Ocean
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South Pacific (Google Maps) |
The world's oceans
(from Okeanos, Greek for river, the ancient Greeks noticed that
a strong current flowed off Gibraltar, and assumed 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 Mariana 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 is 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
- Lapetus Ocean,
the southern hemisphere ocean between Baltica and Avalonia
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, Oceanus
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 Rasa, 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 [were] broken up, and the windows of heaven
were opened (KJV), inundating the world with the waters of the celestial
ocean.
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