Definition of Atoll

Consisting of circular, elliptical, or semicircular

islands made of coral reefs that rise from deep water, atolls

surround central lagoons, typically with no internal landmass.

Some atolls do have small central islands, and these, as

well as parts of the outer circular reef, are in some cases covered

by forests. Most atolls range in diameter from half a

mile to more than 80 miles (1–130 km) and are most common

in the western and central Pacific Ocean basin, and in

the Indian Ocean. The outer margin of the semicircular reef

on atolls is the most active site of coral growth, since it

receives the most nutrients from upwelling waters on the

margin of the atoll. On many atolls, coral growth on the

outer margin is so intense that the corals form an overhanging

ledge from which many blocks of coral break off during

storms, forming a huge talus slope at the base of the atoll.

Volcanic rocks, some of which lie more than half a mile (1

km) below current sea level, underlay atolls. Since corals can

only grow in very shallow water less than 65 feet (20 m)

deep, the volcanic islands must have formed near sea level,

grown coral, and subsided with time, with the corals growing

at the rate that the volcanic islands were sinking.

Charles Darwin proposed such an origin for atolls in

1842 based on his expeditions on the Beagle from 1831 to

1836. He suggested that volcanic islands were first formed

with their peaks exposed above sea level. At this stage, coral

reefs were established as fringing reef complexes around the

volcanic island. He suggested that with time the volcanic

islands subsided and were eroded, but that the growth of the

coral reefs was able to keep up with the subsidence. In this

way, as the volcanic islands sank below sea level, the coral

reefs continued to grow and eventually formed a ring circling

the location of the former volcanic island. When Darwin proposed

this theory in 1842 he did not know that ancient eroded

volcanic mountains underlay the atolls he studied. More

than 100 years later, drilling confirmed his prediction that

volcanic rocks would be found beneath the coralline rocks on

several atolls.

With the advent of plate tectonics in the 1970s, the

cause of the subsidence of the volcanoes became apparent.

When oceanic crust is created at mid-ocean ridges, it is typically

about 1.7 miles (2.7 km) below sea level. With time, as

the oceanic crust moves away from the mid-ocean ridges, it

cools and contracts, sinking to about 2.5 miles (4 km)

below sea level. In many places on the seafloor, small volcanoes

form on the oceanic crust a short time after the main

part of the crust forms at the mid-ocean ridge. These volcanoes

may stick up above sea level a few hundred meters. As

the oceanic crust moves away from the mid-ocean ridges,

these volcanoes subside below sea level. If the volcanoes

happen to be in the tropics where corals can grow, and if

the rate of subsidence is slow enough for the growth of

coral to keep up with subsidence, then atolls may form

where the volcanic island used to be. If corals do not grow

or cannot keep up with subsidence, then the island subsides

below sea level and the top of the island gets scoured by

wave erosion, forming a flat-topped mountain that continues

to subside below sea level. These flat-topped mountains

are known as guyots, many of which were mapped during

exploration of the seafloor associated with military operations

of World War II.

See also CORALS; PLATE TECTONICS; SEAMOUNT.

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