A Moment of Science

Sailing Over a Tidal Wave

If you were sailing far from shore, how would you feel about a tremendous tsunami heading your way?

Small ship on the ocean

Photo: buck82 (flickr)

If a tsunami were to pass under this ship off the coast of Curacao, the sailors might not even feel it if the water is deep enough

Tsunamis are powerful waves caused by underwater earthquakes or eruptions that can crash over ten stories tall into a coastline.

If you were sailing far from shore, how would you feel about a tremendous tsunami heading your way?

Actually, you wouldn’t notice a tsunami at all. In deep water, a tsunami raises the ocean’s surface only about one or two feet—much less than ordinary ocean waves. What’s more, it takes up to an hour for a single tsunami to roll by. How can a wave be so deadly to a coastline, but so tame in the open ocean?

The energy of a wave has more to do with how deep the water churns than how tall the wave rises. Ordinary waves can be rather tall, but they don’t go very deep. What’s more there’s never much distance between one ordinary wave’s crest and the one following it. The distance between waves is called “wavelength,” and it’s another secret of a tsunami’s strength.

While ordinary waves only tug at the surface water, tsunamis churn the water all the way to the deepest ocean floor! They also have very long wavelengths—a hundred miles or more. Although tsunamis raise the surface only a foot or so in deep water, they carry a lot of energy.

When a tsunami approaches a coastline, the water gets shallower. This causes friction at the front of the wave and slows it down. The faster moving back of the wave piles into the slower front, squeezing the whole hundred mile tsunami like an accordion. Just as a rug will crumple and rise if it’s scrunched by an opening door, the tsunami gets taller. By the time it hits the beach, it can be a hundred feet high!

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