Ocean Bottom Relief

The ocean floors can be divided into four major divisions:

Continental Shelf:

The continental shelf is the extended margin of each continent occupied by relatively shallow seas and gulfs.
It is the shallowest part of the ocean showing an average gradient of 1° or even less.
The shelf typically ends at a very steep slope, called the shelf break.
The width of the continental shelves varies from one ocean to another.
The continental shelves are covered with variable thicknesses of sediments brought down by rivers, glaciers, wind, from the land and distributed by waves and currents.

Continental Slope:

The continental slope connects the continental shelf and 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 2-5°.
The depth of the slope region varies between 200 and 3,000 m.
The slope boundary indicates the end of the continents.
Canyons and trenches are observed in this region.

Deep-Sea Plain/Abyssal Plain:

Oceanic Deeps/Trenches:

These areas are the deepest parts of the oceans.
The trenches are relatively steep-sided, narrow basins. They are 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.
The deep trenches have deep-seated earthquake occurrences while in the mid-oceanic ridge areas, the quake foci have shallow depths.

Besides, these divisions there are also some minor relief features on the ocean floors. These are:

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 reach above the ocean’s surface. Iceland, a part of the mid- Atlantic Ridge, is an example.
All along the mid-oceanic ridges, volcanic eruptions are common and they bring huge amounts of lava to the surface in this area.
The rocks equidistant on either side of the crest of mid-oceanic ridges show remarkable similarities in terms of period of formation, chemical compositions and magnetic properties. Rocks closer to the mid-oceanic ridges are normal polarity and are the youngest. The age of the rocks increases as one move away from the crest.
These facts and a detailed analysis of magnetic properties of the rocks on either side of the mid-oceanic ridge led Hess (1961) to propose his hypothesis, known as the “sea floor spreading”.

Seamount:

It is a mountain with pointed summits, rising from the sea floor that does not reach the surface of the ocean.
Seamounts are volcanic in origin.
These 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:

These are deep valleys, some comparable to the Grand Canyon of the Colorado River.
They are sometimes found cutting across the continental shelves and slopes, often extending from the mouths of large rivers.
The Hudson Canyon is the best-known canyon in the world.

Guyots:

It is a flat-topped seamount.
They show evidence of gradual subsidence through stages to become flat-topped submerged mountains.

Atoll:

These are low islands found in the tropical oceans consisting of coral reefs surrounding a central depression.
It may be a part of the sea (lagoon), or sometimes form enclosing a body of fresh, brackish, or highly saline water. Sea-floor Spreading Hypothesis
Hess argued that constant eruptions at the crest of oceanic ridges cause the rupture of the oceanic crust and the new lava wedges into it, pushing the oceanic crust on either side. Thus, the ocean floor thus spreads. The younger age of the oceanic crust as well as the fact that the spreading of one ocean does not because the shrinking of the other, made Hess think about the consumption of the oceanic crust. He further maintained that the ocean floor that gets pushed due to volcanic eruptions at the crest sinks down at the oceanic trenches and gets consumed.