What are Tsunamis? - Causes, Effects & Facts

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  • 0:00 Very Dangerous Waves
  • 0:52 How Do Tsunamis Form?
  • 2:22 Where Do They Typically Occur?
  • 2:40 How Big Are Tsunamis?
  • 5:11 Tsunami Warning System
  • 6:20 Lesson Summary
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Lesson Transcript
Instructor: Charles Spencer

Charles teaches college courses in geology and environmental science, and holds a Ph.D. in Interdisciplinary Studies (geology and physics).

Disastrous tsunamis in Indonesia and Japan have raised awareness of these phenomena. But what are tsunamis, and why do they happen? This lesson will give you the answers.

Very Dangerous Waves

Let's suppose you're walking along the beach when, suddenly, you see the water receding rapidly from the shoreline. What would you do? Run out and collect shells? Or run like heck the other way?

If you chose to collect shells, you might end up in big, big trouble, because rapid withdrawal of the ocean is a warning sign of an approaching tsunami. Tsunami is a Japanese word meaning harbor wave. Tsunamis are sometimes called tidal waves, because the initial change in water level resembles that of a rising or falling tide, but they are created in an entirely different way. Tsunamis share many characteristics with the waves you've encountered at the beach. But it is how they differ from the surf you splash in that makes them much more dangerous, very unpredictable, and often deadly.

How Do Tsunamis Form?

Tsunamis are ripples that form on the ocean surface above where the seafloor is abruptly disturbed, displacing the water above it. Sometimes they consist of single waves, but very often a sequence of waves is created. Anything that causes a seafloor disturbance can produce a tsunami. Earthquakes, volcanic explosions, undersea landslides, and meteor impacts are common causes.

The biggest tsunamis are created when very powerful earthquakes happen at shallow depths below the seafloor (most often at plate subduction zones), because such earthquakes produce large seafloor displacements and vibrations over large areas. The destructive tsunamis that struck Indonesia in 2004 and Japan in 2011 were caused this way.

Large underwater landslides off the flanks of volcanic islands or the continental slope, possibly triggered by earthquakes or volcanic eruptions, have also created destructive tsunamis. In 1998, a 20-foot-high tsunami produced by a submarine landslide struck the coast of Papua New Guinea.

Explosive volcanic eruptions also generate tsunamis, as in 1883 when the Indonesian volcano called Krakatoa exploded. Meteorite impacts in the ocean have created tsunamis during the geologic past. The meteorite that struck the Yucatan Peninsula at the end of the Cretaceous Period produced a tsunami that washed up on the South Coast of the United States.

Where Are Tsunamis Likely to Occur?

Tsunamis have been generated in all of the world's oceans. But they are much more common in the Pacific and Indian Oceans because of the greater number of subduction zones and submarine volcanoes there. Even at that, tsunami waves can travel literally around the world and cause damage at quite distant locations.

How Big Are Tsunamis?

A tsunami's size depends on where you encounter it. Of course, the farther you are from the origin, the smaller the wave is likely to be, because they lose energy traveling across the ocean. But that's not to say tsunamis cannot still be deadly thousands of miles from their origin point. The Indonesian tsunami flooded the coasts of Sri Lanka and India, killing tens of thousands of people, after traveling across the Indian Ocean.

Tsunamis travel away from their origination point at speeds exceeding 500 miles per hour. The width of a single wave can reach a hundred miles, as can the separation between multiple waves. Each wave involves massive amounts of water.

But on the ocean, a tsunami poses almost no danger at all. That's because they are typically just one or two feet high as they race across open water and are nothing more than a broad swell on the ocean surface. You likely wouldn't notice one if it passed under your cruise ship.

It's when a tsunami approaches a coast and slows down because of friction with the shallower seafloor that it grows to damaging size. Because of its high speed, the front of the wave reaches shallow water long before the rest of it does. So, in something akin to a chain reaction accident on a foggy highway, the back of the wave runs into the slower-moving front.

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