Ocean

• Oceanic crust is less than 60-70 million years old, while continental features are over 1 billion years old.
• The ocean is geographically divided into distinct named regions.
• Historically, there were four named oceans: Atlantic, Pacific, Indian, and Arctic. However, a new ocean, the Southern (Antarctic) Ocean, has been recognized as the fifth ocean.
• The Pacific, Atlantic, and Indian are known as the three major oceans.
• Oceans are a source of food (fish, mammals, reptiles, salt, and other marine foodstuffs).
• Tides can be harnessed to provide power.
• Oceanography is the branch of science that studies the physical and biological properties and phenomena of the sea.
• Radar soundings and electrical echo devices are used to find the precise depths of ocean floors and map the relief of oceans.

Ocean Relief Features

• Oceans merge naturally into one another, making it hard to demarcate them.
• Geographers have divided the oceanic part of the Earth into five oceans: Pacific, Atlantic, Indian, Southern, and Arctic.
• Seas, bays, gulfs, and other inlets are parts of these oceans.
• A major portion of the ocean floor is found between 3-6 km below sea level.
• The ocean floors are rugged, with the world’s largest mountain ranges, deepest trenches, and largest plains.
• These features are formed by tectonic, volcanic, and depositional processes, similar to those of the continents.

Major Ocean Relief Features

• The ocean floors can be divided into four major divisions: the Continental Shelf, the Continental Slope, the Deep Sea Plain and the Oceanic Deeps
• Continental Shelf:
  • The continental shelf is a gently sloping seaward extension of the continental plate.
  • These margins are occupied by relatively shallow seas and gulfs.
  • The continental shelf of all oceans covers 7.5% of the total area of the oceans.
  • The gradient is usually around 1° or even less.
  • The shelf typically ends at a very steep slope, called the shelf break.
  • The continental shelves are covered with variable thicknesses of sediments brought down by rivers, glaciers, etc.
  • Massive sedimentary deposits received over a long time by the continental shelves become the source of fossil fuels, such as petroleum.
  • Examples of continental shelves include South-East Asia, the Great Banks around Newfoundland, and the submerged region between Australia and New Guinea.
  • The shelf is formed mainly due to –
    • submergence of a part of a continent, 
    • relative rise in sea level, and 
    • sedimentary deposits brought down by rivers.
  • Various types of shelves exist based on different sediments of terrestrial origin –
    • glaciated shelf (surrounding Greenland), 
    • coral reef shelf (Queensland, Australia), 
    • shelf of a large river (around Nile Delta), 
    • shelf with dendritic valleys (at the mouth of the Hudson River), and 
    • shelf along young mountain ranges (shelves between Hawaiian Islands).
• Continental Slope:
  • A continental slope is the slope between the outer edge of the continental shelf and the deep ocean floor.
  • The continental slope is cut by submarine canyons in many locations.
  • It marks the seaward edge of the continental shelf.
  • The gradient of the slope region varies between 2-5°.
  • It extends between the depth of 180 to 3600 metres.
  • Due to their steepness and increasing distance from the land, continental slopes have very little sediment deposits and fewer sea life compared to the shelf.
  • Along the base of the continental slope is a deposit of sediments forming the continental rise.
  • The width of the continental rise can vary, ranging from narrow to up to 600 km in some regions.
• Continental Rise:
  • The continental slope gradually loses its steepness with depth.
  • When the slope reaches a level of between 0.5° and 1°, it is referred to as the continental rise.
  • With increasing depth, the rise becomes virtually flat and merges with the abyssal plain.
• Deep Sea Plain / Abyssal Plain:
  • Deep sea planes are gently sloping areas of the ocean basins.
  • They are the flattest and smoothest regions of the world due to the terrigenous (marine sediment eroded from the land) and shallow water sediments that buries the irregular topography.
  • These plains cover nearly 40% of the ocean floor.
  • The depths vary between 3,000 and 6,000 m.
  • Fine-grained sediments like clay and silt cover these plains.
  • They have extensive submarine plateaus, ridges, trenches, beams, and oceanic islands that rise above sea level in the midst of oceans.
  • Examples of oceanic islands are the Azores and Ascension Island.
  • 
• Oceanic Deep Trenches:
  • Trenches are narrow, steep-sided basins in the oceans.
  • They are the deepest parts of the oceans and are of tectonic origin, formed during ocean-ocean and ocean-continent convergence.
  • They are 3-5 km deeper than the surrounding ocean floor.
  • Trenches are located at the bases of continental slopes and along island arcs.
  • They are common in the Pacific Ocean and form a ring along the western and eastern margins of the Pacific.
  • The Mariana Trench in the Pacific Ocean is the deepest trench, with a depth of over 11 km.
  • Other ocean deeps include the Mindanao deep, Tonga trench, and Japanese trench.
  • Trenches are associated with active volcanoes and strong earthquakes, making them significant in the study of plate movements.
  • 57 deeps have been explored so far, with 32 in the Pacific Ocean, 19 in the Atlantic Ocean, and 6 in the Indian Ocean.

Minor Ocean Relief Features

• Apart from the above mentioned major relief features of the ocean floor, some minor but significant features predominate in different parts of the oceans.
• 
• Mid-oceanic ridges:
  • A mid-ocean ridge is a seafloor mountain system formed by plate tectonics.
  • It typically has a depth of about 2,600 meters and rises about 2,000 meters above the deepest portion of an ocean basin.
  • This feature is where seafloor spreading takes place along a divergent plate boundary.
• Seamount:
  • A seamount is an underwater mountain formed by volcanic activity. 
  • seamount does not reach the surface of the ocean.
  • These can be 3,000-4,500 m tall.
  • Emperor seamount, an extension of the Hawaiian Islands in the Pacific Ocean, is an example.
• Submarine Canyons:
  • A submarine canyon is a steep-sided valley cut into the seabed of the continental slope.
  • It sometimes extends well onto the continental shelf.
  • It has nearly vertical walls.
  • Occasionally, it can have canyon wall heights of up to 5 km from canyon floor to canyon rim, such as the Great Bahama Canyon.
  • The Hudson Canyon is the best-known submarine canyon in the world.
  • 
• Guyots:
  • Guyot or tablemount is an underwater volcanic mountain with a flat top.
  • It is more than 200 meters below the surface of the sea.
  • Guyots show evidence of gradual subsidence through stages to become flat-topped submerged mountains.
  • More than 10,000 seamounts and guyots exist in the Pacific Ocean alone.
• Atoll:
  • An atoll is a ring-shaped coral reef enclosing a lagoon.
  • The coral rim of the atoll surrounds the lagoon partially or completely.
  • Coral islands or cays may exist on the atoll’s rim.
  • Atolls are found in warm tropical or subtropical oceans and seas where corals can grow.
  • 
• Bank:
  • Banks are formed as a result of the erosional and depositional activity in the continental margins.
  • They are flat-topped elevations located in shallow waters that are navigable.
  • Dogger Bank in the North Sea and Grand Bank in the north-western Atlantic, Newfoundland are famous examples of banks.
  • Banks are known for their productive fisheries.
• Shoal:
  • A shoal is a detached elevation with shallow depths. 
  • Since they project out of water with moderate heights, they are dangerous for navigation.
• Reef:
  • A reef is a mound or rocky elevation formed by living or dead organisms.
  • Coral reefs are common in the Pacific Ocean and associated with seamounts and guyots.
  • The largest reef in the world is off the coast of Queensland, Australia.
  • Reefs can extend above the surface and can be dangerous for navigation.

Significance of Study of Oceanic Relief

• Ocean relief controls the motion of sea-water.
• The oceanic movement in the form of currents, in turn, causes many variations in both oceans and in the atmosphere.
• The bottom relief of oceans also influences navigation and fishing.