Atolls are found in the open sea. They are formed by a prolific group of animals called corals, which when concentrated in masses,
form coral reefs. These reefs are formed during the slow
sinking of an extinct volcano. Entering the water
from the edge of a sandy white beach, one can swim
in shallow water to the edge of the table reef which will then
drop off directly into very deep water. In essence, the volcanic
island is buried beneath the atoll.
Photo: Josh Jones
October 23, 2000
Formation of a Coral Atoll
This is Genevieve Johnson, talking to you from the Odyssey.
Our present search for sperm whales takes the Odyssey around an area in
the central pacific known as the Republic of Kiribas, which encompasses from
west to east, the Gilbert, Phoenix and Line Island groups. Covering 3.5
million sq.km of ocean, Kiribas is made up of 1 raised coral island and 33
low lying coral atolls that straddle the equator.
The warm waters of the equatorial pacific we are currently exploring, favor
the growth of a prolific group of animals called corals. Tiny soft-bodied
animals housed in limestone shelters of their own making, which when
concentrated in masses, form coral reefs. Coral will only grow in clear,
unpolluted, well-aerated shallow water, and as coral atolls are only found
in open ocean, they are often surrounded by very deep water. So the
question arises, how did these corals get a foothold in the Pacific?
Since the beginning of our planets history, volcanic islands have risen from
the sea floor and then disappeared beneath the waves. Once a volcano
becomes extinct, it slowly subsides, dragging its fringing reef, a thin
coral band formed along its coastline, downward into the water. The actively
growing coral keeps pace with the subsidence, as it must to keep thriving in
the upper sunlit waters, continuously building the reef upward and outward.
Eventually a barrier reef forms, separated from the shoreline of the
protruding volcanic peak or seamount. Continual erosion or an increase in
sea level or both, ultimately cause the volcanic crest to be engulfed. A
ring shaped coral reef or atoll is then left encircling a lagoon, providing
the coral continues to grow.
New coral polyps form on top of the skeletons of the
previous generation, burying ancient corals, which in turn overlay an even
older base of volcanic rock. In essence, the volcanic island is buried
beneath the atoll. The atoll becomes a haven for animals, supporting one of
the richest and most diverse ecosystems on the planet.
The Odyssey crew is looking forward to exploring some of these remote
atolls, with their abundance of thriving bird colonies and dazzling array of
Log by Genevieve Johnson
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