WATTERY OCEANS

An ocean (from Ancient Greek Ὠκεανός, transc. Okeanós, the sea of classical antiquity^[1]) is a body of saline water that composes much of a planet's hydrosphere.^[2] On Earth, an ocean is one of the major conventional divisions of the World Ocean. These are, in descending order by area, the Pacific, Atlantic, Indian, Southern (Antarctic), and Arctic Oceans.^[3][4] The word sea is often used interchangeably with "ocean" in American English but, strictly speaking, a sea is a body of saline water (generally a division of the world ocean) partly or fully enclosed by land.^[5]

Saline water covers approximately 360,000,000 km² (140,000,000 sq mi) and is customarily divided into several principal oceans and smaller seas, with the ocean covering approximately 71% of Earth's surface and 90% of the Earth's biosphere.^[6] The ocean contains 97% of Earth's water, and oceanographers have stated that less than 5% of the World Ocean has been explored.^[6] The total volume is approximately 1.35 billion cubic kilometers (320 million cu mi) with an average depth of nearly 3,700 meters (12,100 ft).^[7][8][9]

As the world ocean is the principal component of Earth's hydrosphere, it is integral to life, forms part of the carbon cycle, and influences climate and weather patterns. The world ocean is the habitat of 230,000 known species, but because much of it is unexplored, the number of species that exist in the ocean is much larger, possibly over two million.

Earth's global ocean

Various ways to divide the World Ocean

Oceanic divisions

Further information: Borders of the oceans

1. Epipelagic zone: surface – 200 meters deep 2. Mesopelagic zone: 200 m – 1000 m 3. Bathypelagic zone: 1000 m – 4000 m 4. Abyssopelagic zone: 4000 m – 6000 m 5. Hadal zone (the trenches): 6000 m to the bottom of the ocean

Though generally described as several separate oceans, the global, interconnected body of salt water is sometimes referred to as the World Ocean or global ocean.^[15][16] The concept of a continuous body of water with relatively free interchange among its parts is of fundamental importance to oceanography.^[17]

The major oceanic divisions – listed below in descending order of area and volume – are defined in part by the continents, various archipelagos, and other criteria.^[9][12][18]

#	Ocean	Location	Area (km2) (%)	Volume (km3) (%)	Avg. depth (m)	Coastline (km)
1	Pacific Ocean	Separates Asia and Oceania from the Americas[19][NB]	168,723,000 46.6	669,880,000 50.1	3,970	135,663
2	Atlantic Ocean	Separates the Americas from Europe and Africa[20]	85,133,000 23.5	310,410,900 23.3	3,646	111,866
3	Indian Ocean	Washes upon southern Asia and separates Africa and Australia[21]	70,560,000 19.5	264,000,000 19.8	3,741	66,526
4	Southern Ocean	Sometimes considered an extension of the Pacific, Atlantic and Indian Oceans,[22][23] which encircles Antarctica	21,960,000 6.1	71,800,000 5.4	3,270	17,968
5	Arctic Ocean	Sometimes considered a sea or estuary of the Atlantic,[24][25] which covers much of the Arctic and washes upon northern North America and Eurasia[26]	15,558,000 4.3	18,750,000 1.4	1,205	45,389
Total – World Ocean			361,900,000 100	1,335,000,000 100	3,688	377,412[27]

NB: Volume, area, and average depth figures include NOAA ETOPO1 figures for marginal South China Sea.
Sources: Encyclopedia of Earth,^{[19][20][21][22][26]} International Hydrographic Organization,^[23] Regional Oceanography: an Introduction (Tomczak, 2005),^[24]Encyclopædia Britannica,^[25] and the International Telecommunication Union.^[27]

Oceans are fringed by smaller, adjoining bodies of water such as seas, gulfs, bays, bights, and straits.

Global system

World Distribution of Mid-Oceanic Ridges; USGS

Three main types of plate boundaries.

^{The mid-ocean ridges of the world are connected and form a single global mid-oceanic ridge system that is part of every ocean and the longest mountain range in the world. The continuous mountain range is 65,000 km (40,000 mi) long (several times longer than the Andes, the longest continental mountain range).[28][10]} The origin of Earth's oceans is unknown; oceans are thought to have formed in the Hadean eon and may have been the impetus for the emergence of life.

Extraterrestrial oceans may be composed of water or other elements and compounds. The only confirmed large stable bodies of extraterrestrial surface liquids are the lakes of Titan, although there is evidence for the existence of oceans elsewhere in the Solar System. Early in their geologic histories, Marsand Venus are theorized to have had large water oceans. The Mars ocean hypothesis suggests that nearly a third of the surface of Mars was once covered by water, and a runaway greenhouse effect may have boiled away the global ocean of Venus. Compounds such as salts and ammonia dissolved in water lower its freezing point so that water might exist in large quantities in extraterrestrial environments as brine or convecting ice. Unconfirmed oceans are speculated beneath the surface of many dwarf planets and natural satellites; notably, the ocean of Europa is estimated to have over twice the water volume of Earth. The Solar System's giant planets are also thought to have liquid atmospheric layers of yet to be confirmed compositions. Oceans may also exist on exoplanets and exomoons, including surface oceans of liquid water within a circumstellar habitable zone. Ocean planets are a hypothetical type of planet with a surface completely covered with liquid.^[11][12]

Clouds over the Atlantic Ocean

Earth's oceans
Arctic Atlantic Indian Pacific Southern
World Ocean