Ocean
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. The area of the oceans is 361 million sq km.
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.)
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.
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. [
1] The vast abyssal plains of the deep ocean cover about 40% 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 1.4 × 10
21 kilograms, which is about 0.023% of the Earth's total mass.
 |
Map of large underwater features. (1995, NOAA) |
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) [
2]. 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.
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.
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 |
A picture of the ocean from the west coast of Ireland. |
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.
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.
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. A more recent fly-by of Titan made by Cassini has produced images that strongly suggest hydrocarbon lakes near the polar regions where it is colder. Titan is also thought likely to have a subterranean water ocean under the mix of ice and hydrocarbons that forms its outer crust.
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 [were] broken up, and the windows of heaven were opened (
KJV),inundating the world with the waters of the celestial ocean (see also
deluge (mythology).
*
Oceanography*
International Maritime Organization*
Sea*
Mediterranean sea*
Marginal sea*
Sea salt*
Water*
World Ocean Day*
Marine biology*
Pelagic zoneMatthias Tomczak and J. Stuart Godfrey. 2003.
Regional Oceanography: an Introduction. (see
the site)
*
Ocean Explorer - An educational and reference resource from NOAA
*
Oceanography Image of the Day - from the Woods Hole Oceanographic Institution
*
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?*
Intergovernmental Oceanographic Commission*
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 *
UN Atlas of the Oceans
