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5.1: Continental Drift

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"Doesn't the east coast of South America fit exactly against the west coast of Africa, as if they had once been joined? This is an idea I'll have to pursue." - Alfred Wegener to his future wife, December, 1910.

We can’t really get into Alfred Wegner’s head, but we can imagine that he started his investigations by trying to answer this question: Why do the continents of Africa and South America appear to fit together so well? Is it an accident that they do, or is there some geological reason?

Wegener’s Idea

Alfred Wegener, born in 1880, was a meteorologist and explorer. In 1911, Wegener found a scientific paper that listed identical plant and animal fossils on opposite sides of the Atlantic Ocean. Intrigued, he then searched for and found other cases of identical fossils on opposite sides of oceans. The explanation put out by the scientists of the day was that land bridges had once stretched between these continents.

Instead, Wegener pondered the way Africa and South America appeared to fit together like puzzle pieces. Other scientists had suggested that Africa and South America had once been joined, but Wegener was the idea’s most dogged supporter. Wegener amassed a tremendous amount of evidence to support his hypothesis that the continents had once been joined.

Imagine that you’re Wegener’s colleague. What sort of evidence would you look for to see if the continents had actually been joined and had moved apart?

Wegener’s Evidence

Here is the main evidence that Wegener and his supporters collected for the continental drift hypothesis:

  • The continents appear to fit together.
  • 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 ( Figure below ). Wegener proposed that the organisms had lived side by side, but that the lands had moved apart after they were dead and fossilized. His critics suggested that the organisms moved over long-gone land bridges, but Wegener thought that the organisms could not have been able to travel across the oceans.
    • 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.

Wegener used fossil evidence to support his continental drift hypothesis. The fossils of these organisms are found on lands that are now far apart.

  • 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 below ). Wegener concluded that they formed as a single mountain range that was separated as the continents drifted.

The similarities between the Appalachian and the eastern Greenland mountain ranges are some of the evidence for the continental drift hypothesis.

  • 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 drifted to new locations on the continents. An animation showing that Earth’s climate belts remain in roughly the same position while the continents move is seen here: http://www.scotese.com/paleocli.htm .
  • Wegener thought that mountains formed as continents ran into each other. This got around the problem of the leading hypothesis of the day, which was that Earth had been a molten ball that bulked up in spots as it cooled (the problem with this idea was that the mountains should all be the same age and they were known not to be). An animation showing how the continents split up can be found here: http://www.exploratorium.edu/origins/antarctica/ideas/gondwana2.html .

Summary

  • Alfred Wegener did some background reading and made an observation.
  • Wegener then asked an important question and set about to answer it.
  • He collected a great deal of evidence to support his idea. Wegener’s evidence included the fit of the continents, the distribution of ancient fossils, the placement of similar rocks and structures on the opposite sides of oceans, and indicators of ancient climate found in locations where those climates do not exist today.

Practice

Use these resources to answer the questions that follow.

1. Who was Alfred Wegener?

2. What evidence did Wegener find for Pangaea?

3. What was the response to Wegener's hypothesis?

4. What is the continental drift hypothesis?

5. What do the continental plates consist of?

6. What were formed when Pangaea broke apart?

7. How long ago did the continents reach their present position?

Review

1. How did Wegener become interested in the idea that continents could move?

2. What did he need to do to explore the question and make it into a reasonably hypothesis?

3. How did Wegener use fossil evidence to support his hypothesis?

4. How did Wegener use climate evidence from rocks to support his hypothesis?

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Feb 24, 2012

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Oct 27, 2014
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