Minor relief features: ridges, seamounts, guyots, canyons, vents and coral islands

Detail

1. Mid-oceanic ridges

• Mid-oceanic ridge: an underwater mountain system made of two chains of mountains with a large depression between them.
• The depression is a rift valley: a long, deep trough that forms where the crust is pulled apart.
• Peaks reach up to 2,500 m.

• Some peaks rise above the sea. Iceland is a part of the Mid-Atlantic Ridge that stands above sea level.

• Size: the global ridge system is about 65,000 km long. It is the longest mountain range on Earth.

• How a ridge forms (plate tectonics):
• Two plates move apart (a divergent boundary).
• Magma rises from the mantle into the gap and cools into new ocean crust.
• The new crust is hot and light, so it stands high and builds the ridge. The pulled-apart centre sinks and forms the rift valley.

2. Indian Ocean ridges

Ridge	Type	Where / key fact
Carlsberg Ridge	Spreading (active) ridge	North-west Indian Ocean (Arabian Sea), between India and the Horn of Africa
Central Indian Ridge	Spreading ridge	Runs south from the Carlsberg Ridge through the central Indian Ocean
South-west Indian Ridge	Spreading ridge	Runs towards the south-west, between Africa and Antarctica
South-east Indian Ridge	Spreading ridge	Runs towards the south-east, between Australia and Antarctica
Ninety East Ridge	Aseismic	Runs north–south along about 90°E in the eastern Indian Ocean. It is a hotspot trace (a line of volcanic rock left behind as a plate moved over a fixed hotspot, here the Kerguelen hotspot)
Chagos-Laccadive Ridge	Aseismic	Off India's west coast. Lakshadweep, the Maldives and the Chagos islands sit on it

• Aseismic ridge: a ridge with no earthquakes. It is not a plate boundary. It is an old volcanic track, not a place where plates are moving apart now.
• Trap: Carlsberg, Central Indian, South-west and South-east Indian Ridges are spreading ridges with earthquakes. The Ninety East and Chagos-Laccadive Ridges are aseismic.

3. Hydrothermal vents

• Hydrothermal vent: an opening on the sea floor along a ridge. Seawater heated by magma gushes out of it, full of dissolved minerals.
• How a vent works:
• Cold seawater seeps down through cracks near the ridge.
• Magma below heats it. The hot water dissolves metals and sulphur from the rocks.

• The water rushes back up through the vent. It meets near-freezing seawater, and the minerals settle out as dark particles. This looks like smoke, so these vents are called "black smokers". The minerals build up into tall chimneys [2].

• Discovery: vents and the life around them were first found on the Galápagos Rift (eastern Pacific) in 1977 [2][3].

• Chemosynthesis: making food from chemicals instead of sunlight.
• No sunlight reaches the deep sea floor.
• At vents, bacteria turn chemicals in the vent water into sugar.
• These bacteria live inside giant tubeworms, mussels and clams and feed them. Both sides gain from this partnership (symbiosis) [3].

• This discovery showed that life can exist without sunlight [2].

• Polymetallic sulphides (PMS): mineral deposits rich in metals such as copper, zinc, lead, gold and silver, formed at vents.

4. India's deep-sea mineral contracts

• ISA (International Seabed Authority): the UN-linked body that gives contracts to explore the seabed beyond any country's control.

Year	Mineral	Area
2002	Polymetallic nodules	Central Indian Ocean Basin [4]
2016	Polymetallic sulphides	Central and South-west Indian Ridges [4]
15 September 2025	Polymetallic sulphides	Carlsberg Ridge: about 10,000 sq km, 15-year exclusive exploration contract [4]

• India applied for the Carlsberg Ridge block on 18 January 2024 [4].
• With the 2025 contract, India became the first country with two ISA contracts for PMS exploration [4]. (NCERT-era position: an application was pending for the Carlsberg Ridge.)

5. Seamounts and the Emperor chain

• Seamount: an underwater volcanic mountain with a pointed top. It is 3,000–4,500 m tall and does not reach the sea surface.
• Emperor Seamount Chain: a line of seamounts running north-west from the Hawaiian Islands in the North Pacific.
• How it formed (hotspot process):
• A hotspot is a fixed plume of hot magma rising from deep in the mantle.
• The Pacific Plate slid over the Hawaii hotspot for about 80 million years. Each time, a new volcano formed above the hotspot.

• The plate carried each old volcano away, so it cooled, sank and became a seamount. The result is a chain that is older the farther it is from Hawaii.

• Hawaii-Emperor bend: a sharp turn where the Emperor chain meets the Hawaiian chain.

• Dated to about 42.0 ± 1.4 million years ago (rocks from the Daikakuji and Kinmei Seamounts) [5].
• The usual explanation is that the Pacific Plate changed direction, from northward to a more westward path, about 43 million years ago [6].
• Later research suggests the hotspot itself drifted south quickly (about 4–5 cm a year) between about 81 and 47 million years ago. This also helps explain the shape of the Emperor chain [7].

6. Guyots

• Guyot: a flat-topped seamount.
• How it forms:
• A volcano grows up to sea level.
• Waves cut off and flatten its top.

• The sea floor cools and sinks as it moves away from the ridge, so the flat top ends up deep under water.

• The Pacific alone has an estimated 10,000+ seamounts and guyots.

Feature	Top	Reaches surface?
Seamount	Pointed	No
Guyot	Flat (cut by waves, then sank)	No (it did once, then sank)
Volcanic island	Peak above the sea	Yes

7. Submarine canyons

• Submarine canyon: a deep, steep-sided valley cut across the continental shelf and slope. Canyons are often found off the mouths of big rivers.
• Continental shelf: the shallow, gently sloping edge of a continent under the sea.

• Continental slope: the steeper drop from the shelf edge down to the deep ocean.

• Hudson Canyon: off New York (USA, North Atlantic). It is the best-known example.

• "Swatch of No Ground": a submarine canyon in the Bay of Bengal, off the Ganga-Brahmaputra delta (Bangladesh–India).
• It carries river sediment down to the deep sea.
• This sediment builds the Bengal Fan, the world's largest submarine fan (a fan-shaped heap of sediment on the deep ocean floor).

8. Coral reefs

• Coral reef: a ridge of limestone built by colonies of tiny animals called polyps.
• Polyps live together with algae that live inside them. The algae make food from sunlight and share it with the polyps.

• The polyps make limestone skeletons. Over centuries these pile up into a reef.

• Conditions corals need:

• Warm water, about 20–29°C
• Clear water with no mud
• Shallow, sunlit water, because the algae need light
• Salty water

• So reefs are found mainly between 30°N and 30°S.

• Why reefs are rare on west coasts:

• Cold ocean currents flow along the western coasts of continents.
• They make the water too cold for corals.

9. Types of reef and Darwin's theory

Type	Position	Example
Fringing reef	Attached to the coast	Andamans, Gulf of Mannar
Barrier reef	Away from the coast, with a lagoon (a shallow body of water) in between	Great Barrier Reef (Australia)
Atoll	A ring-shaped reef around a central lagoon, with no land in the middle	Lakshadweep, Maldives

• Atoll: a low island or ring of islands in tropical oceans, made of coral reef around a central lagoon. The lagoon may hold seawater, fresh water, brackish water (slightly salty) or very salty water.
• Darwin's subsidence theory (1842):
• A fringing reef grows around a volcanic island.
• The island slowly sinks (subsidence). The reef keeps growing upward to stay near the sunlit surface.
• As the island sinks, a lagoon opens between it and the reef, and the fringing reef becomes a barrier reef.
• When the island sinks below the sea, only the ring of reef is left. This is an atoll.

• Sequence: fringing reef → barrier reef → atoll.

• Proof: drilling at Enewetak Atoll (Marshall Islands, Pacific) in 1952 found volcanic rock under thick coral. This confirmed Darwin's theory.

10. Coral islands and India's reefs

• Coral island: an island built from reef limestone.
• All of Lakshadweep is coral, made of atolls and reefs.
• The Maldives are a chain of atolls.

• The Andamans are NOT coral islands. They are mainly rock (the tops of an underwater mountain chain) and only have fringing reefs around them.

• India's reef areas: Gulf of Mannar (Tamil Nadu), Palk Bay (Tamil Nadu), Gulf of Kachchh (Gujarat), Lakshadweep, and the Andaman and Nicobar Islands, with small patches at Malvan (Maharashtra).

• India's reefs cover about 2,375 sq km on average [8].

Region	Coral area (sq km) [8]	Main reef type [8]
Andaman and Nicobar Islands	~1,021 (largest)	Fringing
Lakshadweep Islands	~934	Atolls
Gulf of Kachchh	~352	Fringing
Gulf of Mannar and Palk Bay	~76	Fringing

• Coral bleaching: corals lose their algae when the water gets too hot, turn white, and may die.
• The Fourth Global Coral Bleaching Event (GCBE4, 2023–24) hit the Andaman and Nicobar Islands, Gulf of Mannar, Lakshadweep and Gulf of Kachchh [8].
• Bleaching was widespread in Lakshadweep because El Niño raised sea surface temperatures. (El Niño: a periodic warming of the Pacific Ocean that changes weather around the world.) [8]

Prelims Hooks

• Aseismic ridges in the Indian Ocean: Ninety East Ridge and Chagos-Laccadive Ridge. Carlsberg, Central Indian, South-west and South-east Indian Ridges are spreading ridges.
• Iceland is part of the Mid-Atlantic Ridge that stands above sea level. The global ridge system is about 65,000 km long.
• Hydrothermal vents were first found on the Galápagos Rift, 1977. Life there depends on chemosynthesis, not photosynthesis [2].
• India's ISA contracts: nodules, Central Indian Ocean Basin (2002); sulphides, Central and South-west Indian Ridges (2016); sulphides, Carlsberg Ridge (15 September 2025, about 10,000 sq km) [4].
• Seamount has a pointed top; guyot has a flat top. Neither reaches the surface.
• The Hawaii-Emperor bend is about 42 million years old [5]. The Emperor chain runs north-west from Hawaii.
• Swatch of No Ground (Bay of Bengal) feeds the Bengal Fan, the world's largest submarine fan. Hudson Canyon is off New York.
• Darwin's sequence (1842): fringing → barrier → atoll. It was confirmed by drilling at Enewetak (1952).
• Lakshadweep is made of atolls and Andaman and Nicobar has fringing reefs. The Andamans have the largest coral area in India (~1,021 sq km) [8].
• Trap: the Andamans are not coral islands, and Lakshadweep and the Maldives sit on the Chagos-Laccadive Ridge.

Mains Points

• Ridges, vents and India's Blue Economy (GS-III): polymetallic sulphides at Indian Ocean vents hold copper, zinc, gold and silver. India's three ISA contracts (2002, 2016, 2025) give it a strong position in deep-sea minerals [4]. But vent ecosystems are unique and fragile, so mining needs strict environmental rules.
• Hotspot chains as evidence for plate tectonics (GS-I): the Emperor-Hawaii chain gets older moving away from the hotspot. The bend dated to about 42 million years shows a change in plate motion [5][6]. The Ninety East Ridge is a similar hotspot trace in the Indian Ocean.
• Where corals grow (GS-I): corals need warm, clear, shallow, salty water. So reefs lie between 30°N and 30°S and are rare on west coasts chilled by cold currents. Darwin's subsidence theory explains how fringing reefs turn into barrier reefs and then atolls.
• Threats to India's reefs (GS-III): India has about 2,375 sq km of reef [8]. Warming seas and El Niño caused mass bleaching during GCBE4 (2023–24) in all four main reef regions [8]. Low-lying atolls like Lakshadweep and the Maldives also face sea-level rise.