Drifting Continents • Notes
Continental Drift • Some continents appear to be shaped like puzzle pieces. For example the west side of Africa and the east side of South America look like matching puzzle pieces.
Continental Drift • Scientists Alfred Wegener tried to explain why continents are shaped the way that they are. • Until the 1900s scientists thought that the continents had always remained in the same place. • Even then it wouldn’t be until the 1970s until the idea was firmly planted into mainstream society.
Continental Drift • Wegener thought that Earth had one big continent about 300 million years ago. The big continent broke into smaller pieces and formed smaller continents. The continents slowly drifted apart. Wegener called this continental drift. • According to Wegener, the continents has drifted together to form the supercontinent Pangea and a single superocean called Panthalassa.
Continental Drift • Evidence shows that continental drift really happened. Mountain ranges in Africa and South America as well as North America and Africa line up as if they were once part of the same mountain range. • At the time, most scientists believed that mountains formed because Earth was still cooling and shrinking, causing the surface to wrinkle like that of a dried-up apple. • Wegener claimed that if this were the case, then mountains would be everywhere. He proposed that because mountain ranges usually occur near continental edges, then they formed when continents collided with each other.
Formation of the Appalachian Mountains Matching Mountain Ranges across S. America and Africa
Continental Drift • Wegener also used the existence of specific fossils spanning multiple continents that are now separated by vast oceans as evidence that the continents were once connected. • His primary fossil was Glossopteris, a fern-like plant that lived 250 million years ago, and can be found on Africa, S. America, Australia, India and Antarctica. • The existence of Mesosaurus and Lystrosaursacross many continents also supported Wegener’s hyposthesis.
Continental Drift • Wegener also used evidence from climate change to support his hypothesis. As a continent moves away from the equator, its temperature will drop. As a continent moves towards the continent, its temperature will increase. • Tropical plant fossils have been found in islands above the Arctic Circle, meaning that at one time that landmass was in a much warmer climate.
Continental Drift • Large scratches in the bedrock of South America prove that at one point in its history, S. America was once much close to the south pole. These scratches only appear where continental glaciers once existed on top of the land. • Currently both Greenland and Antarctica are covered by continental glaciers. As the glaciers melt in the summer months, they leave behind large scratches or grooves in the rocks over which they once covered.
Wegener’s Hypothesis Rejected • Despite all of his evidence, Wegener could not explain what causes continental drift. He saw that it did indeed happen, but he couldn’t explain how or why. • His original idea had the continents plowing through the ocean floor. (oops) • Because he did not have a fully developed idea, most other scientists of his time thought that he was wrong. • More evidence would be required before Wegener’s ideas would be accepted.