A satellite image of a swirling hurricane Men with gas masks and protective suits on The planetary rover Sojourner A large group of refugees huddled together







Most of Earth’s processes happen gradually, over long periods of time, sometimes eons. Earthquakes happen suddenly. Subterranean forces overcome the resistance of friction along fault lines—ancient cracks in the Earth’s crust—and cause gigantic plates of rock to shudder as they slip past each other.

The major cause of earthquakes is shifting tectonic plates, the dozen or so fragments of crust that float on the Earth’s thick mantle. Most occur at the boundaries, for example the Ring of Fire along the margins of the Pacific Ocean. Mid-plate earthquakes, like the one at New Madrid, are usually large and destructive—and less well understood.

Damage from earthquakes is not limited to buildings, bridges, and dams. They can trigger fires and landslides: When loose soil, such as landfill, loses its ability to bear loads, the ground behaves like quicksand. Buildings can sink and even disappear.

When they occur at sea, earthquakes can generate seismic waves called tsunamis, which travel great distances at speeds equaling those of commercial jetliners. Tsunamis are barely noticeable—often just ripples on the surface—until they approach shores, when they become monsters.

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View animation of how quakes form.

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Earthquakes were poorly understood until the beginning of the 20th century.

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