Major and Minor Ocean Relief Features – UPSC

• Ocean relief features are the landforms of the sea floor: its plains, mountains, valleys and depressions. They are grouped as major features, the broad depth zones of every ocean, and minor features, smaller forms that rise from or cut into them.
• The floor is shaped by the same tectonic, volcanic and depositional forces as the land, but with almost no weathering. Its forms therefore keep their sharp original outlines.
• It is also young. Oceanic crust is made at ridges and destroyed at trenches, so almost none of it is older than about 180 million years, while continental rocks reach 4 billion years.

The World Ocean and Its Divisions

• There is one continuous world ocean, covering about 71% of the Earth’s surface. It is divided into named oceans for historical, geographical and scientific convenience.
  • Four oceans were traditionally named: the Pacific, Atlantic, Indian and Arctic.
  • The Southern Ocean, the water ringing Antarctica south of about 60°S and defined by the Antarctic Circumpolar Current, was recognised by the National Geographic Society as the fifth ocean in 2021.
  • Seas, gulfs and bays are parts of these oceans, partly enclosed by land.

• The floor is mapped by echo sounding, multibeam sonar and satellite altimetry. Only about 29% of it has yet been mapped by direct measurement.
• Major and minor features compared:

Major Relief Features

Continental shelf

• The continental shelf is the gently sloping, shallow submerged edge of a continent, running from the shore to the shelf break.
  • Gradient about 0.1°; depth at the break about 130–140 m on average (traditionally the 100-fathom line); average width about 70–80 km; about 7–8% of the ocean floor.
• Width depends on the coast behind it. It is under 20 km off Chile and Peru, where the Andes rise from the sea, but over 1,000 km off northern Siberia and in the Sunda Shelf.
• Origin is polygenetic: drowning by post-glacial sea-level rise, sediment building out from rivers, wave erosion, faulting and coral growth.
• India: the west-coast shelf is far wider, broadest off Gujarat and Maharashtra, and holds Mumbai High oil field. The east-coast shelf is narrow, especially off the Ganga, Mahanadi, Godavari, Krishna and Kaveri deltas.
• Value: most of the world’s fish catch, offshore oil and gas, placer minerals (ilmenite and monazite sands off Kerala), and sovereign rights under UNCLOS.

Continental slope and rise

• The continental slope descends from the shelf break to about 2,000–3,000 m. It is the true edge of the continent, where continental crust gives way to oceanic crust.
  • It averages about 4° but is far steeper off volcanic islands and active margins. It covers about 8.5% of the floor.
  • Sediment is thin, because material slides down it, and it is cut by submarine canyons.
• At its foot lies the continental rise, a gentle apron (1:100 to 1:800) of coalesced deep-sea fans. It is broad in the Atlantic and Indian Oceans but rare in the Pacific, where trenches trap the sediment.

Deep-sea plain (abyssal plain)

• Abyssal plains are very flat, sediment-covered surfaces at about 3,000–6,000 m, with slopes gentler than 1:1,000. They are the flattest surfaces on Earth and by far the broadest step of the ocean floor.
  • Sediment buries the rough basaltic crust: land-derived turbidites near the margins and pelagic oozes and red clay farther out.
  • They are extensive in the Atlantic and Indian Oceans and limited in the Pacific, where abyssal hills dominate.
• India: the Central Indian Ocean Basin carries polymetallic nodules. India holds an International Seabed Authority exploration contract (2002) there, a legacy of its 1987 registration as a pioneer investor.

Oceanic deeps (trenches)

• Oceanic deeps are the deepest parts of the ocean, mostly long, narrow, steep-sided trenches at 6,000–11,000 m. Broader, gentler depressions are called troughs.
  • Trenches are asymmetric, with a steeper landward wall. They run parallel to young fold mountains or to the convex side of island arcs.
• Origin: they mark subduction zones, where oceanic lithosphere bends down beneath another plate. Hence their deep earthquakes, volcanic arcs and concentration round the Pacific Ring of Fire.
• The deepest point is the Challenger Deep (about 10,935 m) in the Mariana Trench. Other very deep trenches, the Tonga, Philippine, Kuril–Kamchatka and Kermadec, are also in the Pacific.

Minor Relief Features

Ridges, seamounts and guyots

• The mid-ocean ridge system is the longest mountain chain on Earth, about 65,000 km, rising 2–3 km above the floor and covering about a third of it.
  • It forms at divergent plate boundaries, where rising basaltic magma makes new crust. Its crest may carry a rift valley (Mid-Atlantic Ridge) or be a smooth rise (East Pacific Rise).
  • It breaks the surface where volcanism is excessive, as in Iceland and the Azores.
  • India: the Carlsberg and Central Indian Ridges. The Ninety East Ridge and the Chagos–Laccadive Ridge, on which Lakshadweep stands, are aseismic ridges built over hotspots.
• Seamounts are isolated submarine volcanoes, more than 1,000 m high, with peaked summits that stay below sea level. Over 43,000 were catalogued by 2023, many in chains over hotspots, such as the Hawaiian–Emperor chain.
  • India: the Afanasy Nikitin Seamount in the Central Indian Ocean, for whose cobalt-rich crusts India sought an exploration licence in 2024.
• Guyots (tablemounts) are flat-topped seamounts, named by Harry Hammond Hess (1946). A volcanic island is planed flat by waves, then sinks as its plate cools, often to 1–2 km below the surface. They are most numerous in the western and central Pacific.

Trenches, island arcs and fracture zones

• Island arcs are curved chains of volcanic islands on the landward side of a trench, formed where one oceanic plate subducts beneath another. Examples are Japan, the Philippines, the Aleutians and the Marianas.
  • India: the Andaman–Nicobar Islands form the outer ridge of the Sunda arc, in front of the Sunda Trench. Barren and Narcondam Islands are volcanoes of its inner arc.
• Fracture zones are long scarps and troughs at right angles to a ridge, the scars of transform faults that offset the ridge into segments. The Romanche Fracture Zone in the equatorial Atlantic is a well-known example.

Submarine canyons

• Submarine canyons are deep, steep-walled, V-shaped valleys cut into the shelf and slope, often opposite large river mouths. Monterey Canyon (California) is the classic example and Hudson Canyon was the first described.
• Most began as river valleys cut at low sea levels and were deepened by turbidity currents, dense, sediment-laden flows down the slope.
• India: the Swatch of No Ground, a canyon across the shelf south of the Ganga–Brahmaputra delta, feeds the Bengal Fan, the largest submarine fan on Earth. Smaller canyons lie off the Godavari, Krishna, Mahanadi and Palar.

Banks, shoals, reefs and atolls

• A bank is a flat-topped elevation on the continental shelf, with shallow water that is still navigable. Banks are among the richest fishing grounds.
  • Examples are the Dogger Bank (North Sea) and Grand Banks (Newfoundland). In India, the Wadge Bank south of Kanyakumari (about 10,000 km²) and the Angria Bank, a sunken atoll-like bank about 105 km off Vijaydurg.
• A shoal is a detached shallow elevation, of sand, rock or coral, that endangers navigation.
  • Adam’s Bridge, a 48 km chain of limestone shoals between Pamban Island and Mannar, forces large ships to sail round Sri Lanka.
• A reef is a ridge of rock or coral at or near the surface. Coral reefs are fringing, barrier or atoll reefs. The Great Barrier Reef off Queensland is the largest.
  • India: coral reefs of the Gulf of Mannar, Gulf of Kachchh, Andaman–Nicobar and Lakshadweep.
• An atoll is a ring-shaped coral reef enclosing a lagoon, often crowned by low islands. In the subsidence theory of Charles Darwin, it began as a fringing reef that kept growing upward while its volcanic island sank.
  • India: Lakshadweep has 12 atolls, 3 reefs and 5 submerged banks; the Maldives lie on the same ridge.

Significance of Ocean Relief

• Ocean circulation: ridges, rises and sills steer deep currents and separate water masses, which in turn shape climate.
• Navigation and cables: charts of shoals, reefs and banks are vital for shipping. Canyons and slopes guide the routing of submarine cables, which turbidity currents can break.
• Fishing: shelves and banks such as the Wadge Bank yield most of the catch. Seamounts are biological hotspots.
• Resources: oil and gas on shelves (Mumbai High); placers; nodules, crusts and sulphides on the deep floor and ridges. India’s Deep Ocean Mission is building the 6,000 m crewed submersible Matsya-6000 to reach them.
• Hazards: trenches generate great earthquakes and tsunamis. The Sunda Trench produced the 2004 Indian Ocean tsunami, after which INCOIS set up India’s tsunami early-warning centre (2007).
• Science and law: the bottom relief of the oceans records the history of sea-floor spreading, and fixes the outer edge of the legal continental shelf that coastal states can claim.