ATOLL (native name atollon in the Maldive Islands), a horse-shoe
or ring shaped coral reef enclosing a lagoon. The usual
shape is that of a partly submerged dish with a broken edge,
forming the ring of islands, standing upon a conical pedestal.
The dish is formed of coral rock and the shells of various
reef-dwelling mollusca, covered, especially at the seaward edges, with
a film of living coral polyps that continually extend the fringe,
and enlarge the diameter of the atoll. The lagoon tends to deepen
when the land is stationary by the death of the coral animals in
the still water, and the patchy disintegration of the “hard”
coral, while waves and storms tear off blocks of rock and pile
them up at the margin, increasing the height of the islands,
which become covered by vegetation. The lagoon entrance in
the open part of the horse-shoe is always to leeward of prevailing
winds, since the coral growth is there slower than where the waves
constantly renew the polyps’ food supply. The conical pedestal
rising from the depths is frequently a submarine volcanic cone
or island, though any submerged peak may be crowned by an
atoll. For the theory of atoll formation see Coral-reefs.
This page is extract from an ebook created by volunteers. It is for the use of anyone anywhere at no cost and with almost no restrictions whatsoever. You may copy it, give it away or re-use it under the terms of the Project Gutenberg License included online at www.gutenberg.org
A few typographical errors have been corrected. They
appear in the text like this, and the
explanation will appear when the mouse pointer is moved over the marked
passage. Sections in Greek will yield a transliteration
when the pointer is moved over them, and words using diacritic characters in the
Latin Extended Additional block, which may not display in some fonts or browsers, will
display an unaccented version.
Links to other EB articles: Links to articles residing in other EB volumes will
be made available when the respective volumes are introduced online.