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Maps exhibiting the world's oceanic waters. A continuous body of water encircling the Earth
, the world (global) ocean
is divided into a number of principal areas. Five oceanic divisions are usually recognized: Pacific
, and Southern
; the last two listed are sometimes consolidated into the first three.
More than half of this area is over 3,000 metres
(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 30 to 38 ppt. Scientists estimate that 230,000 marine life forms of all types are currently known, but the total could be up to 10 times that number.
Though generally described as several 'separate' oceans, these waters comprise one global, interconnected body of salt water often 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 Eurasia and Africa
- Indian Ocean, which washes upon southern Asia and separates Africa and Australia
- Southern Ocean, which, unlike other oceans, has no landmass separating it from other oceans and is therefore sometimes subsumed as the southern portions of the Pacific, Atlantic, and Indian Oceans, which encircles Antarctica and covers much of the Antarctic
- Arctic Ocean, sometimes considered a sea of the Atlantic, which covers much of the Arctic and washes upon northern North America and Eurasia
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
. 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
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
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
that live there.
The area of the World Ocean is 361×106
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 1,111 kilometres (690 mi). Its average depth is 3,790 metres (12,430 ft), and its maximum depth is 10,923 metres (6.787 mi)
Nearly half of the world's marine waters are over 3,000 metres (9,800 ft) deep.
The vast expanses of deep ocean (anything below 200 metres (660 ft) 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 1,400,000,000,000,000,000 metric tons (1.5×1018
short tons) or 1.4×1021
kg, which is about 0.023% of the Earth's total mass. Less than 3% is freshwater
; the rest is saltwater
, mostly in the ocean.
Main article: Color of water
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' nuclei of red photons from the incoming light, the only known example of color in nature resulting from vibrational, rather than electronic, dynamics.
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
Map of large underwater features. (1995, NOAA
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 10,971 metres (35,994 ft) (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 10 kilometres (6.2 mi) was created in 1995 based on gravitational distortions of the nearby sea surface.
Regions and depths
The major oceanic divisions
Oceanographers 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 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 according to temperature.
is the uppermost region. Its lowermost boundary is at a thermocline
of 12 °C (Template:Convert/f), which, in the tropics generally lies at 700–1,000 metres (2,300–3,300 ft). Next is the bathypelagic
lying between 10-4 °C (Template:Convert/f), typically between 700–1,000 metres (2,300–3,300 ft) and 2,000–4,000 metres (6,600–13,000 ft) Lying along the top of the abyssal plain is the abyssalpelagic
, whose lower boundary lies at about 6,000 metres (20,000 ft). The final zone includes the deep trenches, and is known as the hadalpelagic
. This lies between 6,000–11,000 metres (20,000–36,000 ft) and is the deepest oceanic zone.
Along 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 4,000 metres (13,000 ft). 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.
Main article: marine geology
A summary of the path of the thermohaline circulation/ Great Ocean Conveyor. Blue paths represent deep-water currents, while red paths represent surface 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.
It is often stated that the thermohaline circulation is the primary reason that the climate 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
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).
Main article: Marine biology
Lifeforms native to oceans include:
The oceans are essential to transportation: most of the world's goods move by ship
between the world's seaports
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.
- 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, liquid water is thought to be present under the surface of the Galilean moons Europa
and, with less certainty, Callisto
have been found on Saturn's moon Enceladus
, though these may not involve bodies of liquid water. Other icy moons may have once had internal oceans that have now frozen, such as Triton
. The planets Uranus
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 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.
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.
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- ^ BAD PHYSICS: Misconceptions spread by K-6 Grade School Textbooks
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- ^ "Mystery Ocean Glow Confirmed in Satellite Photos". October 4, 2005. http://www.livescience.com/strangenews/051004_sea_glow.html.
- ^ 11/21/2005, Usa today: A glowing sea, courtesy of algae Quote: "...The water glowed green in the direction of the movement...A: Little microscopic creatures (called Lingulodinium polyedrum) that glow in the dark caused the alluring strange display that night..."
- ^ 05 October 2005, New Scientist: Sea's eerie glow seen from space Quote: "...The ancient mariners were right. Tales of "milky seas" that glow bluish-white at night and extend as far as the horizon have been spun by sailors for centuries. Now this eerie glow has been spotted from space....The glowing area spanned 15,400 square kilometres (5,900 sq mi), an area the size of Connecticut (Image: Steven D Miller, US Naval Research Laboratory)..."
- ^ NASA, DAAC Study: The Incredible Glowing Algae Quote: "...Each year, the North Atlantic Ocean announces springtime by producing “blooms” large enough to be seen from space. These explosive increases in microscopic marine algae, called phytoplankton, appear as sudden bright blossoms in satellite imagery..."
- ^ Seager first=R. (2006). "The Source of Europe's Mild Climate"]. American Scientist. http://www.americanscientist.org/issues/feature/2006/4/the-source-of-europes-mild-climate.
- ^ Rhines and Hakkinen (2003). "Is the Oceanic Heat Transport in the North Atlantic Irrelevant to the Climate in Europe?". ASOF Newsletter. http://www.realclimate.org/Rhines_hakkinen_2003.pdf.
- ^ Hot "ice" may cover recently discovered planet
- ^ David A. Aguilar (2009-12-16). "Astronomers Find Super-Earth Using Amateur, Off-the-Shelf Technology". Harvard-Smithsonian Center for Astrophysics. http://www.cfa.harvard.edu/news/2009/pr200924.html. Retrieved January 23, 2010.
Pope, F. 2009. From eternal darkness springs cast of angels and jellied jewels. in The Times. November 23. 2009 p. 16 - 17.