How geologists divide the world’s oceans

Many geographers subdivide the world’s oceans into the seven seas. There are the North Pacific Ocean, the South Pacific Ocean, the North Atlantic Ocean, the South Atlantic Ocean, the Indian Ocean, the Southern Ocean and the Arctic Ocean. Geologists think rather differently and divide the oceans into the five deep sea basins, the Pacific Ocean Basin, the Atlantic Ocean Basin, the Indian Ocean Basin, the Southern Ocean Basic and the Arctic Ocean Basin.

The Pacific is the largest of all the oceans. The surface extent is greater than that of all the land surfaces in the world plus the continent of Africa twice over. The Pacific Ocean is bordered in the south by the Southern Ocean and the Eurasian land mass and America in the east and west. There is a small opening through the Bering Straits to the Arctic Ocean in the north.

There are two great ocean currents in the Pacific Ocean. There is a clockwise circulation in the Northern Hemisphere and an anti-clockwise circulation in the southern hemisphere. The cold Humboldt Current which flows north along the coast of South America is important to the fishing industry. It causes the coastal upwelling of nutrients, which makes these seas very fertile. From time to time the circulation patterns are disrupted by the El Nino causing the fishing industry to fail.

Ferdinand Magellan named this ocean the Pacific because his voyage was tranquil. The Pacific is often far from tranquil, there are tropical storms and tsunamis, volcanoes and earthquakes. The Pacific Rim is geologically active. It sometimes called the Pacific Ring of Fire. On all landward faces, in the Americas, Japan and along the Asian coast to Sumatra the Pacific Ocean is shrinking by several inches a year. Tectonic forces cause volcanoes, deep ocean trenches and frequent earthquakes. The volcanoes on Hawaii have a different source. It is a local hotspot where upwelling magma has penetrated the Earth crust. There are many seamounts of volcanic origin which do not penetrate the surface of the Pacific ocean. Several have recently been discovered off the coast of California.

The Atlantic is the second largest ocean. Northern and southern boundaries are defined by the Arctic and Southern Oceans. East and West boundaries are defined by Europe, Africa and the Americas. North Atlantic waters rotate in a clockwise direction. Those in the South Atlantic rotate in a counter clockwise direction. The Atlantic is one of the saltiest seas and is noted for its summer hurricane season. The Atlantic is currently widening at the rate of one inch per year due to sea floor spreading at the mid Atlantic ridge.

The Indian Ocean is the third largest ocean. It contains 20% of the world’s seawater. It is bounded in the west by Africa, by India in the north, by Malaysia and Australia in the east and by the Southern Ocean in the south. This is a fairly salty ocean. Few major rivers discharge into the sea and evaporation rates are high. Some of the surface currents are influenced by the monsoons. Surface water currently are dominated by a clockwise gyration in the northern hemisphere and a counter clockwise flow in the southern hemisphere. A deep water current flows in from the Atlantic.

The Southern Ocean is the southernmost ocean. It completely encircles the Antarctic landmass and borders the Indian, Southern Atlantic and Southern Pacific Oceans. Oceanographers find it hard to define its northern extent. Latitude 60 degrees is currently under discussion as the latitude where cold Antarctic waters mix with more temperate waters. The easterly directed Antarctic circumpolar current is a fast moving high volume sea current.

The Arctic Ocean is the smallest, shallowest and least salty of the five ocean basins. It is almost completely enclosed by the Eurasian and North American continents. For much of the year the Arctic Ocean is covered with pack ice. The salinity is low because the sea receives a great deal of freshwater run off from major rivers with little evaporation from the surface and little opportunity to exchange water with the other oceans. Currents in the North Atlantic and in the North Pacific introduce new water. The return current from the Arctic Ocean into the North Atlantic passes to the west of Greenland. Two public  policy issues involve the Arctic Ocean. First is the extent to which the ice cap is shrinking year on year as a result of global warming. Second, is the possibility of opening up the warming region to mineral extraction and international shipping.

Most the world’s supply of seawater is contained in the five great oceans of the world. The very different character of these seas shows that oceanology is a rich science in which much is yet to be discovered.