The Mighty Wave - the Tsunami!

A tsunami (soo-NAH-mee) is one the world's most powerful and destructive natural forces. It can speed across the ocean as fast as a jet plane, swallow up islands and wipe out villages. But what causes these mammoth waves...?

Join National Geographic Kids as we dive into the deep to find out!


How do tsunami's happen?

Meaning 'great harbour wave' in Japanese, tsunamis are sometimes called 'tidal waves', but their strength has nothing to do with the tides. About four out of five tsunamis happen within the Ring Of Fire, a zone in the Pacific Ocean where earthquakes and volcanic eruptions frequently take place.

Ring of Fire in the Pacific Ocean

Giant slabs of the Earth's crust, called tectonic plates, grind together. Sometimes, though, the plates get stuck, the pressure builds up and they suddenly slam into a new position. This causes an earthquake. If an earthquake lifts or drops part of the ocean floor, the water above rises and starts spreading across the ocean, causing a tsunami. Underwater landslides or volcanic eruptions can also displace water (cause water to spread across the ocean) and may lead to a tsunami.

Formation of a Tsunami

How big is a tsunami?

Out in the open ocean, tsunami waves are only about one-metre high because the water is deep. However, as the water becomes shallow, the waves slow down and begin to grow. They can rise 35m or higher – that's the same as a 10-floor block of flats! However, the scariest thing about a tsunami is its wavelength, as this determines how far inland it can travel. Whereas a large wave caused by a storm might have a wavelength of up to 150m, a tsunami could reach up to a fearsome 1,000km! 

Cross Section of a Tsunami

Fast and deadly!

A normal wind wave travels at about 90kmh, but a tsunami can race across the ocean at an incredible 970kmh! Sometimes, before a tsunami hits, there is a huge vacuum effect, sucking water from harbours and beaches. People can see the ocean floor littered with flopping fish and other sea animals. Then a wave blasts onto the shore minutes later, then another and another for two hours or more. There may also be up to one hour between each wave.

Town Before and After Tsunami

Life-saving system!

To save lives, scientists established the Pacific Tsunami Warning System, based in Hawaii, in the USA. Its network of detectors can track quakes that may cause a tsunami. These waves can race from one side of the Pacific Ocean to the other in less than a day, so people need to be warned in time to head for higher ground!

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