He called his hypothesis continental drift.
Besides the way the continents fit together, Wegener and his supporters collected a great deal of evidence for the continental drift hypothesis. For one, identical rocks of the same type and age are found on both sides of the Atlantic Ocean.
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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. Wegener concluded that they formed as a single mountain range that was separated as the continents drifted.
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. Wegener proposed that the organisms had lived side by side, but that the lands had moved apart after they were dead and fossilized. He suggested that the organisms would not have been able to travel across the oceans. For example, the fossils of the seed fern Glossopteris were too heavy to be carried so far by wind.
The reptile Mesosaurus could only swim in fresh water.
Cynognathus and Lystrosaurus were land reptiles and were unable to swim. Grooves and rock deposits left by ancient glaciers are found today on different continents very close to the equator.
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.
To his colleagues, his greatest problem was that he had no plausible mechanism for how the continents could move through the oceans. Based on his polar experiences, Wegener suggested that the continents were like icebreaking ships plowing through ice sheets. The continents moved by centrifugal and tidal forces. What observations would you expect to see on these continents? Figure 9. Early hypotheses proposed that centrifugal forces moved continents.
When Continental Drift Was Considered Pseudoscience
This is the same force that moves the swings outward on a spinning carnival ride. Scientists at the time calculated that centrifugal and tidal forces were too weak to move continents. When one scientist did calculations that assumed that these forces were strong enough to move continents, his result was that if Earth had such strong forces the planet would stop rotating in less than one year. In addition, scientists also thought that the continents that had been plowing through the ocean basins should be much more deformed than they are.
Wegener answered his question of whether Africa and South America had once been joined. But a hypothesis is rarely accepted without a mechanism to drive it. Are you going to support Wegener? Figure This rock then spreads out and cools, sinking back towards the core, where it can be heated again. This circulation of rock through the mantle creates convection cells. Wegener had many thoughts regarding what could be the driving force behind continental drift.
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In a convection cell , material deep beneath the surface is heated so that its density is lowered and it rises. Near the surface it becomes cooler and denser, so it sinks. Holmes thought this could be like a conveyor belt. Where two adjacent convection cells rise to the surface, a continent could break apart with pieces moving in opposite directions. Although this sounds like a great idea, there was no real evidence for it, either. Alfred Wegener died in on an expedition on the Greenland icecap. Answer the question s below to see how well you understand the topics covered in the previous section.
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