How Tsunamis Form
A tsunami begins when an underwater disturbance suddenly displaces a column of ocean water—that is, moves the water from its normal position. Sometimes, landslides trigger tsunamis. A chunk of land may break off the coast of a continent or an island and slide into the ocean. Or a volcano may erupt, spewing ash and molten rock onto the ocean floor. Even a meteorite or asteroid can set off a tsunami by plummeting into the sea.
However, most tsunamis occur because of earthquakes on or beneath the ocean floor. Most earthquakes, in turn, are a result of movements of the enormous tectonic plates that make up Earth's outer shell. Each plate consists of crust and the uppermost layer of mantle. The crust is the outermost layer of Earth. It makes up the continents and the ocean floor. The mantle is a thick layer of hot rock under the crust. The plates slide about on an even hotter layer of mantle. In some cases, plates press against each other. When a plate beneath the ocean presses against a plate that supports a continent, the oceanic plate can lift the continental plate or force it downward. Tectonic movements place a tremendous stress on the plates. So sometimes huge masses of rock suddenly break and shift, and an earthquake occurs.
If rock on or beneath the ocean floor shifts upward, the huge column of water above the earthquake site also shifts upward. But then the water column falls to the surface under the force of gravity, as the ocean seeks to flatten itself out again, or regain its equilibrium (state of balance). If the rock shifts downward instead of upward during the earthquake, the water column also drops down. The sea around the water column then rushes in, trying to reestablish equilibrium. In either case, the ocean's attempt to regain its normal shape produces tsunami waves that begin in the ocean above the epicenter of the earthquake and travel outward in all directions. (The epicenter of an earthquake is the point on the Earth's surface directly above the focus, the point within Earth where the rocks first break.)
As tsunami waves move across the ocean, they look and act different from ordinary, wind-driven waves. A wind-driven wave has a pronounced crest (highest point), but the crest of a tsunami wave is usually indistinguishable from the rest of the wave. Wind-driven waves also have a much shorter wavelength (distance between crests). The average wavelength of wind-driven waves is 100 to 200 meters (300 to 600 feet), but tsunami waves in the deep ocean are often 200 kilometers (125 miles) long. The tremendous wavelengths of such tsunamis make the waves virtually unnoticeable. A tsunami wave can be 1 to 2 meters (3 to 6 feet) high as it begins to move from the point above an earthquake. But its height is spread out over such a large wavelength that the wave cannot be seen or felt by people aboard ships, nor can it be noticed by anyone in an airplane above.
Tsunami waves also differ from wind-driven waves in their period (time between crests). Wind-driven waves usually have periods of 5 to 20 seconds, but the period of a tsunami wave can range from 10 minutes to an hour.
Despite their tremendous size, tsunami waves travel at incredibly high speeds. The speed of a tsunami depends on the depth of the ocean through which it is passing: the deeper the water, the greater its speed. Where the ocean is 6 kilometers (3.7 miles) deep, tsunamis travel faster than a commercial jet plane, about 800 kilometers (500 miles) per hour.
As a tsunami reaches shallower coastal waters, friction with the ocean floor slows it down. In addition, the height of each successive wave increases rapidly, as the wave encounters less and less room between the ocean surface and the ocean floor. As a result, the water piles up in a wall of destruction 30 meters (98 feet) or more in height—but even a tsunami 3 meters (10 feet) high can be extremely destructive.
Although tsunamis are popularly depicted as a giant breaking wave with a crest that towers over the land, most tsunamis actually strike the coast as a plateaulike wall of water or a tidelike flood. But sometimes the first sign of a tsunami is actually a withdrawal of water from the shore. If the tsunami originated when rock sank during an earthquake—causing the water column to drop down at the earthquake site—coastal waters may recede just before the tsunami strikes. The outward flow will expose more shoreline than shows at even the lowest tide. Many coastal dwellers have perished as they rushed to gather fish left high and dry by the departing waters, only to be hit by the incoming wall of water.