Just like the relief features on continents blocks, oceans too have characteristic relief features. The ocean floors can be divided into following divisions:
|Major Relief Features||Minor Relief Features|
|Continental Shelf |
Deep Sea Plain
|Mid oceanic ridge|
Continental shelf is the continuation of coastal land or continental margins and slope gently towards the sea. It is the shallowest part of the ocean and typically ends at a very steep slope, called the shelf break. The average width of continental shelves for the entire world is about 65 km. In other words, continental shelf is a broad, relatively shallow submarine terrace of continental crust forming the edge of a continental landmass. The geology of continental shelves is often similar to that of the adjacent exposed portion of the continent.
ORIGIN OF CONTINENTAL SHELVES
The origin of the continental shelves is due to the various complex processes and their formation may be due to:
- Sediments brought by rivers and sea waves e.g., Shelf surrounding the Nile river delta
- Simple faulting along continental margins
- Abrasion work of sea waves leading to the formation of extensive wave cut platforms
- Submergence of continental lands e.g., Western coast of India
SIGNIFICANCE OF CONTINENTAL SHELVES
- Fishing: Provide one of the richest fishing grounds. Marine food almost entirely comes from continental shelves.
- Minerals & Oils: Potential sites for economic minerals and fossil fuels. E.g., Petroleum and natural gas, polymetallic nodules.
- Supports Marine ecosystem: Significant for growth of phytoplankton and coral reefs, which sustain marine ecosystems and perform important ecological functions.
- Important for ships: Shallowness increases height of tides and hence, facilitates entry of ships at ports. E.g., Rotterdam port
- Mitigation of cyclones by vegetation (like mangroves) growing on continental shelves.
- Enormous potential for tourism & marine aquaculture.
The region extending between continental shelf and deep-sea plain is known as continental slope. It has a steeper slope compared to continental shelf. The lower part of continental slope where it merges into deep sea plain is referred to as continental rise. The slope boundary indicates the end of the continents.
DEEP SEA PLAIN
They are extensive, flat plains found between continental slope and oceanic abyss. These are the gently sloping areas of the ocean basins. These plains are covered with fine grained sediments like clay and silt.
OCEANIC DEEPS OR TRENCHES
These are long, narrow, and deep depressions of the sea floor, with relatively steep sides. They are the deepest parts of the oceans. Most of the oceanic trenches are found along the Circum – Pacific belt like Kurile trench, Tonga trench, etc. They are associated with active volcanoes and strong earthquakes.
MINOR RELIEF FEATURES
Submarine canyons are relatively narrow, deep valleys with vertical side walls and steep slopes resembling the land valleys and are found in continental shelf and slope area. They are usually associated with straight coasts rather than indented ones. They are found in abundance along the eastern coast of America extending from Canada to Cape Hatteras.
MID OCEANIC RIDGE
A Mid Oceanic Ridge are found along diverging plate boundaries where plates move away from each other and the gap is filled up by upwelling magma which solidifies to form a new crust. It 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.
SEAMOUNTS AND GUYOTS
Seamounts are undersea mountains formed by volcanic activity that rise hundreds or thousands of feet from the sea floor. They are found near plate boundaries. It is a mountain with pointed summits, rising from the seafloor that does not reach the surface of the ocean. The Emperor seamount, an extension of the Hawaiian Islands in Pacific Ocean, is a good example. Guyots are flat topped seamounts. Due to movement of ocean floor away from oceanic ridges, the sea floor gradually sinks and the flattened guyots are submerged to become undersea flat-topped peaks.
An atoll is a ring-shaped coral reef, island, or series of islets. The atoll surrounds a body of water called a lagoon. Atolls develop with underwater volcanoes, called seamounts. 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.