Coral reef

Also called: Coral reefs of India · Topic: Oceans: Hydrological Cycle, Relief of the Ocean Floor, Temperature and Salinity · NCERT: Class 7, Ch 1 "Geographical Diversity of India"

Meaning

A coral reef is a ridge of limestone built over centuries by colonies of tiny sea animals called polyps, in warm, clear, shallow and salty tropical seas. Reefs form whole islands such as Lakshadweep and the Maldives and protect coasts. They are also an early warning of a warming ocean, because hot water makes corals bleach and die.

Explanation

How a reef is built

• Polyps and algae live together:
• Polyps are tiny animals. Algae live inside their bodies.

• The algae make food from sunlight and share it with the polyps.

• Polyps build skeletons:

• Each polyp makes a hard limestone skeleton.
• When polyps die, new ones grow on top of the old skeletons.

• Over centuries these skeletons pile up into a reef.

• Coral island: an island built from reef limestone. All of Lakshadweep is made this way.

Conditions corals need

• Warm water: about 20–29°C.
• Clear water: no mud. Mud blocks light and chokes the polyps.
• Shallow, sunlit water: the algae need light to make food.
• Salty water: corals need seawater.
• Result: 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.

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 lagoon water: it may be 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.
• A lagoon opens between the island and the reef, so the fringing reef becomes a barrier reef.
• The island finally sinks below the sea. Only the ring of reef is left, and this is an atoll.

• Sequence: fringing reef → barrier reef → atoll.

• Proof: in 1952, drilling at Enewetak Atoll (Marshall Islands, Pacific) found volcanic rock under thick coral.

Coral bleaching

• Cause: the sea water becomes too hot.
• Process: the corals lose the algae living inside them.
• Result: the corals turn white. Without the algae's food, they may die.

Where it occurs

• India's reef areas:
• Gulf of Mannar (Tamil Nadu)
• Palk Bay (Tamil Nadu)
• Gulf of Kachchh (Gujarat)
• Lakshadweep
• Andaman and Nicobar Islands

• Small patches at Malvan (Maharashtra)

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

Region	Coral area (sq km) [2]	Main reef type [2]
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

• Lakshadweep and the Maldives sit on the Chagos-Laccadive Ridge, an aseismic ridge (a ridge with no earthquakes) off India's west coast. The Maldives are a chain of atolls.
• World: the Great Barrier Reef (Australia) is a barrier reef. Enewetak Atoll (Marshall Islands, Pacific) is the atoll where Darwin's theory was proved.

Don't confuse with

• Coral island vs Andaman Islands: Lakshadweep is made entirely of coral. The Andamans are not coral islands. They are mainly rock, the tops of an underwater mountain chain, with only fringing reefs around them.
• Atoll vs barrier reef: both have a lagoon. A barrier reef still has land inside the lagoon. An atoll is a ring of reef with no land in the middle.
• Coral reef vs guyot: both can sit on old volcanoes. A guyot is a flat-topped seamount that waves cut flat and that then sank deep. A reef is limestone built by living polyps in shallow, sunlit water.
• Largest coral area vs atoll region: the Andaman and Nicobar Islands have India's largest coral area (~1,021 sq km), but its reefs are fringing. Lakshadweep (~934 sq km) is India's atoll region [2].

Prelims Hooks

• Darwin's sequence (1842): fringing → barrier → atoll. It was confirmed by drilling at Enewetak Atoll (1952).
• Coral needs: 20–29°C, clear, shallow, sunlit and salty water. Reefs lie mainly between 30°N and 30°S and are rare on west coasts because of cold currents.
• Region and reef type: Lakshadweep = atolls. Andaman and Nicobar, Gulf of Kachchh, Gulf of Mannar = fringing [2].
• Ranking by coral area: Andaman and Nicobar > Lakshadweep > Gulf of Kachchh > Gulf of Mannar and Palk Bay [2].
• GCBE4 (2023–24) hit the Andaman and Nicobar Islands, Gulf of Mannar, Lakshadweep and Gulf of Kachchh [2].
• Trap: the Andamans are not coral islands. Small reef patches also occur at Malvan (Maharashtra).

Mains Points

• Where corals grow (GS-I):
• Corals need warm, clear, shallow, salty water, so reefs lie between 30°N and 30°S.
• Cold currents keep reefs off west coasts.

• Darwin's subsidence theory explains how a sinking island turns a fringing reef into a barrier reef and then an atoll.

• Bleaching threat to India's reefs (GS-III):

• India has about 2,375 sq km of reef [2].
• During the Fourth Global Coral Bleaching Event (GCBE4, 2023–24), warm seas caused mass bleaching in all four main reef regions [2].

• In Lakshadweep, bleaching was widespread because El Niño (a periodic warming of the Pacific Ocean that changes weather around the world) raised sea surface temperatures [2].

• Low-lying atolls at risk (GS-III):

• Lakshadweep and the Maldives are built of coral and stand only a little above the sea.
• Sea-level rise threatens them, and bleaching weakens the reefs that protect them.

Related concepts

• Seamount
• Emperor Seamount Chain
• Guyot
• Submarine canyon
• Hydrothermal vent
• Atoll
• Coral island

Read more

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