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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.

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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.And at hundreds of miles per hour, the water builds up very quickly and the wave height increases rapidly.

The steepness of the seafloor and whether the coastline funnels the wave towards the head of a bay also contribute. Tsunamis rarely look like the breaking waves you see at the beach, however. Usually they appear as a churning, rapidly rising surge of water that pushes inland almost like a raging river’s floodwaters.A large tsunami wave surges onshore with great force. You can’t swim out of it.

And the water carries lots of debris in it, posing a risk of injury. Once the crest of the wave has made landfall the water recedes, carrying debris back out to sea.If the tsunami consists of more than one wave, another crest will appear, on average ten to fifteen minutes after the first. And the subsequent waves can be larger, deeper, and more powerful than the first. In the case of the 2004 tsunami, the third wave to strike Thailand’s beaches was the largest.And why does the water recede from shore before a tsunami hits? That happens when the trough (or low point) of the wave reaches shore.

Tsunami Warning System

The variables that contribute to the generation and ultimate size of tsunamis combine to make prediction difficult, so we rely on issuing warnings once they happen. The U.S. National Oceanic and Atmospheric Administration oversees monitoring of a network of seafloor instruments and buoys positioned around the Pacific Ocean rim. The instruments are designed to detect the passage of a tsunami wave and relay the information to ground-based stations.A similar network is currently under development in the Indian Ocean, spurred by the tsunami on December 26, 2004, where a lack of warning contributed to the deaths of nearly 300,000 people.

In that case, the short time lag between the earthquake and the tsunami striking Indonesia might have limited the value of a warning, but lives might have been saved at other places farther away.If you take only one thing from this lesson, let it be this: if you are ever on a beach and a tsunami warning is issued, climb! Don’t collect fish. Don’t go surfing. The wave may not arrive, or it might be small. But you absolutely do not want to get caught in a big one.

Lesson Summary

Let’s review.

Tsunamis are ocean waves produced by rapid displacement of the seafloor, most often by an earthquake, a landslide, or a volcanic explosion. They travel away from where they form at high velocities. They are very wide, but only a foot or two high on the open ocean.

The height of a tsunami increases as it approaches shore, when the wave produces a powerful surge of water that can advance many miles inland. A warning network is in place around the Pacific Ocean, and one is currently being built in the Indian Ocean.

Learning Outcomes

Once you watch the video, you should be able to:

  • Recognize what a tsunami is
  • Describe how a tsunami is formed
  • Recall the factors that influence the size of a tsunami
  • Explain how the tsunami warning system works

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