continenta drift

What is Continental Drift?


The theory of continental drift reconciled similar fossil plants and animals now found on widely separated continents. Gondwana is shown here.
Credit: USGS
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Continental drift was the first attempt to explain why similar animal and plant fossils are found on different continents. The theory was set forth in 1912 by Alfred Wegener andpublished in a 1915 book. He wasn't the first person to suggest Earth's continents were once connected in a giant land mass called a supercontinent, but he was the most reviled.
Wegener's theory of continental drift was soundly denounced by geologists. Part of the opposition was because Wegener didn't have a good model to explain how the continents moved back and forth.
Though most of Wegener'sobservations were correct, he was wrong on a couple key points. Wegener thought the continents drifted around the planet by plowing through the crust underlying the seafloor, like icebreakers smashing through ice. He also suggested they sailed at a speedy 10 inches (250 millimeters) per year. At the time, geologists thought Earth's mountains and other topography were created as it cooled and shrunk sincethe planet formed. Scientists now know Earth's outer shell moves about ten times more slowly than Wegener thought, in giant tectonic plates that include the ocean and continents. [Have There Always Been Continents?]
A map of the continents inspired Wegener's quest to explain Earth's geologic history. Trained as a meteorologist, he was intrigued by the interlocking fit of Africa's and SouthAmerica's shorelines. Wegener then assembled an impressive amount of evidence to show that Earth's continents were once connected in a single supercontinent, which he named Pangaea.

Tectonic plates of the Earth.
Credit: USGS
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Evidence for continental drift
Wegener knew that fossil plants and animals such as mesosaurs, a freshwater reptile found only South America and Africaduring the Permian period, could be found on many continents. Previously, scientists had explained the widely separated fossils by suggesting a land bridge connected the continents.
He also matched up rocks on either side of the Atlantic Ocean like puzzle pieces. For example, the Appalachian Mountains (United States) and Caledonian Mountains (Scotland) fit together, as do the Karroo strata inSouth Africa and Santa Catarina rocks in Brazil.
Despite his incredible evidence for continental drift, Wegener never lived to see his theory gain wider acceptance. He died in 1930 at age 50 of a probable heart attack while on a scientific expedition in Greenland.
In the 1950s, the discovery of new oceanic crust forming at seafloor-spreading ridges led researchers to revive the idea of continentaldrift and develop the theory of plate tectonics
Continental Drift and Plate-Tectonics Theory
Source: U.S. Dept. of the Interior, Geological Survey
According to the theory of continental drift, the world was made up of a single continent through most of geologic time. That continent eventually separated and drifted apart, forming into the seven continents we have today. The first comprehensivetheory of continental drift was suggested by the German meteorologist Alfred Wegener in 1912. The hypothesis asserts that the continents consist of lighter rocks that rest on heavier crustal material—similar to the manner in which icebergs float on water. Wegener contended that the relative positions of the continents are not rigidly fixed but are slowly moving—at a rate of about one yard percentury.
According to the generally accepted plate-tectonics theory, scientists believe that Earth's surface is broken into a number of shifting slabs or plates, which average about 50 miles in thickness. These plates move relative to one another above a hotter, deeper, more mobile zone at average rates as great as a few inches per year. Most of the world's active volcanoes are located along or...