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A tsunami is a series of ocean waves that send surges(汹涌) of water, sometimes reaching a height of over 100 feet, onto land. These walls of water can cause widespread destruction when they crash ashore.

These fear-inspiring waves are typically caused by large, undersea earthquakes at plate boundaries. When the ocean floor at a plate boundary rises or falls suddenly, it displaces the water above it and launches the rolling waves that will become a tsunami.

Tsunamis may also be caused by underwater landslides(山崩) or volcanic eruptions. They may even be launched, as they frequently were in Earth’s ancient past, by the impact of a large meteorite(陨石) falling into an ocean.

Tsunamis race across the sea at up to 500 miles an hour — about as fast as a jet airplane. At that pace they can cross the entire area of the Pacific Ocean in less than a day. And their long wavelengths mean they lose very little energy along the way.

In deep oceans, tsunami waves may appear only a foot or so high. But as they approach shoreline and enter shallower water they slow down and begin to grow in energy and height. The tops of the waves move faster than their bottoms do, which causes them to rise steeply.