Though generally described as several 'separate' oceans, the earth has one global, interconnected body of salt water, often referred to as the World Ocean or global ocean.[2][3] This concept of a continuous body of water with relatively free interchange among its parts is of fundamental importance to oceanography.[4]
The oceans are (from largest to smallest):
- Pacific Ocean which separates Asia from the Americas
- Atlantic Ocean which separates the Americas from Europe and Africa
- Indian Ocean which separates Africa from India and India from Australia
- Arctic Ocean which borders the northern continents and encompasses the far North including the North pole.
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.
A Southern ocean, surrounding Antarctica and extending to 60°S was defined in the year 2000.
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 and the Caspian and Black Seas. 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 Bosporous 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.
