| Atolls
are circular to elliptical coral reefs that encircle a lagoon
and are surrounded by deep water of the open ocean (Bates and
Jackson, 1987). Atolls range in diameter from approximately 1
km to more than 130 km and are very common in the western and
central Pacific Ocean. Original foundations were volcanic islands
that eroded away or other topographically high structures on the
seafloor. |
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| Oblique air
photograph of an atoll in French Polynesia. The volcanic core
has eroded down below sea level. The atoll displays a well-developed
windward margin with abundant reef development and a leeward margin
with lesser reef development. |
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Coral atolls
in the Society Islands of the South Pacific. The twin atolls in
the lower right show two well-developed volcanic islands surrounded
by coral reefs. The volcanoes supplied the foundation for development
of the coral reefs. The atoll in the middle has a coral reef surrounding
an eroded volcanic island. The atoll at the upper left shows a
coral reef with no island in the lagoon, indicating that the volcanic
island eroded to below sea level. (Click
on each stage to see illustrative cross section.)
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| Belize
has several large atolls in front of the barrier reef in
deep water. Atolls formed on faulted structural highs. Image
courtesy of Earth Sciences and Image Analysis Laboratory,
NASA Johnson Space Center (http://eol.jsc.nasa.gov
). Photograph ID = ESC_large_ISS002_ISS002-E-5462.JPG . |
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