Water covers 71% of the Earth's surface. Nearly all of that water is seas and oceans. Because the oceans are continuous, it is sometimes hard to tell exactly where one ocean ends and the next begins. However, most classification systems agree that the oceans can be divided into 5 major ocean basins:
* Atlantic Ocean Basin
* Pacific Ocean Basin
* Indian Ocean Basin
* Arctic Ocean Basin
* Southern Ocean Basin
These basins are bordered by the continents and by each other. However, where there is no clear cape, continental shoreline, or undersea geological formation to divide one ocean area from another, agencies often disagree over boundaries. Even the boundaries of the Arctic Ocean Basin, which is a nearly landlocked ocean, are under dispute. The International Hydrographic Organization (IHO), which is responsible for nautical charting, defines the limits of the Arctic Ocean so as not to include any major islands or local bodies of water which have other designations. On the other hand, the CIA Factbook shows the Arctic Ocean as extending all the way into and including Hudson Bay in Canada, as well as the entire Northwest Passage.
Another major point of contention is whether there is a Southern Ocean Basin at all. There is no international consensus on this point. As of the 3rd printing, the IHO does not show a Southern Ocean, but this may change in the future. Those who do consider there to be a Southern Ocean set its northern limits at 55, 50, or even 35 degrees of latitude.
These disagreements are political as much as they are geological. Oceans are traditionally open to free international passage and resource mining. A country's internal waterways are not. Thus, as ocean surface and undersea technologies improve and more of the ocean is ice-free, these disputes are likely to intensify.
Pacific Ocean Basin
The Pacific Ocean Basin is the largest, deepest, and oldest of the 5 ocean basins. The deepest point anywhere on Earth is the Mariana Trench, in the Northwest Pacific Ocean. At 35,797 feet, it is nearly a mile deeper than Mount Everest is high.
The approximate borders of the Pacific Ocean Basin are the west coast of North and South America down to Cape Horn, the south coast of Alaska, the east coast of Asia, the northeast coast of the Indonesian islands and the east coast of New Zealand or Australia. This is the so-called Ring of Fire, because the region has so many violent volcanoes and earthquakes.
The southern border of the Pacific Ocean is under dispute. The IHO takes it down as far as the north shore of Antarctica. The CIA Factbook stops short of Antarctica at 50 degrees south.
The East Pacific Rise is the offshore subduction region next to North and South America. This is where the Pacific Plate meets the continental plates. The resulting pressure has built up high mountains all along the west coast of North and South America, some of which are active stratovolcanoes.
Atlantic Ocean Basin
The Atlantic Ocean Basin is ringed by North and South America to the west, and by Europe and Africa to the east. It also goes north as far as Iceland and the south part of Greenland. The North Atlantic, north of the equator, is the world's most heavily traveled ocean region.
The southern border of the Atlantic Ocean is under dispute. The IHO takes it down as far as the north shore of Antarctica. The CIA Factbook stops short of Antarctica at 50 degrees south.
The entire Atlantic Ocean is divided north to south by the Mid-Atlantic Ridge, which marks the place where the North American and European Plates are moving away from each other. This is the youngest part of the Earth's ocean floor. Iceland is right in the middle. The constant tearing action is the reason for Iceland's constant vulcanism.
Indian Ocean Basin
The Indian Ocean Basin is smaller than the Atlantic Ocean Basin, but larger than the Arctic Ocean Basin. It is bordered by Africa to the west as far south as the Cape of Good Hope, Asia and Indonesia to the north, and Australia to the east.
The southern border of the Indian Ocean is under dispute. The IHO takes it down as far as the north shore of Antarctica. The CIA Factbook stops short of Antarctica at 50 degrees south.
The islands of Indonesia lie right on the edge between several tectonic plates. Depending on which particular plates are involved in a subduction earthquake, the resulting tsunami can travel east across the Indian Ocean or west across the Pacific, but not both. The location of Sumatra usually protects Singapore and Hong Kong from either type of tsunami.
Arctic Ocean Basin
The Arctic Ocean Basin, also known as the North Polar Basin, is the smallest of the 5 ocean basins. It is also the shallowest, with an average depth of just 3,400 feet. Its approximate borders are the north shores of Russia, Alaska, Canada, Greenland, and the Svalbard Archipelago of Norway. Some definitions go down as far south as the northern shores of Norway.
The Lomosov Ridge bisects the Arctic Ocean Basin into 2 major parts: the Eurasian Basin and the Amerasian Basin. The Alpha Ridge and Gakkel Ridge further divide the basin into the Canada Basin, the Makarov Basin, the Fram Basin, and the Nansen Basin.
Until a decade ago, the Arctic Ocean was mostly ice-covered throughout the year. This is no longer true. At the time of writing, roughly 50% of the Arctic Ocean is reliably ice-free in summer, although even the North Pole is sometimes ice free. Loose pack ice is still common in winter as far south as the northernmost shores of North America, Europe, and Asia.
Because of its intermediate location and the fact that it never leaves the continental shelf, some oceanographers do not consider the Arctic Ocean Basin to be a true ocean. Instead, they call it the Arctic Mediterranean Sea or the Arctic Sea. "Mediterranean," here, means that it is a mostly landlocked sea with limited outside water circulation.