SALLY BENSUSEN / SCIENCE PHOTO LIBRARY SALLY BENSUSEN / SCIENCE PHOTO LIBRARY
Tsunami. Illustration showing how a tsunami forms. Although sometimes called tidal waves, tsunamis are not caused by the tide but by abrupt physical displacement of the seabed, which is usually caused by an earthquake (epicentre at left). The sudden disturbance sets a series of waves rolling. At first these are broad and low but extremely fast, moving at up to 800 km per hour. When the tsunami reaches shallow coastal water the front of each wave hits the seabed and slows down, which causes the crest of each to rise. Towering masses of water up to 30 metres in height then crash into the coast, demolishing buildings and sweeping trees and cars out to sea.
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