A ring-shaped coral reef or island, enclosing a central lagoon, found in tropical oceans is called an:

An atoll is a ring-shaped or horseshoe-shaped coral reef, or a chain of coral islands, that encloses a central lagoon partly or completely. Atolls are found in warm tropical seas.

How atolls form (Darwin's subsidence theory):

• Charles Darwin explained the formation of atolls after his voyage on HMS Beagle in the 1830s.
• First, corals grow around the edges of a volcanic island, forming a fringing reef.
• Over time, the volcanic island slowly sinks (subsides) and is worn down by erosion.

The corals keep growing upward towards the sunlight, so the reef remains near the sea surface while a lagoon forms between the reef and the sinking island. This is a barrier reef.

Finally, the island disappears completely below the sea, leaving a ring of coral around a central lagoon. This is an atoll.

Conditions needed for coral growth:

• Warm water, about 20 to 29 degrees Celsius.
• Clear, shallow water (usually less than about 50 metres deep), so that sunlight can reach the algae living in the corals.
• Salty water, with normal ocean salinity.
• Corals are built by tiny animals called coral polyps, which secrete calcium carbonate skeletons and live together with algae called zooxanthellae.

Examples of atolls:

• The Maldives consist of 26 natural atolls.
• Kiribati, Tuvalu and the Marshall Islands in the Pacific are atoll nations.

Threats to atolls:

• Rising sea levels due to climate change threaten low-lying atoll nations, many of which rise only a metre or two above the sea.
• Warming oceans cause coral bleaching, in which corals expel their algae and may die.

Why not the other options?

• ✗A fringing reef grows close to the shore.
• ✗A barrier reef is separated from the coast by a lagoon but still has land inside.
• ✗A seamount is an underwater volcanic mountain.