4.10: Present-Day Oceans

Having learned about processes that create, shape, and destroy ocean floor topography, we can look with greater understanding at the present-day oceans, which are all connected. The Atlantic Ocean connects with both the Indian and Pacific Oceans near Antarctica. The Arctic Ocean connects with the Pacific Ocean only where shallow water covers the continental shelf in the Bering Strait between Alaska and Siberia. In contrast, the Arctic Ocean is connected with the Atlantic Ocean by the deeper and much wider passage through the Nansen Fracture Zone between Greenland and Spitsbergen, Norway (Fig. 4-18). The somewhat deeper connection between the Arctic and Atlantic Oceans allows somewhat restricted water transfer between the two basins (Chap. 8).

Pacific Ocean

The Pacific Ocean is the world’s largest and, on average, deepest ocean. It is almost completely surrounded by narrow continental shelves and deep trenches of subduction zones. It has many islands, including volcanic islands and atolls formed at hot spots, islands in magmatic and sedimentary arcs, and islands that appear to be small pieces of previous continents. 

Aside from hot spots and limited areas of new spreading, notably the East Pacific Rise, the Pacific Ocean crust is generally old, cold, and dense. Hence, it floats low in the asthenosphere. However, the oldest oceanic crust found in the Pacific Ocean is not substantially older than the oldest oceanic crust in other oceans. This is because all of the Pacific Ocean’s older crust has been subducted. Passive margins of continents that once surrounded the Pacific developed into subduction zones long ago. Oceanic crust that once was present near the passive margins also has been subducted. The sediment that once covered the subducted crust was either subducted or compacted into rock and added to island arcs or continents as exotic terrane.

When the Pacific Ocean margins changed from passive margins to subduction zones as the ocean stopped expanding hundreds of millions of years ago, a ring of coastal mountains formed on the surrounding continents. The mountains prevented most rivers on these continents from draining into the Pacific. In addition, trenches in which river-borne sediments accumulated were formed around much of the Pacific. In other areas, magmatic arcs formed, creating marginal seas where sediments discharged by rivers were trapped. All oceanic crust now found in the Pacific Ocean, other than crust formed in marginal basins that are not part of the Pacific Plate, was created after these mountain chains, trenches, and magmatic arcs were formed. Hence, since its formation, the oceanic crust of the Pacific Plate has remained remote from any major rivers and the large amounts of sediment that they can contribute (Chap. 6), and the sedimentation rate on this crust has been slow for hundreds of millions of years. Because it is not significantly older than the crust in other oceans and because the sedimentation rate is low, the Pacific Ocean floor has a relatively thin cover of sediment. As a result, the abyssal plains are not as flat as parts of the Atlantic Ocean’s abyssal plains.

Atlantic Ocean

The Atlantic is a long, narrow ocean that is often considered to include the Arctic Ocean because the Mid-Atlantic Ridge stretches essentially from the North Pole to Antarctica. The Atlantic Ocean is expanding in an east–west direction. An oceanic ridge, the Mid-Atlantic Ridge, runs down its center. Most of the Atlantic coasts of Europe, Africa, and North and South America are passive margins. The Atlantic’s oldest oceanic crust is at two subduction zones: the Puerto Rico–Cayman Trench at the entrance to the Caribbean Sea, and the South Sandwich Trench located northeast of the Antarctic Peninsula.

The Atlantic Ocean has fewer island arcs and hot spots than the Pacific has. Greenland, the world’s largest island, is part of the North American continent and is joined to Canada by the submerged continental shelf of Baffin Bay. Continental shelves are broader in the Atlantic Ocean than in the Pacific Ocean, and they constitute a larger percentage of the total area of the Atlantic than of the Pacific. The Atlantic also has several shallow marginal seas, including the Baltic, Mediterranean, and Caribbean Seas and the Gulf of Mexico. Thick salt deposits and oil and gas reservoirs typical of developing passive margins are present in many locations around the edges of the Atlantic Ocean. Such deposits formed early in the ocean’s history, during periods when the new ocean and its marginal seas were isolated from the rest of the world ocean.

The Atlantic Ocean floor has fewer seamounts and a smoother abyssal plain than the Pacific Ocean floor, in part because of the relative dearth of hot-spot volcanoes in the Atlantic. In addition, topography is buried by very large quantities of sediment deposits that are found on the Atlantic Ocean crust, except on the newly formed oceanic ridge mountains. The large quantities of sediments are derived from the great quantities of sediment that rivers have transported into the Atlantic, as the passive margins on both sides of the ocean have eroded. The Atlantic continues to receive freshwater runoff and some sediment from vast drainage areas of Europe, Africa, and the Americas, particularly in equatorial regions. Two large rivers, the Amazon and the Congo, empty into the equatorial Atlantic. They contribute about one-fourth of the total worldwide river flow to the oceans.

Indian Ocean

The Indian Ocean is the youngest of the three major ocean basins; it was formed only during the past 125 million years by the breakup of Gondwanaland. The plate tectonic features and history of the Indian Ocean are complex. For example, it is not yet known how and why the prominent Ninety East Ridge (Fig. 4-11, Fig. 3-4) that divides the Indo-Australian Plate was formed.

The northern part of the Indian Ocean is dominated by the collision plate boundary between India and Eurasia. The newly formed Himalaya mountains are readily eroded, and large quantities of sediment are transported into the Arabian Sea and the Bay of Bengal by many rivers. Those rivers include three that are among the world’s largest: the Indus, which empties into the Arabian Sea; and the Ganges and Brahmaputra, both of which empty into the Bay of Bengal. The enormous quantities of sediment flowing from India and the Himalayas have accumulated to form massive abyssal fans and extensive, relatively shallow abyssal plains in large areas of the northern Indian Ocean.

To the east of the India–Eurasia collision, along the northeastern edge of the Indian Ocean, is the very active Indonesian subduction zone. Its trenches and island arcs extend between mainland Asia and Australia. To the west of the India–Eurasia collision, the northern Red Sea is a rift zone that becomes an oceanic ridge system at its southern end. This oceanic ridge extends southward into the central Indian Ocean, where it divides. One of the ridges that originates at that point extends southwest around Africa, and the other extends southeast around Australia. The Indian Ocean is opening in a complex manner as the African, Antarctic, and Indo-Australian plates move apart.

Passive margins are present along most of West Africa, most of Australia, and the coast of India. There are few islands in the Indian Ocean other than the many islands that form the Indonesian arc, where subduction is occurring along an oceanic convergent plate boundary. Madagascar, which is now part of Africa, appears to be a fragment of Pangaea that broke away from India when India began to move rapidly northward toward Asia, long after the initial breakup of Pangaea itself.

Marginal Seas

Several arms of the major oceans are partially isolated from the major ocean basins by surrounding landmasses. Such marginal seas are of four types:

One type consists of shallow submerged areas of continental crust. Examples include the Baltic Sea, the North Sea, Baffin Bay, and Hudson Bay.

A second type is formed in back-arc basins behind subduction zones, which often contain deep areas where oceanic crust is present. The marginal sea is separated from the open oceans by islands of the magmatic arc and a submerged ridge that connects the islands. Many such seas contain thick sediment deposits derived primarily from erosion of the newly formed islands of the arc. Examples include the Java Sea behind the Indonesian Arc, the South China Sea behind the Philippines, and the Aleutian Basin in the Bering Sea behind the Aleutian Island chain.

The third type of marginal sea is a narrow remnant of an old closing ocean. The Mediterranean Sea, which lies between the converging African and Eurasian Plates, is a good example. Such seas may have thick sediments.

Finally, some marginal seas are the long and narrow arms formed as a continental divergent plate boundary develops into a new ocean. The primary example is the Red Sea.