64 Tsunami!-High

Tsunami! FOCUS Question What are the causes and effects of tsunamis?

What Is a Tsunami? A giant wall of water swamps a coastal town in mere minutes. This is no regular ocean wave— it's a tsunami! A tsunami (tsoo-NAH-mee) is a group of huge ocean waves that rush onshore. Regular ocean waves are caused by wind, but tsunami waves are caused by sudden movements of the ocean floor. Tsunami waves are fast and dangerous, traveling far across the ocean at up to 966 kilometers per hour (600 mph) before they reach land. That's faster than a jet airplane! Word Wise Tsunamis are not "tidal waves." Tsunamis are caused by geologic movements, not by tides.

Making Waves Underwater earthquakes are the most common cause of tsunamis. Earth's surface is made up of many enormous, rocky plates. Earthquakes happen where two plates rub together and slide past each other. When an earthquake shakes the land under an ocean, the seafloor gets pushed up. This force pushes on the water and can trigger a tsunami. The waves that form move outward in every direction from the source of the earthquake. Several of Earth's plates meet in the Pacific Ocean. Earthquakes and tsunamis occur more frequently there than in other places. Of course, earthquakes don't only happen under the ocean. They sometimes cause landslides that suddenly force rocks and soil into the ocean. This sudden movement is another cause of tsunamis.

Landslides also happen underwater when large amounts of rock and dirt on the ocean floor move suddenly. If this disturbance is strong enough to affect the water above, a tsunami forms. In 1998, a terrible tsunami hit the coast of Papua New Guinea. This tsunami was caused by an underwater landslide. Volcanic eruptions above and below the ocean's surface can also result in powerful waves. When volcanoes erupt, rock and debris fall into the ocean and push the water, causing a tsunami. In 1883, a volcano known as Krakatau erupted in Indonesia, causing a tsunami that harmed people hundreds of kilometers away.

Hazards and Warnings Whatever the cause, a tsunami is a natural hazard— a natural event that causes damage and endangers people. Any low area near a coast is at risk. At first, a tsunami may cause the water near the shore to retreat, making a beach seem bigger and exposing the ocean floor. Then the first wave hits. The danger can last for hours afterward as more waves keep hitting. The waves can flood the land hundreds of meters inland with a wall of fast-moving water, taking many lives in the process. On beaches, they can cause erosion— the destruction of an area as sand, sediment, rocks, and plants are cleared away by the force of the water. Waves can easily sweep up heavy boulders, boats, and cars, carrying them inland. The waves can also destroy buildings, bridges, and roads.

Because they can happen very suddenly, earthquakes, landslides, and volcanic eruptions are hard to predict. As a result, scientists do not know where or when the next tsunami will strike. But once a tsunami starts, they can detect it with special machines. Tsunami warning centers are run by scientists working all around the world. These experts use tsunami detectors placed in the deep ocean to record movements of the water. Then they use computer models to predict where and when the dangerous waves may hit. The centers warn officials of the threat with the hope of reducing the loss of life and property. Fact or Fiction? Objects from space can cause tsunamis. That's a fact! About two million years ago, a huge asteroid struck the ocean near Chile. It caused a tsunami that swamped parts of South America and Antarctica.