Ocean
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Ocean (from Okeanos, a Greek god of sea and water; Greek ωκεανός) covers almost three quarters (71%) of the surface of the Earth.
This global, interconnected body of salt water, called the World Ocean, is divided by the continents and archipelagos into the following five bodies, from the largest to the smallest: the Pacific Ocean, the Atlantic Ocean, the Indian Ocean, the Southern Ocean, and the Arctic Ocean. Official boundaries are defined by the International Hydrographic Organization; the Southern Ocean, though long recognized in maritime tradition, was officially sanctioned only in 2000, and is unique in being defined only by a line of latitude with no landmass boundaries.
Oceanographers, however, may recognize only four oceans, treating the Arctic Ocean (or the Arctic Sea) as a part of the Atlantic Ocean.
Smaller regions of the oceans are called seas, gulfs, straits and other names.
Geologically, an ocean is an area of oceanic crust covered by water. Oceanic crust is the thin layer of solidified volcanic basalt that covers the Earth's mantle where there are no continents. From this point of view, there are three oceans today: the World Ocean, and the Black and Caspian Seas that were formed by the collision of Cimmeria with Laurasia. The Mediterranean Sea is very nearly its own ocean, being connected to the World Ocean through the Strait of Gibraltar, and indeed several times over the last few million years movement of the African Continent has closed the straight off entirely, making the Mediterranean a fourth ocean. (The Black Sea is connected to the Mediterranean through the Bosporus, but this is in effect a natural canal, cut through continental rock some 5000 years ago, rather than a piece of oceanic sea floor like the Strait of Gibraltar.)
The area of the World Ocean is 361 million km?, its volume is 1370 million km?, and its average depth is 3790 m. This does not include seas not connected to the World Ocean, such as the Caspian Sea.
The total mass of the hydrosphere is about 1.4 × 1021 kg, ca. 0.023 % of the Earth's total mass.
See sea water for a detailed discussion of ocean water composition, most notably its salinity.
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Exploration
Ocean_gravity_map.gif
Travel on the surface of the ocean through the use of boats dates back to prehistoric times, but only in modern times has extensive underwater travel become possible.
The deepest point in the ocean is the Mariana Trench located in the Pacific Ocean near the Northern Mariana Islands. It has a maximum depth of 10,923 m (35,838 ft) [1] (http://www.rain.org/ocean/ocean-studies-challenger-deep-mariana-trench.html). It was fully surveyed in 1951 by the British Navy vessel, "Challenger II" which gave its name to the deepest part of the trench, the "Challenger Deep".
Much of the bottom of the world's oceans is unexplored and unmapped. A global image of many underwater features larger than 10 km was created in 1995 based on gravitational distortions of the nearby sea surface.
Climate
One of the most dramatic forms of weather occurs over the oceans: tropical cyclones (also called "typhoons" and "hurricanes" depending upon where the system forms). Ocean currents greatly affect Earth's climate by transferring warm or cold air and precipitation to coastal regions, where they may be carried inland by winds. The Antarctic Circumpolar Current encircles that continent, influencing the area's climate and connecting currents in several oceans.
Ecology
The oceans are home to many forms of life, such as:
- fish
- cetacea such as whales, dolphins and porpoises,
- cephalopods such as the octopus
- crustaceans such as lobsters and shrimp
- marine worms
- plankton
- krill
Economy
The oceans are essential to transportation: a huge portion of the world's goods are moved by ship between the world's seaports. Important ship canals include the Saint Lawrence Seaway, Panama Canal, and Suez Canal.
Extraterrestrial oceans
Earth is the only known planet with liquid water on its surface, and is certainly the only such in our own solar system. However, liquid water is thought to be present under the surface of several natural satellites, particularly the Galilean moons of Europa, and, with less certainty, its fellows Callisto and Ganymede. Other icy moons may have once had internal oceans that have now frozen, such as Triton. The planets Uranus and Neptune may also possess large oceans of liquid water under their thick atmospheres, though their internal structure is not well understood at this time.
There is currently much debate over whether Mars once had an ocean of water in its northern hemisphere, and over what happened to it if it did; recent findings by the Mars Exploration Rover mission indicate it had some long-term standing water in at least one location, but its extent is not known.
Liquid hydrocarbons are thought to be present on the surface of Titan, though it may be more accurate to describe them as "lakes" rather than an "ocean". The distribution of these liquid regions will hopefully be better known after the full analysis of data from the Huygens probe of the Cassini-Huygens space mission, which dropped onto Titan's surface in January 2005. Titan is also thought likely to have a subterranean water ocean under the mix of ice and hydrocarbons that forms its outer crust.
See Also
External links
- Science taps into ocean secrets (http://news.bbc.co.uk/2/hi/science/nature/4033555.stm)
- Why is the ocean salty? (http://www.palomar.edu/oceanography/salty_ocean.htm)
- Official IHO boundaries of Oceans and Seas (http://ioc.unesco.org/oceanteacher/resourcekit/M3/Formats/Geography/OceansSeas.htm)
- The Hydrogen Expedition (http://www.thehydrogenexpedition.com) The first circumnavigation of the globe in a hydrogen fuel cell powered boat
- Coreocean (http://www.coreocean.org)
- NOPP - The National Oceanographic Partnership Program (http://www.nopp.org/)