You've heard about them, and maybe seen them in movies as they smash apart ocean liners, or wash whole cities into the sea. What causes tidal waves, and how are they different from the ordinary waves that wash against our coastlines?
Tidal waves actually have nothing to do with tides, and nothing to do with ordinary ocean waves. They're more properly called "tsunamis," which is Japanese for "harbor waves." It's a good name because, although tidal waves lie relatively flat on the open ocean, they can pile up tremendously high when they hit the shallower water of a continental shelf or harbor.
Ordinary, ocean waves are caused by the action of wind, like ripples on a wind-blown pond. Even the impressive surf of the Pacific got its start as wind blowing across the open water. The height of such waves depends on how fast the wind is blowing, and how much open water the wind travels across.
Tsunamis, however, have nothing to do with wind. They're caused by the sudden movement of the Earth's crust during an underwater earthquake or a volcanic eruption. Imagine a child's plastic wading pool filled with water in the back yard. Ordinary waves are like the little surface ripples from the afternoon breeze. A tsunami is what you'd get if you kicked the side of the pool.
Tsunamis can travel thousands of miles, and still send water crashing a hundred feet high into a harbor or coastal town. A 1960 earthquake in Chile sent tsunamis across the Pacific at three hundred miles per hour, destroying harbors in Hawaii and Japan.