Natural disasters are responsible for 60,000 deaths, on average, every year. A tsunami — a series of large ocean waves — is one of the most destructive and life-threatening natural disasters humankind experiences. The Indian tsunami in 2004 killed approximately 225,000 people across a dozen countries. It’s important to understand the cause of a tsunami in order to prepare for and minimize the impact when one strikes.

What is a tsunami?

Big wave crashing over street
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A tsunami is generally understood as a large ocean wave, because it’s the initial surge of water meeting land that people witness and remember. In reality, a tsunami is actually a series of many waves known as a “wave train.” It’s the collective ripple of water that makes tsunamis so destructive.

Tsunami wave trains are created when water is pushed, pulled, or displaced by a natural occurrence like an earthquake or landslide. Imagine dropping a pebble into a pond. The ripple created by the impact is a miniature tsunami.

Tsunamis are often measured by the height of their wave(s), and most reach between 10 and 100 feet high. When a tsunami’s waves breach that 100-foot crest, they’re better classified as mega-tsunamis. The largest recorded tsunami wave washed through Lituya Bay in Alaska in 1958. The wave, caused by a landslide, measured a staggering 1,720 feet high.

Tsunamis are caused by a few different things

Simulated tsunami wave showing roaring water against beach shore
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Tsunamis are most often caused by a large movement occurring on the ocean floor. This movement displaces enough water to cause a series of waves to radiate out from the epicenter of the movement (remember the pebble in the pond). There are a number of things that cause this movement and water displacement.

Highway cracked due to earthquake
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Underwater earthquakes are the most common cause of tsunamis. In an underground earthquake, tectonic plates stick and rupture around areas called subduction zones. A subduction zone is a meeting of two oceanic plates in which a thick, dense plate is forced below a thin, lighter plate. An earthquake occurs when the lower plate sticks and then cracks, causing a massive buildup of energy to suddenly release. It’s the release of this energy and the following displacement of water that causes the tsunami.

Volcano erupting into a clear, blue sky
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A volcanic eruption is another underwater occurrence that often leads to a surface-level tsunami. A volcano, in a nutshell, is a vent in the earth’s surface that exposes the gases and magma below. Volcanoes form around subduction zones and hotspots, and a volcanic eruption releases a tremendous amount of energy.

While people are most familiar with volcanoes erupting above water,  underwater volcanoes are responsible for 75% of the total lava that erupts every year. Tsunamis are caused when an underwater volcano erupts and displaces the water around it.

Landslide rocks and debris blocking roadway in Gran Canaria, Spain
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Landslides occur both on the surface of the planet and underwater. A landslide is just what it sounds like: land that is moved and slides down and out of place, disturbing everything in its path. Surface landslides that cause tsunamis happen when large masses of earth and rock break free and crash into the water. It was a surface landslide that caused the record-setting tsunami in Alaska in 1958. Similarly, underwater landslides cause tsunamis from subsequent earth and water displacement.

Tourist standing near large crater in the earth, Winslow, Arizona
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It is theorized (but has never been witnessed) that the impact of a meteor on the ocean’s surface would be enough to cause a tsunami. In fact, many believe it was the combination of a meteor impact and the resulting tsunami that wiped out the dinosaurs.

The idea is so prevalent that astrophysicist Neil deGrasse Tyson predicted an asteroid orbiting Earth in 2019 (asteroids become meteors when they enter the atmosphere) could cause a similar tsunami if it impacted an ocean.

What makes tsunamis so dangerous?

Building destruction and damage after a large earthquake and tsunami
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While tsunamis are measured by the height of their waves, they’re also often measured by the magnitude of the responsible earthquake, the length of the waves, and the speed at which the waves travel.

Tsunami waves in the deep ocean can reach speeds rivaling a commercial jet plane (500 miles per hour), and they can stretch as wide as 60 to 100 miles. What’s more, these wave ripples can be spaced as far as an hour apart from each other.

While tsunami waves slow considerably when they reach shallower water, they’re still traveling 20 to 30 miles per hour upon landfall. Pair that with the sheer weight of water, and disaster is inevitable when a tsunami hits land.

The pressure of a breaking wave is anywhere from 250 to 6,000 pounds per square foot, but you’re not likely to have a tsunami wave break over you. Let’s do some math, though: A gallon of dense seawater weighs roughly 10 pounds. The average bathtub holds around 40 gallons, so that’s 400 pounds. Man-made wave pools like the one you’ll find at a water park often hold 200,000 to 300,000 gallons of water. Now, imagine the weight and force behind a 60-mile tsunami wave, and you get a rough idea of what makes tsunamis so dangerous.