Ocean

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Oceans (from Okeanos in Greek, the ancient Greeks noticing the strong current that flowed off Gibraltar and assuming it was a great river) cover almost three quarters (71%) of the surface of the Earth, and nearly half of the world's marine waters are over 3,000 meters (9,800 ft) deep.

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". (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.)

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. Stardust 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 (139 million sq mi), its volume is 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. ^* 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 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 (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".

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.

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 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:

Panthalassa, the vast world ocean that surrounded the Pangaea supercontinent.
Tethys Ocean, the ocean between the ancient continents of Gondwana and Laurasia.
Iapetus Ocean, the southern hemisphere ocean between Baltica and Avalonia.

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". Cassini-Huygens space mission, which dropped the Huygens probe onto Titan's surface in January 2005, found that Titan is currently without such lakes but that it may gain and lose them periodically. 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, Samudra in Hindu mythology (compare also Jörmungandr, the sea serpent living in that outer ocean in 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 ^* broken up, and the windows of heaven were opened (KJV),

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

References

Matthias Tomczak and J. Stuart Godfrey. 2003. Regional Oceanography: an Introduction. (see the site)

External links

Ocean Explorer - An educational and reference resource from NOAA
Science taps into ocean secrets
The Last Days of the Ocean - a Mother Jones special report on the state of the seas
Why is the ocean salty?
Official IHO boundaries of Oceans and Seas
The Hydrogen Expedition The first circumnavigation of the globe in a hydrogen fuel cell powered boat
Oceana - Protecting the World's Oceans
CORE - Consortium for Oceanographic Research and Education
NOPP - National Oceanographic Partnership Program
NOSB - National Ocean Sciences Bowl
CoML - Census of Marine Life
World Ocean Observatory
Greenpeace Defending our Oceans
Ocean Voyager, a five-part journey to save the seas, created by Mother Jones magazine
The Ocean Conservancy - Advocates for Wild, Healthy Oceans
Ocean Data on-line

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