"Ocean" comes from the Greek word Oceanus. The somewhat arbitrarily divided oceans on the planet Earth are actually one interconnected body of salt water. Oceans cover about 70 percent of the Earth's surface and until the year 2000 there were four recognized oceans: the Arctic, Atlantic, Indian and Pacific. In 2000 the International Hydrographic Organization identified a new ocean, the Southern Ocean which surrounds Antarctica.
The Pacific Ocean is the largest and is often referred to as two oceans: the North Pacific and the South Pacific, divided by the equator. It is located between the continents of North America, South America, Asia, Antarctica and Australia.
The Atlantic Ocean is the next largest and also the youngest ocean. It is also considered to be two oceans divided by the equator, the North Atlantic and the South Atlantic. It is located between North and South America, Europe, Africa and Antarctica.
The Indian Ocean is the third largest. It is surrounded by Asia, Africa, the Arabian Peninsula, the Malay Peninsula, Australia and Antarctica.
The Arctic Ocean is the smallest and it lies almost entirely above the Arctic Circle.
Oceans cover 361 million square kilometers and have a volume of over 1340 million cubic kilometers. The average ocean depth is 2.3 miles (3711 meters).
Waters branching off the oceans are called seas, which are bodies of water partly enclosed by land, such as the South China Sea, the Caribbean Sea and the Mediterranean Sea.
Each ocean is, in effect, a basin with a spreading center. The spreading centers are the places where the tectonic plates of the Earth’s surface are moving apart and new surface is being added from the mantle below the sea floor. These spreading centers have large mountain ridges, some of which are characterized by volcanic activity.
Ocean waters arrive from rivers where small amounts of mineral salts have been picked up. Water leaves the oceans only by evaporation, but even after evaporation, the salt remains. Because of this, the oceans get saltier as time passes and now are approximately 3.5 percent salt. The least salty oceans are in the polar regions because melting polar ice and rain dilute the salinity.
Ocean waters are always in motion, affecting weather and temperature. Oceans absorb solar radiation and currents distribute this heat energy around the globe. Ocean waters are driven by two great wind systems: the Trade Winds and the Westerlies. The Trade Winds, which occur close to the equator, blow the surface waters westward. The Westerlies, in the more temperate zones, cause surface waters to blow back toward the east. This results in a circular motion of surface water in each ocean basin. In the northern hemisphere this circular motion is clockwise and in the southern hemisphere it is counterclockwise. Surface and deep-water currents change the climate by moving cold water from the poles toward the tropics and warm water from the tropics toward the poles.
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