An important piece of plate tectonic theory is the continental drift idea. This was developed in the early part of the 20th century, mostly by a single scientist, Alfred Wegener. His hypothesis states that continents move around on Earth's
An important piece of plate tectonic theory is the continental drift idea. This was developed in the early part of the 20th century, mostly by a single scientist, Alfred Wegener. His hypothesis states that continents move around on Earth's surface and that they were once joined together as a single supercontinent (Figure 6.5). Wegener's idea eventually helped to form the theory of plate tectonics, but while Wegener was alive, scientists did not believe that the continents could move.
- Be able to explain the continental drift hypothesis.
- Describe the evidence Wegener used to support his continental drift idea.
- Describe how the north magnetic pole appeared to move, and how that is evidence for continental drift.
The Continental Drift Idea
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. In this case, your continent puzzle piece includes all of the continental crust for that continent and reflects the true size and shape of the continent. Can you fit the pieces together? The easiest link is between the eastern Americas and western Africa and Europe, but the rest can fit together too!
Alfred Wegener, an early 20th century German meteorologist believed that the continents could fit together. He proposed that the continents were not stationary but that they had moved during the planet's history. He suggested that at one time, all of the continents had been united into a single supercontinent. He named the supercontinent Pangaea, meaning entire earth in ancient Greek. Wegener further 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 fit of the continents, Wegener and his supporters collected a great deal of evidence for the continental drift hypothesis. Wegener found that this evidence was best explained if the continents had at one time been joined together.
Wegener discovered that identical rocks could be found on both sides of the Atlantic Ocean. These rocks were the same type and the same age. Wegener understood that the rocks had formed side-by-side and that the land has since moved apart. Wegener also matched up mountain ranges that had the same rock types, structures, and ages, but that 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.
Wegener also found evidence from ancient fossils (Figure 6.6). He found fossils of the same species of extinct plants and animals in rocks of the same age, but on continents that are now widely separated. Wegener suggested that the continents could not have been in their current positions because the organisms would not have been able to travel across the oceans. For example, fossils of the seed fern Glossopteris are found across all of the southern continents. But the plants' seeds were too heavy to be carried across the ocean by wind. Mesosaurus fossils are found in South America and South Africa, but the reptile only could only swim in fresh water. Cynognathus and Lystrosaurus were reptiles that lived on land. Both of these animals were unable to swim, let alone swim across wide seas! Their fossils have been found across South America, Africa, India and Antarctica. Wegener proposed that the organisms had lived side by side, but that the lands had moved apart after they were dead and fossilized.
Wegener also looked at evidence from ancient glaciers. Large glaciers are most commonly found in frigid climates, usually in the far northern and southern latitudes. Using the distribution of grooves and rock deposits left by ancient glaciers on many different continents, Wegener traced the glaciers back to where they must have started. He discovered that if the continents were in their current positions, the glaciers would have formed in the middle of the ocean very close to the equator. Wegener knew that this was impossible! However, if the continents had moved, the glaciers would have been centered over the southern land mass much closer to the South Pole.
Wegener also found evidence for his hypothesis from warm climate zones. Cora