What is a Tsunami?

by Read Listen Learn

A tsunami (or 'tidal wave') is a very high wave that comes in from the sea and causes death and destruction on the coast. Luckily, they don't happen very often but, why do they happen at all?

Normal waves can be up to fifteen metres or more, but this is out at sea. Waves get smaller as they come nearer the beach. Made by the wind and the Earth turning, all normal waves lose size and energy as the water under them gets shallower. A tsunami is born in a different way. An earthquake is when two giant pieces of the Earth's surface, touching each other, suddenly move in opposite directions along a very deep crack in the ground, called a 'fissure'. If strong enough, an earthquake can cause buildings to fall down and fissures a few metres wide to appear in the ground. An earthquake at sea also makes these fissures on the sea floor. This, in turn, releases massive bubbles of air to the surface of the sea and, so, a giant wave sets off just like the ripple you see when you drop a coin into a big bowl of water.

The tsunami gets smaller and slower as it travels but even land hundreds of miles from the seaquake can be very badly hit. People on the beach see the sea go back hundreds of metres very quickly as the tsunami nears the coast. Then comes the water itself as a huge wave. The best idea is to get to high ground, one hundred metres above sea level, but, if it's too late for that, then climbing a tall tree may help people to survive.

It can be a bad idea to be in a building, especially on lower floors, because it may fall down, pushed by the giant wall of water. Also, rooms may fill up with water, drowning everyone inside.

After some minutes, the waters will begin to go back to the sea. There could be other giant waves following and it takes several hours before people can be sure that it is safe to be on the ground. At this point, other problems begin: broken pipes for gas, water or sewage cause fire, disease and problems getting clean drinking water. There is, of course, great destruction to cars, buildings and roads; a lot of people are drowned.

The biggest tsunami in history was the Indian Ocean Tsunami on 26 December, 2004. It killed about 230,000 people in fourteen different countries in and around the Indian Ocean. Most of the dead were local but, because the tsunami came at Christmas, many were tourists on beach holidays too.