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Ocean

2007 Schools Wikipedia Selection. Related subjects: Geology and geophysics

   The world's oceans as seen from the South Pacific Ocean, before the
   definition of the Southern Ocean in 2000
   The world's oceans as seen from the South Pacific Ocean, before the
   definition of the Southern Ocean in 2000
     Earth's oceans
   ( World Ocean)
     * Arctic Ocean
     * Atlantic Ocean
     * Indian Ocean
     * Pacific Ocean
     * Southern Ocean

   An ocean (from Ωκεανός, Okeanos (Oceanus) in Greek) is a principal
   component of the hydrosphere: a major body of saline water that, in
   totality, covers about 71% of the Earth's surface (or an area of some
   361 million square kilometers). Nearly half of the world's marine
   (oceanic) waters are over 3,000 meters (9,800 ft) deep. Average oceanic
   salinity is around 35 parts per thousand (ppt) (3.5%), and nearly all
   seawater has a salinity in the range of 31 to 38 ppt.

   Though somewhat arbitrarily divided into several 'separate' oceans,
   these waters comprise one global, interconnected body of salt water
   often referred to as the World Ocean or global ocean. The major oceanic
   divisions are defined in part by the continents, various archipelagos,
   and a number of other criteria; these divisions are (in descending
   order of size) the Pacific Ocean, the Atlantic Ocean, the Indian Ocean,
   the Southern Ocean (which is sometimes reckoned instead as just the
   southern portions of the Pacific, Atlantic, and Indian Oceans), and the
   Arctic Ocean (which is sometimes considered a sea of the Atlantic).
   Smaller regions of the oceans are called seas, gulfs, bays and other
   names.

   There are also some smaller bodies of salt water that are inland and
   not interconnected with the World Ocean: e.g., the Caspian Sea, Aral
   Sea, the Great Salt Lake. These are not considered to be oceans or
   parts of oceans, though some are called 'seas'.

   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, the
   Caspian and the Black Seas, the latter two of which were formed by the
   collision of Cimmeria with Laurasia. The Mediterranean Sea is very
   nearly a discrete 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 strait off
   entirely. The Black Sea is connected to the Mediterranean through the
   Bosporus, but this is in effect a natural canal cut through continental
   rock some 7,000 years ago, rather than a piece of oceanic sea floor
   like the Strait of Gibraltar.

Physical properties

   The area of the World Ocean is 361 million square kilometers (139
   million sq mi), its volume is over 1,340 million cubic kilometers (319
   million cu mi), and its average depth is 3,711 meters (12,175 ft).
   Nearly half of the world's marine waters are over 3,000 meters (9,800
   ft) deep. The vast abyssal plains of the deep ocean cover about 40% of
   the Earth's surface. 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 kilograms, which
   is about 0.023% of the Earth's total mass.

Exploration

   Map of large underwater features. (1995, NOAA)
   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 Marianas Trench located in the
   Pacific Ocean near the Northern Mariana Islands. It has a maximum depth
   of 10,923 meters (35,838 ft) . It was fully surveyed in 1951 by the
   British naval vessel, "Challenger II" which gave its name to the
   deepest part of the trench, the " Challenger Deep". In 1960, the
   Trieste successfully reached the bottom of the trench, manned by a crew
   of two men.

   Much of the bottom of the world's oceans are unexplored and unmapped. A
   global image of many underwater features larger than 10 kilometers (6
   mi) was created in 1995 based on gravitational distortions of the
   nearby sea surface.

Regions

   The major oceanic divisions
   The major oceanic divisions

   Oceans are divided into numerous regions depending on the physical and
   biological conditions of these areas. The pelagic zone includes all
   open ocean regions, and can be subdivided into further regions
   categorised by depth and light abundance. The photic zone covers the
   oceans from surface level to 200 metres down. This is the region where
   the photosynthesis most commonly occurs and therefore contains the
   largest biodiversity in the ocean. Since plants can only survive with
   photosynthesis any life found lower than this must either rely on
   material floating down from above (see marine snow) or find another
   primary source; this often comes in the form of hydrothermal vents in
   what is known as the aphotic zone (all depths exceeding 200m). The
   pelagic part of the photic zone is known as the epipelagic. The pelagic
   part of the aphotic zone can be further divided into regions that
   succeed each other vertically. The mesopelagic is the uppermost region,
   with its lowermost boundary at a thermocline of 10°C, which, in the
   tropics generally lies between 700 and 1,000m. After that is the
   bathypelagic lying between 10°C and 4°C, or between 700 or 1,000m and
   2,000 or 4,000m. Lying along the top of the abyssal plain is the
   abyssalpelagic, whose lower boundary lies at about 6,000m. The final
   zone falls into the oceanic trenches, and is known as the hadalpelagic.
   This lies between 6,000m and 10,000m and is the deepest oceanic zone.

   Along with pelagic aphotic zones there are also benthic aphotic zones,
   these correspond to the three deepest zones. The bathyal zone covers
   the continental slope and the rise down to about 4,000m. The abyssal
   zone covers the abyssal plains between 4,000 and 6,000m. Lastly, the
   hadal zone corresponds to the hadalpelagic zone which is found in the
   oceanic trenches. The pelagic zone can also be split into two
   subregions, the neritic zone and the oceanic zone. The neritic
   encompasses the water mass directly above the continental shelves,
   while the oceanic zone includes all the completely open water. In
   contrast, the littoral zone covers the region between low and high tide
   and represents the transitional area between marine and terrestrial
   conditions. It is also known as the intertidal zone because it is the
   area where tide level affects the conditions of the region.

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 the majority of plant and animal life on Earth.
   These lifeforms include:
     * Radiata
     * 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: most 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. They
   are also an important source of valuable foodstuffs for the fishing
   industry. Some of these are shrimp, fish, crabs and lobsters.

Ancient oceans

   Continental drift has reconfigured the Earth's oceans, joining and
   splitting ancient oceans to form the current oceans. Ancient oceans
   include:
     * Bridge River Ocean, the ocean between the ancient Insular Islands
       and North America.
     * Iapetus Ocean, the southern hemisphere ocean between Baltica and
       Avalonia.
     * Panthalassa, the vast world ocean that surrounded the Pangaea
       supercontinent.
     * Rheic Ocean
     * Slide Mountain Ocean, the ocean between the ancient Intermontane
       Islands and North America.
     * Tethys Ocean, the ocean between the ancient continents of Gondwana
       and Laurasia.
     * Khanty Ocean, the ocean between Baltica and Siberia.
     * Mirovia, the ocean that surrounded the Rodinia supercontinent.
     * Paleo-Tethys Ocean, the ocean between Gondwana and the Hunic
       terranes.
     * Proto-Tethys Ocean,
     * Pan-African Ocean, the ocean that surrounded the Pannotia
       supercontinent.
     * Superocean, the ocean that surrounds a global supercontinent.
     * Ural Ocean, the ocean between Siberia and Baltica.

Extraterrestrial oceans

   Earth is the only known planet with liquid water on its surface and is
   certainly the only one 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, Callisto and Ganymede. Geysers have been found on Enceladus.
   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 Cassini-Huygens space mission initially discovered only
   what appeared to be dry lakebeds and empty river channels, suggesting
   that Titan had lost what surface liquids it might have had. A more
   recent fly-by of Titan made by Cassini has produced radar images that
   strongly suggest hydrocarbon lakes near the polar regions where it is
   colder. Titan is also thought likely to have a subterranean water ocean
   under the mix of ice and hydrocarbons that forms its outer crust.

Mythology

   The original concept of "ocean" goes back to notions of Mesopotamian
   and Indo-European mythology, imagining the world to be encircled by a
   great river. Okeanos, "Ωκεανός" in Greek, reflects the ancient Greek
   observation that a strong current flowed off Gibraltar and their
   subsequent assumption that it was a great river. (Compare also Samudra
   from Hindu mythology and Jörmungandr from Norse mythology). The world
   was imagined to be enclosed by a celestial ocean above the heavens, and
   an ocean of the underworld below (compare Rasā, Varuna). This is
   evidenced for example in the account of Noah's flood in Genesis 7:11,
   where

          all the fountains of the great deep [were] broken up, and the
          windows of heaven were opened ( KJV),

   inundating the world with the waters of the celestial ocean (see also
   deluge (mythology)).

Trivia

   One common misconception today is that the oceans are blue primarily
   because the sky is blue. In fact, water has a very slight blue colour
   that can only be seen in large volumes. While the sky's reflection does
   contribute to the blue appearance of the surface, it is not the primary
   cause. The primary cause is the absorption by the water molecules'
   nuclei of red photons from the incoming light, the only known example
   of colour in nature resulting from nuclear, rather than electronic,
   dynamics.

   Retrieved from " http://en.wikipedia.org/wiki/Ocean"
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   with only minor checks and changes (see www.wikipedia.org for details
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