Tsunamis are caused by earthquakes at sea. Earthquakes happen when the plates that make up the Earth's surface suddenly move against each other.
A tsunami forms when energy from an earthquake vertically jolts the seabed by several metres, displacing hundreds of cubic kilometres of water.
Large waves begin moving through the ocean, away from the
In deep water, the tsunami moves at great speeds. When it reaches shallow water near coastal areas, the tsunami slows but increases in height.
In the devastating tsunami of December 2004, many coastal areas in the Indian Ocean had almost no warning of the approaching tsunami.
The only sign came just before it struck, when the waterline suddenly retreated, exposing hundreds of metres of beach and seabed.
The several waves of the tsunami came at intervals of between five and 40 minutes. In Kalutara, on the Sri Lankan coast, the water reached at least 1km (0.6 miles) inland, causing widespread destruction.
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