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NOAA
What Is a Tsunami?
A tsunami (pronounced tsoo-NAH-mee) is a series of waves, made in an ocean or other body of water by an earthquake, landslide, volcanic eruption, or meteorite impact. Tsunamis can cause huge destruction when they hit coastlines. Some people call tsunamis "tidal waves", but these large waves really have little to do with tides, so the term "tidal wave" does not really suit them.
Tsunami waves are different from the waves you can usually find rolling into the coast of a lake or ocean. Those waves are made by wind offshore and are quite small compared with tsunami waves. A tsunami wave in the open ocean can be more than 100 km across. That's roughly the length of 1000 American football fields! Tsunami waves are huge and can travel very quickly, at about 700 km/hr, but they are only about one meter high in the open ocean.
As a tsunami wave travels into the shallower water near the coast, it slows and grows in height. Even though a tsunami may be barely visible at sea, it may grow to be many meters high near the coast and have a tremendous amount of energy. When it finally reaches the coast, a tsunami may appear as a rapidly rising or falling tide or a series of waves with a maximum height of up to 30 meters.
A few minutes before a tsunami wave hits, the water near shore may move away, exposing the ocean floor. Often the first wave may not be the largest, and additional waves may arrive at the coast every 10 to 60 minutes. They move much faster than a person can run. The danger from a tsunami can last for several hours after the arrival of the first wave. Unlike other waves, tsunami waves typically do not curl and break.
Coasts affected by a tsunami will be severely eroded. A tsunami can cause flooding hundreds of meters inland. The water moves with such force that it is capable of crushing homes and other buildings.
