Coral Reef Types and Their Development
Geography › Coral Reefs, Ocean Resources and Law of the Sea · Topic 12.1
A coral reef is a limestone ridge built by colonies of tiny animals called coral polyps, which secrete calcium carbonate skeletons. The three main types are fringing reefs, which are attached to the shore, barrier reefs, which are separated from the land by a lagoon, and atolls, which are rings of coral with a central lagoon. Darwin's subsidence theory explains how one type develops into the next.
6 min read · UPSC Prelims · Geography · Coral Reefs, Ocean Resources and Law of the Sea
At a Glance
- Builder Coral polyps; calcium carbonate skeleton
- Fringing reef Attached to the coast
- Barrier reef Separated from land by a lagoon
- Atoll Ring-shaped reef with a lagoon
- Largest Great Barrier Reef, Australia
- Exam link UPSC Prelims, Geography: Coral Reefs, Ocean Resources and Law of the Sea
Where Does It Fit?
This is the first topic of the category on coral reefs, ocean resources and the law of the sea. The previous macro ended with the physical properties of sea water, and this one begins with the living structures that depend on them. The next topic covers the conditions needed for coral growth. Previous topic: Ocean Salinity. Next topic: Conditions for Coral Growth and Distribution of Coral Reefs.
What Is a Coral Reef and How Is It Built?
In short: a coral reef is a rock-like structure built over thousands of years by millions of tiny animals.
Corals are marine animals related to sea anemones and jellyfish. Each individual is a small, soft polyp with a ring of tentacles around a mouth. Reef-building or stony corals take calcium and carbonate from sea water and secrete a hard skeleton of calcium carbonate, called limestone, around themselves. Colonies of millions of polyps live together, and when old polyps die, new ones grow on their skeletons. Over thousands of years this builds up a reef, a ridge of limestone close to the sea surface. Other organisms, such as coralline algae, shellfish and sponges, add to the structure by cementing the reef together. The polyps live in partnership with microscopic algae that live in their tissues, as in Coral Bleaching and Zooxanthellae.
Coral reefs cover less than one per cent of the sea floor, but support about a quarter of all marine species, so they are known as the rainforests of the sea. They also protect coasts from waves, supply fish and drive tourism, as in Energy and Biotic Resources of the Oceans.
What Are the Types of Coral Reefs?
| Type | Description | Examples |
|---|---|---|
| Fringing reef | Grows directly from the shore or a short distance away, with a narrow, shallow channel between reef and land; the most common and the youngest type | Red Sea coasts, the Andaman and Nicobar Islands, parts of the Gulf of Mannar, East Africa and the Caribbean |
| Barrier reef | A long, broad ridge separated from the land by a deep, wide lagoon; may be many kilometres from the coast | The Great Barrier Reef off Queensland, Australia, which is the largest coral structure in the world, about 2,300 km long; the Belize Barrier Reef; the barrier reef of New Caledonia |
| Atoll | A circular, oval or horseshoe-shaped ring of reef enclosing a lagoon, with no central land; often built on a sunken volcano | Maldives, Lakshadweep, Tuvalu, Marshall Islands, Kiribati; Bikini and Eniwetok |
| Patch reef and bank reef | Small, isolated mounds in a lagoon or on the shelf | Lagoons of the Great Barrier Reef |
The Great Barrier Reef, a UNESCO World Heritage Site, consists of thousands of individual reefs and hundreds of islands, over an area of about 344,000 square kilometres, and is the largest living structure on Earth, visible from space.
How Do Reefs Develop: Darwin’s Theory and Alternatives?
- Fringing stage: corals begin to grow in the shallow water around a new volcanic island, forming a fringing reef.
- Barrier stage: the island slowly sinks, because of the cooling and subsidence of the crust. The corals grow upward to remain near the light, and the outer edge grows faster in clear, oxygen-rich water, so the reef is separated from the island by a lagoon, becoming a barrier reef.
- Atoll stage: when the island sinks completely beneath the sea, the reef remains as a ring around a lagoon, an atoll.
This is the subsidence theory of Charles Darwin, published in 1842 after his voyage on the Beagle. Deep drilling at Eniwetok atoll in the Marshall Islands in the 1950s found volcanic rock more than 1,200 m below the coral, which supported his idea. The glacial control theory of Reginald Daly suggested that in the ice age the sea level fell and waves cut flat platforms on the islands, and when the sea rose again, corals grew on those platforms. Today, scientists accept that both subsidence and sea-level changes played a part, as in Minor Ocean Relief Features.
India has four main reef areas: the Gulf of Kachchh, the Gulf of Mannar and Palk Bay, the Andaman and Nicobar Islands and the Lakshadweep, which are atolls on the Chagos-Laccadive Ridge. As an analytical point, atolls and reefs are both products of slow geological processes and highly sensitive to rapid change, so their health is an early indicator for the effects of warming on the ocean.
Why Is This Topic Important?
- Facts Types and examples asked
- Theory Darwin’s subsidence theory
- India Reef areas of India
What Came Next?
The next topic describes the conditions needed for coral growth and the distribution of reefs. Next topic: Conditions for Coral Growth and Distribution of Coral Reefs.
Key People and Terms
- Coral polyp Tiny animal that builds a reef
- Fringing reef Reef attached to the shore
- Barrier reef Reef separated from land by a lagoon
- Atoll Ring reef around a lagoon
- Lagoon Shallow water enclosed by a reef
- Subsidence Sinking of land
Exam Corner
Points to Remember
- Corals build calcium carbonate skeletons.
- Darwin’s order: fringing, barrier, atoll.
- The Great Barrier Reef is about 2,300 km long.
- Reefs cover under 1 per cent of the sea floor but hold about a quarter of marine species.
- Lakshadweep consists of atolls.
Do Not Confuse With
- Barrier reef and atoll Reef with an island inside versus no land
- Fringing and barrier reef Next to the coast versus separated by a lagoon
- Coral and coral reef The animal versus the structure
Memory Hook
Fringe the island, it sinks and the lagoon opens; barrier turns to ring and the atoll remains.
Mains Angle
Questions ask for types and the theory. These points add depth.
- Types Fringing, barrier, atoll
- Theory Darwin and Daly
- Value Biodiversity, coastal protection
A Question You May Face
An original practice question, not a past paper question.
“Describe the types of coral reefs and explain the formation of atolls according to Darwin’s subsidence theory.”
How to Answer
- Builders Polyps and algae.
- Types Fringing, barrier, atoll.
- Theory Subsidence.
- India Four reef regions.
GS Relevance
Frequently Asked Questions
What is a coral reef?
A coral reef is a ridge of limestone built over thousands of years by colonies of tiny marine animals called coral polyps. The polyps secrete calcium carbonate skeletons, and new polyps grow on the old ones, forming a living structure that covers a small area of the sea floor but supports many species.
What are the three main types of coral reefs?
The three main types are fringing reefs, which grow close to the shore; barrier reefs, which are separated from the land by a deep lagoon, such as the Great Barrier Reef; and atolls, which are ring-shaped reefs enclosing a lagoon, such as the Maldives and Lakshadweep.
What is Darwin's theory of coral reef formation?
Darwin's subsidence theory states that a fringing reef forms around a volcanic island, and as the island sinks slowly, the corals grow upward, becoming a barrier reef with a lagoon. When the island has completely disappeared below the sea, a ring-shaped reef remains, called an atoll.
Which is the largest coral reef system in the world?
The Great Barrier Reef off the northeastern coast of Australia is the largest coral reef system, about 2,300 kilometres long and covering around 344,000 square kilometres. It is made of thousands of reefs and hundreds of islands and is a UNESCO World Heritage Site.
Where are coral reefs found in India?
Coral reefs in India occur in four areas: the Gulf of Kachchh in Gujarat, the Gulf of Mannar and Palk Bay in Tamil Nadu, the Andaman and Nicobar Islands, and the Lakshadweep islands, which are atolls. The Gulf of Mannar was declared a Biosphere Reserve.
PYQ Practice — Statement Analysis
1 An atoll is a ring-shaped coral reef enclosing a lagoon.
It often sits on a sunken volcano.
2 A fringing reef is separated from the shore by a wide, deep lagoon.
That is a barrier reef.
3 Lakshadweep consists of coral atolls.
They lie on the Chagos-Laccadive Ridge.
4 Darwin proposed the subsidence theory of coral reef formation.
He published it in 1842.
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