A tsunami is a very large coastal wave that is most often the result of an underwater earthquake. The word tsunami comes from the Japanese meaning a ‘harbour wave’.

 
Causes
Most large tsunamis occur at convergent plate boundaries where two tectonic plates are crashing into each other. As the two plates collide one plate is forced down underneath the other. As this happens the leading edge of the top plate snags on the bottom plate and pressure starts to build. Eventually the pressure becomes too much and the pressure is released in the form of an earthquake. This type of earthquake is called a megathrust earthquake which is the most powerful type of earthquake. When these megathrust earthquakes occur under water large amount of water is displaced and a tsunami wave occurs.

 
Tsunami Wave
Normal waves occur as a result of friction between wind and sea and only affect the surface area. When tsunamis occur a shock wave is sent throughout the full depth of the ocean and this has a number of different affects. Tsunami waves are very small in the open ocean, measuring only one or two meters, and only manifest themselves as they approach the shallower seas of the coastline. As a tsunami reaches the coastline they start to slow down as the bottom of the wave comes in contact with shallower sea bed. The top of the wave however continues at a high speed and as a result the wave stacks up on itself and becomes higher and higher before eventually falling over itself and breaking onto the coastline.

Destruction
There are normally two phases of destruction when a tsunami hits, the first as the wave crashes onto coastal areas and the second as the water from the wave pulls back into the sea bringing with is large amounts of debris from the land. Tsunamis can range in size from 1 or two metres to Mega-tsunamis hundreds of metres high.
Mega-tsunamis

Most mega-tsunamis are caused by major landslides falling into bodies of water. The largest tsunami ever recorded was in Lituya Bay, Alaska and measured 520 metres tall and was as a result of a massive landslide falling into Lituya Bay. Spanish Arch in Galway was very badly damaged by a tsunami as a result of a massive earthquake that occurred just of the Lisbon coast in Portugal in 1775.