By Memorie Yasuda

It's no ordinary day at the beach when ocean water suddenly pulls out to sea, leaving flopping fish and stranded boats along the shore. When a huge wave hits just moments later, plowing over small buildings and everything in its path, that's not business as usual either.

But it happened on December 26, 2004, when a series of waves appeared out of the blue and killed more than 250,000 people who were living at or visiting coastal areas of the Indian Ocean.

These deadly trains of waves are called tsunamis (tsoo-NAH-mees)-a Japanese word for "great harbor wave." Tsunamis are capable of moving across entire oceans at speeds greater than 500 miles an hour. That's as fast as a jet airplane. The recent Indian Ocean tsunami, which was triggered by the great Sumatra earthquake, raced around the world from the epicenter of the earthquake and arrived in places as far away as New Jersey 32 hours later.

What is a Tsunami?

Although often called tidal waves, tsunamis have nothing to do with tides and they are not driven by the wind like everyday waves at the beach. They are caused by movements of the earth under the sea–such as as earthquakes, undersea landslides, and undersea volcanic eruptions-or by impacts of objects from outer space–such as asteroids or comets. Like a pebble dropped in a pond, these events unleash a series of waves that radiate outward in widening circles. All of these events generate tsunamis by displacing water rapidly.

While out in the open ocean, tsunamis are so small that you wouldn't notice if one crossed your path. They are rarely more than one to two feet tall and the waves are spaced more than 100 miles apart.

When tsunamis pass through the ocean, their effects extend all the way down to the deepest parts of the ocean–more than six miles down. Tsunamis carry and transmit a huge amount of energy across entire oceans. They only begin to lose their energy as they enter shallow water and slow down. The path of a tsunami is influenced by the shape of the seafloor below.

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As tsunamis approach the shore they increase in height. Waves of the 2004 Indian Ocean tsunamis reached heights more than 10 meters (30 feet). As they spilled onto land, the waves looked more like a thick river of ooze than waves with a good curl to surf.

When the crest (highest part) of a tsunami hits land, water rushes inland and pushes against everything in its path. When the trough (lowest part) reaches the coast, water is pulled offshore and carries material and debris out to sea. The first tsunami wave to arrive can cause water to either recede or move landward.

Even though tsunamis slow down when they get close to shore, they still move at speeds of tens of miles an hour, which is much faster than you can run. So a person should never walk out onto the exposed seafloor after the water has receded because another tsunami wave will arrive within minutes. At the time of the 2004 Indian Ocean tsunami, a vacationing 10-year-old British girl saw water rapidly being pulled out to sea and recognized this as the sign of an impending tsunami wave. She was able to alert other beachgoers and save hundreds of lives.

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