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What is a tsunami?
A tsunami is a series of massive waves that is usually produced as a result of an earthquake taking place at sea. The severity of the tsunami depends on a number of factors and these will dictate just what is the impact of a tsunami.
Earthquakes are not the only source of disruption in the ocean that can cause a tsunami, but they are the predominant cause. Events that cause a large displacement of water can cause a tsunami, for example, an underwater volcanic eruption; a coastal landslide; collapse of the caldera of a volcano, causing a sudden lowering of the overlying water level; entry to the ocean of a pyroclastic flow; destruction of a volcanic island (this was the case in the tsunami caused by the destruction of KraKatoa in 1883); or atmospheric pressure waves produced by a large scale eruption, in a near coastal region. For more information on volcanoes, you may be interested in reading What Comprises Volcanic Ash and the related article How Volcanic Ash Affects the Environment, to learn about another one of the results of volcanic eruptions.
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What Causes an Earthquake?
Earthquakes are caused when two tectonic plates move together. Frictional forces prevent the tectonic plates from moving past each other easily, but eventually, the frictional forces are overcome and the plates move past each other in a process called subduction. There is a tremendous release of energy when subduction occurs which makes earthquakes so destructive.
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Environmental Impact Of A Tsunami
The answer to the question, “what is the environmental impact of a tsunami?" clearly depends on the nature and magnitude of the triggering event. In the Krakatoa event in 1882, the cataclysmic destruction of the island caused a series of tsunami waves with a peak height of 37m (120 feet), but because the triggering event was localised on the volcanic eruption, the devastation was confined to a relatively local area. Nevertheless, 36417 people lost their lives in the event.
The devastating tsunami that centred on Aceh province in Indonesia on 26th December 2004 left at least 300 000 people dead and a further half million homeless. The earthquake epicentre was 100km away from Aceh and the quake measured 9 on the (logarithmic) Richter scale. The waves caused massive destruction to coastal communities and the infrastructure in a wide region; damage and fatalities also occurred across the Indian Ocean in Africa as the tsunami waves propagated through the ocean. A cubic metre of water weighs a metric ton; so it is not difficult to imagine the devastation that a wall of water at least 30m high would cause and get an idea of the tremendous destructive power that came onshore.
In addition to the tragic human loss of life and devastation to habitations and infrastructure of a major tsunami event, the environment can be damaged by the effect of the ingress of salt water into agricultural lands. The vegetation was also extremely badly impacted by the physical force of the waves, and the depth of the resultant flooding as the waves receded. Mangroves, coral reefs and aquaculture were also damaged by the event. It would be safe to say that the impact of a major tsunami on the environment is profound.
The United Nations Environment Programme (see reference 4) conducted an environmental assessment on the affected region (a very wide geographical area) in the immediate aftermath of the disaster; it makes for grim reading.
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- Geoscience Australia: http://www.ga.gov.au/hazards/tsunami/causes.jsp
- Krakatoa: http://www.drgeorgepc.com/Tsunami1883Krakatoa.html
- Plate Tectonics: http://www.platetectonics.com/book/page_12.asp
- United Nations Environment Programme: http://www.unep.org/tsunami/reports/iucn_land.pdf