An
ocean (from
Greek , "''okeanos''"
Oceanus) is a major body of
saline water, and a principal component of the
hydrosphere. Approximately 71% of the
Earth's
surface (~) is covered by ocean, a
continuous body of water that is customarily divided into several principal oceans and smaller
seas.
More than half of this area is over 3,000 metres (9,800 ft) deep. Average oceanic salinity is around 35 parts per thousand (‰) (3.5%), and nearly all seawater has a salinity in the range of 30 to 38 ‰. Scientists estimate that 230,000 marine species are currently known, but the total could be up to 10 times that number.
Overview
Though generally described as several 'separate' oceans, these waters comprise one global, interconnected body of salt water sometimes referred to as the
World Ocean or global ocean. This concept of a continuous body of water with relatively free interchange among its parts is of fundamental importance to
oceanography.
The major oceanic divisions are defined in part by the continents, various archipelagos, and other criteria. These divisions are (in descending order of size):
Pacific Ocean, which separates Asia and Australia from the Americas
Atlantic Ocean, which separates the Americas from Europe and Africa
Indian Ocean, which washes upon southern Asia and separates Africa and Australia
Southern Ocean, sometimes considered an extension of the Pacific, Atlantic and Indian Oceans, which encircles
Antarctica.
Arctic Ocean, sometimes considered a sea of the Atlantic, which covers much of the Arctic and washes upon northern North America and Eurasia.
The Pacific and Atlantic may be further subdivided by the equator into northern and southern portions. Smaller regions of the oceans are called seas, gulfs, bays, 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. Continental crust is thicker but less dense. From this perspective, the earth has three oceans: the World Ocean, the Caspian Sea, and Black Sea. The latter two were formed by the collision of Cimmeria with Laurasia. The Mediterranean Sea is at times a discrete ocean, because tectonic plate movement has repeatedly broken its connection to the World Ocean through the Strait of Gibraltar. The Black Sea is connected to the Mediterranean through the Bosporus, but the Bosporus is 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.
Despite their names, smaller landlocked bodies of saltwater that are ''not'' connected with the World Ocean, such as the Aral Sea, are actually salt lakes.
Borders of the oceans
''For a detailed list of the borders of the oceans, see
here.''
Ocean and life
The ocean has a significant effect on the
biosphere. Oceanic
evaporation, as a phase of the
water cycle, is the source of most
rainfall, and ocean temperatures determine
climate and
wind patterns that affect life on land.
Life within the ocean
evolved 3 billion years prior to life on land. Both the depth and distance from shore strongly influence the amount and kinds of
plants and
animals that live there.
Physical properties
The area of the World Ocean is 361 million square kilometres (139 million square miles)
Its volume is approximately 1.3 billion cubic kilometres (310 million cu mi). This can be thought of as a cube of water with an edge length of . Its average depth is , and its maximum depth is Nearly half of the world's marine waters are over deep. The vast expanses of deep ocean (anything below ) cover about 66% 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 or 1.4×1021 kg, which is about 0.023 percent of the Earth's total mass. Less than 3 percent is freshwater; the rest is saltwater, mostly in the ocean.
Color
A common misconception is that the oceans are blue primarily because the sky is blue. In fact, water has a very slight blue color 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 of red photons from the incoming light, the only known example of color in nature resulting from vibrational, rather than electronic, dynamics.
Glow
Sailors and other mariners have reported that the ocean often emits a visible glow, or luminescence, which extends for miles at night. In 2005, scientists announced that for the first time, they had obtained photographic evidence of this glow. It may be caused by
bioluminescence.
Exploration
Ocean travel by boat 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. Its maximum depth has been estimated to be (plus or minus 11 meters; see the Mariana Trench article for discussion of the various estimates of the maximum depth.) The British naval vessel, ''Challenger II'' surveyed the trench in 1951 and named 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 ocean bottom remains unexplored and unmapped. A global image of many underwater features larger than was created in 1995 based on gravitational distortions of the nearby sea surface.
Regions and depths
400px|thumb|The major oceanic divisions|alt=Drawing showing divisions according to depth and distance from shoreOceanographers divide the ocean into regions depending on physical and biological conditions of these areas. The
pelagic zone includes all open ocean regions, and can be divided into further regions categorized by depth and light abundance. The
photic zone covers the oceans from surface level to 200 metres down. This is the region where photosynthesis can occur and therefore is the most
biodiverse. Since plants require photosynthesis, life found deeper than this must either rely on material sinking from above (see
marine snow) or find another energy source;
hydrothermal vents are the primary option in what is known as the
aphotic zone (depths exceeding 200 m). 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 according to temperature.
The
mesopelagic is the uppermost region. Its lowermost boundary is at a
thermocline of , which, in the tropics generally lies at . Next is the
bathypelagic lying between , typically between and Lying along the top of the
abyssal plain is the
abyssalpelagic, whose lower boundary lies at about . The last zone includes the deep trenches, and is known as the
hadalpelagic. This lies between and is the deepest oceanic zone.
200px|thumb|Major sea divisions, including sea life that lives in that divisionAlong with pelagic aphotic zones there are also benthic aphotic zones. These correspond to the three deepest zones of the deep-sea. The bathyal zone covers the continental slope down to about . The abyssal zone covers the abyssal plains between 4,000 and 6,000 m. 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.
Geology
The ocean floor spreads from
mid-ocean ridges where two
plates adjoin. Where two plates move towards each other, one plate
subducts under another plate (oceanic or continental) leading to an
oceanic trench.
Climate effects
Ocean currents greatly affect the Earth's
climate by transferring heat from the tropics to the polar regions, and transferring warm or cold air and precipitation to coastal regions, where winds may carry them inland. Surface heat and freshwater
fluxes create global
density gradients that drive the
thermohaline circulation part of large-scale ocean circulation. It plays an important role in supplying heat to the polar regions, and thus in sea ice regulation. Changes in the thermohaline circulation are thought to have significant impacts on the Earth's radiation budget. Insofar as the thermohaline circulation governs the rate at which deep waters reach the surface, it may also significantly influence atmospheric carbon dioxide concentrations.
For a discussion of the possibilities of changes to the thermohaline circulation under global warming, see shutdown of thermohaline circulation.
It is often stated that the thermohaline circulation is the primary reason that the climate of Western Europe is so temperate. An alternate hypothesis claims that this is largely incorrect, and that Europe is warm mostly because it lies downwind of an ocean basin, and because atmospheric waves bring warm air north from the subtropics.
The Antarctic Circumpolar Current encircles that continent, influencing the area's climate and connecting currents in several oceans.
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).
Biology
Lifeforms native to oceans include:
Radiata
Fish
Cetacea such as whales, dolphins and porpoises,
Cephalopods such as octopus and squid
Crustaceans such as lobsters, clams, shrimp and krill
Marine worms
Plankton
Echinoderms such as brittle stars, starfish, sea cucumbers and sand dollars.
Economy
The oceans are essential to transportation: most of the world's goods move by
ship between the world's
seaports.
Oceans are also the major supply source for the fishing industry. Some of the more major ones are shrimp, fish, crabs and lobster.
Ancient oceans
Continental drift continually reconfigures the oceans, joining and splitting bodies of water. 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.
Poseidon Ocean
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
:See also ''
Extraterrestrial liquid water''
Earth is the only known
planet with liquid water on its surface and is certainly the only one in our own
solar system. However, a layer of liquid water thick enough to decouple the crust from the mantle is thought to be present under the surfaces of the moons
Titan,
Europa and, with less certainty,
Callisto and
Ganymede. A similar
magma ocean is thought to be present on
Io.
Geysers have been found on Saturn's moon
Enceladus, though these may not involve bodies of liquid water. Other icy moons and
trans-neptunian objects may also have internal oceans, or have once had internal oceans that have now frozen. 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.
There is currently much debate over whether Mars once had an ocean in its northern hemisphere, and over what happened to it; recent findings by the Mars Exploration Rover mission indicate Mars had long-term standing water in at least one location, but its extent is not known.
Astronomers believe that Venus had liquid water and perhaps oceans in its very early history. If they existed, all later vanished via resurfacing.
Liquid hydrocarbons (not water) are thought to be present on the surface of Titan, though ''lakes'' may be a more accurate term. 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. Cassini's more recent fly-by of Titan offers radar images that strongly suggest hydrocarbon lakes near the colder polar regions. Titan is thought to have a subterranean water ocean under the ice and hydrocarbon mix that forms its outer crust.
Beyond the solar system, the planet Gliese 581 c is at the right distance from its sun to support liquid surface water. However, its greenhouse effect would make it too hot for oceans to exist on the surface. On Gliese 581 d the greenhouse effect may bring temperatures suitable for surface oceans. Astronomers dispute whether HD 209458 b has water vapour in its atmosphere. Gliese 436 b is believed to have "hot ice." Neither of these planets are cool enough for liquid water—but if water molecules exist there, they are also likely to be found on planets at a suitable temperature. GJ 1214 b, detected by transit, found evidence that this planet has oceans made of exotic form of ice VII, making up 75% of all the planet's mass.
Culture
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 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.
Artworks which depict maritime themes are known as marine art, a term which particularly applies to common styles of European painting of the 17th to 19th centuries.
See also
References
Matthias Tomczak and J. Stuart Godfrey. 2003. ''Regional Oceanography: an Introduction''. (see the site)
"Origins of the oceans and continents". ''UN Atlas of the Oceans.''
Further reading
Pope, F. 2009. From eternal darkness springs cast of angels and jellied jewels. ''in'' The Times. November 23. 2009 p. 16–17.
External links
National Oceanic and Atmospheric Administration
Marine forecasts for any ocean point – Buoyweather is a helpful link for marine safety
Council on Foreign Relations, Interactive Guide to Oceans Governance
Smithsonian Ocean Portal
Oceans
Category:Greek loanwords
Category:Coastal and oceanic landforms
Category:Nature
am:ውቅያኖስ
ang:Ȝārsecȝ
ar:محيط (جغرافيا)
an:Ocián
arc:ܐܘܩܝܢܘܣ
gn:Paraguasu
az:Okean
bn:মহাসাগর
zh-min-nan:Hái-iûⁿ
map-bms:Samudra
ba:Океан
be:Акіян
be-x-old:Акіян
bcl:Kadagatan
bi:Dipsi
bs:Okean
br:Meurvor
bg:Океан
ca:Oceà
cv:Океан
cs:Oceán
cy:Cefnfor
da:Verdenshave
de:Ozean
nv:Tó niteel
et:Ookean
el:Ωκεανός
es:Océano
eo:Oceano
eu:Ozeano
fa:اقیانوس
fo:Hav
fr:Océan
fy:Oseaan
fur:Ocean
gl:Océano
gan:大洋
ko:대양
hy:Օվկիանոս
hi:महासागर
hr:Ocean
io:Oceano
ig:Oke Òrìmìlì
id:Samudra
ia:Oceano
ie:Ocean
iu:ᐃᒫᓗᒃ/imaaluk
is:Haf
it:Oceano
he:אוקיינוס
jv:Samodra
kn:ಮಹಾಸಾಗರ
krc:Океан
ka:ოკეანე
kk:Мұхит
sw:Bahari
ht:Oseyan
lo:ມະຫາສະໝຸດ
la:Oceanus
lv:Okeāns
lt:Vandenynas
jbo:braxamsi
lmo:Ucean
hu:Óceán
mk:Океан
mg:Ranomasina
ml:സമുദ്രം
mr:महासागर
arz:محيط
ms:Lautan
mn:Далай
my:သမုဒ္ဒရာ
nl:Oceaan
new:चमुत्तिरम् (सन् २००२या संकिपा)
ja:大洋
no:Hav#Verdenshavene
nn:Verdshav
oc:Ocean
uz:Okean
pnb:بحر
pap:Oceano
ps:سمندر
km:មហាសមុទ្រ
nds:Ozean
pl:Ocean
pt:Oceano
kaa:Okean
ro:Ocean
qu:Mama qucha
rue:Океан
ru:Океан
sah:Океан
sco:Ocean
sq:Oqeani
scn:Ocèanu
simple:Ocean
sk:Oceán (časť svetového oceánu)
sl:Ocean
szl:Uocean
ckb:ئوقیانووس
sr:Океан
sh:Ocean
su:Jaladri
fi:Valtameri
sv:Ocean
tl:Karagatan
ta:பெருங்கடல்
tt:Океан
te:మహాసముద్రం
th:มหาสมุทร
tg:Уқёнус
chr:ᎠᎺᏉᎯ
tr:Okyanus
tk:Okean
uk:Океан
ur:سمندر
ug:ئوكيانىيە
vec:Oceano
vi:Đại dương
fiu-vro:Suurmeri
wa:Oceyan
zh-classical:洋
war:Kalawdan
wo:Mbàmbulaan
yi:אקעאן
yo:Òkun
zh-yue:大洋
bat-smg:Ondėnīns
zh:海洋