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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 3000 m 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 mantleBlack 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 7000 years ago, rather than a piece of oceanic sea floor like the Strait of Gibraltar.) where there are no continents. Origins: There are thought to have been two 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; the other is the large numbers of comets which impacted with the Earth and added their water to it. Physical properties: The area of the World Ocean is 361 million km², its volume is 1340 million km³, and its average depth is 3711 m. Nearly half of the world's marine waters are over 3000 m 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 kg, ca. 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 m (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 km 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: a huge portion 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 Pangaeac superontinent.
- Tethys Ocean, the ocean between the ancient continents of Gondwana and Laurasia.
- Iapetus Ocean, the southern hemisphere ocean between Baltica and Avalonia.
Extraterrestrial oceans:Earth is the only known planet with liquid water on its surface, and is certainly the only such 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. GeysersEnceladus. Other icy moons may have once had internal oceans that have now frozen, such as Triton. The planets Uranus and Neptune have been found on 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 further 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).
Sunrise Diving, Phuket, Thailand.
PADI 5 Star National Geographic Instructor Development Center. 198/12 Rat-U-Thit Road, Patong, Phuket, Thailand. Phone: (+66) 076292052 Fax: (+66) 076293034
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