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Chapter 1, Section 3

 

Chapter 1, Section 3

Continental Drift

In 1910, a young German scientist named Alfred Wegener (vay guh nur) became curious about the relationship of the continents. He hypothesized that Earth’s continents had moved! Wegener’s hypothesis was that all the continents were once joined together in a single landmass and have since drifted apart. Wegener’s idea that the continents slowly moved over Earth’s surface became known as continental drift.

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Continental Drift

According to Wegener, the continents drifted together to form the supercontinent Pangaea (pan jee uh).Pangaea means “all lands.” According to Wegener, Pangaea existed about 300 million years ago. This was the time when reptiles and winged insects first appeared. Tropical forests, which later formed coal deposits, covered large parts of Earth’s surface.

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Over tens of millions of years, Pangaea began to break apart. The pieces of Pangaea slowly moved toward their present-day locations. These pieces became the continents as they are today.

Wegener gathered evidence from different scientific fields to support his ideas about continental drift. He studied land features, fossils, and evidence of climate change. In 1915, Wegener published his evidence for continental drift in a book called The Origin of Continents and Oceans.

Evidence From Land Features

As shown in Figure 12, mountains and other features on the continents provided evidence for continental drift. For example, when Wegener pieced together maps of Africa and South America, he noticed that mountain ranges on both continents line up. He noticed that European coal fields match up with coal fields in North America.

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Figure 12Glossopteris, Mesosaurus Fossils and rocks found on different continents provide evidence that Earth’s landmasses once were joined together in the supercontinent Pangaea. Inferring What do the matching mountain ranges in Africa and South America show, according to Wegener’s hypothesis?

Evidence From Fossils

Wegener also used fossils to support his argument for continental drift. A fossil is any trace of an ancient organism that has been preserved in rock. For example, Glossopteris (glaw sahp tuh ris), was a fernlike plant that lived 250 million years ago. Glossopteris fossils have been found in rocks in Africa, South America, Australia, India, and Antarctica. The occurrence of Glossopteris on these widely separated landmasses convinced Wegener that Pangaea had existed.

Other examples include fossils of the freshwater reptiles Mesosaurus and Lystrosaurus. These fossils have also been found in places now separated by oceans. Neither reptile could have swum great distances across salt water. Wegener inferred that these reptiles lived on a single landmass that has since split apart.

Evidence From Climate

Wegener used evidence of climate change to support his hypothesis. As a continent moves toward the equator, its climate becomes warmer. As a continent moves toward the poles, its climate becomes colder. But the continent carries with it the fossils and rocks that formed at its previous locations. For example, fossils of tropical plants are found on Spitsbergen, an island in the Arctic Ocean. When these plants lived about 300 million years ago, the island must have had a warm and mild climate. According to Wegener, Spitsbergen must have been located closer to the equator.

Geologists found evidence that when it was warm in Spitsbergen, the climate was much colder in South Africa. Deep scratches in rocks showed that continental glaciers once covered South Africa. Continental glaciers are thick layers of ice that cover hundreds of thousands of square kilometers. But the climate of South Africa is too mild today for continental glaciers to form. Wegener concluded that when Pangaea existed, South Africa was much closer to the South Pole. According to Wegener, the climates of Spitsbergen and South Africa changed because these landmasses had moved.

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