Encyclopedia
Oceans cover almost three quarters of the surface of the
Earth, and nearly half of the world's marine waters are over 3,000 meters deep. The area of the oceans is 361 million sq km.
This global, interconnected body of
salt water, called the World Ocean, is generally divided by the
continents and
archipelagos into the following bodies, from the largest to the smallest: the
Pacific Ocean, the
Atlantic Ocean, the
Indian Ocean, the
Southern Ocean, and the
Arctic 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 strait off entirely, making the Mediterranean a fourth "ocean".
Origins
There are thought to have been two or three primary sources for the primordial water that formed Earth's oceans, with debate as to their relative importance. One is outgassing of steam from the Earth's interior, which contributed to the atmosphere and, once the young planet had sufficiently cooled, produced rain; another being that the numerous
comets which impacted with the Earth and added their water to it.
Another theory is that Earth once had a ring of ice exactly like
Saturn's and as that ring melted the water gravitated towards the planet, and rained down.
Panspermia theories about Earth's first creatures are linked to what exactly composed the ring besides ice.
Physical properties
The area of the World Ocean is 361 million square kilometers , its volume is 1,340 million cubic kilometers , and its average depth is 3,711 meters . Nearly half of the world's marine waters are over 3,000 meters 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
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 . 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".
Much of the bottom of the world's oceans are unexplored and unmapped. A global image of many underwater features larger than 10 kilometers was created in 1995 based on gravitational distortions of the nearby sea surface.
Regions
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 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 or find another primary source, this often comes in the form of
hydrothermal vents, this is known as the aphotic zone and is defined as being deeper than 200m down. 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, whos 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 to this the
littoral zone covers the region between low and high tide and represents the transitional area between marine and terrestrial conditions. Also known as the
intertidal zone due to it being 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 .
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:
...
such as
whales,
dolphins and
porpoises,
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 via the
fishing industry.
Ancient oceans
Continental drift has reconfigured the Earth's oceans, joining and splitting ancient oceans to form the current oceans. Ancient oceans include:
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 were 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, "O?ea???" in
Greek,
Samudra in
Hindu mythology . The world was imagined to be enclosed by a celestial ocean above the heavens, and an ocean of the underworld below . 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 ,
inundating the world with the waters of the celestial ocean
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 .
See also
References
- Matthias Tomczak and J. Stuart Godfrey. 2003. Regional Oceanography: an Introduction.
External links
- - An educational and reference resource from NOAA
- - from the Woods Hole Oceanographic Institution
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- - a Mother Jones special report on the state of the seas
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- , a five-part journey to save the seas, created by Mother Jones magazine
- - Advocates for Wild, Healthy Oceans
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