Before the 20th century, people did not know that the continents moved around the planet. Continental drift is such a slow process that you can't see land masses shift with the naked eye. Because the continents never stop moving, however, the world map you know today will not look the same in the distant future.
Continental Motion: First Clues
In 1915, Alfred Wegener published "The Origin of Continents and Oceans," a book that shares his theories about continental drift. He wasn't the first to notice how Africa and South America seemed to fit together like jigsaw puzzle pieces. But he was the first to present scientific evidence that showed that these continents were once one land mass.
Scientists discovered the remains of a mesosaurus in two places: South America and the southern part of Africa. Because this extinct reptile could not have swum between the two continents, a likely explanation for its presence in both places is that they were once a single land mass. In the 1950s, new discoveries in fields such as paleomagnetism caused most scientists to accept the fact that continents move. Not only does tectonic motion separate land masses, but it causes earthquakes, makes volcanoes erupt and builds mountains.
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A supercontinent is a land mass composed of other continents. Geologists believe that all the Earth's continents once formed Pangaea, a supercontinent that existed about 225 million years ago. Because the continents are now unique entities, you also see separate oceans, such as the Atlantic and the Pacific.
It's All About the Plates
Plate tectonic theory explains why continents continue to move. The planet's outer shell consists of plates that move a few centimeters a year. Heat from the Earth's interior causes this motion to happen via convection currents in the mantle. Over a period of millions of years, this slow motion caused the single supercontinent to split into the seven continents you see today.
Plate Activity Changes the Earth's Crust
Most plate movement happens in boundaries that lie between different plates. When plates move away from one another, new crust forms at divergent boundaries. Conversely, tectonic motion destroys crust when one plate moves beneath another at convergent boundaries. At transform boundaries where plates simply move past each other horizontally, the motion does not create or destroy crust. Geologists also observe plate boundary zones where boundaries between plates are not defined well.
View Tectonic Motion in Action
Visit Krafla Volcano in Iceland, and you'll see cracks in the ground that get wider within a few months. Surface cracking between 1975 and 1984 caused displacements in the ground of about 7 meters (22 feet). Scientists can track plate motion on a small scale using laser instruments to take surveys. Satellites help scientists to take precise measurements of locations on Earth to observe how they move. They call this process space geodesy.