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Volcano Learning Zone > Volcanic Hazards>Tsunamis

Tsunamis are waves or surges on the sea caused by the disturbance of the sea bed or by material falling into the sea. Tsunamis can move at 100s mph and inundate coastal areas.If a volcanic eruption quickly deposits a large amount of material in a body of water,such as a landslide, then a displacement of water occurs which radiates outwards. In the sea this called a Tsunami but can occur in lakes too. The Tsunami will move through deeper water with a long wavelength and will be undetectable by boats.When the tsunami reaches shore as the water shallows wavelength will shorten and the height of the wave increase. The Tsunami will surge ashore rather then break like a normal sea wave. A warning sign of a Tsunami is the the sea retreats exposing the seabed before the wave suddenly advances.A common misconception is that tsunami is do with the tide , this not so!

Krakatoa May 1883

Debris on Spirit Lake with mt St Helens in the background USGSKrakatoa is a volcano near Java in Indonesia. In 1883 it erupted with the loudest sound heard in modern history. A series of eruptions produced eruption columns which collapsed producing pyroclastic flows which plunged into the sea displacing creating tsunmais. The tsunamis were 40m high and killed 90% of the 36,000 killed by the eruption.Cruelly those who surived the tsunamis by rushg to higher ground ere killed by pyroclastic floes which had raced out over the sea on a cushion of steam.

Mt St Helens

Mount St Helens in 1980 unleasheded a wave 250m high into Spirit Lake when the northern slope of the mountain slid into Spirit Lake. The wave was 6 times higher than the Krakatoa wave.