Earth's Systems: What are waves?
By Encyclopaedia Britannica, adapted by Newsela staff
Text Level 6 03/28/2017 Word Count 786
The destructive power of a Tsunami
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Next to their vastness, the most striking feature of the oceans and other large bodies of water is the constant motion of their surfaces. Waves are ripples, ridges and hollows moving over the water. They are the cause of this choppy, rolling, or otherwise disturbed appearance.
Wind is the most common cause of waves. Waves generated by the wind may range in height from less than 1 inch to as much as 60 feet. Waves breaking against a shore are called surf. Other waves are caused by such geologic disturbances as earthquakes and volcanic eruptions beneath the oceans. Waves formed by underwater earthquakes are known as tsunamis. Tsunamis are sometimes incorrectly called tidal waves, but they have no relationship to the tides. Near seacoasts, tsunamis may become very large and cause great destruction, but in the deep open sea they cannot be detected by the eye. |
Wave Measurement
Waves are characterized by their dimensions. The highest part, or top, of a wave is called the crest, or peak, and the lowest part, or hollow, is called the trough. A wave's dimensions are its height, which is the vertical distance between the crest and the trough; its wavelength, which is the distance between one wave and the next, measured from crest to crest or trough to trough; and its period, which is the time required for the wave to pass a fixed point.
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When the wavelength measures less than the depth of the water, the waves formed are called deepwater waves, or short waves. Wind-generated waves on the open sea or very large lakes, such as Lake Michigan in the United States, are waves of this kind.
When the wavelength is greater than the depth of the water, shallow-water waves, or long waves, are formed. Tsunamis are shallow-water waves because they may have wavelengths of 100 miles or more, while the ocean's greatest confirmed depth is only about 6.8 miles.
In the area where they are generated to their maximum height by the wind, waves are called seas, or forced waves. They tend to be very steep and to break or curl over. As waves travel away from their generating area, they become longer and smoother. In this state they are called swell, or free waves. Waves that originate in stormy Antarctic waters may appear as swell off tropical shores thousands of miles away. The speed at which a wave travels depends upon its period and the depth of the water over which it moves.
When the wavelength is greater than the depth of the water, shallow-water waves, or long waves, are formed. Tsunamis are shallow-water waves because they may have wavelengths of 100 miles or more, while the ocean's greatest confirmed depth is only about 6.8 miles.
In the area where they are generated to their maximum height by the wind, waves are called seas, or forced waves. They tend to be very steep and to break or curl over. As waves travel away from their generating area, they become longer and smoother. In this state they are called swell, or free waves. Waves that originate in stormy Antarctic waters may appear as swell off tropical shores thousands of miles away. The speed at which a wave travels depends upon its period and the depth of the water over which it moves.
Wave Forms
Waves encountered in the oceans are very irregular. No two are identical. Because of the complexity of the forces that operate to form waves, scientists often find it convenient to visualize and to speak of waves in terms of an ideal wave form.
In the ideal wave, water particles follow a nearly circular path in a vertical plane: they move upward as the wave approaches, downward as the crest passes, and then return to their original position. In the real ocean where seas are irregular, however, the motion of water particles in a wave is also irregular, and their path is very completely circular.
In the ideal wave, water particles follow a nearly circular path in a vertical plane: they move upward as the wave approaches, downward as the crest passes, and then return to their original position. In the real ocean where seas are irregular, however, the motion of water particles in a wave is also irregular, and their path is very completely circular.
To the eye, progressive waves give the effect of water traveling from one point to another. For example, when a pebble is dropped into a quiet pool, water appears to travel toward the edge of the pool from the point where the pebble struck. In reality, the water travels very little; what is seen is the wave traveling along the surface of the water. A leaf floating on the pool would merely bob up and down as each wave passed beneath it and would progress very little along the water's surface.
Another type of wave is the standing wave. Standing waves may be formed by two similar progressive waves traveling across the same area at the same speed but in opposite directions. A standing wave can be constructed in a fish tank by slightly tilting the tank, then setting it back on its stand. The water levels in the ends of the tank will rise and fall opposite each other. Meanwhile, the water in the middle of the tank will remain at a fairly constant level.
This article was originally published by Encyclopaedia Britannica, Inc. Newsela adapted the original article and is responsible for all revisions and for any errors that may result from the adaptation.
Another type of wave is the standing wave. Standing waves may be formed by two similar progressive waves traveling across the same area at the same speed but in opposite directions. A standing wave can be constructed in a fish tank by slightly tilting the tank, then setting it back on its stand. The water levels in the ends of the tank will rise and fall opposite each other. Meanwhile, the water in the middle of the tank will remain at a fairly constant level.
This article was originally published by Encyclopaedia Britannica, Inc. Newsela adapted the original article and is responsible for all revisions and for any errors that may result from the adaptation.