Atoll: The Coral Island That Embraces a Lagoon

An **atoll** is a ring-shaped coral reef, island, or chain of islets that encircles a central lagoon—partially or completely. It represents the final stage of a volcanic island's subsidence and reef growth, as theorized by Charles Darwin.

Formation Process (Darwin's Subsidence Theory)

1. Fringing reef: Coral colonizes shallow shores of a volcanic island
2. Barrier reef: As island subsides (or sea level rises), reef grows upward, separated from shore by a lagoon
3. Atoll: Island subsides completely below sea level; reef continues growing upward, forming a ring with central lagoon

Key Characteristics

• Structure: Living coral reef rim (windward side higher); reef flat; lagoon (few meters to 100+ m deep)
• Islets (motus): Accumulated coral rubble and sand on reef flat; vegetated; often inhabited
• Passes (ava): Natural channels through reef connecting lagoon to ocean; critical for navigation and water exchange

Global Distribution

• Pacific: Tuamotus (French Polynesia — 77 atolls), Marshall Islands, Kiribati, Tokelau, Caroline Islands
• Indian Ocean: Maldives (26 atolls), Chagos, Seychelles (outer islands), Cocos (Keeling)
• Atlantic: Few — Belize's atolls (Glover's Reef, Lighthouse, Turneffe), Bermuda (pseudo-atoll)

Ecological and Human Significance

• Biodiversity hotspots: coral, fish, seabirds, turtles
• Subsistence and commercial fisheries
• Tourism (diving, snorkeling, overwater bungalows)
• Extreme climate vulnerability: sea-level rise, bleaching, storm intensification, freshwater lens contamination
• Nuclear testing legacy: Bikini, Enewetak, Moruroa, Fangataufa

Famous Examples

Bikini Atoll, Kwajalein (largest lagoon), Funafuti (Tuvalu capital), Male (Maldives capital — highly modified), Aldabra (UNESCO, giant tortoises).