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Geography of the Ocean and the Structure of Planet Water

The Ocean / NEXT: Geological Makeup of Marine Environments »

The Global Ocean

The five oceans from smallest to largest are: the Arctic, Southern, Indian, Atlantic and Pacific.

These five "oceans" are connected and are actually one huge body of water, called the global ocean or just the ocean. If you were to add the smaller seas like the Barents, Beaufort, Chukchi, Kara, Laptev, East Siberian, Lincoln, Wandel, Greenland and Norwegian, etc. you would have a total area of the ocean of about 361,000,000 km² (which is ~71% of the Earth's surface), a total volume of roughly 1,370,000,000 km³, and an average depth of 3,790 m. Our hydrosphere (ocean plus all freshwater in ground water, lakes, rivers, snow, ice and the atmosphere) makes up about 0.023% of the total mass of the Earth.

The Water Cycle

The word “Ocean” comes from Okeanos, the Greek god of the Ocean.

Global gravity map

Terrigenous, pelagic, and authigenic materials composes most of ocean sediments. Erosion, weathering and volcanic activity on land washes out to sea and creates the sand, mud, and rock particles that make up terrigenous deposits. Consequently, terrigenous deposits are confined to narrow marginal bands close to land like continental shelves and are deepest near the mouths of large rivers or desert coasts. Pelagic deposits derived from seawater are red clays and the skeletal remains of organisms that have died and sunk to the ocean floor. These include pelagic red clays and globigerina, pteropod and siliceous oozes. Most of the ocean floor is actually covered in these organic remains with a depth ranging anywhere from 60 to 3,300 m deep, but they are thickest in convergence belts and upwelling zones. Authigenic deposits are made up of particles like manganese nodules and include montmorillonite and phillipsite and can be found in places where the sedimentation process occurs very slowly or currents sort out the deposits.

The Arctic Ocean

The Arctic Ocean is divided by an underwater ocean ridge called the Lomonosov ridge into the 4,000-4,500 m deep Eurasian or Nasin basin and the 4,000 m deep North American or Hyperborean basin. The topography of the Arctic Ocean bottom varies consisting of fault-block ridges, abyssal plains, and ocean deeps and basins that have an average depth of 1,038 m due to the continental shelf on the Eurasian side.

The greatest inflow of water to the Arctic Ocean comes from the Atlantic via the Norwegian Current, (which then travels along the Eurasian coast) although water also enters from the Pacific via the Bering Strait. The greatest outflow comes from the East Greenland Current. Ice used to cover most of the Arctic Ocean year round (this is now changing drastically due to global warming). When the ice melts, salinity and subfreezing temperatures vary. The subfreezing temperatures cool the air traveling towards the equator, mixing with warmer air at middle latitudes, resulting in rain and snow. Marine life is thought to be relatively scarce in the cold waters of the Arctic Ocean except in the open, southerly waters. Air traffic is common over the Arctic because it is the shortest route between the Pacific coast of North America and Europe. For boats, major ports are the Russian cities known as Murmansk and Arkhangelsk (Archangel).

An iceberg off Antarctica

The Southern Ocean

The Southern Ocean is the world's fourth-largest body of water. It encircles Antarctica and is actually divided among the Atlantic, Indian, and Pacific Oceans. Most people of North America and Continental Europe have no name for the area and regard the area as parts of the Atlantic, Pacific, and Indian Oceans simply extending to Antarctica. However, because mariners have long referred to this area as the “Southern Ocean” it was accepted as an ocean in 2000 by the International Hydrographic Organization. This ocean is predominantly deep water, averaging 4,000-5,000 m deep, and includes the Antarctic continental shelf, an unusually deep and narrow area with an edge of 400-800 m deep (over 270-670 m deeper than average). The lowest point is 7,235 m deep at the southern end of the South Sandwich Trench. There is a seven-fold increase in the size of the Antarctic ice pack between March and September (though this is also changing due to global warming), ranging from 2,600,000 km² to 18,800,000 km². The world's largest ocean current, the Antarctic Circumpolar Current (21,000 km in length) moves perpetually eastward here and transports 130,000,000 m³ of water per second—100 times the flow of all the world's rivers combined.

Current Environmental Issues

An increase in solar ultraviolet radiation originating from the Antarctic ozone hole is reducing marine primary productivity, or phytoplankton, by as much as 15% and is actually damaging the DNA of some fish.

Unregulated (5-6 times more than the regulated fishery) and unreported illegal fishing of Patagonian toothfish has been occurring, an activity that will have a long-term affect on the sustainability of the remaining stock. This illegal fishing also increases the mortality of seabirds who get caught in the long-lines used for toothfish.

The now-protected seal population is making a strong comeback after its severe overexploitation in the 18th and 19th centuries.

International Environmental Agreements

Most recently designated as its own ocean, the Southern Ocean is required to follow all international agreements regarding the world's oceans. The boundaries between the oceans are set by the International Hydrographic Organization which has determined that the Southern Ocean extends from the coast of Antarctica to 60° S latitude. The Southern Ocean is also subject to these agreements that are specific to the region according to the International Whaling Commission, a commission that prohibits commercial whaling south of 40° S latitude. The Convention on the Conservation of Antarctic Seals limits sealing and the Convention on the Conservation of Antarctic Marine Living Resources regulates fishing in this area of the world. Many nations prohibit mineral resource exploration and exploitation south of the fluctuating Antarctic Polar Front, or Antarctic Convergence. The Antarctic Convergence is located in the middle of the Antarctic Circumpolar Current and serves as the dividing line between the extremely cold polar surface waters to the south and the warmer waters located to the north.

The Indian Ocean

The Indian Ocean is the third-largest in the world and makes up approximately 20% of the Earth's water surface. It is bounded by southern Asia in the north, the Arabian Peninsula and Africa in the west, the Malay Peninsula, Sundra Islands and Australia in the east and the Southern Ocean in the south. The 20° east meridian separates the Indian Ocean from the Atlantic Ocean and the 147° east meridian separates it from the Pacific Ocean. The Indian Ocean stretches to about 30° N latitude in the Persian Gulf at its northernmost extent. At the southern tips of Africa and Australia, it is almost 10,000 km (or 6,200 miles) wide and its area is 73,556,000 km² (or 28,400,000 sq miles) when the Red Sea and the Persian Gulf are included. The volume of this massive body of water has been estimated at 292,131,000 km³ (or 70,086,000 mi³). Other features include small islands around the continental rims such as Madagascar (the world's fourth largest island), Comoros, Seychelles, Maldives, Mauritius, Sri Lanka, and Indonesia. The Indian Ocean is an important transit route between Asia and Africa, a geographical feature that has fueled some strong historical conflicts. Because the Indian Ocean is so enormous, no nation had ruled it until the beginning of the 1800s when Britain was able to dominate much of the surrounding land.

Underneath the surface of the Indian Ocean lies the convergence of the African, Indian and Antarctic crustal plates- their junctures marked by the Y-shaped branches of the Mid-Oceanic Ridge and a stem running south from the edge of the continental shelf near Mumbai, India. The resulting ridges subdivide the eastern, western and southern basins into smaller basins. The Indian Ocean consists of narrow 200 km (125 mile) continental shelves with the exception of a shelf of width that exceeds 1,000 km (600 miles) off of Australia's western coast. On average, the depth of this ocean is 3,890 m with the deepest point being the Java Trench at 7,450 m. Northwards of 50° S latitude, 86% of the major basin is covered by pelagic sediment and more than half is globigerina ooze. The rest is layered with terrigenous sediments and almost all the extreme southern latitudes are covered in glacial outwash.

Large rivers flowing into the Indian Ocean include the Zambezi, Arvandrud/Shatt-al-Arab, Indus, Ganges, Brahmaputra and the Irrawaddy. Monsoons control the currents in this oceanographic region. One current flows clockwise in the northern hemisphere and the other flows counterclockwise south of the equator. These two large, circular currents make up most of the flow pattern. When the winter monsoon occurs, the currents in the north are reversed.

The circulation of deep water is mostly controlled by water flowing in from the Atlantic Ocean, the Red Sea, and Antarctic currents. The surface temperature is 22°C (72°F) north of 20° S latitude and exceeds 28°C (82°F) to the east. Surface temperatures drop quickly south of 40° south latitude. The highest salinity occurs in the Arabian Sea and in a belt between southern Africa and southwestern Australia although average salinity of surface water ranges from 32 to 37 parts per 1,000 (ppt). South of 65° S latitude, pack ice and icebergs can be found year-round, although they don't usually make it further than the northern limit of 45° S latitude.

The Atlantic Ocean

The Earth's second-largest ocean is the Atlantic, a name derived from the “Sea of Atlas” in Greek mythology. It covers approximately one-fifth of the entire global ocean. Water drains into the Atlantic from a land area four times the size of both the Pacific and Indian oceans. The area of the Atlantic excluding the seas next to it is 82,400,000 km² and the volume is 323,617,637 km³. Including the adjacent seas the area is 106,400,000 km², and the volume is 354,700,000 km³. Including the adjacent seas, the Atlantic averages 3,332 m (10,932 ft) deep. Excluding the neighboring seas the Atlantic has an average depth of 3,926 m (12,881 ft). The deepest area is found in the Puerto Rico Trench at 8,605 m or 28,232 ft. The Atlantic varies in width anywhere from a narrow 2,848 km between Brazil and Liberia to a wide 4,830 km between the United States and northern Africa.

The geography of this ocean can be visualized by imagining a large S-shape basin extending north to south and divided into North Atlantic and South Atlantic by counter currents at the equator (about 8° N latitude). In the west, the Atlantic stretches all the way to North and South America. In the east, the Atlantic is connected to the Pacific Ocean by the Arctic Ocean in the north and the south. Incredible human labor created the Panama Canal, which now connects the Atlantic and the Pacific Oceans. The 20° east meridian divides the Atlantic from the Indian Ocean in the east. The Arctic Ocean is separated from the Atlantic by a line from Greenland to southernmost Svalbard to northern Norway. The lowest point of the Atlantic is 4,665 m deep in the Fram Basin.

The coasts of the Atlantic are marked by scores of bays, gulfs, and seas, including the Caribbean Sea, the Gulf of Mexico, Gulf of St. Lawrence, Mediterranean Sea, Black Sea, North Sea, Baltic Sea and the Norwegian-Greenland Sea. Islands include Svalbard, Greenland, Iceland, Rockall, Great Britain, Ireland, Fernando de Noronha, the Azores, the Madeira Islands, the Canaries, the Cape Verde Islands, Newfoundland, Bermuda, the West Indies, Ascension, St. Helena, Tristan da Cunha, the Falkland Islands and South Georgia Island.

A giant submarine mountain range called the Mid-Atlantic Ridge extends from Iceland in the north to about 58° S latitude, becoming very wide at approximately 1,600 km. A rift valley, or valley formed by faults, extends along most of the length of the Mid-Atlantic ridge and the depth of this ridge is less than 2,700 m in many places with mountain peaks that rise up to form islands above water. A smaller submarine ridge in the south Atlantic is known as the Walvis Ridge.

The Atlantic is divided by the Mid-Atlantic Ridge into two huge troughs between 3,700 to 5,500 m deep. Transverse ridges that run crosswise between the continents and the Mid-Atlantic Ridge divides the ocean floor into many different basins. Large basins include the Guiana, North American, Cape Verde, and Canaries basins in the North Atlantic. In the South Atlantic large basins include the Angola, Cape, Argentina and Brazil basins.

The deep ocean floor is mostly flat but there are quite a few seamounts, guyots and deeps or trenches. The deepest trench in the North Atlantic is the Puerto Rico Trench at 8,605 m, in the South Atlantic it is the South Sandwich Trench at 8,428 m and near the equator is the Romanche Trench at 7,454 m. The deepest point in the Atlantic is at 8,605 m and is called the Milwaukee Deep, an area located in the Puerto Rico Trench. Off the eastern coast of Canada is the Laurentian Abyss. The shelves that run along the edges of the continents make up approximately 11% of the bottom topography in addition to several deep channels that cut across the continental rise.

The Pacific Ocean

The Pacific is the world's largest body of water and was named by the Portuguese explorer Ferdinand Magellan who found the Pacific very peaceful (“pacifique”, means peaceful in French) for most of his journey from the Straits of Magellan to the Philippines. In contrast to its name, the islands of the “peaceful ocean” are often slammed by typhoons and hurricanes. The countries that border the Pacific, or the Pacific Rim, often experience volcanoes and earthquakes. Entire towns have been wiped out by Tsunamis, the large waves caused by an underwater earthquake.

The Pacific Ocean covers a third of the Earth's surface, has an area of 179.7 million km² and extends about 15,500 km from the Bering Sea in the Arctic all the way to the icy waters of Antarctica's Ross Sea in the South. The Pacific is widest eastwards at 5° N latitude where it reaches all the way from Indonesia to the Columbian Coast, a distance of 19,800 km. Its farthest western point is most likely the Strait of Malacca. The Pacific Ocean also contains the lowest point on earth and deepest part of the Ocean known as the Mariana Trench, an area that is 10,911 m below sea level. There are 25,000 Pacific islands in the Pacific Ocean—more than any other ocean.

Hawaii sunset at the beach

Most of these islands are located south of the equator. The largest seas of the Pacific include: the Celebes Sea, Coral Sea, East China Sea, Sea of Japan, South China Sea, Sulu Sea, Tasman Sea, and Yellow Sea. The Pacific and Indian Oceans are connected by the Straits of Malacca in the west while the Pacific and Atlantic Ocean are connected by the Straits of Magellan in the east.

Most of the ocean floor in the central Pacific basin is fairly level and has a mean depth of 4,270 m. Most variation in the ocean floor consists of steep-sided, flat-topped submarine peaks called seamounts. Mountain arcs, known as the Solomon Islands and New Zealand, soar above the surface in the west. Mountain arcs also form deep trenches like the Mariana Trench, the Philippine Trench, and the Tonga Trench adjacent to the outer edges of the wide western Pacific continental shelf. The East Pacific Rise is about 3,000 km wide and rises about 3 km above the adjacent ocean floor. It is located along the eastern edge of the Pacific Basin, a component of the world-wide mid-oceanic ridge. Most sediments in the Pacific Ocean are authigenic or pelagic in origin due to the relatively small land area draining into this enormous body of water.

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