Evidence for Continental Drift

Continental drift refers to the idea that the present continents once formed a single. giant continent called Pangaea, and since that time have been slowly drifting apart. In 1915 the originator of this idea, the German meteorologist Alfred Wegener, was impressed by the close resemblance of coastlines of continents on opposite sides of the Atlantic Ocean, particularly South America and Africa. However,the configuration of the coastlines results from erosional and depositional processes and therefore is constantly being modified, so even if Wegener was right and the continents had separated in the distant past, it is not likely that the coastlines would fit exactly. But decades later it was shown that the continents fit together well along the continental slope (the broad underwater shelf on the edges of continents), where crosion is minimal, and recent studies have confirmed the close fit between continents when they are reassembled to form Pangaea.

If the continents were at one time joined, then the rocks and mountain ranges of the same age in adjoining locations on the opposite continents should closely match. Such is the case for the continents thought to have together formed the southern supercontinent Gondwana when Pangaca broke up into a northern and a southern supercontinent, mostly during the Jurassic period (200-146 million years ago) Antarctica, South America, Africa,Australia-New Guinea, and India comprised Gondwana. Marine, nonmarine, and glacial rock sequences of the Pennsylvanian epoch(325-299 million years ago)to the Jurassic period are almost identical for all five Gondwana continents, strongly indicating that they were joined at one time. The trends of several major mountain ranges also support the hypothesis of continental drift. The folded Appalachian Mountains of North America, for example, trend northeastward through the eastern United States and Canada and terminate abruptly at the Newfoundland coastline. Mountain ranges of the same age and deformational style occur in eastern Greenland, Ireland Great Britain, and Norway. Even though these mountain ranges are currently separated by the Atlantic Ocean, they form an essentially continuous mountain range when the continents are positioned next to each other.

During the later part of the Paleozoic era(544-255 million years ago massive glaciers covered large continental areas of the Southern Hemisphere, leaving behind layers of till(sediment deposited by glaciers) and striations (scratch marks) in the bedrock beneath the till. Fossils and sedimentary rocks of the same age from the Northern Hemisphere, however, give no indication of glaciation.Fossil plants found in coals indicate that the Northern Hemisphere had a tropical climate during the time that the Southern Hemisphere was glaciated.

All the Gondwana continents except Antarctica are currently located near the equator in subtropical to tropical climates. Mapping of glacial striations in bedrock in Australia India, and South America indicates that the glaciers moved from the areas of present-day oceans onto land. This would be highly unlikely because large continental glaciers (such as occurred on the Gondwana continents during the late Paleozoic era) flow outward from their central area of accumulation toward the sea. If the continents had not moved in the past, one would have to explain how glaciers moved from the oceans onto land and how large-scale continental glaciers formed near the equator. But if the continents are reassemble as a single landmass with South Africa located at the South Polc. the direction of movement of late Paleozoic continental glaciers makes sense. Furthermore, this geographic arrangement places the northern continents nearer the tropics, which is consistent with the fossil and climatologic evidence.

Finally, some of the most compelling evidence for continental drift comes from the fossil record. For example, fossils of Glossopteris, a group of woody shrubs, are found in equivalent Pennsylvanian and Permian coal deposits (299-251 million years ago) on all five Gondwana continents, Glassopteris shrubs produced seeds too large to have been carried by winds, and even if the seeds had floated across the ocean, they would not have remained viable for any length of time in salt water. The present-day- climates of the southern continents range from tropical to polar and are much too diverse to support the type of plants in the Glossopteris flora. These continents must once have been joined so that these widely separated localities were all in the same latitudinal climatic belt.?

1.Continental drift refers to the idea that the present continents once formed a single. giant continent called Pangaea, and since that time have been slowly drifting apart. In 1915 the originator of this idea, the German meteorologist Alfred Wegener, was impressed by the close resemblance of coastlines of continents on opposite sides of the Atlantic Ocean, particularly South America and Africa. However,the configuration of the coastlines results from erosional and depositional processes and therefore is constantly being modified, so even if Wegener was right and the continents had separated in the distant past, it is not likely that the coastlines would fit exactly. But decades later it was shown that the continents fit together well along the continental slope (the broad underwater shelf on the edges of continents), where crosion is minimal, and recent studies have confirmed the close fit between continents when they are reassembled to form Pangaea.?