The ocean tides are generated by the gravitational pulls of the moon and sun. However, most of the tidal effect results from the gravitational pull of the moon, which tugs at the side of the earth facing it, causing the earth to bulge slightly in that direction. Because water is less dense than the solid earth it bulges out more, forming a tidal crest on the side facing the moon. On the opposite side, the earth is pulled toward the moon more than the ocean, leaving a bulge or crest of water on the opposite side.

Inasmuch as the moon is the controlling body, the tidal crest moves at the same rate as the moon's orbit—about 1,000 miles per hour at the equator. For any given place there are two high and two low tides during a complete rotation. Because the moon and earth's rotation are not synchronous, the time difference between the two high tides is about 12 hours and 25 minutes, not 12 hours. Since the relative positions of the sun, moon, and earth are known with precision, tidal charts can be produced in advance to show the exact times of high and low tide at any given place.

Other factors, such as the eccentricity and inclination of the moon's orbit in relation to the earth's, the irregular configuration of the ocean basin, the Coriolis force (caused by the rotation of the earth), friction as water moves, and the influence of the sun, complicate tidal rates and fluctuations.

For example, in deep water the tidal range is about 1.5 m. As the tide approaches the continental shelf it slows down because of friction, resulting in a higher tide. When the earth, sun, and moon are aligned in a row, a so-called spring tide occurs—that is, higher than normal tides caused by the enhanced combination of their gravitational pulls.

However, when the three bodies are at right angles to one another, the tidal effects of the sun and moon tend to cancel each other, producing a low, or neap, tide.

A rising tide is often called a flood tide; it lasts for 6 hours and 13 minutes, the time it takes to reach its maximum height. For a short time the water may seem motionless or slack; then the tide reverses and the water level drops, forming an ebb tide lasting the same amount of time—6 hours and 13 minutes.

In estuaries, such as the Hudson, for exam ple, the flow of the ebb tide is strengthened by the addition of river water that also diminishes the strength of the flood tide as it flows against the incoming water. As tides move up and down and flow back and forth in bays, estuaries, and the shallow continental shelf, they often transport a considerable amount of sediment.

In the open ocean the tidal current travels at about a quarter of a knot. But when it reaches land, the resulting tidal currents increase in speed, in some cases to 10 knots, especially in estuaries, bays, and straits where they become restricted. Along coastlines that become progressively more narrow, water is squeezed together causing the incoming tide to rise higher and higher. In the Bay of Fundy, between Nova Scotia and New Brunswick, Canada, this funneling effect produces tides up to 20 m in height.

However, the height of a tide is not as easily predictable as its timing, because it is controlled, in part, by meteorological conditions. For example, wind as well as atmospheric pressure cause substantial rises and falls in predicated height.

Both the solid part of the earth and the atmosphere respond to the same tidal forces as the ocean. As would be expected, the effect of tides on the denser solid earth is much less than in the ocean, while it is greater than in the atmosphere, where it is manifested as small changes in atmospheric pressure in any given place.

Tides also affect animals, such as some molluscs and arthropods that live along the shoreline, where their life activities are adjusted to the tidal cycles. Alternating fluctuations in temperature, changes in food supply, salinity, possible predation, and movement of sand—all these mold their behavior and physiological responses. These animals have adapted to a constantly changing envi ronment, and it has been shown that their tidal rhythms continue even after the organism is put into a laboratory. A good example is the fiddler crab, commonly found scavenging and mating during low tide. As high tide approaches the crabs retreat into their burrows until the next low tide approaches, when their internal clocks tell them to begin to leave their burrows.

—Sidney Horenstein

See also: Beaches; Coral Reefs; Lagoons; Oceans Bibliography

Laing, David. 1991. The Earth System: An Introduction to Earth Science. Dubuque, IA: Wm. C. Brown; Lynch, D. K. 1982. "Tidal Bores." Scientific American 247 (4): 146-156; Montgomery, Carla. 1996. Fundamentals of Geology, 3rd ed. New York: McGraw Hill Professional Publishing; Plummer, Charles C., David McGeary and Diane Carlson. 2002. Physical Geology, 9th ed. New York: McGraw-Hill.

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