Photo: Great Nicobar Island after tsunami

Photograph by Deshakalyan Chowdhury/AFP/Getty Images

A tsunami is a series of ocean waves that sends surges of water, sometimes reaching heights of over 100 feet (30.5 meters), onto land. These walls of water can cause widespread destruction when they crash ashore.

These awe-inspiring waves are typically caused by large, undersea earthquakes at tectonic plate boundaries. When the ocean floor at a plate boundary rises or falls suddenly it displaces the water above it and launches the rolling waves that will become a tsunami.

Most tsunamis, about 80 percent, happen within the Pacific Ocean’s “Ring of Fire,” a geologically active area where tectonic shifts make volcanoes and earthquakes common.

Tsunamis may also be caused by underwater landslides or volcanic eruptions. They may even be launched, as they frequently were in Earth’s ancient past, by the impact of a large meteorite plunging into an ocean.

Tsunamis race across the sea at up to 500 miles (805 kilometers) an hour—about as fast as a jet airplane. At that pace they can cross the entire expanse of the Pacific Ocean in less than a day. And their long wavelengths mean they lose very little energy along the way.

In deep ocean, tsunami waves may appear only a foot or so high. But as they approach shoreline and enter shallower water they slow down and begin to grow in energy and height. The tops of the waves move faster than their bottoms do, which causes them to rise precipitously.

A tsunami’s trough, the low point beneath the wave’s crest, often reaches shore first. When it does, it produces a vacuum effect that sucks coastal water seaward and exposes harbor and sea floors. This retreating of sea water is an important warning sign of a tsunami, because the wave’s crest and its enormous volume of water typically hit shore five minutes or so later. Recognizing this phenomenon can save lives.

A tsunami is usually composed of a series of waves, called a wave train, so its destructive force may be compounded as successive waves reach shore. People experiencing a tsunami should remember that the danger may not have passed with the first wave and should await official word that it is safe to return to vulnerable locations.

Some tsunamis do not appear on shore as massive breaking waves but instead resemble a quickly surging tide that inundates coastal areas.

The best defense against any tsunami is early warning that allows people to seek higher ground. The Pacific Tsunami Warning System, a coalition of 26 nations headquartered in Hawaii, maintains a web of seismic equipment and water level gauges to identify tsunamis at sea. Similar systems are proposed to protect coastal areas worldwide.

Share

More About Tsunamis

  • Nuclear reactor picture: cargo containers in Sendai – for gallery on earthquake effects, including nuclear emergency

    Photos: Japan Tsunami, the Aftermath

    The day after Japan's biggest earthquake, cities smoldered, soldiers lent helping hands, and a nuclear reactor exploded.

  • Japan tsunami and earthquake picture: houses, some on fire, swept away by tsunami waves in Japan

    Photos: Quake, Tsunami Shock Japan

    The biggest earthquake in Japan's history Friday sparked three-story waves, hundreds of casualties, and towering infernos.

  • The aftermath of a tidal wave.

    Tsunami Facts: Killer Waves Explained

    In light of Friday's tsunami following the Japan earthquake, find out how the killer waves are caused, what the warning signs are, and how to respond when a tsunami threatens.

  • Two men watch waves in San Francisco, California, after a large earthquake in Japan.

    Tsunami Waves Hit U.S.

    The deadly earthquake that struck Japan Friday sent a tsunami racing across the Pacific Ocean, reaching Hawaii, the Pacific Northwest, and California.

  • Video: Tsunami 101

    Find out how a tsunami is born ... and how it destroys.

  • A rescue worker searches for victims of Tuesday's eruption of Mount Merapi.

    Indonesia Tsunami, Eruption Linked?

    The two nearly simultaneous events might have been triggered by the same earthquake, which may have been a rare "slow" temblor, experts say.

  • Port-au-Prince Bay, Haiti, ten days after a massive earthquake.

    L.A. Has Higher Tsunami Risk?

    Southern California, Seattle, and Taiwan are some of the places where tsunamis may be more likely than thought, a new study says.

How to Feed Our Growing Planet

  • hub_tease2.jpg

    Feed the World

    National Geographic explores how we can feed the growing population without overwhelming the planet in our food series.

See blogs, stories, photos, and news » »

@NatGeoGreen on Twitter

The World's Water

  • change-the-course-dry-co.jpg

    Help Save the Colorado River

    NG's new Change the Course campaign launches. When individuals pledge to use less water in their own lives, our partners carry out restoration work in the Colorado River Basin.

  • water-grabs-mali-4x3.jpg

    Water Grabbers: A Global Rush on Freshwater

    A special series on how grabbing water from poor people and future generations threatens global food security, environmental sustainability, and local cultures.

Learn More About Freshwater »