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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 km², its volume is 1370 million km³, 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 × 10
21 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.
Exploration
Map of large underwater features. (1995, [[NOAA)]]
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) 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:
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.
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
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bg:Световен океан
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de:Ozean
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he:אוקיינוס
la:Oceanus
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pt:Oceano
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Category:Forms of water
Category:Oceanography