define hypothesis of continental drift

The Continental Drift Theory: Revolutionary and Significant

• Plate Tectonics
• Types Of Rocks
• Landforms and Geologic Features
• Weather & Climate

Opposition to Continental Drift Theory

Data supporting continental drift theory.

• Wegener's Search for Scientific Truth

Acceptance of Continental Drift Theory

• MLA, Harvard Graduate School of Design

Continental drift was a revolutionary scientific theory developed in the years 1908-1912 by Alfred Wegener (1880-1930), a German meteorologist, climatologist, and geophysicist, that put forth the hypothesis that the continents had all originally been a part of one enormous landmass or supercontinent about 240 million years ago before breaking apart and drifting to their current locations. Based on the work of previous scientists who had theorized about horizontal movement of the continents over the Earth's surface during different periods of geologic time, and based on his own observations drawing from different fields of science, Wegener postulated that about 200 million years ago, a supercontinent that he called Pangaea (which means "all lands" in Greek) began to break up. Over millions of years the pieces separated, first into two smaller supercontinents, Laurasia and Gondwanaland, during the Jurassic period and then by the end of the Cretaceous period into the continents we know today.

Wegener first presented his ideas in 1912 and then published them in 1915 in his controversial book, "The Origins of Continents and Oceans , " which was received with great skepticism and even hostility. He revised and published subsequent editions of his book in 1920,1922, and 1929. The book (Dover translation of the 1929 fourth German edition) is still available today on Amazon and elsewhere.

Wegener's theory, although not completely correct, and by his own admission, incomplete, sought to explain why similar species of animals and plants, fossil remains, and rock formations exist on disparate lands separated by great distances of sea. It was an important and influential step that ultimately led to the development of the theory of plate tectonics , which is how scientists understand the structure, history, and dynamics of the Earth’s crust.

There was much opposition to Wegener's theory for several reasons. For one, he was not an expert in the field of science in which he was making a hypothesis , and for another, his radical theory threatened conventional and accepted ideas of the time. Furthermore, because he was making observations that were multidisciplinary, there were more scientists to find fault with them.

There were also alternative theories to counter Wegener’s continental drift theory. A commonly held theory to explain the presence of fossils on disparate lands was that there was once a network of land bridges connecting the continents that had sunk into the sea as part of a general cooling and contraction of the earth. Wegener, however, refuted this theory maintaining that continents were made of a less dense rock than that of the deep-sea floor and so would have risen to the surface again once the force weighing them down had been lifted. Since this had not occurred, according to Wegener, the only logical alternative was that the continents themselves had been joined and had since drifted apart.

Another theory was that the fossils of temperate species found in the arctic regions were carried there by warm water currents. Scientists debunked these theories, but at the time they helped stall Wegener’s theory from gaining acceptance.

In addition, many of the geologists who were Wegener's contemporaries were contractionists. They believed that the Earth was in the process of cooling and shrinking, an idea they used to explain the formation of mountains, much like wrinkles on a prune. Wegener, though, pointed out that if this were true, mountains would be scattered evenly all over the Earth's surface rather than lined up in narrow bands, usually at the edge of a continent. He also offered a more plausible explanation for mountain ranges. He said they formed when the edge of a drifting continent crumpled and folded — as when India hit Asia and formed the Himalayas.

One of the biggest flaws of Wegener’s continental drift theory was that he did not have a viable explanation for how continental drift could have occurred. He proposed two different mechanisms, but each was weak and could be disproven. One was based on the centrifugal force caused by the rotation of the Earth, and the other was based on the tidal attraction of the sun and the moon.

Though much of what Wegener theorized was correct, the few things that were wrong were held against him and prevented him from seeing his theory accepted by the scientific community during his lifetime. However, what he got right paved the way for plate tectonics theory.

Fossil remains of similar organisms on widely disparate continents support the theories of continental drift and plate tectonics. Similar fossil remains, such as those of the Triassic land reptile Lystrosaurus and the fossil plant Glossopteris , exist in South America, Africa, India, Antarctica, and Australia, which were the continents comprising Gondwanaland, one of the supercontinents that broke off from Pangaea about 200 million years ago. Another fossil type, that of the ancient reptile Mesosaurus , is only found in southern Africa and South America.  Mesosaurus was a freshwater reptile only one meter long that could not have swum the Atlantic Ocean, indicating that there was once a contiguous landmass that provided a habitat for it of freshwater lakes and rivers.

Wegener found evidence of tropical plant fossils and coal deposits in the frigid arctic near the North Pole, as well as evidence of glaciation on the plains of Africa, suggesting a different configuration and placement of the continents than their present one.

Wegener observed that the continents and their rock strata fit together like pieces of a jigsaw puzzle, particularly the east coast of South America and the west coast of Africa, specifically the Karoo strata in South Africa and Santa Catarina rocks in Brazil. South America and Africa were not the only continents with similar geology , though. Wegener discovered that the Appalachian Mountains of the eastern United States, for instance, were geologically related to the Caledonian Mountains of Scotland. 

Wegener's Search for Scientific Truth

According to Wegener, scientists still did not appear to understand sufficiently that all earth sciences must contribute evidence toward unveiling the state of our planet in earlier times, and that the truth of the matter could only be reached by combing all this evidence. Only by combing the information furnished by all the earth sciences would there be hope to determine "truth," that is to say, to find the picture that sets out all the known facts in the best arrangement and that therefore has the highest degree of probability. Further, Wegener believed that scientists always need to be prepared for a possibility that a new discovery, no matter what science furnishes it, may modify the conclusions we draw.

Wegener had faith in his theory and persisted in using an interdisciplinary approach, drawing on the fields of geology, geography, biology, and paleontology, believing that to be the way to strengthen his case and to keep up the discussion about his theory. His book, "The Origins of Continents and Oceans , " also helped when it was published in multiple languages in 1922, which brought it worldwide and ongoing attention within the scientific community. When Wegener gained new information, he added to or revised his theory, and published new editions. He kept the discussion of the plausibility of the continental drift theory going until his untimely death in 1930 during a meteorologic expedition in Greenland.

The story of the continental drift theory and its contribution to scientific truth is a fascinating example of how the scientific process works and how scientific theory evolves. Science is based on hypothesis, theory, testing, and interpretation of data, but the interpretation can be skewed by the perspective of the scientist and his or her own field of specialty, or denial of facts altogether. As with any new theory or discovery, there are those who will resist it and those who embrace it. But through Wegener’s persistence, perseverance, and open-mindedness to the contributions of others, the theory of continental drift evolved into the widely accepted theory today of plate tectonics. With any great discovery it is through the sifting of data and facts contributed by multiple scientific sources, and ongoing refinements of the theory, that scientific truth emerges.

When Wegener died, discussion of continental drift died with him for a while. It was resurrected, however, with the study of seismology and further exploration of the ocean floors in the 1950s and 1960s that showed mid-ocean ridges, evidence in the seafloor of the Earth's changing magnetic field, and proof of seafloor spreading and mantle convection, leading to the theory of plate tectonics. This was the mechanism that was missing in Wegener's original theory of continental drift. By the late 1960s, plate tectonics was commonly accepted by geologists as accurate.

But the discovery of seafloor spreading disproved a part of Wegener's theory, because it wasn't just the continents that were moving through static oceans, as he had originally thought, but rather entire tectonic plates, consisting of the continents, ocean floors, and parts of the upper mantle. In a process similar to that of a conveyor belt, hot rock rises from the mid-ocean ridges and then sinks down as it cools and becomes denser, creating convection currents that cause movement of the tectonic plates.

The theories of continental drift and plate tectonics are the foundation of modern geology. Scientists believe that there were several supercontinents like Pangaea that formed and broke apart over the course of Earth's 4.5-billion year lifespan. Scientists also now recognize that Earth is constantly changing and that even today, the continents are still moving and changing. For example, the Himalayas, formed by the collision of the Indian plate and the Eurasian plate is still growing, because plate tectonics is still pushing the Indian plate into the Eurasian plate. We may even be heading toward the creation of another supercontinent in 75-80 million years due to the continued movement of tectonic plates.

But scientists are also realizing that plate tectonics does not work merely as a mechanical process but as a complex feedback system, with even things such as climate affecting the movement of the plates, creating yet another quiet revolution in the theory of plate tectonics variable in our understanding of our complex planet.

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Continental Drift: The groundbreaking theory of moving continents

Continental drift theory introduced the idea of moving continents.

• Continental drift theory

Evolving theories

• Continental drift evidence

Additional resources

Continental drift was a revolutionary theory explaining that continents shift position on Earth's surface. The theory was proposed by geophysicist and meteorologist Alfred Wegener in 1912, but was rejected by mainstream science at the time. Scientists confirmed some of Wegener's ideas decades later, which are now part of the widely accepted theory of plate tectonics .

Wegener's continental drift theory introduced the idea of moving continents to geoscience. He proposed that Earth must have once been a single supercontinent before breaking up to form several different continents. This explained how similar rock formations and plant and animal fossils could exist on separated continents. Modern science recognizes this ancient supercontinent called Pangaea did exist before breaking up about 200 million years ago, as Wegener theorized. 

Related: Massive supercontinent will form hundreds of millions of years from now  

Why did scientists reject Wegener's continental drift theory?

Geologists roundly denounced Wegener's continental drift theory after he published the details in a 1915 book called " The Origin of Continents and Oceans ." Part of the opposition was because Wegener didn't have a good model to explain how the continents moved, something scientists later explained under the umbrella of plate tectonics — the theory that Earth's crust is fractured into plates that move over a rocky inner layer called the mantle . 

"There's an irony that the key objection to continent drift was that there is no mechanism, and plate tectonics was accepted without a mechanism," to move the continents, Henry Frankel (1944–2019), an emeritus professor at the University of Missouri-Kansas City and author of the four volume " The Continental Drift Controversy " (Cambridge University Press, 2012) previously told Live Science.

Though most of Wegener's observations about fossils and rocks were correct, he was outlandishly wrong on a couple of key points. For instance, Wegener thought the continents might have plowed through the ocean crust like icebreakers smashing through ice. 

When Wegener proposed continental drift, many geologists were contractionists. They thought Earth's incredible mountains were created because our planet had been cooling and shrinking since its formation, Frankel said. And to account for the identical fossils discovered on continents such as South America and Africa, scientists invoked ancient land bridges, now vanished beneath the sea. 

Researchers argued over the land bridges right up until the plate tectonics theory was developed from the 1950s to the 1970s, Frankel said. For instance, as geophysicists began to realize that continental rocks were too light to sink down to the ocean floor, prominent paleontologists instead wrongly suggested that the similarities between fossils had been overestimated, Frankel said.

Plate tectonics is like a modern update to continental drift. In the 1960s, scientists discovered plate edges through magnetic surveys of the ocean floor and through the seismic listening networks built to monitor nuclear testing, according to Encyclopedia Britannica . Alternating patterns of magnetic anomalies on the ocean floor indicated seafloor spreading , where new plate material is born. Magnetic minerals aligned in ancient rocks on continents also showed that the continents have shifted relative to one another. 

Related: Plate tectonics are 3.6 billion years old, oldest minerals on Earth reveal  

What evidence is there for continental drift?

A map of the continents inspired Wegener's quest to explain Earth's geologic history. He was intrigued by the interlocking fit of Africa's and South America's shorelines. Wegener then assembled an impressive amount of continental drift evidence to show that Earth's continents were once connected in a single supercontinent.

Wegener knew that fossil plants and animals such as mesosaurs , a freshwater reptile found only in South America and Africa during the Permian period, could be found on many continents. He also matched up rock formations on either side of the Atlantic Ocean like puzzle pieces. For example, the Appalachian Mountains (United States) and Caledonian Mountains (Scotland) fit together, as do the Karoo strata in South Africa and Santa Catarina rocks in Brazil.

In fact, plates moving together created the highest mountains in the world, the Himalayans, and the mountains are still growing due to the plates pushing together, even now, according to National Geographic . Despite his incredible continental drift evidence, Wegener never lived to see his theory gain wider acceptance. He died in 1930 at age 50 just two days after his birthday while on a scientific expedition in Greenland , according to the University of Berkeley .

• Learn more about the history of continental drift and plate tectonics from the U.S. Geological Survey . 
• Learn more about Alfred Wegener from the NASA Earth Observatory . 
• Watch this short video on YouTube about plate tectonics and continental drift, from National Geographic . 

This article was updated on Dec. 14, 2021, by Live Science Staff Writer Patrick Pester. Additional reporting by Alina Bradford, Live Science contributor.  

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What is Continental Drift?

In 1912, Alfred Wegener published a theory to explain why the Earth looked like a huge jigsaw. He believed the continents were once joined, forming a supercontinent he called Pangaea. Over 180 million years ago, this supercontinent began to “break up” due to continental drift.

Continental drift

During the 20th Century, scientists developed the theory of Plate Tectonics. The theory suggests that the crust of the Earth is split up into seven large plates (see map below) and a few smaller ones, all of which can slowly move around on the Earth’s surface. They lie on the ductile mantle that allows them to move. There are several explanations for the movement of the Earth’s plates, and these are explored in the Why do plates move? page.

The Earth’s main tectonic plates

What is the evidence for continental drift?

Wegener’s evidence for continental drift was that:

• the same types of fossilised plants and animals are found in South America and Africa;
• the east coast of South America fits the west coast of Africa like a jigsaw puzzle;
• rock formations and mountain chains match in South America and Africa;
• similar mineral deposits and natural resources, such as coal, exist along the east coast of Africa and the west coast of South America.

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Continental Drift

Understanding continental drift.

Continental drift is a theory that has revolutionized the way we understand the Earth’s surface. At the heart of this concept is the idea that the continents are not fixed, but instead, have been moving across the Earth’s surface over millions of years. First proposed by Alfred Wegener in 1912, continental drift has since become a cornerstone of modern geophysics, leading to the development of the tectonic plate theory.

Causes of Continental Drift

As tectonic plates that form the Earth’s crust float on the more malleable mantle beneath, they carry continents with them. Whenever these plates interact, they can move apart (diverge), grind past each other (transform), or converge where one plate is forced below another (subduction). Each type of interaction has a significant role in shaping the Earth’s surface and is a driving factor in continental drift.

Effects of Continental Drift

Evidence of continental drift.

The evidence for continental drift is extensive and multifaceted. One of the most striking pieces of evidence comes from the apparent fit of South America’s east coast and West Africa’s west coast, suggesting that these continents were once joined. Fossil records also play a crucial role in supporting the theory, with similar plant and animal fossils being found on continents that are now widely separated by oceans. These fossils suggest the continents were once connected, providing a land bridge for species migration.

Understanding The Mechanics of Plate Tectonics

Building on the foundation of continental drift, the theory of plate tectonics provides a more comprehensive understanding of how the Earth’s plates move and interact. It describes the Earth’s lithosphere (the crust and upper mantle) as being divided into a number of large and small plates that float on the underlying semi-molten asthenosphere. The relative movement of these plates can cause dramatic geological events.

Challenges and Revisions in Theory

Impact of continental drift on human life, related posts:.

Continental Drift versus Plate Tectonics

A scientific idea that was initially ridiculed paved the way for the theory of plate tectonics, which explains how Earth’s continents move.

Earth Science, Geology, Geography, Physical Geography

Alfred Wegener in Greenland

Plate tectonics is the theory that Earth's land masses are in constant motion. The realization that Earth's land masses move was first proposed by Alfred Wegener, which he called continental drift. He is shown here in Greenland.

Photograph from Pictoral Press

We don’t perceive that the continents we live on are moving. After all, it’s not as if an airplane flight between Europe and Africa takes five hours one year but only three hours the next. But the continents actually are shifting, very slowly, relative to one another. In the early 20th century, a scientific theory called continental drift was proposed about this migration of the continents . That theory was initially ridiculed, but it paved the way for another theory called plate tectonics that scientists have now accepted to explain how Earth’s continents move. The story begins with Alfred Wegener (1880–1930), a German meteorologist and geophysicist who noticed something curious when he looked at a map of the world. Wegener observed that the continents of South America and Africa looked like they would fit together remarkably well—take away the Atlantic Ocean and these two massive landforms would lock neatly together. He also noted that similar fossils were found on continents separated by oceans, additional evidence that perhaps the landforms had once been joined. He hypothesized that all of the modern-day continents had previously been clumped together in a super continent he called Pangaea (from ancient Greek, meaning “all lands” or “all the Earth”). Over millions of years, Wegener suggested, the continents had drifted apart. He did not know what drove this movement, however. Wegener first presented his idea of continental drift in 1912, but it was widely ridiculed and soon, mostly, forgotten. Wegener never lived to see his theory accepted—he died at the age of 50 while on an expedition in Greenland. Only decades later, in the 1960s, did the idea of continental drift resurface. That’s when technologies adapted from warfare made it possible to more thoroughly study Earth. Those advances included seismometers used to monitor ground shaking caused by nuclear testing and magnetometers to detect submarines. With seismometers , researchers discovered that earthquakes tended to occur in specific places rather than equally all over Earth. And scientists studying the seafloor with magnetometers found evidence of surprising magnetic variations near undersea ridges: alternating stripes of rock recorded a flip-flopping of Earth’s magnetic field . Together, these observations were consistent with a new theory proposed by researchers who built on Wegener’s original idea of continental drift —the theory of plate tectonics . According to this theory , Earth’s crust is broken into roughly 20 sections called tectonic plates on which the continents ride. When these plates press together and then move suddenly, energy is released in the form of earthquakes. That is why earthquakes do not occur everywhere on Earth—they’re clustered around the boundaries of tectonic plates. Plate tectonics also explains the stripes of rock on the seafloor with alternating magnetic properties: As buoyant, molten rock rises up from deep within Earth, it emerges from the space between spreading tectonic plates and hardens, creating a ridge. Because some minerals within rocks record the orientation of Earth’s magnetic poles and this orientation flips every 100,000 years or so, rocks near ocean ridges exhibit alternating magnetic stripes. Plate tectonics explains why Earth’s continents are moving; the theory of continental drift did not provide an explanation. Therefore, the theory of plate tectonics is more complete. It has gained widespread acceptance among scientists. This shift from one theory to another is an example of the scientific process: As more observations are made and measurements are collected, scientists revise their theories to be more accurate and consistent with the natural world. By running computer simulations of how Earth’s tectonic plates are moving, researchers can estimate where the planet's continents will likely be in the future. Because tectonic plates move very slowly—only a few centimeters per year, on average—it takes a long time to observe changes. Scientists have found that the planet’s continents will likely again be joined together in about 250 million years. Researchers have dubbed this future continental configuration “ Pangaea Proxima.” One intriguing aspect of Pangaea Proxima is that it will likely contain a new mountain range with some of the world’s highest mountains. That is because as Africa continues to migrate north it will collide with Europe, a collision that will probably create a Himalaya-scale mountain range. However, Christopher Scotese, one of the scientists who developed these simulations , cautions that it is difficult to predict exactly how the continents will be arranged in millions of years. “We don’t really know the future, obviously,” Scotese told NASA. “All we can do is make predictions of how plate motions will continue, what new things might happen, and where it will all end up.”

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30 Continental Drift: founding block of the Plate Tectonics Theory

The continental drift hypothesis, the foundation of Plate Tectonics theory was developed in the early part of the twentieth century, mostly by Alfred Wegener who proposed that continents move around on Earth’s surface and that they were once joined together as a single supercontinent called Pangaea. His hypothesis did not stand a trial at that time because nobody could explain the mechanism of the movement of continental plates.

Only with discoveries of ocean structure in 1960-1970s scientists understood the mechanism of plate movement.

See below a short documentary about the discovery of Plate Tectonics theory.

The Continental Drift Idea

Figure 1. The continents fit together like pieces of a puzzle. This is how they looked 250 million years ago.

Alfred Wegener, a German climatologist, proposed that the continents were once united into a single supercontinent named Pangaea, meaning all earth  in ancient Greek . He suggested that Pangaea broke up long ago and that the continents then moved to their current positions. He called his hypothesis  continental drift.

Evidence for Continental Drift

Besides the way the continents fit together, Wegener and his supporters collected a great deal of evidence for the continental drift hypothesis.

• Identical rocks, of the same type and age, are found on both sides of the Atlantic Ocean. Wegener said the rocks had formed side-by-side and that the land had since moved apart.
• Mountain ranges with the same rock types, structures, and ages are now on opposite sides of the Atlantic Ocean. The Appalachians of the eastern United States and Canada, for example, are just like mountain ranges in eastern Greenland, Ireland, Great Britain, and Norway (figure 2). Wegener concluded that they formed as a single mountain range that was separated as the continents drifted.

Figure 2. The similarities between the Appalachian and the eastern Greenland mountain ranges are evidences for the continental drift hypothesis.

• Fossils of the seed fern  Glossopteris  were too heavy to be carried so far by wind.
• Mesosaurus  was a swimming reptile but could only swim in fresh water.
• Cynognathus  and  Lystrosaurus  were land reptiles and were unable to swim.

• Grooves and rock deposits left by ancient glaciers are found today on different continents very close to the equator. This would indicate that the glaciers either formed in the middle of the ocean and/or covered most of the Earth. Today glaciers only form on land and nearer the poles. Wegener thought that the glaciers were centered over the southern land mass close to the South Pole and the continents moved to their present positions later on.
• Coral reefs and coal-forming swamps are found in tropical and subtropical environments, but ancient coal seams and coral reefs are found in locations where it is much too cold today. Wegener suggested that these creatures were alive in warm climate zones and that the fossils and coal later had drifted to new locations on the continents.

Although Wegener’s evidence was sound, but most geologists at the time rejected his hypothesis of continental drift. Why do you think they did not accept continental drift?

Scientists argued that there was no way to explain how solid continents could plow through solid oceanic crust. Wegener’s idea was nearly forgotten until technological advances presented even more evidence that the continents moved and gave scientists the tools to develop a mechanism for Wegener’s drifting continents.

Magnetic Polarity Evidence

Figure 4. Earth’s magnetic field is like a magnet with its north pole near the geographic North Pole and the south pole near the geographic South Pole.

Puzzling new evidence came in the 1950s from studies on the Earth’s magnetic history (figure 4).

Scientists used  magnetometers , devices capable of measuring the magnetic field intensity, to look at the magnetic properties of rocks in many locations .

Magnetic Pole Moving?

Geologic Approach to Magnetic Pole Wandering

All igneous rocks have magnetite, a mineral consisting mainly of iron. Igneous rocks can be found in many parts of the world.

Magnetite  crystals are like tiny magnets that point to the north magnetic pole as they crystallize from magma. The crystals record both the direction and strength of the  magnetic field  at the time. The direction is known as the field’s  magnetic polarity.

Geologists noted important things about the magnetic polarity of different aged rocks on the same continent:

• Magnetite crystals in fresh volcanic rocks point to the current magnetic north pole  (figure 5) no matter what continent or where on the continent the rocks are located.

• Older rocks that are the same age and are located on the same continent point to the same location, different from the current north magnetic pole.
• Older rock that are of different ages do not point to the same locations or to the current magnetic north pole.

In other words, although the magnetite crystals were pointing to the magnetic north pole, the location of the pole seemed to wander.  Scientists were amazed to find that the north magnetic pole changed location through time (figure 6).

Figure 6. The location of the north magnetic north pole 80 million years before present (mybp), then 60, 40, 20, and now.

There are three possible explanations for this:

1. The continents remained fixed and the north magnetic pole moved.

2. The north magnetic pole stood still and the continents moved.

3. Both the continents and the north pole moved.

During studies of magnetism in various countries geologists noted that for rocks of the same age on different continents , the  magnets pointed to different magnetic north poles .

• 400-million-year-old magnetite in Europe pointed to a different north magnetic pole than the same-aged magnetite in North America.
• 250 million years ago, the north poles were also different for the two continents.

The scientists looked again at the three possible explanations. Only one can be correct.

If the continents had remained fixed while the north magnetic pole moved, there must have been two separate north poles.

Since there is only one north pole,   the only reasonable explanation is that the north magnetic pole has remained fixed but that the continents have moved.

To test this, geologists fitted the continents together as Wegener had done. It worked!

There has only been one magnetic north pole and the continents have drifted (see figure below). The view on this map is from the north down (polar projection).

The maps above show the polar wandering curve through Pangaea as well as through the present day configuration, which provides further evidence for moving tectonic plates.

This evidence for continental drift gave geologists renewed interest in understanding how continents could move about on the planet’s surface.

Lesson Summary

• In the early part of the 20th century, scientists began to put together evidence that the continents could move around on Earth’s surface.
• The evidence for continental drift included the fit of the continents; the distribution of ancient fossils, rocks, and mountain ranges; and the locations of ancient climatic zones.
• Although the evidence for continental drift was extremely strong, scientists rejected the idea because no mechanism for how solid continents could move around on the solid earth was developed.
• The discovery of apparent polar wander renewed scientists interest in continental drift.

Geology 101 for Lehman College (CUNY) Copyright © by Yuri Gorokhovich and Lumen Learning is licensed under a Creative Commons Attribution-NonCommercial-ShareAlike 4.0 International License , except where otherwise noted.

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Plate Tectonics

Theory of continental drift.

The continental drift hypothesis was developed in the early part of the 20th century, mostly by Alfred Wegener. Wegener said that continents move around on Earth’s surface and that they were once joined together as a single supercontinent. While Wegener was alive, scientists did not believe that the continents could move.Find a map of the continents and cut each one out. Better yet, use a map where the edges of the continents show the continental shelf. That’s the true size and shape of a continent and many can be pieced together like a puzzle. The easiest link is between the eastern Americas and western Africa and Europe, but the rest can fit together too.

Alfred Wegener proposed that the continents were once united into a single supercontinent named Pangaea, meaning all earth in ancient Greek. He suggested that Pangaea broke up long ago and that the continents then moved to their current positions. He called his hypothesis continental drift.

Evidence for Continental Drift

Besides the way the continents fit together, Wegener and his supporters collected a great deal of evidence for the continental drift hypothesis. For one, identical rocks of the same type and age are found on both sides of the Atlantic Ocean. Wegener said the rocks had formed side-by-side and that the land had since moved apart.Mountain ranges with the same rock types, structures, and ages are now on opposite sides of the Atlantic Ocean. The Appalachians of the eastern United States and Canada, for example, are just like mountain ranges in eastern Greenland, Ireland, Great Britain, and Norway. Wegener concluded that they formed as a single mountain range that was separated as the continents drifted.Ancient fossils of the same species of extinct plants and animals are found in rocks of the same age but are on continents that are now widely separated. Wegener proposed that the organisms had lived side by side, but that the lands had moved apart after they were dead and fossilized. He suggested that the organisms would not have been able to travel across the oceans. For example, the fossils of the seed fern Glossopteris were too heavy to be carried so far by wind. The reptile Mesosaurus could only swim in fresh water. was a swimming reptile but could only swim in fresh water. Cynognathus and Lystrosaurus were land reptiles and were unable to swim.Grooves and rock deposits left by ancient glaciers are found today on different continents very close to the equator. This would indicate that the glaciers either formed in the middle of the ocean and/or covered most of the Earth. Today glaciers only form on land and nearer the poles. Wegener thought that the glaciers were centered over the southern land mass close to the South Pole and the continents moved to their present positions later on.Coral reefs and coal-forming swamps are found in tropical and subtropical environments, but ancient coal seams and coral reefs are found in locations where it is much too cold today. Wegener suggested that these creatures were alive in warm climate zones and that the fossils and coal later had drifted to new locations on the continents.Although Wegener’s evidence was sound, most geologists at the time rejected his hypothesis of continental drift. Scientists argued that there was no way to explain how solid continents could plow through solid oceanic crust. Wegener’s idea was nearly forgotten until technological advances presented even more evidence that the continents moved and gave scientists the tools to develop a mechanism for Wegener’s drifting continents.

Magnetic Polarity on the Same Continent with Rocks of Different Ages

Magnetic polarity on different continents with rocks of the same age.

• Dynamic Earth: Introduction to Physical Geography. Authored by : R. Adam Dastrup. Located at : http://www.opengeography.org/physical-geography.html . Project : Open Geography Education. License : CC BY-SA: Attribution-ShareAlike
• Fossil Map. Authored by : Osvaldocangaspadilla. Provided by : USGS. Located at : https://commons.wikimedia.org/wiki/File:Snider-Pellegrini_Wegener_fossil_map.svg . License : Public Domain: No Known Copyright

Alfred Wegener: Building a Case for Continental Drift

Continental Drift

The Theory of Continental Drift is defined as the movement of the Earth’s continents relative to each other, thereby appearing to drift together across the oceanic bed. Although Alfred Wegener was able to produce a viable hypothesis with evidence and specifically state the theory, it should be noted there were previous geologists and scientists who thought similar to Wegener. For example, between 1889 and 1909 Roberto Mantovani speculated that all continents had once been conjoined in a “supercontinent,” and even developed an expanding Earth hypothesis.

A depiction of the continents joined together as the “supercontinent,” Pangea, and the path that led them to the positions they reside in today.

Alfred Wegener’s curiosity toward the possibility of continental drift came in 1910 after he noticed how Earth’s continents resembled pieces of a jigsaw puzzle. For example, he noted how South America coast correctly lined up with the coast of Northwest Africa. It wasn’t until 1911, when Wegener came across several scientific documents listing fossils of identical plants and animals found on opposite sides of the Atlantic that his passion for the subject truly showed. Reflecting on this monumental moment in his life, he wrote, “A conviction of the fundamental soundness of the idea took root in my mind.” Alfred Wegener knew massive amounts of evidence needed to be collected in order to justify such a fantastic idea—because with no practical driving force behind it the theory loses most of its credit. In order to maximize evidence for his theory and overlook the absence of a mechanism, he decided to draw from a variety of scientific fields including geology, geography, biology, and paleontology.

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What is the theory of continental drift?

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Continental Drift and Plate Tectonics

The slow discovery of plate tectonics was one of the greatest breakthroughs of modern science, and it changed our understanding of how this planet functions. But the concept of plate tectonics and the theory of continental drift that preceded it were quite radical. It took almost a century to develop and confirm the theory, and it was not widely accepted even among scientists until the 1960s. This is a brief story of how the theory evolved and why there was so much resistance. (additional text under video)

The Earth’s crust is divided into seven large and a number of smaller chunks called “plates.” At the boundaries where these plates meet, there are different types of activity, together known as plate tectonics. The mechanisms at the plate boundaries are responsible for some of the most important functions of our planet.

For example:

• movement of the continents over time and distribution of oceans
• emergence of mountain ranges like the Andes and the Himalayas
• volcanoes and earthquakes
• circulation of minerals from Earth’s core to the surface
• and important features of our atmosphere.

Geologists today know that plate tectonics make Earth an extraordinary place –a planet that can support life. No other planet in our solar system has plate tectonics.

DEVELOPMENT OF THE THEORY

Millions of years ago the present-day continents were one huge land mass. Over time that massive formation broke apart and the individual chunks drifted apart and regrouped in different ways several times. But an understanding of how the continents became separate entities took centuries.

If you look at continents like South America and Africa, for example, you will notice that they seem to fit together. Map-makers and geologists observed that out long ago. But scientists also discovered ancient fossils that were very similar on widely separated continents. And they noticed that rock formations seemed to match up across different continents, suggesting that continents were once joined. But how could huge land masses possibly move? There was no easy answer.

For a long time, there were two major theories. One was that the positions of the continents formed early in Earth’s history and they never moved. The other major theory was that geological cycles caused periodic contractions of the earth, which in turn caused dry land to sink in some places while the seafloor rose in others. The first theory –that the continents had never moved— was largely dismissed over time as geological studies improved. But the second theory – contraction cycles—lasted for decades.

CONTRACTION CYCLES

In the early 1900s an Austrian geologist, Edward Suess, suggested that the long cooling period of early Earth caused the planet to contract over time. These contractions wrinkled the planet -rather like a dried up apple-- and reduced the surface. This periodic wrinkling led to a cycle in which huge areas of the land might collapse into the early oceans, while submerged areas of Earth’s crust rose to become land masses. This rising-sinking pattern would create new ocean basins and continents and the pressure of the contractions would form huge mountain ranges. Suess also proposed that the continents in the southern hemisphere had once formed a huge landmass which he called Gondwana. The submerge-and-surface cycle gradually broke up Gondwana, resulting in the modern continents of South America, Southern Africa, India, Antarctica and Australia. To explain the similarity of fossils across these continents, Suess proposed that for some time as during the breakup of Gondwana the land masses were probably connected by land bridges.

There were some variations on the Contraction Theory, but the important point is that this rise-and-fall concept developed long before scientists learned that active tectonic plates on Earth’s crust actually moved continents.

CONTINENTAL DRIFT

In 1910, an American geologist, Frank Bursley Taylor, proposed that the obvious “fit” between the South American and Africa continents indicated an original landmass that somehow drifted apart over eons. From his study of mountain ranges on these continents, he concluded that such huge masses must have been caused by the collision of continents. The movement of the continents, according to Taylor, must be caused by the gravitational pull of the moon during the Cretaceous period, 145 million years ago. Taylor could not convince the American scientists.

At about the same time, a German meteorologist, Alfred Wegener, offered an alternative to the contraction theory. There were, he noted, two important problems. One was that the heating-cooling cycle fundamental to the contraction theory was not possible because radioactive decay deep in the Earth produced continuous heat. Another problem was that the geological formations underlying the huge mountain ranges showed compression patterns that Suess’s theory could not explain.

Wegener developed several key ideas, together known as the theory of Continental Drift. He suggested that 200-300 million years ago, in the Paleozoic and Mesozoic eras, there was a single supercontinent, which he called Pangea (which means “all Earth”). Paleoclimate changes during this long period led to the breakup of Pangea into Gondwana and Laurasia supercontinents. Wegener explained continental drift theory in his famous 1915 book The Origin of Continents and Oceans. In this work he presented supporting evidence from fossils, geological formations and paleoclimate indicators.

But the Continental Drift theory worked out independently by Taylor and Wegener, was ignored or rejected. Very little was known at the time about the earth’s crust, especially since most of it was under the sea. So it was difficult for scientists in the early 1900s to understand how massive continents could glide along the surface of the planet. Continental drift was controversial –or rejected—for decades.

PLATE TECTONICS

But over time, further evidence supporting continental drift accumulated. In 1937, Alex du Toit, a South African geologist, made a detailed investigation of geological matchups in the continental shelves of southern Africa and South America. He explained his discoveries and the very clear links between the continents that originated in Gonwanda In his 1937 book Our Wandering Continents. Du Toit’s evidence was conclusive.

In the 1920s, British geologist Arthur Holmes moved the plate tectonics theory forward with two important contributions. He introduced radiometric dating of minerals, ensuring accuracy of the analysis of geological formations and the types of minerals associated with geological deposits. He also elaborated on the continental drift theory by proposing that plate junctions lay under the seabed, and that “convection cells” in the Earth’s mantle generated intense heat energy that could move the crust. This hypothesis was a major advance toward a viable explanation of continental movement.

However, despite the accumulating evidence for continental drift and the force of plate tectonics that moved the continents, many geologists had difficulty accepting how the huge continents could move. Part of the problem was poor communication between European and American scientists in a period of international political tensions. But intellectual resistance to a radical theory of continental shift was the enduring problem -and it was independent of political context.

Steady technological innovations during the World War II period advanced the investigations of the ocean floor. And those detailed investigations revealed rifts in the seabed that were unambiguous indications that the Earth’s crust did move. The problem for several more decades was understanding the source of such massive movements. In the 1960s further investigations provided geological details of the rift patterns. The major studies were done by Harry Hess, Fred Vine and Drummond Matthews. According to their hypothesis, the Earth’s crust is divided into plates that shift over time. The movement causes the seafloor to spread where the plates meet. They found that the rifts were associated with periodic reversals in Earth’s magnetic field, and those reversals were recorded in the seafloor crust. Geologists can calculate when and where plate movements occurred over millions of years.

In the 1960s, these and other scientific advances confirmed at last the theory of “plate tectonics,” –the source of the energy and mechanics of plate movement on the Earth’s crust. Continental drift over millions of years was caused by plate tectonics. And plate tectonics also explained how the movement of the plates create volcanoes and earthquakes, and how the collision between continents gave rise to huge mountain ranges.

TYPES OF PLATE BOUNDARIES

Ultimately, geologists established three basic types of plate boundaries with their respective effects. They are called Convergent, Divergent, and Transform boundaries.

•In Divergent boundaries two plates move away from each other. This causes earthquakes along the boundaries, and magma (molten rock) from deep in the Earth’s mantle rises to the surface, dragging minerals and gases up to be incorporated in new crust.

•In Convergent boundaries the plates move toward each other, and the collision causes subduction zones, where one plate slides under another, dragging material down into Earth’s mantle. This causes earthquakes, volcanoes and the rise mountains. Subduction is important for circulation of minerals and gases.

•In Transform boundaries or “shearing” the plates slide past each other. Typically, this splits apart and grinds up rock formations. Undersea canyons are examples of this process.

Despite the solid evidence of plate tectonics, some very prominent scientists continued to reject the notion that continents could drift. One of the most famous deniers was Harold Jeffreys. Long past the time when plate tectonics became part of standard geological theory, Jeffreys argued that there was “no force strong enough” to move continents across the crust of the planet.

Today, geologists understand also the link between plate tectonics and the movement of land masses, and also zones of earthquakes and volcanoes. And because of plate tectonics, atmospheric gases and minerals that are essential for the development and sustaining life on this planet are recycled.

It took a long time, but today plate tectonics is a central component of geological science.

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