Early in the 20th century, Alfred Wegener suggested that the all the world’s continents had once been part of a single supercontinent, which he called Pangea. Over hundreds of millions of years, the continents drifted apart—and even now they continued to drift (very slowly) across the face of the planet. Other geologists were dubious, but over time they found enough evidence to prove Wegener correct.

This theory is now called plate tectonics. The plates "float"on the asthenosphere, a layer of soft, molten rock, and are pushed about by convection currents rising up from the mantle below. The plates move just an inch or two each year. But that's enough to create enormous stress when two plates run into each other.

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