Earth's hard outer shell, known as the lithosphere, is not continuous across the surface of the planet. Instead, it consists of 12 rigid plates between 60 km (37 mi) and 200 km (124 mi) thick that are composed of continental crust, oceanic crust, and the upper part of the mantle. These plates "float" on the underlying and more flexible layer known as the aesthenosphere. When two plates converge, pressure forces sections of crust to pile up, and mountain systems and volcanoes form on the overriding plate. Meanwhile, the subducting plate plunges deep into the mantle. Large earthquakes with epicenters located at undersea subduction zones are the most frequent causes of powerful ocean waves known as tsunamis.

Tsunamis carry the energy produced by earthquakes or, less frequently, other Earth disturbances such as volcanoes, landslides, and meteorites. When tsunami waves break on land, this energy is released and can cause catastrophic damage. Certain physical characteristics of tsunami waves along with the waves' behavior upon reaching land explain a tsunami's enormous capacity for destruction.

The deeper the water, the faster tsunami waves travel. Despite their high speed, in the open ocean tsunami waves have a low amplitude (height) and long wavelength (distance between crests). Because waves lose energy at a rate inversely related to their wavelength, tsunami waves typically experience very little energy loss as they travel across the ocean.

When a tsunami wave nears land, its wave profile changes drastically. In the open ocean, the wave energy extends from the surface to the bottom, but in shallower water, the wave gets compressed. As the wave's leading edge interacts with the rising seafloor, it slows. The water moving in behind piles up, so that when the wave finally reaches the shore, it may have risen to tens of meters (tens of yards) in height.

If the trough (or low part) of the wave is first to reach the shore, water at the shore gets drawn seaward before the peak (or high part) arrives. To help understand why this happens, imagine a string arranged in a wave pattern on a table. If you shorten the wavelength, the amplitude increases and the ends of the string pull inward. Likewise, as a tsunami wave approaches shore, its wavelength shortens, pulling water from all directions, including the shoreline.

Because water is very heavy -- a cubic meter weighs a metric ton -- a tsunami wave is capable of inflicting immense destruction on land. The presence of reefs and steep coastal shelves, however, can act as a breakwater to lessen the force of the wave.

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