Ocean

views updated

Ocean

Oceans are large bodies of saltwater that surround Earth's continents and occupy the basins between them. Ocean basins are the part of the seafloor that lies beyond the margins of the continents, generally in water deeper than 600 ft (183 m). Therefore, an ocean is both larger in area and deeper than a sea.

Origin of ocean water

As Earth formed in a cloud of gas and dust more than 4.5 billion years ago, a huge amount of lighter elements, including hydrogen (H) and oxygen (O), became trapped inside the planet as the gases condensed and formed molten rock. Materials of different densities separated out; in the young planet's molten interior, heavy elements sank and light elements rose.

Gases rose through thousands of miles of molten and melting rock, to erupt on the surface through volcanoes and fissures.

Within the planet and above the surface, oxygen combined with hydrogen to form water (H20). Enormous quantities of water—enough to fill oceans if it were liquid—shrouded the globe as an incredibly dense atmosphere of water vapor. Near the top of the atmosphere, where heat could be lost to outer space , water vapor condensed to liquid and fell back into the water vapor layer below, cooling the layer. This atmospheric cooling process continued until the first raindrops fell to Earth's surface and flashed into steam.

Many geologists argue this process may have happened several times, because planetoids (rocks the size of moons or asteroids) were still colliding with the early earth until about 3.9 billion years ago. Monstrous planetoid impacts would have vaporized all the water on the planet's surface. Earth has been changed so much by plate tectonics that no vestige of its original appearance remains. Unlike Earth, the faces of the moon , Mars , and Mercury bear the marks of the turbulent earliest history of the solar system . Hellas Basin on Mars and Caloris Basin on Mercury are the scars of planet-shattering impacts. One of these giant craters can be seen from anywhere on Earth: the moon's large round dark "eye," called Mare Imbrium.

Lithospheric plates and the origin of the seafloor

The oceanic plates beneath the ocean waters and above the earth's mantle are made up of igneous rock, formed from the magma , or molten rock, at mid-oceanic ridges. If you think of the plates as the skin of the earth, then the mid-oceanic ridge system is a 38,000-mile-long (64,000 km) cut in that skin. This cut bleeds lava, and never heals, because as soon as a "scab" of solidified lava develops over it, the two sides are pulled apart—allowing more molten rock to ooze forth into the ocean water. In this way, plates of oceanic crust are formed; one long thin section at a time bonds to the most recently made edges.

Hydrologic cycle

It may be that most of the water on Earth today has been cycling between the oceans, the land, and Earth's atmosphere for more than four billion years. However, small amounts of "new" water escape the planet's interior, from volcanoes, even today.

Weather effects of ocean waters

Water possesses the unusual property of being able to absorb a large amount of heat energy before its temperature changes. It follows that water must lose a large amount of heat energy before it cools noticeably. The net result of this phenomenon is that water, more than air or earth, tends to remain at the temperature at which it is already. Water is not given to sudden, wild extremes of temperature. Therefore water has a strong moderating effect on climates. Where there is water, there are more moderate temperatures. A "maritime" (meaning "ocean-like") climate means a moist climate that rarely experiences temperature extremes.

Nearly all coasts experience this maritime effect, but it is especially apparent along coasts where there are large-scale oceanic currents . The moderating effects of the Gulf Stream are a good example. Caribbean sunshine warms the waters of the Gulf Stream in the tropics. This warm water then flows up the east coast of the United States and finally crosses the Atlantic to the coast of Western Europe . This warm current is why England's climate is so much warmer than areas at about the same latitude in North America . However, when the Gulf Stream is diverted southward, Western Europe experiences extreme cold-the last such event, during the fourteenth through nineteenth century, is known as the "Little Ice Age."

Opening and closing of ocean basins

Oceans, like most natural phenomena, exist across a span of time called a "life cycle." For a new ocean to be born, the earth's crust beneath an ocean or a continent must be torn, or rifted, apart.

An ocean basin ceases to exist because its lithosphere gets entirely subducted (that is what usually happens) or obducted (rare and localized). An ocean basin no longer grows when its mid-oceanic ridge gets pulled down into a subduction zone, or gets crammed into a mountain range on the side of a continent. If it is not growing any larger in area, then it can not replace the area it loses to subduction and obduction. Eventually the processes of subduction and obduction put all the oceanic crust of the dying ocean basin either under bordering continents (by subduction) or on top of the bordering continents (obduction). This life cycle of an ocean basin is the same no matter how long it takes or how large the ocean gets to be.

What happens to the water in the dying ocean? Remember, this process takes tens of millions of years. The water flows gradually into other oceans as the basin shrinks, and also departs through evaporation and precipitation .

Resources

books

Borgese, E., ed. Ocean Frontiers. New York; Harry N. Abrams Inc., 1992.

Brower, K. Realms of the Sea. Washington DC; National Geographic Society, 1991.

Carson, R. The Sea Around Us. New York, Oxford University Press, 1989.

Earle, Sylvia A., and Eric Lindstrom. Atlas of the Ocean: TheDeep Frontier. Washington, DC: National Geographic Press, 2001.

Fischer, G., and G. Wefer. Use of Proxies in Paleoceanography. Berlin Heidelberg: Springer-Verlag, 1999.

Hamblin, W.K., and Christiansen, E.H. Earth's Dynamic Systems. 9th ed. Upper Saddle River: Prentice Hall, 2001.

Pinet, Paul R. Invitation to Oceanography. Boston: Jones & Bartlett, 2003.

Sverdrup, Keith A., et al. An Introduction to the World'sOceans. New York: McGraw-Hill, 2002.

Thurman, Harold V., and Alan P. Trujillo. Essentials ofOceanography. 7th ed. Englewood Cliffs, NJ: Prentice Hall, 2001.

periodicals

Lee, Thomas. "Eleventh AMS Conference on Satellite Meteorology and Oceanography." Bulletin of the American Meteorological Society 83, no. 11 (2002): 1645-1648.

other

National Oceanic & Atmospheric Administration (NOAA). <http://www.noaa.gov>

Clinton Crowley

KEY TERMS

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

Continental shelf: —A relatively shallow, gently sloping, submarine area at the edges of continents and large islands, extending from the shoreline to the continental slope.
Maritime climate: —A moist climate that is neither too hot nor too cold, caused by the moderating effect of water on temperatures.
Pelagic sediment: —Sediment that exists in the open ocean, away from land.
Pillow lava: —The form that basaltic lava takes when it is erupted deep under water.
Trenches: —The deep, trough-like depressions in the ocean floor that oceanic crust descends into when it is destroyed.
Turbidity currents: —Local, rapid-moving currents that result from water heavy with suspended sediment mixing with lighter, clearer water. Causes of turbidity currents are earthquakes or when too much sediment piles up on a steep underwater slope. They can move like avalanches.