A wave is defined as a rhythmic movement that carries energy through a substance. Waves can travel through virtually any material. For example, they pass through rocks as vibrations during earthquakes. They also travel through air as sound waves, such as when someone honks a car horn. In all types of waves, the particles that make up a substance transfer energy to the surrounding particles as they bump into each other. While the energy carried by a wave can travel great distances, the particles that transmit that energy actually move very little.

Water waves are no different. Although it may appear that the water that forms a wave far offshore is the same water that crashes onto the beach minutes later, closer inspection proves otherwise. With each oncoming wave, a water molecule is pushed slightly up and forward. Then, as the high point — or crest — of the wave passes, the molecule moves down and back toward the wave's low point, or trough. With every successive wave, the molecule follows the same circular path, bumping into other molecules and transferring the wave's energy along the way. Each molecule moves only very slightly forward in the wave's direction.

Wind blowing over the surface of water creates waves. Wave size depends on the speed of the wind, the length of time the wind blows, and a measure called fetch — the distance the wind blows over open water. High-speed winds that blow for long periods of time over a long fetch create the largest waves. The largest wave ever recorded grew out of a powerful storm in the North Pacific. It was 34 meters (112 feet) high.

Most mid-ocean waves are small swells measuring a meter (three feet) or less from trough to crest. In offshore waves, the distance between the crest of one wave and the crest of another — the wavelength — tends to be large, typically a few hundred meters (a thousand feet). However, as waves approach the shore and begin to interact with the ocean bottom, these characteristics change.

A wave touches the ocean bottom where the water depth is about one-half the wavelength. As it moves forward into increasingly shallow water, the bottom of the wave slows due to friction with the ocean bottom, while the top of the wave continues to move at its original speed and is pushed higher. This causes the crest of the wave to become unstable and eventually topple over, or break. The breaking of waves scrapes sediments off the ocean bottom near the shore and deposits them along the shorelines of sandy beaches.

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