volcano monitoring falls into four broad categories: seismic, deformation, gas, and thermal monitoring ( Table 1.1 ). Seismic monitoring tools,

TABLE 1.1 Ground-Based Instrumentation for Monitoring Volcanoes

including seismometers and infrasound sensors, are used to detect vibrations caused by breakage of rock and movement of fluids and to assess the evolution of eruptive activity. Ambient seismic noise monitoring can image subsurface reservoirs and document changes in wave speed that may reflect stress. changes. Deformation monitoring tools, including tiltmeters, borehole strainmeters, the Global Navigation Satellite System (GNSS, which includes the Global Positioning System [GPS]), lidar, radar, and gravimeters, are used to detect the motion of magma and other fluids in the subsurface. Some of these tools, such as GNSS and lidar, are also used to detect erupted products, including ash clouds, pyroclastic density currents, and volcanic bombs. Gas monitoring tools, including a range of sensors ( Table 1.1 ), and direct sampling of gases and fluids are used to detect magma intrusions and changes in magma–hydrothermal interactions. Thermal monitoring tools, such as infrared cameras, are used to detect dome growth and lava breakouts. Continuous video or photographic observations are also commonly used and, despite their simplicity, most directly document volcanic activity. Less commonly used monitoring technologies, such as self-potential, electromagnetic techniques, and lightning detection are used to constrain fluid movement and to detect

ash clouds. In addition, unmanned aerial vehicles (e.g., aircraft and drones) are increasingly being used to collect data. Rapid sample collection and analysis is also becoming more common as a monitoring tool at volcano observatories. A schematic of ground-based monitoring techniques is shown in Figure 1.6 .

Monitoring Volcanoes from Space

Satellite-borne sensors and instruments provide synoptic observations during volcanic eruptions when collecting data from the ground is too hazardous or where volcanoes are too remote for regular observation. Repeat-pass data collected over years or decades provide a powerful means for detecting surface changes on active volcanoes. Improvements in instrument sensitivity, data availability, and the computational capacity required to process large volumes of data have led to a dramatic increase in “satellite volcano science.”

Although no satellite-borne sensor currently in orbit has been specifically designed for volcano monitoring, a number of sensors measure volcano-relevant

TABLE 1.2 Satellite-Borne Sensor Suite for Volcano Monitoring

NOTE: AIRS, Atmospheric Infrared Sounder; ALOS, Advanced Land Observing Satellite; ASTER, Advanced Spaceborne Thermal Emission and Reflection Radiometer; AVHRR, Advanced Very High Resolution Radiometer; CALIPSO, Cloud-Aerosol Lidar and Infrared Pathfinder Satellite Observation; COSMO-SkyMed, Constellation of Small Satellites for Mediterranean Basin Observation; GOES, Geostationary Operational Environmental Satellite; IASI, Infrared Atmospheric Sounding Interferometer; MISR, Multi-angle Imaging SpectroRadiometer; MLS, Microwave Limb Sounder; MODIS, Moderate Resolution Imaging Spectroradiometer; OMI, Ozone Monitoring Instrument; OMPS, Ozone Mapping and Profiler Suite; SAGE, Stratospheric Aerosol and Gas Experiment.

parameters, including heat flux, gas and ash emissions, and deformation ( Table 1.2 ). Thermal infrared data are used to detect eruption onset and cessation, calculate lava effusion rates, map lava flows, and estimate ash column heights during explosive eruptions. In some cases, satellites may capture thermal precursors to eruptions, although low-temperature phenomena are challenging to detect. Both high-temporal/low-spatial-resolution (geostationary orbit) and high-spatial/low-temporal-resolution (polar orbit) thermal infrared observations are needed for global volcano monitoring.

Satellite-borne sensors are particularly effective for observing the emission and dispersion of volcanic gas and ash plumes in the atmosphere. Although several volcanic gas species can be detected from space (including SO 2 , BrO, OClO, H 2 S, HCl, and CO; Carn et al., 2016 ), SO 2 is the most readily measured, and it is also responsible for much of the impact of eruptions on climate. Satellite measurements of SO 2 are valuable for detecting eruptions, estimating global volcanic fluxes and recycling of other volatile species, and tracking volcanic clouds that may be hazardous to aviation in near real time. Volcanic ash cloud altitude is most accurately determined by spaceborne lidar, although spatial coverage is limited. Techniques for measuring volcanic CO 2 from space are under development and could lead to earlier detection of preeruptive volcanic degassing.

Interferometric synthetic aperture radar (InSAR) enables global-scale background monitoring of volcano deformation ( Figure 1.7 ). InSAR provides much higher spatial resolution than GPS, but lower accuracy and temporal resolution. However, orbit repeat times will diminish as more InSAR missions are launched, such as the European Space Agency’s recently deployed Sentinel-1 satellite and the NASA–Indian Space Research Organisation synthetic aperture radar mission planned for launch in 2020.

1.5 ERUPTION BEHAVIOR

Eruptions range from violently explosive to gently effusive, from short lived (hours to days) to persistent over decades or centuries, from sustained to intermittent, and from steady to unsteady ( Siebert et al., 2015 ). Eruptions may initiate from processes within the magmatic system ( Section 1.3 ) or be triggered by processes and properties external to the volcano, such as precipitation, landslides, and earthquakes. The eruption behavior of a volcano may change over time. No classification scheme captures this full diversity of behaviors (see Bonadonna et al., 2016 ), but some common schemes to describe the style, magnitude, and intensity of eruptions are summarized below.

Eruption Magnitude and Intensity

The size of eruptions is usually described in terms of total erupted mass (or volume), often referred to as magnitude, and mass eruption rate, often referred to as intensity. Pyle (2015) quantified magnitude and eruption intensity as follows:

magnitude = log 10 (mass, in kg) – 7, and

intensity = log 10 (mass eruption rate, in kg/s) + 3.

The Volcano Explosivity Index (VEI) introduced by Newhall and Self (1982) assigns eruptions to a VEI class based primarily on measures of either magnitude (erupted mass or volume) or intensity (mass eruption rate and/or eruption plume height), with more weight given to magnitude. The VEI classes are summarized in Figure 1.8 . The VEI classification is still in use, despite its many limitations, such as its reliance on only a few types of measurements and its poor fit for small to moderate eruptions (see Bonadonna et al., 2016 ).

Smaller VEI events are relatively common, whereas larger VEI events are exponentially less frequent ( Siebert et al., 2015 ). For example, on average about three VEI 3 eruptions occur each year, whereas there is a 5 percent chance of a VEI 5 eruption and a 0.2 percent chance of a VEI 7 (e.g., Crater Lake, Oregon) event in any year.

Eruption Style

The style of an eruption encompasses factors such as eruption duration and steadiness, magnitude, gas flux, fountain or column height, and involvement of magma and/or external source of water (phreatic and phreatomagmatic eruptions). Eruptions are first divided into effusive (lava producing) and explosive (pyroclast producing) styles, although individual eruptions can be simultaneously effusive and weakly explosive, and can pass rapidly and repeatedly between eruption styles. Explosive eruptions are further subdivided into styles that are sustained on time scales of hours to days and styles that are short lived ( Table 1.3 ).

Classification of eruption style is often qualitative and based on historical accounts of characteristic eruptions from type-volcanoes. However, many type-volcanoes exhibit a range of eruption styles over time (e.g., progressing between Strombolian, Vulcanian, and Plinian behavior; see Fee et al., 2010 ), which has given rise to terms such as subplinian or violent Strombolian.

1.6 ERUPTION HAZARDS

Eruption hazards are diverse ( Figure 1.9 ) and may extend more than thousands of kilometers from an active volcano. From the perspective of risk and impact, it is useful to distinguish between near-source and distal hazards. Near-source hazards are far more unpredictable than distal hazards.

Near-source hazards include those that are airborne, such as tephra fallout, volcanic gases, and volcanic projectiles, and those that are transported laterally on or near the ground surface, such as pyroclastic density currents, lava flows, and lahars. Pyroclastic density currents are hot volcanic flows containing mixtures of gas and micron- to meter-sized volcanic particles. They can travel at velocities exceeding 100 km per hour. The heat combined with the high density of material within these flows obliterates objects in their path, making them the most destructive of volcanic hazards. Lava flows also destroy everything in their path, but usually move slowly enough to allow people to get out of the way. Lahars are mixtures of volcanic debris, sediment, and water that can travel many tens of kilometers along valleys and river channels. They may be triggered during an eruption by interaction between volcanic prod-

TABLE 1.3 Characteristics of Different Eruption Styles

ucts and snow, ice, rain, or groundwater. Lahars can be more devastating than the eruption itself. Ballistic blocks are large projectiles that typically fall within 1–5 km from vents.

The largest eruptions create distal hazards. Explosive eruptions produce plumes that are capable of dispersing ash hundreds to thousands of kilometers from the volcano. The thickness of ash deposited depends on the intensity and duration of the eruption and the wind direction. Airborne ash and ash fall are the most severe distal hazards and are likely to affect many more people than near-source hazards. They cause respiratory problems and roof collapse, and also affect transport networks and infrastructure needed to support emergency response. Volcanic ash is a serious risk to air traffic. Several jets fully loaded with passengers have temporarily lost power on all engines after encountering dilute ash clouds (e.g., Guffanti et al., 2010 ). Large lava flows, such as the 1783 Laki eruption in Iceland, emit volcanic gases that create respiratory problems and acidic rain more than 1,000 km from the eruption. Observed impacts of basaltic eruptions in Hawaii and Iceland include regional volcanic haze (“vog”) and acid rain that affect both agriculture and human health (e.g., Thordarson and Self, 2003 ) and fluorine can contaminate grazing land and water supplies (e.g., Cronin et al., 2003 ). Diffuse degassing of CO 2 can lead to deadly concentrations with fatal consequences such as occurred at Mammoth Lakes, California, or cause lakes to erupt, leading to massive CO 2 releases that suffocate people (e.g., Lake Nyos, Cameroon).

Secondary hazards can be more devastating than the initial eruption. Examples include lahars initiated by storms, earthquakes, landslides, and tsunamis from eruptions or flank collapse; volcanic ash remobilized by wind to affect human health and aviation for extended periods of time; and flooding because rain can no longer infiltrate the ground.

1.7 MODELING VOLCANIC ERUPTIONS

Volcanic processes are governed by the laws of mass, momentum, and energy conservation. It is possible to develop models for magmatic and volcanic phenomena based on these laws, given sufficient information on mechanical and thermodynamic properties of the different components and how they interact with each other. Models are being developed for all processes in volcanic systems, including melt transport in the mantle, the evolution of magma bodies within the crust, the ascent of magmas to the surface, and the fate of magma that erupts effusively or explosively.

A central challenge for developing models is that volcanic eruptions are complex multiphase and multicomponent systems that involve interacting processes over a wide range of length and time scales. For example, during storage and ascent, the composition, temperature, and physical properties of magma and host rocks evolve. Bubbles and crystals nucleate and grow in this magma and, in turn, greatly influence the properties of the magmas and lavas. In explosive eruptions, magma fragmentation creates a hot mixture of gas and particles with a wide range of sizes and densities. Magma also interacts with its surroundings: the deformable rocks that surround the magma chamber and conduit, the potentially volatile groundwater and surface water, a changing landscape over which pyroclastic density currents and lava flows travel, and the atmosphere through which eruption columns rise.

Models for volcanic phenomena that involve a small number of processes and that are relatively amenable to direct observation, such as volcanic plumes, are relatively straightforward to develop and test. In contrast, phenomena that occur underground are more difficult to model because there are more interacting processes. In those cases, direct validation is much more challenging and in many cases impossible. Forecasting ash dispersal using plume models is more straightforward and testable than forecasting the onset, duration, and style of eruption using models that seek to explain geophysical and geochemical precursors. In all cases, however, the use of even imperfect models helps improve the understanding of volcanic systems.

Modeling approaches can be divided into three categories:

• Reduced models make simplifying assumptions about dynamics, heat transfer, and geometry to develop first-order explanations for key properties and processes, such as the velocity of lava flows and pyroclastic density currents, the height of eruption columns, the magma chamber size and depth, the dispersal of tephra, and the ascent of magma in conduits. Well-calibrated or tested reduced models offer a straightforward ap-

proach for combining observations and models in real time in an operational setting (e.g., ash dispersal forecasting for aviation safety). Models may not need to be complex if they capture the most important processes, although simplifications require testing against more comprehensive models and observations.

• Multiphase and multiphysics models improve scientific understanding of complex processes by invoking fewer assumptions and idealizations than reduced models ( Figure 1.10 ), but at the expense of increased complexity and computational demands. They also require additional components, such as a model for how magma in magma chambers and conduits deforms when stressed; a model for turbulence in pyroclastic density currents and plumes; terms that describe the thermal and mechanical exchange among gases, crystals, and particles; and a description of ash aggregation in eruption columns. A central challenge for multiphysics models is integrating small-scale processes with large-scale dynamics. Many of the models used in volcano science build on understanding developed in other science and engineering fields and for other ap-

plications. Multiphysics and multiscale models benefit from rapidly expanding computational capabilities.

• Laboratory experiments simulate processes for which the geometry and physical and thermal processes and properties can be scaled ( Mader et al., 2004 ). Such experiments provide insights on fundamental processes, such as crystal dynamics in flowing magmas, entrainment in eruption columns, propagation of dikes, and sedimentation from pyroclastic density currents ( Figure 1.11 ). Experiments have also been used successfully to develop the subsystem models used in numerical simulations, and to validate computer simulations for known inputs and properties.

The great diversity of existing models reflects to a large extent the many interacting processes that operate in volcanic eruptions and the corresponding simplifying assumptions currently required to construct such models. The challenge in developing models is often highlighted in discrepancies between models and observations of natural systems. Nevertheless, eruption models reveal essential processes governing volcanic eruptions, and they provide a basis for interpreting measurements from prehistoric and active eruptions and for closing observational gaps. Mathematical models offer a guide for what observations will be most useful. They may also be used to make quantitative and testable predictions, supporting forecasting and hazard assessment.

Volcanic eruptions are common, with more than 50 volcanic eruptions in the United States alone in the past 31 years. These eruptions can have devastating economic and social consequences, even at great distances from the volcano. Fortunately many eruptions are preceded by unrest that can be detected using ground, airborne, and spaceborne instruments. Data from these instruments, combined with basic understanding of how volcanoes work, form the basis for forecasting eruptions—where, when, how big, how long, and the consequences.

Accurate forecasts of the likelihood and magnitude of an eruption in a specified timeframe are rooted in a scientific understanding of the processes that govern the storage, ascent, and eruption of magma. Yet our understanding of volcanic systems is incomplete and biased by the limited number of volcanoes and eruption styles observed with advanced instrumentation. Volcanic Eruptions and Their Repose, Unrest, Precursors, and Timing identifies key science questions, research and observation priorities, and approaches for building a volcano science community capable of tackling them. This report presents goals for making major advances in volcano science.

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Plate tectonics, volcanoes and earthquakes.

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The Earth rumbles and a hiss of steam issues from the top of Mt Ruapehu. Are these two events related? Is the earthquake caused by the volcano? Or is the steam caused by the earthquake?

Tectonic plates

When Alfred Wegener first proposed the idea of continental drift (the precursor idea to plate tectonic theory), it didn’t quite explain the full story. While he correctly showed that Africa and South America fitted together, his model wasn’t able to explain the violent forces that occur around the Earth’s crust.

It wasn’t until the 1960s that a full explanation began to develop – the theory of plate tectonics. This theory explained many pieces of the puzzle that scientists had observed, for example, continental fit, matching geology, past glaciation, movements of the ocean floor and the location of fossils of ancient animals and plants.

Scientists now believe that the crust of the Earth consists of rigid interconnecting plates (6 major plates and a few smaller ones). Plates are thought to float on the partially molten mantle, moving away from oceanic ridges where new plate material is produced and moving past each other or colliding along plate boundaries. Earthquakes and volcanoes are related to this movement.

Explore plate tectonics further.

Colliding plates

Where plates come into contact, energy is released. Plates sliding past each other cause friction and heat. Subducting plates melt into the mantle, and diverging plates create new crust material.

Subducting plates, where one tectonic plate is being driven under another, are associated with volcanoes and earthquakes. This activity is focused along the edge of the plate boundary where two plates come into contact, forming regions such as the Pacific Ring of Fire – a chain of earthquake and volcanic activity around the edge of the Pacific Ocean – which generates 75% of the world’s volcanoes and 80% of the world’s earthquakes.

Diverging plates

When plates move away from each other, the space between them gets filled with material, which rises to the surface, cools and forms mid-oceanic ridges. The Pacific Ocean is growing wider by about 18 cm per year as the plates diverge and the mid-oceanic ridge is built up.

Subducting plates and volcanoes

Plate material that is produced along the ocean floor is generally quite dense and relatively heavy. Continental plates don’t tend to get subducted. When oceanic plate is pushed from the mid-ocean ridge towards a plate boundary with a continental plate, it tends to subduct or dive below the continental crust. In this process, water is also being subducted with the oceanic plate. Friction increases the heat along such boundaries, which causes this material to melt and mix the oceanic plate material, the continental plate material and the water.

Andesite volcanoes tend to form at these subduction boundaries. This may have something to do with differences in plate densities and the release of gases, such as water vapour. As more heat is being produced through the subduction process, the mix of more and less volatile ingredients causes changes in density and pressure, which are linked to volcanic activity.

Volcanoes and earthquakes

Volcanoes and earthquakes are often found in the same place, but are they related? Does one trigger the other? The answer seems to be yes – but not always. They are sometimes linked but are often independent events.

When a volcano erupts, the pressure of the rising magma forcing its way through the crust to the surface will often trigger earthquake activity. Scientists have been able to demonstrate this link and also know what type of earthquake to look for.

Conversely, an earthquake may trigger subsequent eruptions. As the crust changes and moves in a major earthquake, fissures or cracks can form that may act as pipelines for magma and future volcanoes. This is harder to monitor and test and is an area of active research.

Related content

Use the teaching resource Earthquakes New Zealand to learn about seven activities designed to help students develop an understanding about earthquakes in New Zealand, including why we get them and how we measure them.

There are many volcanoes in the Pacific, including the very large Hunga Tonga–Hunga Haʻapa underwater one. Read about the devastating Hunga Tonga–Hunga Haʻapa volcanic eruption in January 2022, the tsunami that followed, and what we might expect next? This article looks at what we’ve learnt a year later about this violent eruption and predicting future submarine volcanic eruptions.

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Earthquakes and Volcanoes

Earthquakes and volcanic eruptions are incredible and dramatic natural events. On this page you can learn more about the science behind them:

• Bárðarbunga-Holuhraun Eruption Learn about the 2014 Bárðarbunga eruption and the massive Holuhraun lava field that it created.
• Fascinating Facts Discover interesting facts that you might not know about earthquakes and volcanoes.

Bárðarbunga-Holuhraun Eruption

Iceland sits on crack in the surface of the Earth where two tectonic plates (large blocks that make up the outer most layer of the Earth) are ripping apart from each other. These two blocks are moving apart at around 2 cm per year (approximately the rate that your finger nails grow) and are slowly widening the Atlantic Ocean; increasing the distance from Europe to North America. As the tectonic plates split apart magma (molten rock) wells up from below forming volcanoes, which creates new land in between. This rifting process is happening all the way down the middle of the Atlantic Ocean beneath the sea. But at Iceland we see the process on land because a hot upwelling from deep within the Earth also sits beneath Iceland, pushing it upwards out of the sea and making the area even more volcanically active.

Iceland has over 30 active volcanic systems, which have a central volcano in the middle with long cracks spreading outwards to the north and south (fissure swarms). Magma (molten rock) can erupt from the central volcano itself, or travel through the cracks of the fissure swarm to create long lines of eruptions far from the volcano itself.

The main volcanic rift zones in Iceland, with Bárðarbunga volcano beneath the Vatnajökull ice sheet.

When you think of Icelandic volcanoes, the one you will probably remember is the eruption of the volcano with the long, hard to pronounce name that had news readers getting their tongues in a twist: Eyjafjallajökull, which sits under a glacier in southern Iceland. This volcano erupted in 2010 and was all over the news because it produced so much ash that it grounded 100,000 flights and closed European air space. The main reason why this eruption was so ash heavy was due to the interaction of the hot magma erupting under the cold overlying ice. But a couple of years later in 2014 there was another eruption in Iceland, at a volcano called Bárðarbunga. It was the largest eruption in Iceland in over 200 years, and was 10 times bigger than than the eruption of Eyjafjallajökull, but it didn't ground a single flight.

The eruption of Eyjafjallajökull in 2010 created a large ash plume that blew across western Europe .

Bárðarbunga volcano is right in the middle of Iceland in the remote and uninhabited highlands. It sits underneath the massive Vatnajökull glacier, covered by around 800 m of thick ice. Scientists first knew something interesting was happening at Bárðarbunga in August of 2014 when hundreds of tiny earthquakes were detected within the volcano deep below the ice. Teams rushed out to deploy more seismic instruments to record the earthquakes and determine where they were happening more accurately. Scientists were flown up onto the glacier in helicopters, before speeding around on snow scooters to get the essential instruments out as quickly as possible.

Bárðarbunga volcano beneath the Vatnajökull ice sheet.

Over the next two weeks the country watched closely as the 30,000 tiny earthquakes tracked the underground movement of the magma as it cracked a 46 km long path through the earth’s crust. The earthquake activity moved gradually northwards, away from the volcano at 6 km down within the earth, as authorities and scientists tried to guess where and when the magma would move upwards and erupt at the surface. Would the eruption happen under the glacier producing an ash heavy eruption like Eyjafjallajökull, or would it travel far enough away from the glacier that it wouldn't interact with the ice? Or maybe it wouldn't erupt at all, and the molten rock would stay deep within the earth, cooling and solidifying to form an igneous intrusion? The magma movement was also tracked using GPS and satellite measurements. The GPS were able to measure ground movement caused by the magma pushing into the earth and shoving material outwards to either side by up to 1.5 m. This helped scientists to understand the shape that the magma was flowing in—a vertical sheet of molten rock known as a dyke, about 5 m wide.

3-D visualisation of earthquake epicentres through time as the magmatic dyke propagated from the Bárðarbunga central volcano to the Holuhraun eruption site.

The dyke advanced in rapid bursts with an average speed of 2 km per hour, reaching speeds of up to 4.7 km per hour. In the end the dyke travelled 46 km away from the feeder volcano Bárðarbunga before erupting at the surface on a river flood plain away from the glacier. The eruption, which was named Holuhraun, saw runny fluid lava thrown up to 200 m into the air (higher than Big Ben!), in pulsating fire fountains, before flowing away as rivers of molten rock. There was no central eruption point, instead the eruption happened all the way down the length of a 1.5 km long eruptive fissure that over time concentrated down to a few craters, and finally to one main crater called Baugur. The amount of thermal energy the eruption produced was tremendous; equivalent to the energy of a Hiroshima sized atomic bomb being set off every 2 minutes!

The Bárðarbunga-Holuhraun eruption in action, with a lava flow extending across the sandur and volcanic gases issuing from the volcanic vent.

Footage of the 2014 Bárðarbunga-Holuhraun eruption.

The eruption lasted for 6 months, eventually stopping in February 2015. By then it had produced 1.6 km 3 of lava, covering an area of 85 km 2 – large enough to cover the city of Cambridge twice ( how much of your area would it have covered? ). Because there was no interaction with ice, only a minute amount of ash was produced. However it did release around 11 M tones of SO 2 – more than the whole of Europe produces in a year! This was the main hazard of the eruption as the gas was blown around on the wind affecting large numbers of the population. Ground level concentrations of SO 2 exceeded health limits over most of Iceland for days to weeks, with the high concentrations of SO 2 can cause breathing difficulties and eye irritation.

The extent of the Holuhraun lava flow in red.

It's very important to carefully monitor volcanoes to try and understand what kind of eruption they might have and what hazards that could present. The Holuhraun eruption was not hazardous to planes since it was an effusive eruption and it didn’t create a huge eruption column, but if the eruption had happened under the ice it would have been a completely different story. Measuring the tiny earthquakes that happen in volcanoes, gives us a powerful tool we can use to see inside the earth and track where molten rock is moving beneath the surface. Keeping track of these observations can help inform public bodies, aiding their decision making and their planning of hazard mitigation strategies.

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Fascinating Facts

Iceland facts.

• The population of Iceland is 330,000 , or approximately the population of Reading.
• Two-thirds of the Icelandic population live in the capital city, Reykjavik .
• More than one million people visit Iceland each year on holiday, about three times its population.
• All of Iceland's heating and electricity is renewable, either from hydroelectric or geothermal methods.
• The UK went to war with Iceland once over fish!
• Good morning in Icelandic is ‘Góðan dag’ .
• An earthquake swarm precedes eruption of the Icelandic volcano Hekla by 30 minutes, which is unhelpful if you are on the volcano, but enough time to get the news helicopters in the air from Reykjavik!
• There are about 130 volcanoes in Iceland.

Bárðarbunga- Holuhraun eruption facts

• The Holuhraun lava flow covers an area of 84 km 2 .
• During eruption, lava fountains sprayed lava up to 150 m high , which is taller than Big Ben!
• The eruption released vast amounts of thermal energy, equivalent to: releasing a Hiroshima atomic bomb every minute (in the first month), and every two minutes (on average over the six-month-long eruption).
• The Holuhraun lava flow released 100 million tonnes of sulphur dioxide, which is equivalent to the annual emissions of Europe.
• More than 30,000  small earthquakes were detected accompanying the magma intrusion.
• The peak rate of magma release was around  500 tonnes per second .
• Members of the Volcano Seismology Group from the University of Cambridge were the first people to see the eruption with their own eyes. They tracked the eruption in collaboration with the Icelandic Met Office and the University of Iceland during the first weeks of the eruption before they had to (reluctantly) go home.
• Instruments were placed so close to the eruption site that they had to be rescued from the advancing lava.
• The intrusion, and subsequent lava flow, was monitored in real-time by more than 100 seismometers, more than 30 GPS instruments, three satellites, and four webcams.

Eyjafjallajökull eruption facts

• The Eyjafjallajökull eruption ejected ash 35,000 feet into the air.
• The Eyjafjallajökull ash cloud grounded over 100,000 flights, which cost the aviation industry an estimated   £1.1 billion .
• To volcano’s name is pronounced: "EY - A - FYAT - LA - YO - KUTL" .
• The eruption caused several glacial floods (jökulhaups) when hot magma melted ice in the overlying glacier.
• Small earthquakes were detected by the Cambridge Volcano Seismology Group down to a depth of 30 km below the surface, as molten rock stored at depth migrated during the eruption.

Earthquake and seismology facts

• The magnitude scale of earthquakes is logarithmic, so every step up the scale is an earthquake ten times bigger. A magnitude 2.0 event is, therefore, ten million times smaller than the 2004 boxing day earthquake (magnitude 9.0).
• Humans have only ever dug to a depth of 12 km; less than 1% of the Earth's total thickness. The only reason we know anything about the internal structure of the Earth is because earthquake waves travel through it, and become modified by the material that they travel through.
• The United States Geological Survey (USGS) estimates that several million earthquakes occur in the Earth each year. Many go undetected because they hit remote areas or have very small magnitudes. The NEIC (National Earthquake Information Centre) now locates about 50 earthquakes each day (larger than magnitude 4.5), or around 20,000 a year.
• We get earthquakes in the UK, a few every couple of days. However, they are just very small, generally less than magnitude 2, which means that they are just too small for people to feel.
• The largest earthquake to ever happen in the UK was a magnitude 6.1 off the east coast of England in the North Sea.
• The most damaging earthquake in the UK in recent history was in 1884 near Colchester.  We don’t know exactly how large it was (probably around magnitude 4.5), but historical records show that some 1200 buildings required repairs, and three to five people were killed.

Volcano facts

• There are about 1500 active volcanoes in the world.
• 300 million, or one-in-twenty, people in the world live within the 'danger range' of an active volcano.
• About 1,500 different volcanoes have erupted over the past 10,000 years, but only about 60 erupt each year. On any given day, there are about 20 volcanoes erupting somewhere in the world.
• Many volcanoes are underwater and erupt onto the sea floor.

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Related Environment essays

Earthquakes are some of the most destructive natural disasters. An earthquake consists of rapid vibration of the earth surface. Since its occurrence and enormous capacity is unpredictable by the ordinary human beings, it has been creating fear in human beings since the ancient times. A single shock would last for a few seconds, but when a series of smaller quakes occurs, it could last for about five minutes. It occurs quite frequently across the world but most of the times earthquakes are not very strong to be witnessed by people. However, huge earthquakes have devastating effects. This essay seeks to illustrate the causes and effects of earthquakes.

There are two types of earthquakes: tectonic and volcanic. Tectonic earthquakes occur when the earth is subjected to immense strain making it to eventually move. The earth crust comprises of several plates which float on the mantle. Considering that the plates can move, they can drift apart, towards each other, or slide against each other causing a subduction zone. According to Abrams & Morton (2007), convergent boundaries occur when too much pressure is generated between two plates over a long time. With time, one of the plates bends under the other due to extreme force. As much as the movement is too slow, pressure eventually builds up in the rocks. Eventually an earthquake happens when the pressure can no longer be held by the rocks. The fault ruptures causing the plates to move a long distance in a very short time. The collision triggers large forces in the plates resulting to the occurrence of earthquakes. The best example of a plate tectonic quake is that which occurred in the San Andreas Fault. In this case, the North American plate and the Pacific plate were moving towards the same direction but one was moving faster than the other.

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On the other hand, the volcanic earthquakes are caused by the explosive volcanic eruption. This could either occur at the sea bed or on the land. There are many faults across the world. The location of such faults is a major determinant of where an earthquake will occur because they are the main causes of earthquakes. The type of the fault also determines how often earthquakes occur (Abrams & Morton, 2007). When the earth crust is submitted to tensional forces, it becomes thinner and weak. It causes a hot spot in the mantle which leads the magma to produce pressure and penetrate into the lithosphere and eventually erupt. Earthquakes are triggered by the forceful movement of magma as it finds its way onto the earth surface.

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Earthquakes have various effects including damage to infrastructure, change in geological features and impact on the living things in the areas they occur. Rae (2008) asserts that the rumbling of the earth usually shakes tall buildings, dams and bridges making them fall down. These structures could fall on human beings and animals and kill them or affect the transport system. It becomes very difficult to transport the food, water and health aid to the affected people when bridges are destroyed.

They can also trigger geomorphologic changes. For instance, they could cause the earth crust to move either horizontally or vertically. This could cause a rising, tilting, or dropping of the earth surface and eventually causing landslides or floods. They could also damage gas and power lines and cause fires (Rae, 2008). Additionally, earthquakes that occur in the sea can cause tidal waves or tsunamis: Long and high walls of water travelling at a very high speed. Tsunamis can destroy an entire city or population living next to the coastline. They can also lead to various health effects such as the spread of waterborne diseases that end up killing human beings and animals.

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How are volcanoes and earthquakes related.

Some, but not all, earthquakes are related to volcanoes. For example, most earthquakes are along the edges of tectonic plates. This is where most volcanoes are too. However, most earthquakes are caused by the interaction of the plates not the movement of magma.

Most earthquakes directly beneath a volcano are caused by the movement of magma. The magma exerts pressure on the rocks until it cracks the rock. Then the magma squirts into the crack and starts building pressure again. Every time the rock cracks it makes a small earthquake. These earthquakes are usually too weak to be felt but can be detected and recorded by sensitive instruments. Once the plumbing system of the volcano is open and magma is flowing through it, constant earthquake waves, called harmonic tremor, are recorded (but not felt).

The distribution of earthquakes provides information about magma pathways and the structure of volcanoes. The red dots show earthquakes associated with magma movement. They define the east and southwest rifts of Kilauea. The blue dots show earthquakes associated the sliding of the south flank of Kilauea. Photograph courtesy of U.S. Geological Survey.

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Essay on earthquakes: top 5 essays on earthquakes | geography.

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Here is a compilation of essays on ‘Earthquakes’ for class 6, 7, 8, 9, 10, 11 and 12. Find paragraphs, long and short essays on ‘Earthquakes’ especially written for school and college students.

Essay on Earthquakes

Essay Contents:

• Essay on the World Distribution of Earthquakes

Essay # 1. Introduction to Earthquake:

An earthquake is a major demonstration of the power of the tectonic forces caused by endogenetic thermal conditions of the interior of the earth. ‘An earthquake is a motion of the ground surface, ranging from a faint tremor to a wild motion capable of shaking buildings apart and causing gaping fissures to open in the ground.

The earthquake is a form of energy of wave motion transmitted through the surface layer of the earth in widening circles from a point of sudden energy release, the focus’. ‘An earthquake is a vibration or oscillation of the surface of the earth caused by a transient distur­bance of the elastic or gravitational equilibrium of the rocks at or beneath the earth the surface.’

The magnitude or intensity of energy released by an earthquake is measured by the Richter Scale devised by Charles F. Richter in 1935. The number indicating magnitude or intensity (M) on Richter scale ranges between 0 and 9 but in fact the scale has no upper limit of number because it is a logarithmic scale.

It is estimated that the total annual energy released by all earthquakes is about 10 25 ergs, most of this is from a small number of earthquakes of magnitude over 7. The 1934 Bihar earthquake measuring 8.4 and Good Friday Earthquake of March 27, 1964 in Alaska (USA) meas­uring 8.4 to 8.6 on Richter scale are among the greatest earthquakes of the world ever recorded.

The place of the origin of an earthquake is called focus which is always hidden inside the earth but its depth varies from place to place. The deepest earth­quake may have its focus at a depth of even 700 km below the ground surface but some of the major Himalayan earthquakes, such as the Bihar-Nepal earth­quake of August 21, 1988, have their focus around 20- 30 km deep.

The place on the ground surface, which is perpendicular to the buried ‘focus’ or ‘hypocentre’, recording the seismic waves for the first time is called epicentre. The waves generated by an earthquake are called ‘seismic waves’ which are recorded by an in­strument called seismograph or seismometer at the epicentre. The science, that deals with the seismic waves, is called seismology.

Essay # 2. Causes of Earthquakes :

Earthquakes are caused mainly due to disequi­librium in any part of the crust of the earth. A number of causes have been assigned to cause disequilibrium or isostatic imbalance in the earth’s crust such as volcanic eruptions, faulting and folding, up-warping and down-warping, gaseous expansion and contraction inside the earth, hydrostatic pressure of man-made water bodies like reservoirs and lakes, and plate move­ments.

If we look at the world distribution of earth­quakes (fig. 10.2) it appears that the earthquake belts are closely associated with the weaker zones and isostatically disturbed areas of the globe. It was gener­ally believed that isostatically balanced and old and stable rigid masses were free from seismic events but the devastating earthquake of Koyna on 11 December, 1967, in Satara district of Maharashtra, Latur-Kilari earthquake of Sept. 30, 1993 of Maharashtra, dis­ proved this old connotation and made us believe that no part of the earth is immune from seismic events. A host of possible causes have been suggested to cause disequilibrium in the earth’s crust which trigger earth tremors of various sorts.

i. Vulcanicity:

Volcanic activity is considered to be one of the major causes of earthquakes. In fact, vulcanicity and seismic events are so intimately related to each other that they become cause and effect for each other. In other words, each volcanic eruption is followed by earthquakes and many of the severe earthquakes cause volcanic eruptions.

In fact, earth tremors are major precursor events of possible volcanic eruption in im­mediate future in any region. The explosive violent gases during the process of vulcanicity try to escape upward and hence they push the crustal surface from below with great force and thus is caused severe earth tremor of high magnitude.

Whenever these gases be­come successful in breaking the weak crustal surface they appear on the earth’s surface with violent explo­sion and great force causing devastating volcanic erup­tion which causes sudden disequilibrium in the crustal surface to invite severe earth tremors. It may be pointed out that the magnitude of such earthquakes depends upon the intensity of volcanic eruptions.

The violent eruption of Krakatoa volcano (between Java and Sumatra) caused such a severe earthquake the impact of which was experienced as far away as Cape Horn (some 12,800 km away). The devastating earth­quake generated 30 to 40 m high tsunamis waves which killed 36,000 people in the coastal areas of Java and Sumatra.

ii. Faulting and Elastic Rebound Theory :

The horizontal and vertical movements caused by endogenetic forces result in the formation of faults and folds which in turn cause isostatic disequilibrium in the crustal rocks which ultimately causes earth­quakes of varying magnitudes depending on the nature and magnitude of dislocation of rock blocks caused by faulting and folding. In fact, sudden dislocation of rock blocks caused by both tensile and compressive forces triggers immediate earth tremors due to sudden maladjustment of rock blocks.

The 1950-earthquake of Assam was believed to have been caused due to dis­equilibrium in crustal rocks introduced by crustal frac­ture. The 1934-earthquake of Bihar was also consid­ered to have been triggered by faulting activity under­neath. Underground active fault zone was suggested as one of the possible causes of Koyna earthquake (Maharashtra) of December 11, 1967.

The occurrence of severe devastating earth­quake of San Francisco (USA) in 1906 led H.F. Reid, one of the official investigators of the San Fransisco earthquake disaster, to advance his famous and much appreciated elastic rebound theory to explain the mode and causes of earthquakes mainly caused by fractures and faults in the earth’s crust and upper mantle.

Ac­cording to Reid the underground rocks are elastic like rubber and expand when stretched and pulled. The stretching and pulling of crustal rocks due to tensile forces is slow process. The rocks continue to be stretched so long as the tensile forces do not exceed the elasticity of the rocks but as the tensile forces exceed the rocks elasticity, they are broken and the broken rock blocks try immediately to occupy their previous positions so that they may adjust themselves. All these processes occur so rapidly that the equilibrium of the concerned crustal surface is suddenly disturbed and hence earth tremors are caused.

Reid’s elastic rebound theory very well ex­plains the occurrences of seismic events in Californian valley which is very much frequented by faulting activity. The famous earthquake of 1872 of California was caused due to creation of a massive fault in the Oven Valley. Similarly, the Californian earthquake of April 18, 1906, was caused due to the formation of 640 km long San Andreas Fault. The 1923 earthquake of Sagami Bay of Japan was also believed to have been triggered by big fault.

N. Krishna Brahman and Janardhan G. Niyogi, the two scientists of the National Geophysical Re­search Institute, have opined that the seismic events near Bhatsa Dam and Koyna Dam are very much active due to active faulting beneath the Deccan Traps. They have claimed to have identified two active rift faults in Maharashtra beneath the Deccan Traps viz. Kurduvadi rift and Koyna rift.

According to them Koyna rift begins from Kaladgi in Karnataka and runs for a distance of 540 km through Koyna and terminates 40 km west of Nasik. The 390 km long Kurduvadi rift begins from 40 km south-west of Solapur and after running through Kurduvadi it merges with the Koyna rift to the north of Pune. According to them Bhatsa Dam is located at the junction of Tawi and Koyna faults.

They are of the opinion that gradual increase in the seismic events in Bhatsa Dam area since 1983 is because of active faulting beneath the basaltic crust. The 1950 Assam earthquake, 1934 Bihar earthquake and 2001 Bhuj earthquake (Gujarat) of India were caused mainly by faulting.

iii. Hydrostatic Pressure and Anthropogenic Causes :

Though the earthquakes are natural phenomena and are caused by the endogenetic forces coming from within the earth but certain human activities such as pumping of groundwater and oil, deep underground mining, blasting of rocks by dynamites for construc­tional purposes (e.g., for the construction of dams and reservoirs, roads etc.), nuclear explosion, storage of huge volume of water in big reservoirs etc. also cause earth tremors of serious consequences.

The introduc­tion of additional artificial superincumbent load through the construction of large dams and impounding of enormous volume of water in big reservoirs behind the dams cause disequilibrium of already isostatically ad­justed rocks below the reservoirs or further augment the already fragile structures due to faults and fractures underneath.

Many major seismic events have been cor­related with dams and reservoirs all over the world such as earthquake of 1931 in Greece due to Marathon Dam constructed in 1929; initiation of earth tremors since 1936 around Hoover Dam (USA) due to creation of Mead Lake in 1935; Koyna earthquake of 1967 (in Satara district of Maharashtra) due to Koyna reservoir constructed in 1962; other examples of earthquakes caused by dams and reservoirs are of Monteynard and Grandvale in France, Mangla in Pakistan, Kariba in Zambia, Manic in Canada, Hendrick Verwoerd in South Africa, Nourek in earst-while USSR, Kurobe in Japan etc.

It may be pointed out that the intensity of earthquake has been positively correlated with the levels of water in the reservoirs. The earthquakes caused by hydrostatic pressure of reservoirs are called ‘reservoir-induced earthquakes’.

iv. Plate Tectonic Theory :

Recently, plate tectonic theory has been ac­cepted as the most plausible explanation of the causes of earthquakes. As per theory of the plate tectonics the crust or the earth is composed of solid and moving plates having either continental crust or oceanic crust or even both continental-oceanic crust.

The earth’s crust consists of 6 major plates (Eurasian plate, Ameri­can plate, African plate, Indian plate, Pacific plate and Antarctic plate) and 20 minor plates. These plates are constantly moving in relation to each Other due to thermal convective currents originating deep within the earth.

Thus, all the tectonic events take place along the boundaries of these moving plates. From the stand point of movement and tectonic events and creation and destruction of geomaterials the plate boundaries are divided into:

(i) Constructive plate boundaries,

(ii) Destructive plate boundaries, and

(iii) Conservative plate boundaries.

Constructive plate boundaries repre­sent the trailing ends of divergent plates which move in opposite directions from the mid-oceanic ridges, de­structive plate boundaries are those where two conver­gent plates collide against each other and the heavier plate boundary is sub-ducted below the relatively lighter plate boundary and conservative plate boundaries are those where two plates slip past each other without any collision.

Major tectonic events associated with these plate boundaries are ruptures and faults along the constructive plate boundaries, faulting and folding along the destructive plate boundaries and transform faults along the conservative plate boundaries. All sorts of disequilibrium are caused due to different types of plate motions and consequently earthquakes of varying magnitudes are caused.

Normally, moderate earthquakes are caused along the constructive plate boundaries because the rate of rupture of the crust and consequent movement of plates away from the mid-oceanic ridges is rather slow and the rate of upwelling of lavas due to fissure flow is also slow. Consequently, shallow focus earthquakes are caused along the constructive plate boundaries or say along the mid-oceanic ridges.

The depth of ‘focus’ of earthquakes associated with the constructive plate boundaries ranges between 25 km to 35 km but a few earthquakes have also been found to have occurred at the depth of 60 km. It is, thus, obvious that the earth­quakes occurring along the mid-Atlantic Ridge, mid- Indian Oceanic Ridge and East Pacific Rise are caused because of movement of plates in opposite directions (divergence) and consequent formation of faults and ruptures and upwelling of magma or fissure flow of basaltic lavas (fig. 10.1).

Earthquakes of high magni­tude and deep focus are caused along the convergent or destructive plate boundaries because of collision of two convergent plates and consequent subduction of one plate boundary along the Benioff zone. Here mountain building, faulting and violent volcanic erup­tions (central explosive type of eruptions) cause severe and disastrous earthquakes having the focus at the depth up to 700 km.

This process, convergence of plates and related plate collision, explains the maxi­mum occurrence of earthquakes of varying magnitudes along the Fire Ring of the Pacific or the Circum-Pacific Belt (along the western and eastern margins of the Pacific Ocean or say along the western coastal margins of North and South Americas and thus the Rockies to Andes Mountain Belt and along the eastern coastal margins of Asia and island arcs and festoons parallel to the Asiatic coast).

The earthquakes of the Mid-Conti- nental Belt along the Alpine-Himalayan chains are caused due to collision of Eurasian plates and African and Indian plates. The earthquakes of the western marginal areas of North and South Americas are caused because of subduction of Pacific plate beneath the American plate and the resultant tectonic forces whereas the earthquakes of the eastern margins of Asia are originated because of the subduction of Pacific plate under Asiatic plate.

Similarly, the subduction of Afri­can plate below European plate and the subduction of Indian plate under Asiastic plate cause earthquakes of the mid-continental belt. The severe earthquake of Bhuj of Jan. 26, 2001 (Gujarat, India) was caused due to reactivated subsurface faults due to subduction of Indian plate below Asiatic plate.

Creation of transform faults along the conserva­tive plate boundaries explains the occurrence of severe earthquakes of California (USA). Here one part of California moves north-eastward while the other part moves south-westward along the fault plane and thus is formed transform fault which causes earthquakes.

Essay # 3. Classification of Earthquakes :

It has become apparent after the discussion of the causes of seismic events that there is wide range of variation in the nature and magnitude of earthquakes. Each earthquake differs from the other and thus it becomes difficult to classify all the earthquakes into certain categories.

Inspite of these limitations earth­quakes are classified on the basis of common charac­teristics as given below.

i. Classification on the basis of Causative Factors :

(A) Natural earthquakes are those which are caused by natural processes i.e., due to endogenetic forces.

These are further divided into four subcategories:

(i) Volcanic earthquakes are caused due to vol­canic eruptions of explosive and fissure types. Gener­ally, volcanic earthquakes are confined to volcanic areas. The intensity and magnitude of such earth­quakes depend on the intensity and magnitude of volcanic eruptions. Examples, severe earthquakes caused by violent explosions of Krakatao volcano in 1883 and Etna volcano in 1968.

(ii) Tectonic earthquakes are caused due to dis­location of rock blocks during faulting activity. Such earthquakes are very severe and disastrous. Examples, 1872 earthquake and 1906 earthquake of California (USA), 1923 earthquake of Sagami Bay (Japan), 2001 earthquake of Gujarat etc.

(iii) Isostatic earthquakes are triggered due to sudden disturbance in the isostatic balance at regional scale due to imbalance in the geological processes. Generally, the earthquakes of active zones of mountain building are included in this cat­egory.

(iv) Plutonic earthquakes are infact deep-focus earthquakes which occur at greater depths. The centres (foci) of these earthquakes are generally located within the depths ranging from 240 km to 670 km.

(B) Artificial or man-induced earthquakes or anthropogenic earthquakes are caused by human ac­tivities such as pumping of water and mineral oil from underground aquifers and oil reserves respectively, deep underground mining, blasting of rocks by dyna­mites for constructional purposes (e.g., for the con­struction of dams and reservoirs, roads etc.), nuclear explosion, storage of huge volume of water in big reservoirs etc.

Examples, 1931 earthquake of Greece due to Marathon Dam, 1936 earthquake of Hoover Dam (USA) due to Lake Mead, Koyna earthquake (Maharashtra, India) of 1967 due to Koyna reservoir etc.

ii. Classification on the basis of Focus :

Guttenberg has divided the world seismic cen­tres on the basis of the depths of their foci into 3 types viz.:

(i) Moderate earthquakes—foci are located at the depths from the ground surface (0 km) to 50 km,

(ii) Intermediate earthquakes-seismic foci at the depths between 50 km and 250 km and

(iii) Deep focus earthquakes-seismic foci at the depths between 250 km and 700 km. Moderate and intermediate earthquakes are also called as shallow focus and intermediate focus earthquakes respectively.

iii. Classification on the basis of Human Casualties:

Earthquakes are grouped into 3 categories on the basis of their hazardous impacts in terms of human casualties:

(i) Moderately hazardous earthquakes- When human deaths caused by severe seismic tremors are below 50,000 mark. Examples, Kamakura earth­quake of Japan of 1293 A.D. (22,000 deaths), Tabas earthquake of Iran of 1978 A.D. (25,000 deaths), Armenian earthquake of erstwhile USSR of 1988 (26,000 deaths), Lisbon earthquake of Portugal in 1531 A.D. (30,000 deaths), Chile earthquake of 1939 A.D. (40,000 deaths), Quito earthquake of Ecudador in 1797 A.D. (41,000 deaths), Calabria earthquakes of Italy in 1783 A.D. (50,000 deaths), North Iranian earthquake of 1990 A.D. (50,000 deaths) etc.

(ii) Highly hazardous earthquakes causing human deaths ranging between 51,000 and 1,00,000 occurred in 1268 (in Silicia, Asia Minor, death toll, 60,000), in 1667 (in Shemaka, Caucasia, death toll 60,000), in 1693 (Catania, Italy, 93,000 deaths), in 1693 (Naples, Italy, 93,000 deaths), in 1932 (Kansu, China, human deaths, 70,000), in 1935 (Quetta, Baluchistan, death toll, 60,000), in 1970 (Chimbote, Peru, 67,000 deaths), in 2001 (Bhuj, Gujarat, 50,000-1,00,000 death) etc.

(iii) Most hazardous earthquakes causing human casualitis above 1,00,000 mark occurred in the year 1290 (in Chihli, China, 1,00,000 deaths), in 1556 (in Shen-Shu, China, 8,30,000 deaths), in 1737 (Kolkata, India, 3,00,000 deaths), in 1908 (in Messina, Italy, 1,60,000 deaths), in 1920 (in Kansu, China 1,80,000 deaths), in 1923 (in Tokyo, Japan, 1,63,000 deaths), in 1967 (in Tang-Shan, China 7,50,000) deaths etc.

Essay # 4. Hazardous Effects of Earthquakes:

It may be pointed out that the intensity of earth­quakes and their hazardous impacts are determined not on the basis of the magnitude of seismic intensity as determined by Richter scale but are decided on the basis of quantum of damages done by a specific earth­quake to human lives and property.

An earthquake becomes hazard and desaster only when it strikes the populated area. The direct and indirect disastrous ef­fects of earthquakes include deformation of ground surfaces, damage and destruction of human structures such as buildings, rails, roads, bridges, dams, factories, destruction of towns and cities, loss of human and animal lives and property, violent devastating fires, landslides, floods, disturbances in groundwater condi­tions etc.

i. Slope Instability and Failures and Landslides:

The shocks produced by earthquakes particularly in those hilly and mountainous areas which are composed of weaker lithologies and are tectonically sensitive and weak cause slope instability and slope failure and ultimately cause landslides and debris falls which damage settlements and transport systems on the lower slope segments.

The shocks generated by Peruvian earthquake of May, 1970 triggered off the collapse of ice caps seated on the peak of high mountain called Huascaran of 6654 m height near the town of Yungay in Peru.

The huge masses of falling ice dislodged thousands of tonnes of rock mass from the said moun­tain and thus was generated a gigantic debris flow down the slope of Huascaran mountain travelling at the speed of 320 km per hour. The enormous mass of debris flow covered a distance of 15 km within few minutes and buried many buildings and human struc­tures of Yungay town and killed about 25,000 people.

ii. Damage to Human Structures:

Earthquakes inflict great damage to human structures such as build­ings, roads, rails, factories, dams, bridges, and thus cause heavy loss of human property. It may be pointed out that in the ground surface composed of unconsolidated geomaterials, such as alluvium, colluvium, artificially infilled and levelled depres­sions, swamp deposits reclaimed through the dumping of coarse sands and city garbages the vibrations of earthquakes last longer and the amplitudes of seismic waves are greater than in the structures of consolidated materials, and bedrocks. Thus, the earthquakes cause more damages in the areas of unconsolidated ground than their counterparts in the regions of solid structures and bedrocks.

Two major earthquakes of Bihar-Nepal border in 1934 and 1988 can explain the impact of earthquake disasters on human structures and human lives. The damage caused by the Bihar earthquake of 15 January, 1934, measuring 8.4 on Richter scale, include 10,700 human deaths, landslides and slumping in an area of 250 km length and 60 km width, ruptures and faults in the ground surface etc. which caused irreparable dam­age to human structures.

The Darbhanga (Bihar) earth­quake of 21 August, 1988 measuring only 6.5 magni­tude on Richter scale (1000 times smaller than the great earthquake of 1934 in intensity) damaged 25,000 houses due to unconsolidated Gangetic alluvium which in fact acted as a seismic amplifier. The disastrous earthquake of Mexico city of 1985 (September) caused total collapse of 400 buildings, damage to 6,000 build­ings and moderate damage to 50,000 buildings.

Be­sides, the infrastructures of the city were seriously damaged, for example, water pipes were broken, tel­ecommunication lines and systems were severely dam­aged, power and water supplies were disrupted, inner vehicular transport was halted etc.

The severe earth­quake of 9 February, 1971 in the San Fernando valley, located to the north-west of Los Angeles (USA) caused total collapse of Olive New Hospital in Sylmar. This damage shocked everybody because this building was constructed in conformity with the earthquake resist­ance standards. Uttar Kashi (Uttaranchal) earthquake of 1991 and Latur-Kilari quake (Maharashtra) of 1993 (India) flattened many buildings.

iii. Damages to the Towns and Cities:

Earthquakes have their worst effects on towns and cities because of highest density of buildings and large agglomerations of human populations. The earth tremors of higher magnitudes shake the ground to such an extent that large buildings collapse and men and women are hurried under large debris and rubbles of collapsed structural materials of buildings, ground water pipes are bent and damaged and thus water supply is totally disrupted, electric poles are uprooted and electric and telephone wires and cables are heavily damaged caus­ing total disruption of electric supply, obstruction and destruction of sewer systems causes epidemics, road blocks throw the transport systems out of gear etc.

Kolkata city was severely damaged due to se­vere earthquake of 11 October, 1737 as thousands of buildings were severely damaged and 3,00,000 people were killed. The sad tale of the destruction of Mexico city due to the earthquake of 1985 has already been described. Recent Bhuj earthquake of Gujarat (Jan. 26, 2001) flattered towns of Anjar and Bhuj destroying more than 90 percent buildings.

iv. Loss of Human Lives and Property:

It may be pointed out that it is not the intensity (magnitude of Richter scale) of earthquake alone which matters more as regards the human casualities but it is the density of human population and houses which matter more in terms of human deaths and loss of property.

For exam­ple, the Kangra earthquake of India in 1905 recorded 8.6 magnitude on Richter scale but it could cause deaths of only 20,000 people whereas 1976 Tang-Shan earthquake of China measuring 7.8 to 8.1 on Richter scale killed 7,50,000 people.

More than 40,000 people lost their lives in the devastating earthquake of Turkey (August 17,1999) which recorded 7.4 on Richter scale. The loss of human lives caused by earthquakes has been enumerated in the preceding section on the clas­sification of earthquakes based on human casualities (see also tables 10.1, 10.2, 10.3).

The strong vibrations caused by se­vere earthquakes strongly shake the buildings and thus strong oscillations cause severe fires in houses, mines and factories because of overturning of cooking gas cylinders, contact of live electric wires, churning of blast furnaces, displacement of other electric and fire- related appliances. For example, the house wives were cooking their lunches in the kitchens when disastrous killer earthquake struck in the vicinity of Tokyo and Sagami Bay in 1923.

Consequently, severe fire broke out which claimed the lives of 38,000 people out of total fatalities of 1,63,000 caused by the earthquake through various processes. This earthquake resulted into total loss of property worth 2,500 million US dollars. The severe earthquake of San Fransisco (USA), which occurred on April 18, 1906, caused widespread fires in several parts of the city.

No water could be made available immediately to extinguish the fire because water pipes were also broken and displaced by the earthquake. Two biggest oil refineries of Turkey were completely devastated due to fire caused by the killer earthquake of August 17, 1999 (7.4).

vi. Deformation of Ground Surface:

Severe earth tremors and resultant vibrations caused by severe earth­quakes result in the deformation of ground surface because of rise and subsidence of ground surface and faulting activity. For example, the Alaska (USA) earth­quake of 1964 caused displacement of ground surface upto 10-15 metres.

The 1897-Assam earthquake caused a large fault measuring 10.6 m (35 feet) wide and 19.3 km (12 miles) long. Several faults were created in the mouth areas of the Mississippi river because of the earthquakes of 1811, 1812 and 1813 in the Mississippi valley. The alluvial-filled areas of the flood plains of the Mississippi were fractured at many places which forced ground surface at few places to collapse. This process resulted in the formation of lakes and marshes.

The ground surface was greatly deformed in the delta area of the Indus River (in Pakistan) due to the earthquake of 1819 as an area of 4,500 square kilome­tres was submerged beneath sea water and this land area disappeared for ever. It may be pointed out that subsidence in one area is followed by emergence of the land in other area.

This also happened in the Indus delta area as a large area measuring 80 km in length and 26 km in width was raised by 3 m from the surrounding area. Similarly, the coastal land of Chile was raised from 6m to 13 m because of the earthquake of 1835. The seafloor of Sagami Bay of Japan was subsided from 305 m to 457 m because of the earthquake of 1923.

vii. Flash Floods:

Strong seismic events result in the damages of dams and cause severe flash floods. Severe floods are also caused because of blocking of water flow of rivers due to rock blocks and debris produced by severe tremors on the hill slopes facing the river valleys. Sometimes, the blockade of the rivers is so immense that even the main course of the river is changed.

The 1950 earthquake of Assam produced barrier in the Dihang river, the tributary of the Brahmaputra River, due to accumulation of huge debris caused by landslides triggered by earth tremors and thus caused severe flash floods in the upstream sec­tions. Similarly, the dam on Subansiri River broke in and resultant flash flood submerged an area of 770 square kilometres.

viii. Tsunamis:

The seismic waves, caused by the earthquakes travelling through sea water, generate high sea waves and cause great loss of life and prop­erty. Since the Pacific Ocean is girdled by the ring of earthquakes and volcanoes tsunamis are more com­mon in the Pacific with a minimum frequency of 2 tsunamis per year. The Kutch earthquake of June 16, 1819 generated strong tsunamis which submerged the coastal areas and inflicted great damage to ships and country-made boats of the fishermen.

The land area measuring 24 km in length was raised upward because of tectonic movement triggered by the said earthquake which provided shelter to the stranded and marooned people. This is why the people called this raised land as Allah’s Bund (bund created by the God). The great tsunamis caused by the Lisbon earthquake of the year 1755 (in Portugal) generated about 12 m high sea waves which damaged most parts of Lisbon city and killed 30,000 to 60,000 people.

The impact of this earthquake was so enormous that the waters of inland lakes like Looh Lomond and Looh Ness continued to oscillate for several hours. The strong tsunamis triggered by Lisbon earthquake also caused 3.5 m to 4.5 m high waves as far away as the West Indies. The earthquake caused by violent volcanic eruption of Karakatoa in 1883 caused enormous tsunamis which generated 36.5 m high sea waves which ravaged the coastal areas of Java and Sumatra and killed 36,000 people.

Tsunami: Historical Perspective:

The waves generated in the oceans triggered by high magnitude earthquakes in the ocean floors (ex­ceeding 7.5 on Richter scale), or by violent central volcanic eruptions, or by massive landslides of the coastal lands or of submerged continental shelves and slopes or in deep oceanic trenches, are called tsunami, which is a Japa­nese word meaning thereby harbour waves.

The tsu­namis are long waves (with longer wavelengths of 100 km or more) which travel at the speed of hundreds of kilometers per hour but are of shallow in depth in deeper oceans and seas. As these waves approach coastal land, the depth of oceanic water decreases but the height of tsunamis increases enormously and when they strike the coast, they cause havoc in the coastal areas.

The best example of tsunami induced by violent volcanic eruption is from Krakatao eruption which occurred in 1883. Severe earthquake caused by Krakatao eruption generated furious tsunami waves ranging in 30 to 40 meters in height (average being 120 feet or 36.5 m). These waves were so violent that they ravaged the coasts of Java and Sumatra and killed 36,000 people.

Since the Pacific Ocean is girdled by conver­gent plate boundaries and the ring of earthquakes and volcanoes, tsunamis are more common in the Pacific with a minimum frequency of 2 tsunamis per year. The great tsunamis caused by the Lisbon earthquake (Por­tugal) of the year 1755 generated about 12 m high sea waves which damaged most parts of Lisbon city and killed 30,000 to 60,000 people.

The Kutch earthquake of June 16, 1819 generated strong tsunamis which submerged the coastal areas. The land area measuring 24 km in length was raised upward because of tectonic movements. The raised land was called as Allah’s Bund (bund created by the God).

The following are the significant tsunamis in the second half of the 20th century and 21st century:

(1) Aleutian tsunami:

April 1,1946, gener­ated by Aleutian earthquake of the magnitude of 7.8 on Richter scale, the resultant tsunami with a height of 35 m killed many people in Alaskan and Hawaiian coastal areas.

(2) Kamchatka tsunami:

Nov. 4,1952, earth­quake of the magnitude of 8.2, generated Pacific-wide tsunami with a wave height of 15 m.

(3) Aleutian tsunami:

March 9, 1957, earth­quake of the magnitude of 8.3 on Richter scale, gener­ated a Pacific-wide tsunami of 16 m height and ad­versely affected Hawaii islands.

(4) Chilean tsunami:

May 22, 1960, a strong earthquake of the magnitude of 8.6 on Richter scale, generated Pacific-wide tsunamis and claimed 2,300 human lives in Chile.

(5) Alaskan tsunami:

March 28,1964, a strong earthquake of the magnitude of 8.4 on Richter scale, generated 15 m high tsunami and killed more than 120 people in Alaska.

(6) Papua New Guirea tsunami:

July 17, 1998, a moderate intensity (7.00n Richter scale) sub­marine earthquake followed by massive submarine landslides generated 30m high tsunami killing thou­sands of people living along the lagoon.

(7) Sumatra tsunami:

December 26, 2004, a powerful earthquake of the magnitude of 9 on Richter scale, off the coast of Sumatra with its epicenter at Simeulue in the Indian Ocean occurred at 00:58:53 (GMT), 7:58:53 (Indonesian Local Time) or 6.28 a.m. (Indian Standard Time, 1ST) and generated a powerful tsunami with a wavelength of 160 km and initial speed of 960 km/hr. The deep oceanic earthquake was caused due to sudden subduction of Indian plate below Burma plate upto 20 meters in a boundary line of 1000 km or even more (2000 km upto southern China).

This tec­tonic movement caused 10 m rise in the oceanic bed which suddenly displaced immense volune of water causing killer tsunami. This earthquake was largest (highest on Richter scale) since 1950 and the 4th largest since 1900 A.D. The Andaman and Nicobar group of islands were only 128 km (80 miles) away from the epicenter (Simeulue) and the east coasts of India were about 1920 km (1200 miles) away from the epicenter.

The furious tsunami with a height of about 10 m adversely affected 12 countries bordering the Indian Ocean; worst affected areas included Tamil Nadu coast and Andman-Nicobar Islands of India, Sri Lanka, Indonesia and Thailand. The strong tsunami took about 3 hours to strike Tamil Nadu coast. The killer tsunami claimed more than200,000 human lives in the affected countries wherein Indonesia, Sri Lanka and India stood 1st, 2nd and 3rd in the number of human casualties.

Japan Tsunami, 2011 :

Date : March, 11, 2011; time : Japan time = 2.46 A. M., 1ST = 6.15 A. M.; undersea earth quake of 8.9 magnitude; epicenter 130 km off the coast of Sendai City near Lameng Village and 380 km north-east of Tokyo, at the depth of 10 km on sea bed; tsunami wave height 10m; more than 10,000 people killed; many cities like Miyako, Miyagi, Kesennuma were flattened; Sendai airport was inundated with heaps of cars, trucks, buses and mud deposits; aircrafts including fighter planes standing on airport were washed out by gushing tsunami waves; rotation speed of the earth increased by 16 microseconds; day length decreased by 1.6 microseconds; Honshu island was displaced by 2.4 m due to monstrous quake; earth rotational axis was displaced by 10 centimeters; 2100 km stretch of eastern coastlines having several villages, cities and towns were battered by killer tsunami; nuclear power plants in Fukushima severely damaged resulting into leakage of killer radiactive radiation; more than 5 lakh people in the radius of 20 km from Fukushima power plants were evacuated and shifted to safer places.

Essay # 5. World Distribution of Earthquakes :

If we look at the world distribution map of earthquakes (fig. 10.2) it appears that the seismic centres are closely related to certain zones of the globe. Earthquakes are, in fact, associated with the weaker and isostatically disturbed areas of the globe.

Most of the world earthquakes occur in:

(i) The zones of young folded mountains,

(ii) The zones of faulting and fracturing,

(iii) The zones representing the junction of continental and oceanic margins,

(iv) The zones of active volcanoes, and

(v) Along different plate bounda­ries.

The world map of the distribution of earth­quakes prepared by the seismologists on the basis of computer analysis and simulation of 30,000 earth­quakes that occurred between 1961 and 1967 very much coincides with the traditional map of world distribution of earthquakes (fig. 10.2) e.g.,

(1) Circum- Pacific Belt surrounding the Pacific Ocean,

(2) Mid- Continental Belt representing epicentres located along the Alpine-Himalayan Chains of Eurasia and northern Africa and epicentres of East African Fault Zones, and

(3) Mid-Atlantic Belt representing the earthquakes located along the mid-Atlantic Ridge and its offshoots. ‘The high-quality seismicity maps showed that narrow belts of epicentres coincide almost exactly with the crest of mid-Atlantic (Ridge).

The east Pacific, and the other oceanic ridges, where plates separate. Earthquake epicenters are also aligned along the transform faults, where plates slide past each other. But the earthquakes that occur at depths greater than about 100 km typically occur near margins where plates collide. It is a basic tenet of the theory of plate tectonics that these deep earthquakes actually define the positions of sub-ducted plates which are plunging back into the mantle beneath an overriding plate.

(1) Circum-Pacific Belt includes the epicentres of the coastal margins of North and South Americas and East Asia representing the eastern and western margins of the Pacific Ocean respectively. This belt accounts for about 65 per cent of the total earthquakes of the world.

This belt presents 4 ideal conditions for the occurrences of earthquakes viz.:

(i) Junction of continental and oceanic margins,

(ii) Zone of young folded mountains,

(iii) Zone of active volcanoes, and

(iv) Subduction zone of destructive or convergent plate boundaries.

The western marginal zones of North and South Americas are represented by Rockies and Andes folded mountain chains respectively. These zones are isostatically very sensitive zones because they are also the zones of convergent plate boundaries where the Pacific Oceanic plate is being continuously subducted below the American plates. Besides, these zones are also the areas of strong volcanic activity.

The earth­quakes associated with the eastern coastal margins of Asia and the island arcs and festoons (Kamchatka, Sakhalin, Japan, Philippines) are caused due to the collision of the Pacific and Asiatic plates and conse­quent vulcanicity. Japan records about 1500 seismic shocks every year.

The recent earthquake of Mexico city in 1985 reveals the impact of collision of convergent (destruc­tive) plate boundaries on the occurrences of earth­quakes. The damage done by the devastating earth­quake included death of 5,000 people, disappearance of 2,000 persons, injuries to 40,000 people, destruc­tion of 4000 buildings, damages to 6,000 buildings, lesser damage to 50,000 buildings etc.

(2) Mid-continental belt is also known as Medi­terranean Belt or Alpine-Himalayan Belt which repre­sents the collision or subduction zones of continental plates. About 21 per cent of the total seismic events of the world are recorded in this belt.

This belt includes the epicentres of the Alpine mountains and their off­shoots in Europe, Mediterranean Sea, northern Africa, eastern Africa and the Himalayan mountains and Bur­mese hills. This belt represents the weaker zones of folded mountains where isostatic and fault-induced earthquakes are caused due to subduction of African and Indian plates below Eurasian plate.

The Indian seismic foci are grouped into 3 zones viz.:

(i) Himalayan region,

(ii) Plain region, and

(iii) Plateau region.

The Himalayan region is a zone of maximum intensity in terms of the magnitude of seis­mic tremors because this zone is located in the subduc­tion zones of the Asiatic and Indian plates where the process of mountain building is still in progress. Uttar Kashi earthquake of October 20, 1991 and Chamoli earthquake of 29 March, 1999 (all in Uttaranchal of India) are latest examples. The plain seismic region is a zone of comparatively moderate intensity.

Even the earthquakes of Assam are also included in this zone. The significant earthquakes recorded in the past in this region are 1934 earthquakes of Bihar, Assam earth­quake of 1950, Kolkata earthquake of 1737 and Darbhanga earthquake (Bihar) of 1988. The peninsular Indian region is considered to be a zone of minimum intensity.

The Indian earthquakes along the Himalayas and foothill zones may be explained in terms of plate tectonics. The Asiatic plate is moving southward whereas the Indian plate is moving northward and hence the northern margin of the Indian plate is being subducted below the Asiatic plate.

The collision of Asiatic and Indian plates and resultant subduction of Indian plate and consequent folding and faulting and gradual rise of the Himalayas at the rate of 50 mm per year cause earthquakes of northern India, Tibet and Nepal.

Ac­cording to J.G. Negi, P.K. Agrawal and O.P. Pandey (as reported in Hindu, September 8, 1988) the Indian subcontinent has deformed at places due to the Indian Ocean floor spreading process. India folds at places and when the energy reaches the elastic limit the rocks break up and trigger strike-slip and thrust fault earth­quakes. The Himalayan fault zone is not actually one fault but a broad system of interactive faults. It consists of a complex grid of faults extending all along this colliding zone.

The earthquake belt extends through Sulaiman and Kirthar shear zones in the west, the Himalayas in the north and Burmese arc in the east. These tectonic events caused by plate movements cause earthquakes in the northern and north-eastern parts of India. Even the earthquakes of Peninsular India have been related to the active faults below deccan traps.

On the basis of magnitude of damage risk India is divided into five damage risk zones:

1. Zone I of least damage risk includes the places of some parts of Punjab and Haryana, plain areas of Uttar Pradesh, portions of plains of Bihar and west Bengal, delta area of the Godavari, coastal plain areas of Maharashtra and Kerala, desert areas of Rajasthan and most areas of Gujarat except Kutch area.

2. Zone II of low damage risk includes southern Punjab and Haryana, southern parts of plains of Uttar Pradesh, eastern Rajasthan, coastal districts of Orissa, Tamil Nadu etc.

3. Zone III of moderate damage risk represents the areas of southern and south-eastern Rajasthan, most of Madhya Pradesh, Maharashtra and Karnataka, southern Bihar(Jharkhand), northern and north-western Orissa etc.

4. Zone IV of high damage risk covers Jammu and Kashmir, Himachal Pradesh, northern Punjab and Haryana, Delhi, eastern Uttar Pradesh, ‘tarai’ and ‘bhabar’ regions and Himalayan regions of Uttaranchal and Bihar and Sikkim areas.

5. Zone V of very high damage risk includes parts of Jammu and Kashmir, some parts of Himachal Pradesh, Uttaranchal, western north Bihar (including Munger-Darbhanga), entire north eastern India and Kutch areas of Gujarat.

Though the plains of west Bengal comes under the zone of least damage risk but the devastating severe earthquake of Kolkata of 11 October, 1737, killing 300,000 people, puts a question mark against this concept. The zone of very high damage risk of Kutch region of Gujarat registered most devastating killer earthquake on Jan. 26, 2001 in its seismic history of past 182 years killing 50,000 to 100,000 people. The epicenter was located near Bhuj town.

Bhuj Earthquake (2001):

While the people of India were busy in celebrat­ing the first republic day on Jan. 26, 2001 of the new century in different parts of the country and the pro­gramme of display of might of armed forces of the country was in progress in New Delhi, the nature demonstrated its might by rocking Kutch region of Gujarat when a severe earthquake struck at 8.45 A.M. and shook the region for almost a minute.

Within no time the villages and towns were flattened, high rise buildings collapsed, many villages and towns became heaps of debris, communication and power lines were completely disrupted, transport system was thrown out of gear and settlements became ruins. This was the second most devastating quake in the earthquake his­tory of India after 1737 killer earthquake of Kolkata (300,000 people dead). The epicentre of this earth­quake was located near Bhuj town (population, 150,000).

A moderate quake measuring 4.2 on Richter scale was registered on 24 December, 2000. The epicentre of this precursor quake was located only 22 km away from Bhuj town but no attention was paid to this precursor seismic event either by experts or by govt., agencies. The Bhuj quake of Jan. 26, 2001 was measured 6.9 on Richter scale by the Indian Meteorological Depart­ment (IMD) while the quake was measured 7.9 which was subsequently upgraded to 8.1 by the U.S.A. France and China.

National Geophysical Research Institute (NGRI) of India and Bhabha Atomic Research Centre (BARC) also confirmed the American measurement (8.1). According to Indian Meteorological Department the main reason for the difference in the magnitude of the quake was the application of different methodolo­gies for the measurement of seismic magnitude by different countries and organizations.

It may be pointed out that the IMD uses body wave for the measurement of seismic magnitude while the USA uses shock waves for this purpose. This severe devastating earthquake killed 50,000 to 100,000 people and adversely affected 5,000,000 people. Bhachau and Anjar towns were totally flattened, 90, 60 and 50 per cent houses collapsed in Bhuj, Rajkot and Ahmedabad respec­tively.

If we look at the past seismic history of Gujarat, it appears that a severe earthquake occurs every 30 years e.g., Bhawnagar earthquake, 1872; Kutch earth­quake, 1903; Dwarka earthquake, 1940; Broach earth­quake, 1970 and Bhuj earthquake, 2001. Between 1845 and 1956 sixty six moderate earthquakes were registered in Kutch area but no one was killed, five severe and one very severe earthquakes rocked the area.

In fact, the sequence of destruction of Kutch began with the severe earthquake of June 19, 1819 (7.1 on Richter scale) when 2000 people were killed, Bhuj town was destroyed, famous mosquake of Ahmedabad was damaged, a 100 km long ridge known as Allah Bund was created (most of which is now in Sind of Pakistan, only 15 km ridge is in India) was formed etc.

The main reasons for the recent Bhuj earth­quake of2001 are: sea floor spreading of Indian Ocean at the rate of 5 cm per year, gradual northward move­ment of Indian plate and reactivated faults below the surface. Two major connecting faults have been lo­cated in Kutch region. A 200 km long and 100 km wide fault runs east-west between Bhuj and Ahmedabad.

The second fault measuring 500 km in length and 100 km in width runs in north-south direction through Ahmedabad, Mehsana and Baroda and is known as Combay Graben. These subterranean faults intersect each other near Viramgam, Santhalpur and Radhanpur towns and become the pivot of seismic events when­ever these are activated due to plate movement.

(3) Mid-Atlantic Ridge Belt includes the epicen­tres located along the mid-Atlantic Ridge and several islands nearer the ridge. This belt records moderate and shallow focus earthquakes which are essentially caused due to the creation of transform faults and fractures because of splitting of plates and their move­ment in opposite directions. Thus, the spreading of sea floor and fissure type of volcanic eruption cause earth­quakes of moderate intensity.

It may be pointed out that the earthquakes that occur along the plate margins (boundaries) are well explained on the basis of plate tectonic theory but the earthquakes originating within the plates are difficult to be explained on the basis of this revolutionary theory.

For example, the earthquakes of New Madrid, Charleston, Boston, Tang-Shan, Koyna etc. are a few examples of intraplate earth­quakes. Similarly, ‘the seismicity of the Indian Shield as revealed from Kutch (1819), Koyna (1967), Bhadrachalam (1969) and Broach (1970) cannot be explained easily by plate tectonics since they occurred away from plate boundary’.

Related Articles:

• Concept of Seismic Zoning | Seismology | Geology
• Essay on Tsunami: Top 8 Essays | Natural Disasters | Geography
• Earthquake: Causes, Effects and Distribution of Earthquake
• Essay on Natural Disasters: Top 12 Essays | Geography

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Where do volcanoes and earthquakes happen?

Volcanoes and earthquakes typically happen where tectonic plates meet.

The map below shows the location of active volcanoes and major earthquakes.

The global distribution of earthquakes and volcanoes

The distribution of volcanoes and earthquakes is not random. Volcanoes and earthquakes occur in narrow bands that coincide with tectonic plate margins (compare the map above with the one below). Earthquakes and volcanoes occur both on land and in the sea. A large band of volcanoes and earthquakes happen around the edge of the Pacific Ocean, known as the Pacific Ring of Fire. A band of volcanoes and earthquakes extend from north to south along the Mid-Atlantic Ridge. Some earthquakes and volcanoes occur away from plate margins at volcanic hotspots, where the Earth’s crust is particularly thin. An example of this is Hawaii in the Pacific Ocean.

Earthquakes occur at all three types of plate margins: constructive , destructive and conservative . Volcanoes are found at constructive  and  destructive plate margins. An exception to this are  hot spot volcanoes .

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Volcanoes, explained

These fiery peaks have belched up molten rock, hot ash, and gas since Earth formed billions of years ago.

Volcanoes are Earth's geologic architects. They've created more than 80 percent of our planet's surface, laying the foundation that has allowed life to thrive. Their explosive force crafts mountains as well as craters. Lava rivers spread into bleak landscapes. But as time ticks by, the elements break down these volcanic rocks, liberating nutrients from their stony prisons and creating remarkably fertile soils that have allowed civilizations to flourish.

There are volcanoes on every continent, even Antarctica. Some 1,500 volcanoes are still considered potentially active around the world today; 161 of those—over 10 percent—sit within the boundaries of the United States .

But each volcano is different. Some burst to life in explosive eruptions, like the 1991 eruption of Mount Pinatubo , and others burp rivers of lava in what's known as an effusive eruption, like the 2018 activity of Hawaii's Kilauea volcano. These differences are all thanks to the chemistry driving the molten activity. Effusive eruptions are more common when the magma is less viscous, or runny, which allows gas to escape and the magma to flow down the volcano's slopes. Explosive eruptions, however, happen when viscous molten rock traps the gasses, building pressure until it violently breaks free.

How do volcanoes form?

The majority of volcanoes in the world form along the boundaries of Earth's tectonic plates—massive expanses of our planet's lithosphere that continually shift, bumping into one another. When tectonic plates collide, one often plunges deep below the other in what's known as a subduction zone .

As the descending landmass sinks deep into the Earth, temperatures and pressures climb, releasing water from the rocks. The water slightly reduces the melting point of the overlying rock, forming magma that can work its way to the surface—the spark of life to reawaken a slumbering volcano.

Not all volcanoes are related to subduction, however. Another way volcanoes can form is what's known as hotspot volcanism. In this situation, a zone of magmatic activity —or a hotspot—in the middle of a tectonic plate can push up through the crust to form a volcano. Although the hotspot itself is thought to be largely stationary, the tectonic plates continue their slow march, building a line of volcanoes or islands on the surface. This mechanism is thought to be behind the Hawaii volcanic chain .

Where are all these volcanoes?

Some 75 percent of the world's active volcanoes are positioned around the ring of fire , a 25,000-mile long, horseshoe-shaped zone that stretches from the southern tip of South America across the West Coast of North America, through the Bering Sea to Japan, and on to New Zealand.

This region is where the edges of the Pacific and Nazca plates butt up against an array of other tectonic plates. Importantly, however, the volcanoes of the ring aren't geologically connected . In other words, a volcanic eruption in Indonesia is not related to one in Alaska, and it could not stir the infamous Yellowstone supervolcano .

What are some of the dangers from a volcano?

Volcanic eruptions pose many dangers aside from lava flows. It's important to heed local authorities' advice during active eruptions and evacuate regions when necessary.

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One particular danger is pyroclastic flows, avalanches of hot rocks, ash, and toxic gas that race down slopes at speeds as high as 450 miles an hour . Such an event was responsible for wiping out the people of Pompeii and Herculaneum after Mount Vesuvius erupted in A.D. 79 .

Similarly, volcanic mudflows called lahars can be very destructive. These fast-flowing waves of mud and debris can race down a volcano's flanks, burying entire towns.

Ash is another volcanic danger. Unlike the soft, fluffy bits of charred wood left after a campfire, volcanic ash is made of sharp fragments of rocks and volcanic glass each less than two millimeters across. The ash forms as the gasses within rising magma expand, shattering the cooling rocks as they burst from the volcano's mouth. It's not only dangerous to inhale , it's heavy and builds up quickly. Volcanic ash can collapse weak structures, cause power outages, and is a challenge to shovel away post-eruption.

Can we predict volcanic eruptions?

Volcanoes give some warning of pending eruption, making it vital for scientists to closely monitor any volcanoes near large population centers. Warning signs include small earthquakes, swelling or bulging of the volcano's sides, and increased emission of gasses from its vents. None of those signs necessarily mean an eruption is imminent, but they can help scientists evaluate the state of the volcano when magma is building.

However, it's impossible to say exactly when, or even if, any given volcano will erupt. Volcanoes don't run on a timetable like a train. This means it's impossible for one to be “overdue” for eruption —no matter what news headlines say.

What is the largest eruption in history?

The deadliest eruption in recorded history was the 1815 explosion of Mount Tabora in Indonesia. The blast was one of the most powerful ever documented and created a caldera —essentially a crater—4 miles across and more than 3,600 feet deep. A superheated plume of hot ash and gas shot 28 miles into the sky, producing numerous pyroclastic flows when it collapsed.

The eruption and its immediate dangers killed around 10,000 people. But that wasn't its only impact. The volcanic ash and gas injected into the atmosphere obscured the sun and increased the reflectivity of Earth, cooling its surface and causing what's known as the year without a summer. Starvation and disease during this time killed some 82,000 more people, and the gloomy conditions are often credited as the inspiration for gothic horror tales, such as Mary Shelley's Frankenstein .

Although there have been several big eruptions in recorded history, volcanic eruptions today are no more frequent than there were a decade or even a century ago. At least a dozen volcanoes erupt on any given day. As monitoring capacity for—and interest in—volcanic eruptions increases, coverage of the activity more frequently appears in the news and on social media. As Erik Klemetti, associate professor of geosciences at Denison University, writes in The Washington Post : “The world is not more volcanically active, we’re just more volcanically aware.”

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The surface of the Earth is made up of tectonic plates that lie beneath both the land and oceans of our planet. The movements of these plates can build mountains or cause volcanoes to erupt. The clash of these plates can also cause violent earthquakes, where Earth’s surface shakes. Earthquakes are more common in some parts of the world than others, because some places, like California, sit on top of the meeting point, or fault, of two plates. When those plates scrape against each other and cause an earthquake, the results can be deadly and devastating.

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volcanic eruption , an eruption of molten rock, hot rock fragments, and hot gases through a volcano , which is a vent in a planet’s or satellite’s crust. Volcanic eruptions can cause disastrous loss of life and property. They range from relatively gentle eruptions, as typically seen in Hawaiian volcanoes , to massively destructive ones, such as the eruption of Vesuvius that destroyed Pompeii in 79 ce . Volcanic eruptions have captured the imagination of people over millennia , and they feature in several mythologies as well as works of fiction. These eruptions also play a role in climate change , with expelled gases such as carbon dioxide contributing to global warming , while ash, dust, and gases such as sulfur dioxide can drive global temperatures down.

Volcanic eruptions occur as a result of heat moving under Earth’s surface . They often begin with an accumulation of gas-rich magma (molten underground rock) in reservoirs near Earth’s surface, though they may be preceded by emissions of steam and gas from small vents in the ground. Small earthquakes , which may be caused by a rising plug of dense, viscous magma oscillating against a sheath of more permeable magma, may also signal volcanic eruptions, especially explosive ones.

In some cases, magma rises in conduits to the surface as a thin and fluid lava , either flowing out continuously or shooting straight up in glowing fountains or curtains. The eruptions of Hawaii’s volcanoes fall into this category. In other cases, entrapped gases tear the magma into shreds and hurl viscous clots of lava into the air. In more violent eruptions, the magma conduit is hollowed out by an explosive blast, and solid fragments are ejected in a great cloud of ash-laden gas that rises tens of thousands of metres into the air. An example of this phenomenon is the 1980 eruption of Mount Saint Helens . Many explosive eruptions are accompanied by a pyroclastic flow , a fluidized mixture of hot gas and incandescent particles that sweeps down a volcano’s flanks, incinerating everything in its path. If the expelled ash or gases collect on a high snowfield or glacier , they may melt large quantities of ice , and the result can be a disastrous flood or landslide that rushes down a volcano’s slopes.

Volcanic eruptions can also result in secondary damage , beyond the direct loss to life and property from the eruption itself. Volcanic ash can cause respiratory illnesses such as silicosis and can be particularly harmful to infants and people with chronic lung diseases. Gases such as hydrogen chloride , carbon monoxide , and hydrogen fluoride can cause both short- and long-term problems. Eruptions can cause economic harm that affects workers’ livelihoods and can force mass migrations of people in affected regions. The 2010 eruption of Iceland’s Eyjafjallajökull also demonstrated the threat posed to jet aircraft by high clouds of volcanic ash; this eruption led aviation authorities to ground flights across northern and central Europe for several days.

Volcanoes can be classified by the manner in which they erupt. These six types of volcanic eruptions , starting with the least explosive, make up one classification system: Icelandic, Hawaiian, Strombolian, Vulcanian, Pelean, and Plinian. Each name corresponds to a region or to a specific volcano or historical eruption that exemplifies the type.

Volcanic eruptions are not limited to Earth. Jupiter’s moon Io is subject to strong gravitational forces due to Jupiter’s mass as well as interaction with Jupiter ’s other moons Europa and Callisto . These forces cause distortions in Io’s shape and make it the most volcanically active body in the solar system . Mars is also known for several volcanoes, with Olympus Mons being the largest known volcano in the solar system. The volcanoes of Mars are shield volcanoes , which have a relatively flat profile, as Mars’s low gravity allows for longer and more widespread lava flows. (According to some estimates, Olympus Mons has been built up by eruptions for more than a billion years, which has resulted in its 700-km [435-mile] diameter.)

Volcanoes and volcanic eruptions feature in several mythologies and cultural traditions, especially in regions with high volcanic activity . The word volcano is derived from the Latin Volcanus , or Vulcan , the name of the ancient Roman god of fire. The Māori people of New Zealand (Aotearoa)—which lies on the Pacific Ring of Fire —have, in their traditions, a god of volcanoes and earthquakes called Rūaumoko. Pele is the Hawaiian goddess of fire and volcanoes. In Norse mythology , Muspelheim is a hot, glowing land in the south, guarded by Surt, the fire giant.

Volcanic activity is a common feature in fiction too. In the Inferno section of The Divine Comedy (c. 1308–21), Dante describes Hell as an inverted cone, with the last and innermost circle a fiery lake, symbolic of a volcanic crater. In A Journey to the Centre of the Earth (1864) by Jules Verne , the protagonists commence their journey at the Snæfellsjökull volcano in Iceland, and they return to the surface by way of an eruption at Stromboli in the Mediterranean. Verne also set his The Mysterious Island (1874) on a volcanic island, and it ends with the volcano erupting, obliterating the island. In J.R.R. Tolkien ’s The Lord of the Rings trilogy (1954–55), the Dark Lord Sauron forges the One Ring of Power in the fiery pits of the volcano Mount Doom, where Frodo and Sam go to destroy the ring. The journey to Mount Doom is the primary plot element in the series.

The 1815 eruption of Mount Tambora in Indonesia—the largest volcanic eruption in recorded history—had a tremendous impact on the world’s climate as well as its culture . Tambora expelled as much as 150 cubic km (roughly 36 cubic miles) of ash, pumice and other rock, and aerosols into the atmosphere . These materials blocked substantial amounts of sunlight from reaching Earth’s surface, eventually reducing the average global temperature by as much as 3 °C (5.4 °F). The following year was called the “year without a summer.” It was during this bleak period that Mary Shelley and her literary circle were confined indoors in Geneva, when she conceived of her masterpiece Frankenstein (1818).

When Krakatoa , also in Indonesia, erupted in 1883, the Norwegian artist Edvard Munch , out on a walk, saw the sky turn blood red halfway across the world and later was inspired to paint The Scream in 1893.

Films have also included their fair share of volcanic eruptions as crucial plot points, from the eruption that ends the tribal conflict in One Million Years B.C. (1966) to the triggered-to-erupt volcanic base of the villain Blofeld in the James Bond film You Only Live Twice (1967) to the very obviously named Volcano (1997), in which Tommy Lee Jones and Anne Heche fight off lava from an erupting volcano in Los Angeles. Famous eruptions in history, such as those of Vesuvius and Mount Saint Helens, have featured in numerous films and TV programs.

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