Relief of the Ocean Floor

The ocean floor is not a flat, featureless plain. Just as the continents have mountains, valleys and plains, the land beneath the sea is rugged and varied, holding the world's largest mountain ranges, its deepest trenches and its most extensive plains. These features are shaped by the same tectonic, volcanic and depositional forces that build the continents.

The oceans occupy the great depressions of the earth's outer layer. Unlike the continents, which are clearly separated, the oceans merge into one another so naturally that drawing boundaries between them is difficult. Geographers have still divided the oceanic part of the earth into five oceans: the Pacific, the Atlantic, the Indian, the Southern and the Arctic. The various seas, bays, gulfs and other inlets are simply parts of these large oceans. A major portion of the ocean floor lies between 3 and 6 km below sea level.

Major Divisions of the Ocean Floor

The ocean floor is divided into four major divisions, each with distinct characteristics.

Continental Shelf

The continental shelf is the extended margin of each continent, covered by relatively shallow seas and gulfs. It is the shallowest part of the ocean, with an average gradient of 1° or even less. The shelf typically ends at a very steep slope called the shelf break.

The width of continental shelves varies greatly from one ocean to another. The average width is about 80 km, but this hides enormous variation:

• Shelves are almost absent or very narrow along some margins, such as the coasts of Chile and the west coast of Sumatra.
• The Siberian shelf in the Arctic Ocean, the largest in the world, stretches to 1,500 km in width.

The depth of the shelves also varies, from as shallow as 30 m in some areas to as deep as 600 m in others. The shelves are covered with sediments of variable thickness, brought down by rivers, glaciers and wind from the land, and then distributed by waves and currents. Massive sedimentary deposits accumulated over long periods on the continental shelves become the source of fossil fuels.

Continental Slope

The continental slope connects the continental shelf to the ocean basins. It begins where the bottom of the continental shelf sharply drops off into a steep slope. The gradient of the slope region varies between 2° and 5°, and its depth varies between 200 and 3,000 m. The slope boundary marks the end of the continents. Canyons and trenches are observed in this region.

Deep Sea Plain

Deep sea plains are gently sloping areas of the ocean basins. They are the flattest and smoothest regions of the world, with depths varying between 3,000 and 6,000 m. These plains are covered with fine-grained sediments like clay and silt.

Oceanic Deeps or Trenches

These are the deepest parts of the oceans. Trenches are relatively steep-sided, narrow basins, some 3–5 km deeper than the surrounding ocean floor. They occur at the bases of continental slopes and along island arcs, and are associated with active volcanoes and strong earthquakes. This makes them very significant in the study of plate movements.

As many as 57 deeps have been explored so far: 32 in the Pacific Ocean, 19 in the Atlantic Ocean and 6 in the Indian Ocean.

Minor Relief Features

Beyond the major divisions, several minor but significant features predominate in different parts of the oceans.

Mid-Oceanic Ridges

A mid-oceanic ridge is composed of two chains of mountains separated by a large depression. The mountain ranges can have peaks as high as 2,500 m, and some even rise above the ocean's surface. Iceland, a part of the Mid-Atlantic Ridge, is an example.

Seamount

A seamount is a mountain with pointed summits, rising from the seafloor, that does not reach the surface of the ocean. Seamounts are volcanic in origin and can be 3,000–4,500 m tall. The Emperor Seamount, an extension of the Hawaiian Islands in the Pacific Ocean, is a good example.

Submarine Canyons …