Science Textbook pgs. 326-330 1.24.12
"Five hundred years ago, the sea voyages of Columbus and other explorers changed the map of the world. The continents of Europe, Asia, and Africa were already known to mapmakers. Soon mapmakers were also showing the outlines of the continents of North and South America. Looking at these world maps, many people wondered why the coasts of several continents matched so neatly.
Look at a modern world map. Notice how the coasts of Africa and South America look as if they could fit together like jigsaw-puzzle pieces. Could the continents have once been a single landmass? In the 1700s, the first geologists thought that the continents had remained fixed in their positions throughout Earth’s history. Early in the 1900s, however, one scientist began to think in a new way about this riddle of the continents. His idea changed the way people look at the map of the world.
In 1910, a young German scientist named Alfred Wegener (vay guh nur) became curious about the relationship of the continents. He formed a hypothesis that Earth’s continents had moved!Wegener’s hypothesis was that all the continents had once been joined together in a single landmass and have since drifted apart.
Wegener named this supercontinent Pangaea (pan jee uh), meaning “all lands.” According to Wegener, Pangaea existed about 300 million years ago. This was the time when the fossil record contains the first evidence that reptiles and winged insects lived on Earth. Also, tropical forests, which later formed coal deposits, covered much of Earth.
World’s Continents Today’s continents provide clues about Earth’s history. Observing Which coastlines of continents seem to match up like jigsaw-puzzle pieces?
Wegener hypothesized that over tens of millions of years, Pangaea began to break apart. The pieces of Pangaea slowly moved toward their present-day locations, becoming the continents as they are today. Wegener’s idea that the continents slowly moved over Earth’s surface became known as continental drift.
Continental Drift
Have you ever tried to persuade a friend to accept a new idea? You probably had to provide some convincing evidence. Wegener gathered evidence from different scientific fields to support his ideas about continental drift. In particular, he studied landforms, fossils, and evidence that showed how Earth’s climate had changed over many millions of years. Wegener published all his evidence for continental drift in a book called The Origin of Continents and Oceans, first published in 1915.
Evidence From Landforms Wegener thought that mountain ranges and other features on the continents provided evidence for continental drift. When he pieced together maps of Africa and South America, he saw a mountain range running from east to west in South Africa that lined up with a mountain range in Argentina. European coal fields matched up with similar coal fields in North America. Wegener compared matching these features to reassembling a torn-up newspaper. If the pieces could be put back together, the “words” would match.
Evidence From Fossils Wegener also used fossil evidence as support for continental drift. A fossil is a trace of an organism that has been preserved in rock. For example, fossils of the reptilesMesosaurus and Lystrosaurus had been found in places now separated by oceans. Neither reptile could have swum long distances across salt water. Therefore, Wegener concluded that these reptiles once lived on a single landmass. Another example wasGlossopteris (glaw sahp tuh ris), a fernlike plant that lived 250 million years ago. Glossopteris fossils had been found in rocks in Africa, South America, Australia, India, and Antarctica. The occurrence of Glossopteris on these widely separated landmasses convinced Wegener that the continents had once been united.Fossil Evidence Fossils of the freshwater reptile Mesosaurus found in Africa and South America provide evidence of continental drift.
The seedlike structures of Glossopteris could not have traveled across the distances that separate the continents today. The “seeds” were too large for the wind to carry and too fragile to have survived an ocean trip. How did Glossopteris come to live on such widely separated continents? Wegener inferred that the continents at that time were joined as the supercontinent Pangaea.
Evidence From Climate Wegener also used evidence of climate change to support his hypothesis of continental drift. Spitsbergen is an island in the Arctic Ocean north of Norway. This island is ice-covered and has a harsh polar climate. But fossils of tropical plants had been found on Spitsbergen. When these plants lived about 300 million years ago, the island must have had a warm and mild climate. Wegener concluded that Spitsbergen must have been located closer to the equator at that time.
Thousands of kilometers to the south, geologists had found evidence that at the same time it was warm in Spitsbergen, the climate was much colder in South Africa. This evidence included deep scratches in rocks that 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, these clues provide evidence that continental drift happened. The climates of Spitsbergen and South Africa changed because the positions of these places on Earth’s surface changed. 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 location.
Scientists Reject Wegener’s Hypothesis
Wegener did more than provide a hypothesis about continental drift. He attempted to explain how drift took place. He even offered a new explanation for how mountains form. Wegener thought that when drifting continents collide, their edges crumple and fold. The folding continents slowly push up huge chunks of rock to form great mountains.
However, Wegener could not provide a satisfactory explanation for the force that pushes or pulls the continents.Because Wegener could not identify the cause of continental drift, most geologists rejected his idea. In addition, for geologists to accept Wegener’s idea, they would need to change their own explanations of what caused continents and mountains to form.
Many geologists in the early 1900s thought that Earth was slowly cooling and shrinking. According to this idea, mountains formed when the crust wrinkled like the skin of a dried-up apple. Wegener said that if the apple hypothesis were correct, then mountains should be found all over Earth’s surface. But mountains usually occur in narrow bands along the edges of continents. Wegener thought that his own hypothesis better explained where mountains occur and how they form.
For nearly half a century, from the 1920s to the 1960s, most scientists paid little attention to the idea of continental drift. Then new evidence about Earth’s structure led scientists to reconsider Wegener’s bold hypothesis."
1. What was Wegener’s hypothesis of continental drift? 2. How did Wegener use evidence based on fossils to support his hypothesis that the continents had moved?
3.What was the main reason scientists rejected Wegener’s hypothesis of continental drift?
4. Thinking Critically- Inferring Coal deposits have also been found beneath the ice of Antarctica. But coal only forms in warm swamps. Use Wegener’s hypothesis to explain how coal could be found so near the poles.
Look at a modern world map. Notice how the coasts of Africa and South America look as if they could fit together like jigsaw-puzzle pieces. Could the continents have once been a single landmass? In the 1700s, the first geologists thought that the continents had remained fixed in their positions throughout Earth’s history. Early in the 1900s, however, one scientist began to think in a new way about this riddle of the continents. His idea changed the way people look at the map of the world.
In 1910, a young German scientist named Alfred Wegener (vay guh nur) became curious about the relationship of the continents. He formed a hypothesis that Earth’s continents had moved!Wegener’s hypothesis was that all the continents had once been joined together in a single landmass and have since drifted apart.
Wegener named this supercontinent Pangaea (pan jee uh), meaning “all lands.” According to Wegener, Pangaea existed about 300 million years ago. This was the time when the fossil record contains the first evidence that reptiles and winged insects lived on Earth. Also, tropical forests, which later formed coal deposits, covered much of Earth.
World’s Continents Today’s continents provide clues about Earth’s history. Observing Which coastlines of continents seem to match up like jigsaw-puzzle pieces?
Wegener hypothesized that over tens of millions of years, Pangaea began to break apart. The pieces of Pangaea slowly moved toward their present-day locations, becoming the continents as they are today. Wegener’s idea that the continents slowly moved over Earth’s surface became known as continental drift.
Continental Drift
Have you ever tried to persuade a friend to accept a new idea? You probably had to provide some convincing evidence. Wegener gathered evidence from different scientific fields to support his ideas about continental drift. In particular, he studied landforms, fossils, and evidence that showed how Earth’s climate had changed over many millions of years. Wegener published all his evidence for continental drift in a book called The Origin of Continents and Oceans, first published in 1915.
Evidence From Landforms Wegener thought that mountain ranges and other features on the continents provided evidence for continental drift. When he pieced together maps of Africa and South America, he saw a mountain range running from east to west in South Africa that lined up with a mountain range in Argentina. European coal fields matched up with similar coal fields in North America. Wegener compared matching these features to reassembling a torn-up newspaper. If the pieces could be put back together, the “words” would match.
Evidence From Fossils Wegener also used fossil evidence as support for continental drift. A fossil is a trace of an organism that has been preserved in rock. For example, fossils of the reptilesMesosaurus and Lystrosaurus had been found in places now separated by oceans. Neither reptile could have swum long distances across salt water. Therefore, Wegener concluded that these reptiles once lived on a single landmass. Another example wasGlossopteris (glaw sahp tuh ris), a fernlike plant that lived 250 million years ago. Glossopteris fossils had been found in rocks in Africa, South America, Australia, India, and Antarctica. The occurrence of Glossopteris on these widely separated landmasses convinced Wegener that the continents had once been united.Fossil Evidence Fossils of the freshwater reptile Mesosaurus found in Africa and South America provide evidence of continental drift.
The seedlike structures of Glossopteris could not have traveled across the distances that separate the continents today. The “seeds” were too large for the wind to carry and too fragile to have survived an ocean trip. How did Glossopteris come to live on such widely separated continents? Wegener inferred that the continents at that time were joined as the supercontinent Pangaea.
Evidence From Climate Wegener also used evidence of climate change to support his hypothesis of continental drift. Spitsbergen is an island in the Arctic Ocean north of Norway. This island is ice-covered and has a harsh polar climate. But fossils of tropical plants had been found on Spitsbergen. When these plants lived about 300 million years ago, the island must have had a warm and mild climate. Wegener concluded that Spitsbergen must have been located closer to the equator at that time.
Thousands of kilometers to the south, geologists had found evidence that at the same time it was warm in Spitsbergen, the climate was much colder in South Africa. This evidence included deep scratches in rocks that 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, these clues provide evidence that continental drift happened. The climates of Spitsbergen and South Africa changed because the positions of these places on Earth’s surface changed. 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 location.
Scientists Reject Wegener’s Hypothesis
Wegener did more than provide a hypothesis about continental drift. He attempted to explain how drift took place. He even offered a new explanation for how mountains form. Wegener thought that when drifting continents collide, their edges crumple and fold. The folding continents slowly push up huge chunks of rock to form great mountains.
However, Wegener could not provide a satisfactory explanation for the force that pushes or pulls the continents.Because Wegener could not identify the cause of continental drift, most geologists rejected his idea. In addition, for geologists to accept Wegener’s idea, they would need to change their own explanations of what caused continents and mountains to form.
Many geologists in the early 1900s thought that Earth was slowly cooling and shrinking. According to this idea, mountains formed when the crust wrinkled like the skin of a dried-up apple. Wegener said that if the apple hypothesis were correct, then mountains should be found all over Earth’s surface. But mountains usually occur in narrow bands along the edges of continents. Wegener thought that his own hypothesis better explained where mountains occur and how they form.
For nearly half a century, from the 1920s to the 1960s, most scientists paid little attention to the idea of continental drift. Then new evidence about Earth’s structure led scientists to reconsider Wegener’s bold hypothesis."
1. What was Wegener’s hypothesis of continental drift? 2. How did Wegener use evidence based on fossils to support his hypothesis that the continents had moved?
3.What was the main reason scientists rejected Wegener’s hypothesis of continental drift?
4. Thinking Critically- Inferring Coal deposits have also been found beneath the ice of Antarctica. But coal only forms in warm swamps. Use Wegener’s hypothesis to explain how coal could be found so near the poles.