Geography Notes On – Oceanic Trenches – For W.B.C.S. Examination.

ভূগোল নোট  – মহাসাগরীয় খাত – WBCS পরীক্ষা।

• The trenches are relatively steep sided, narrow basins (Depressions). These areas are the deepest parts of the oceans.Continue Reading Geography Notes On – Oceanic Trenches – For W.B.C.S. Examination.
• They are of tectonic origin and are formed during ocean – ocean convergence and ocean continent convergence.
• They are some 3-5 km deeper than the surrounding ocean floor.
• The trenches lie along the fringes of the deep-sea plain at the bases of continental slopes and along island arcs.
• The trenches run parallel to the bordering fold mountains or the island chains.
• The trenches are very common in the Pacific Ocean and form an almost continuous ring along the western and eastern margins of the Pacific.
• The Mariana Trench off the Guam Islands in the Pacific Ocean is the deepest trench with, a depth of more than 11 kilometres.
• They are associated with active volcanoes and strong earthquakes (Deep Focus Earthquakes like in Japan). This makes them very significant in the study of plate movements.
• As many as 57 deeps have been explored so far; of which 32 are in the Pacific Ocean; 19 in the Atlantic Ocean and 6 in the Indian Ocean.
• A mid-oceanic ridge is composed of two chains of mountains separated by a large depression. [Divergent Boundary]
• The mountain ranges can have peaks as high as 2,500 m and some even reach above the ocean’s surface.
• Running for a total length of 75,000 km, these ridges form the largest mountain systems on earth.
• These ridges are either broad, like a plateau, gently sloping or in the form of steep-sided narrow mountains.
• These oceanic ridge systems are of tectonic origin and provide evidence in support of the theory of Plate Tectonics.
• Iceland, a part of the mid-Atlantic Ridge, is an example.
  • Seamount: It is a mountain with pointed summits, rising from the seafloor 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 [Hotspot] in the Pacific Ocean, is a good example.
  • Guyots: The flat topped mountains (seamounts) are known as guyots.
  • Seamounts and guyots are very common in the Pacific Ocean where they are estimated to number around 10,000.

  Submarine Canyons

  • CANYON: a deep gorge, especially one with a river flowing through it
  • GORGE: a steep, narrow valley or ravine
  • VALLEY: a low area between hills or mountains or a depression, typically with a river or stream flowing through it.

• 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.

Broadly, there are three types of submarine canyons—

• Small gorges which begin at the edge of the continental shelf and extend down the slope to very great depths, e.g., Oceanographer Canyons near New England.
• Those which begin at the mouth of a river and extend over the shelf, such as the Zaire, the Mississippi and the Indus canyons.
• Those which have a dendritic appearance and are deeply cut into the edge of the shelf and the slope, like the canyons off the coast of southern California. The Hudson Canyon is the best known canyon in the world.
• The largest canyons in the world occur in the Bering Sea off Alaska. They are the Bering, Pribilof and Zhemchung canyons.

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