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Ocean Topography

UPSC Past papers

Ocean Topography

The ocean surface has highs and lows, similar to the hills and valleys of Earth’s land surface, called as ocean surface topography or dynamic topography. These are mapped using measurements of sea surface height relative to Earth’s geoid. Ocean surface topography is specifically the distance between the height of the ocean surface from the geoid. Ocean surface topography is caused by ocean waves, tides, currents, and the loading of atmospheric pressure. The main purpose to measure ocean surface topography is to understand the large-scale circulation of the ocean.
Submarine Relief
The submarine relief may be classified into following categories:
1. Continental Shelf
2. Continental Slope
3. Continental Rise
4. Deep-Ocean Basin (Abyssal Plains)
5. Submarine Ridges
6. Oceanic Trenches
7. Submarine Canyons
8. Island Arcs

1. Continental Shelf
– The submerged margins of continental mass extending from the shore to the first prominent break are sloped usually at a depth of about 120 m.
– Continental shelf occupies about 7.6% of the ocean floor.
– The width of the continental shelf varies considerably. The largest shelf – the Siberian Shelf in the Arctic Ocean – stretches to 1,500 kilometers in width.
– Continental shelves are well known for oil and natural gas and mineral deposits. Coral reefs and bio-classic materials are also common on continental shelves.
– The coastal states exercise sovereign rights over the continental shelf for the purpose of exploring it & exploiting its natural resources.

2. Continental Slope
– The slope that extends from a continental shelf down to the ocean deep is the continental slope.
– Continental slopes occupy about 8.5% of the total area of the oceans.
– The gradient of the slope is lowest off stable coasts without major rivers and highest off coasts with young mountain ranges and narrow continental shelves.
– The boundary between the continental slope and the continental shelf is known as the shelf break.
– These are characterized by submarine canyons.

3. Continental Rise
– The continental rise is found between the continental slope and the abyssal plain. It represents the final stage in the boundary between continents and the abyssal plain.
– Accumulated sediments can be found at the bottom of the continental rise.
– The general slope of the continental rise is between 0.5 degrees and 1.0 degrees. Deposition of sediments at the mouth of submarine canyons may form enormous fan-shaped accumulations called submarine fans.
– Submarine fans form part of the continental rise. Beyond the continental rise stretches the abyssal plain, an extremely deep and flat area of the sea floor.
– Together, the continental shelf, continental slope, and continental rise are called the continental margin. Continental margins constitute about 28% of the oceanic area.

4. Abyssal Plains
– An abyssal plain is a deep ocean floor, usually found at depths between 3,000 metres and 6,000 metres. Abyssal plains cover more than 50% of the Earth’s surface. They are among the flattest, smoothest and least explored regions on Earth.
– The creation of the abyssal plain is the end result of spreading of the seafloor (plate tectonics) and melting of the lower oceanic crust.
– They result from the blanketing of an originally uneven surface of oceanic crust by fine-grained sediments, mainly clay and silt.
– In addition to their high biodiversity, abyssal plains are of great current and future commercial and strategic interest.
– Sediments of certain abyssal plains contain abundant mineral resources, notably polymetallic nodules. These potato-sized concretions of manganese, iron, nickel, cobalt, and copper, distributed on the seafloor at depths of greater than 4000 meters, are of significant commercial interest.

5. Submarine Ridges
– A mid-ocean ridge is an underwater mountain system formed by plate tectonics. The various mountains are linked in chains, typically having a valley known as a rift.
– This type of oceanic mountain ridge is characteristic of an oceanic spreading center, which is responsible for seafloor spreading.
– These ridges are either broad, like a plateau, gently sloping or in the form of steep-sided narrow mountains. Running for a total length of 75,000 km, these ridges form the largest mountain systems on the earth.
– A mid-ocean ridge demarcates the boundary between two tectonic plates, and consequently is termed a divergent plate boundary.
– The mid-ocean ridges of the world are connected and form the Ocean Ridge, a single global mid-oceanic ridge system that is part of every ocean, making it the longest mountain range in the world. The continuous mountain range is 65,000 km long (several times longer than the Andes, the longest continental mountain range), and the total length of the oceanic ridge system is 80,000 km.

6. Oceanic Trenches
– The oceanic trenches are hemispheric-scale long but narrow topographic depressions of the sea floor. They are also the deepest parts of the ocean floor.
– They are a distinctive morphological feature of convergent plate boundaries, along which lithospheric plates move towards each other at rates varying from a few mm to over ten cm per year. A trench marks the position at which the flexed, subducting slab begins to descend beneath another lithospheric slab.
– Trenches are generally parallel to a volcanic island arc.
– The bottom temperature of trenches is very cold (between 1° to 4°C).
– Great earthquakes and Tsunamis are born in the ocean trenches.

7. Submarine Canyons
– A submarine canyon is a steep-sided V-shaped valley cut into the seabed of the continental slope, sometimes extending well onto the continental shelf, having nearly vertical walls.
– These serve as channels for the flow of turbidity currents across the seafloor. Turbidity currents are flows of dense, sediment laden waters that are supplied by rivers, or generated on the seabed by storms, submarine landslides, earthquakes, and other soil disturbances. Turbidity currents finally deposit sediment onto the abyssal plain, where the particles settle out.
– The formation of submarine canyons occurs as the result of two main processes: 1) erosion by turbidity current erosion; and 2) slumping and mass wasting of the continental slope.
– Submarine canyons are more common on the steep slopes found on active margins compared to those on the gentler slopes found on passive margins. Canyons are steeper, shorter, more dendritic and more closely spaced on active than on passive continental margins.

8. Island Arcs
– An island arc is a type of archipelago, often composed of a chain of volcanoes, situated parallel and close to a boundary between two converging tectonic plates.
– Most island arcs consist of two parallel, arcuate rows of islands. The inner row of such a double arc is composed of a string of explosive volcanoes, while the outer row is made up of non-volcanic islands. In the case of single arcs, many of the constituent islands are volcanically active.
– An island arc typically has a land mass or a partially enclosed, unusually shallow sea on its concave side. Along the convex side there almost invariably exists a long, narrow deep-sea trench.
– Destructive earthquakes occur frequently at the site of island arcs.