What Is a Ringlike Coral Reef Enclosing a Lagoon Called?

It is called an **atoll**.

Definition

An **atoll** is a **ring-shaped coral reef, island, or series of islets** that **encircles a central lagoon** (partially or completely). The reef sits atop a **submerged volcanic seamount**; as the volcano **subsides/erodes**, the coral **grows upward**, maintaining its position in the photic zone, eventually forming the ring.

Key Features

• Lagoon: **Shallow, sheltered, often stunning turquoise** — may be open (passes/channels) or closed to ocean.
• Reef Flat/Rim: **Living coral, coral rubble, sand** — often **awash at low tide**.
• Islets (Motu): **Low, sandy, vegetated (coconut, pandanus)** — on reef rim. **Habitation, airstrips, coconut plantations**.
• Passes (Ava): **Gaps in reef** allowing boat access, water exchange — **critical for lagoon health, navigation, predator access**.

Formation (Darwin's Theory — Confirmed)

1. **Fringing reef** on volcanic island.
2. **Barrier reef** as island subsides, lagoon widens.
3. **Atoll** when island **fully subsides**, only reef ring remains.

Distribution & Examples

• Pacific: **Tuamotus (French Polynesia — 78 atolls), Marshall Islands (Kwajalein — world's largest lagoon, 2,174 km²), Kiribati, Tokelau, Caroline Islands.**
• Indian Ocean: **Maldives (26 atolls, 1,192 islands), Chagos (Diego Garcia), Seychelles (outer islands), Lakshadweep (India).**
• Atlantic: **Bermuda (pseudo-atoll, high-latitude), Rocas (Brazil), Belize Barrier Reef (atoll-like).**

Vulnerability

**Sea-level rise, warming (bleaching), acidification, storms, overfishing, nuclear testing legacy (Bikini, Moruroa).** **Existential threat** to atoll nations (Kiribati, Tuvalu, Marshall Islands, Maldives).

Kwajalein (Marshall Islands) = **largest atoll by lagoon area**. **Kiritimati (Christmas Island, Kiribati)** = **largest atoll by land area**.