These destructive surges of water are caused by undersea earthquakes at tectonic plate boundaries. Water is displaced as the ocean floor at a plate boundary rises or falls suddenly.
Tsunamis are also caused by underwater volcanic eruptions.
In the deep ocean, tsunami waves are less than a metre high. However, as they approach a shoreline and enter shallower water they slow down and grow in height and energy. Waves can travel at speeds of upwards of 800 kilometres an hour and have been recorded as high as 30 metres when they hit a coastline. A wave train – a succession of waves – is commonplace.
Here is a video showing the impact of recent tsunamis. It is a truly frightening experience.
While recent tsunamis have affected Asian countries, Europe too has had a few notable incidents, in particular after a massive volcanic eruption on the Greek island of Thera – present day Santorini. This event, some 3,600 years ago and probably the largest ever in the world, caused a huge tsunami throughout the Mediterranean. One of the areas affected was Crete some 100 kilometres away. The Minoan culture was destroyed as a consequence of the Santorini tsunamis. Here is a digital reproduction of the Santorini tsunami.
There have been many reports that the legendary city of Atlantis located north of the Spanish city of Cadiz was also destroyed by a tsunami.