About 100 years ago, German scientist Alfred Wegener noticed that the coasts of western Africa and eastern South America matched up like pieces of a jigsaw puzzle. Although he was not the first to notice this, Wegener was the first to formally present evidence suggesting that the two continents had once been connected. Wegener's hypothesis was called "continental drift".
Wegener was convinced that the two continents were once part of an enormous super-continent called Pangaea, meaning “all earth,” that split apart over hundreds of millions of years. He knew that the two areas had many geological and biological similarities. For example, fossils of the ancient reptile mesosaurus are only found in southern Africa and South America. Mesosaurus, which lived in freshwater lakes and rivers, was only about one meter long and would not have been able to swim across the salty Atlantic Ocean.
For a very long time, other scientists did not accept Wegener’s theory of continental drift. But today, scientists know that the Earth's lithosphere is broken into massive tectonic plates that float on the asthenosphere, and are always moving and interacting. This is called the theory of plate tectonics.
So then what is causing the tectonic plates to move? Convection currents in the Earth's mantle. As a result of the uneven heating of the Earth's plastic mantle (asthenosphere), hotter, less dense mantle is continually rising, and cooler, more dense mantle is continually sinking. This transfer (and cycling) of heated material is called convection. Convection can occur in any gas or liquid where there is uneven heating.
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The constant movement changes the Earth's surface, rearranging and reshaping its landmasses, creating mountains, new sea floor, volcanoes, Earthquakes, and land rifts. In fact, the continents are still moving. North America and Europe are moving away from each other at the rate of about 2.5 centimeters per year. In another 360 million years, it is even possible that another super-continent may form someday!