Though generally described as several 'separate' oceans,these waters comprise one global,interconnected body of salt water sometimes referred to as the World Ocean or global ocean.[3][4] This concept of a continuous body of water with relatively free interchange among its parts is of fundamental importance to oceanography.[5]
The major oceanic divisions are defined in part by the continents,various archipelagos,and other criteria.These divisions are (in descending order of size):
* Pacific Ocean,which separates Asia and Australia from the Americas
* Atlantic Ocean,which separates the Americas from Eurasia and Africa
* Indian Ocean,which washes upon southern Asia and separates Africa and Australia
* Southern Ocean,which,unlike other oceans,has no landmass separating it from other oceans and is therefore sometimes subsumed as the southern portions of the Pacific,Atlantic,and Indian Oceans,which encircles Antarctica and covers much of the Antarctic
* Arctic Ocean,sometimes considered a sea of the Atlantic,which covers much of the Arctic and washes upon northern North America and Eurasia
The Pacific and Atlantic may be further subdivided by the equator into northern and southern portions.Smaller regions of the oceans are called seas,gulfs,bays,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.Continental crust is thicker but less dense.From this perspective,the earth has three oceans:the World Ocean,the Caspian Sea[citation needed],and Black Sea.The latter two were formed by the collision of Cimmeria with Laurasia.The Mediterranean Sea is at times a discrete ocean,because tectonic plate movement has repeatedly broken its connection to the World Ocean through the Strait of Gibraltar.The Black Sea is connected to the Mediterranean through the Bosporus,but the Bosporus is 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.
Despite their names,smaller landlocked bodies of saltwater that are not connected with the World Ocean,such as the Aral Sea,are actually salt lakes.