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Theory:

Oceanic Deeps
Oceanic deeps or Trenches are the deepest part of the oceans. It occupies about \(7\)% of the total relief of the ocean floor. They are long, narrow, steep-sided and flat-floored depressions on the ocean floor. The ocean temperature in the trench is a bit cooler than the freezing temperature. They are sediment-free, and most trenches are V-shaped. The epicentre of the great earthquakes is all located in the trenches.
Dragon Hole is the deepest known underwater sinkhole in the world. The local fishermen call it the ‘eye’ of the South China Sea.
Oceanic Ridge
Mid-ocean ridges are continuous submarine mountain chains created by the upwelling of basaltic lava and lateral rifting of ocean crust. They form a rift valley system that encircles the Earth along a total length of over \(75,000 km\). Mid-ocean ridges are made of young basaltic rock formed in the divergent boundary where two tectonic plates move apart. It is probably the most extensive single feature of the Earth’s topography. Two of the most well-known midocean ridges are the East Pacific Ridge and the Mid-Atlantic Ridge. The Mid-Atlantic Ridge is the largest unbroken oceanic ridge.
 
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Fathoms: A nautical measurement of the depth of water in the ocean.
Isobath: An imaginary line on a map joining the points of equal depths.
Isohaline: An imaginary line on a map joining the points of equal salinity in oceans.