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How Many Oceans on Earth?


Q: We have been discussing how many oceans and how many seas there are on earth. There are varying opinions and we would really like to get this matter cleared up. We have been told that there are only 4 oceans (not 5 as many think, the southern ocean not being an official ocean) and 7 seas.

From Ms Steph Baldwin (November 2009)

Reply by Dr Ted Nield

Exactly what constitutes a sea and what an ocean is arguable a geological question, since according to a geologist a true ocean is underlain by oceanic crust, as opposed to a continental shelf, which is underlain by continental rocks. This is broadly in accordance with the United Nations Convention on the Law of the Sea ( which establishes national mineral rights in maritime areas and defines these on geological grounds.

The rocks of the Earth come in two basic kinds - the denser basalts that are made at mid-ocean spreading ridges, and lighter continental crust which is on average much older but which is produced as geological processes refine the primordial material of the Earth's mantle and change its chemistry through time. Continental rocks make up continents, which ride high on the Earth because they are less dense and therefore float higher in the Earth's Mantle. Oceanic rocks, being denser, sit at a lower level. The usual analogy is between balsa wood, which floats high on water, and teak, which is so dense it barely rises above the surface.

So this would mean that the true ocean basins of the world, underlain by oceanic crust, are the Atlantic, Pacific, Indian, Mediterranean and Arctic. The "Southern Ocean" is as you say not officially recognised - but incorporates parts of the Atlantic, Pacific and Indian oceans. However the "nomenclature" of oceans is geography, rather than geology.

Many smaller areas of the ocean basins are referred to as "seas". There is no proper definition of a Sea of which I am aware, though it is possible that geographers have ruled on this matter. The Caribbean Sea is part of the Atlantic Ocean, and the same goes for such smaller areas as the Andaman Sea, the Tyrrhenian Sea, and so on. The Sargasso Sea has a defined geography but has no land borders.

Oceanographers (as distinct from geologists) recognise the different oceans of course but tend to speak of the "global ocean" to emphasize that all oceans are interconnected - even those with restricted outlets, like the Mediterranean and the Baltic - the waters of the former being more saline (by evaporation) and of the latter being more brackish (because of runoff from the land).