The most common cause of surface ocean waves is air movement (the wind). Waves within the ocean can also be caused by tides, interactions between waves, submatine earthquakes or volcanic activity, and atmospheric disturbances (storms). Wave size depends on wind speed, wind duration, and the distance of water over which the wind blows. The longer the distance the wind travels over water, or the harder it blows, the higher the waves.
As the wind blows over the water, it tries to drag the surface of the water with it. The surface water cannot move as fast as air, so the water rises. After it rises, the water is pulled back down by gravity. The falling water's momentum is carried below the surface, and water pressure from below pushes this swell back up again. This tug of war between gravity and water pressure creates wave motion.
Capillary waves are small waves caused by breezes of less than 2 knots (1 knot equals 1.15 miles per hour). A wind speed of 13 knots produces waves that are so steep that they tip over, creating whitecaps. For a whitecap to form, the wave height must be at least one-seventh the distance between wave crests.
The largest waves are generated not by the wind, but by large submarine earthquakes. Waves produced in this fashion are called tsunamis [tsoo-NAH-meez], or tidal waves. A tsunami is set in motion by a vertical shift in the ocean floor, which pushes the water ahead of it.
Tsunamis start out small and grow larger as they near land. They travel at speeds of up to 500 miles per hour (800 kilometers per hour) and measure 100 to 200 miles (161 to 322 kilometers) in length. It is typical for a tsunami to measure 60 to 100 feet (18 to 30 meters) in height by the time it reaches land.
Tsunamis occur most often in the Pacific Ocean. Several tsunamis have affected Alaska and Hawaii. In 1958, a 200-foot- (60-meter-) tall tsunami, generated by a minor earthquake and resultant rockfall into the sea, crashed into Lituya Bay, Alaska. It destroyed great tracts of forestland as far as 1,700 feet (520 meters) above sea level.
Sources: Engelbert, Phillis. The Complete Weather Resource, vol.1, pp. 147-49; vol2., p. 308; Flatow, Ira. Rainbows, Curve Balk, and Other Wonders of the Natural World Explained, pp. 28-32; Groves, Don. The Ocean Book, p. 29; Lee, Sally. Predicting Violent Storms, pp. 99-100.