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.
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