Atoll

Geography glossary

Topic: Oceans: Hydrological Cycle, Relief of the Ocean Floor, Temperature and Salinity · NCERT: Class 11, Ch 12 "Water (Oceans)"

Meaning

An atoll is a low, ring-shaped coral reef, or a ring of small coral islands, in tropical oceans that surrounds a central lagoon, with no land in the middle. It matters because it is the last stage of Darwin's theory of how reefs form. All of India's Lakshadweep islands and the whole of the Maldives are built from atolls, and these low islands are now at risk from warming seas and rising sea level.

Explanation

What an atoll is made of

  • Coral reef: a ridge of limestone built by colonies of tiny animals called polyps.
  • Algae live inside the polyps. They make food from sunlight and share it with the polyps.

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

  • Parts of an atoll:

  • Reef ring: a circle of coral reef, often topped by low islands made of coral.
  • Lagoon: the shallow water inside the ring. It may hold seawater, fresh water, brackish water (slightly salty) or very salty water.
  • No central land: this is what makes an atoll different from a barrier reef.

How an atoll forms: Darwin's subsidence theory (1842)

  • Step 1: Fringing reef. A reef grows right along the coast of a volcanic island.
  • Step 2: Barrier reef. The island slowly sinks. This sinking is called subsidence.
  • The reef keeps growing upward so it stays in shallow, sunlit water.

  • A lagoon opens up between the sinking island and the reef.

  • Step 3: Atoll. The island sinks completely below the sea.

  • Only the ring of reef is left around the lagoon.

  • Sequence: fringing reef → barrier reef → atoll.

  • Proof: in 1952, scientists drilled at Enewetak Atoll (Marshall Islands, Pacific Ocean). They found volcanic rock under thick layers of coral. This confirmed Darwin's theory.

Conditions corals need

  • Warm water of about 20–29°C.
  • Clear water with no mud, because mud blocks light and smothers polyps.
  • Shallow, sunlit water, because the algae inside the polyps need light.
  • Salty water.
  • Result: reefs and atolls 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 and make the water too cold for corals.

Where it occurs

  • India: Lakshadweep (Arabian Sea, off the west coast)
  • All of Lakshadweep is coral. It is made of atolls and reefs.

  • It has about 934 sq km of coral, mainly atolls [2]. This is India's second-largest coral area [2].

  • Maldives (Indian Ocean, south-west of India): a chain of atolls.

  • Base of these atolls: Lakshadweep, the Maldives and the Chagos islands all sit on the Chagos-Laccadive Ridge. This is an aseismic ridge (an old volcanic ridge with no earthquakes, not a plate boundary).
  • Pacific Ocean: Enewetak Atoll (Marshall Islands). This is where the 1952 drilling proved Darwin's theory.
  • Not atolls in India:
  • The Andaman and Nicobar Islands have the largest coral area in India (~1,021 sq km), but their reefs are mainly fringing reefs [2].
  • The Gulf of Kachchh (~352 sq km) and the Gulf of Mannar and Palk Bay (~76 sq km) also have mainly fringing reefs [2].

Don't confuse with

  • Fringing reef: it is attached directly to the coast, with no wide lagoon between, as in the Andamans and the Gulf of Mannar. An atoll has no coast to attach to. It is a ring around open lagoon water.
  • Barrier reef: it lies away from the coast, with a lagoon in between, but land still stands in the middle, as at the Great Barrier Reef (Australia). An atoll has no land in the middle.
  • Andaman Islands: they are not coral islands. They are mainly rock, the tops of an underwater mountain chain, with only fringing reefs around them. Lakshadweep is fully coral and made of atolls.
  • Guyot: this is also a sunken volcano, but it is a flat-topped seamount cut by waves and then drowned deep under water, with no living reef ring at the surface. An atoll's reef keeps growing up to stay near the sunlit surface.

Prelims Hooks

  • Darwin's subsidence theory (1842) gives the order fringing reef → barrier reef → atoll.
  • Darwin's theory was confirmed by drilling at Enewetak Atoll (Marshall Islands), 1952, which found volcanic rock under the coral.
  • Main reef type by region: Lakshadweep has atolls, while Andaman and Nicobar, Gulf of Kachchh, and Gulf of Mannar and Palk Bay have fringing reefs [2].
  • Trap: the Andamans have the largest coral area in India (~1,021 sq km), but they are not coral islands and are not atolls [2].
  • Lakshadweep and the Maldives sit on the Chagos-Laccadive Ridge, which is aseismic, not a spreading ridge.
  • An atoll lagoon can hold seawater, fresh water, brackish water or very salty water. "Always seawater" is a wrong option.

Mains Points

  • Atolls as evidence of a sinking sea floor (GS-I):
  • Darwin explained atolls as reefs that kept growing upward while a volcanic island sank.
  • Drilling at Enewetak (1952) found volcanic rock under the coral and proved the theory.

  • Atolls therefore show that parts of the ocean floor slowly subside over long periods.

  • Warming seas and coral bleaching (GS-III):

  • Coral bleaching happens when corals lose their algae in very hot water. They turn white and may die.
  • During the Fourth Global Coral Bleaching Event (GCBE4, 2023–24), bleaching was widespread in Lakshadweep [2].
  • It was caused by El Niño (a periodic warming of the Pacific Ocean that changes weather worldwide), which raised sea surface temperatures [2].

  • If the corals die, the reef stops growing and the atoll loses its natural defence against waves.

  • Sea-level rise and low-lying atolls (GS-III):

  • Atoll islands like Lakshadweep and the Maldives stand only a little above the sea.
  • Rising sea level threatens their land, fresh water and people.
  • Policy responses include protecting reefs, controlling coastal pollution and planning for climate change in island territories.

Related concepts

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Sources

  1. 1Class 11, Ch 12 "Water (Oceans)" (primary)
  2. 2Ministry of Earth Sciences / PIB release on coral reefs and the Fourth Global Coral Bleaching Eventmoes.gov.in · tier 1