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Walls of Water

By Nelida Gonzalez Cutler

At 7:58 a.m. on December 26, the earth's crust trembled. The powerful undersea quake set off shock waves that were felt more than 3,000 miles away. Walls of water called a tsunami swept across the Indian Ocean.

A tsunami is not one wave. It is a series of very long waves. Sometimes there are hundreds of miles between the waves' crests. The waves race along the ocean at speeds that can reach 600 miles an hour. In deep, open water, a tsunami may pass unnoticed. The height of the waves may be only one to two feet. But when the water's depth decreases, the length of the waves become shorter and the height increases. The waves can crest to more than 30 feet. Tsunamis crash onto the shore with powerful, destructive force.

From the shore, tsunamis appear similar to a rising or falling tide. But tsunamis are not tidal waves, because they are not affected by the gravitational pull of the moon. The swells of water are set off by disturbances that suddenly change the sea level. Quakes, landslides and underwater volcanic eruptions can all cause tsunamis.

Slipping and Sliding
The earth's curst is like a giant jigsaw puzzle. The pieces, called tectonic plates, are constantly and slowly shifting. An earthquake occurs when two plates grind against each other and suddenly slip.

The floor of the Indian Ocean--the Indian plate--is moving north at around 2.5 inches per year. That's about twice the rate that your fingernails grow. Scientists estimate that on December 26, the Indian plate and the Burma plate moved about 50 feet at once! It was this sudden shift that sent the killer waves racing across the Indian Ocean.

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