Atoll, coral reef enclosing a lagoon. Atolls encompass ribbons of reef that might not constantly be round however whose vast configuration is a closed form as much as dozens of kilometres across, enclosing a lagoon that can be about 50 metres (a hundred and sixty feet) deep or greater.
Most of the reef itself is a submarine feature, growing from the abyssal flooring of the sea to simply below high-tide stage. Around the rim alongside the pinnacle of the reef there are commonly low, flat islands or greater non-stop strips of low, flat land. Some of those islands were settled with the aid of using oceanic peoples just like the Maldivians, Polynesians, and Micronesians for plenty centuries.
The beginning of atolls has constantly involved sailors and naturalists, who early favored that, despite the fact that reef-constructing organisms inhabit simplest the shallowest depths of the sea (approximately a hundred metres [330 feet]), the reefs rose from a whole lot deeper. The present day clarification of atolls consists of the idea of Charles Darwin, who recommended that atolls represented the very last level of a persevering with upgrowth of reef round a sinking extinct volcanic island that had lengthy on account that disappeared from view.
Reefs have a tendency to develop outward from a fringing-reef level in the direction of the higher situations of open water and additionally develop upward if the rules below are sinking. After heaps of years the actively developing reef shape turns into separated from the volcanic coastline with the aid of using an intervening stretch of lagoon water. This is the barrier-reef level. The volcanic island finally subsides from view, leaving a reef whose uppermost element is sort of a saucer whose rim reaches as much as sea stage and whose deeper principal location is a lagoon.
Different styles of reefs and volcanic islands are discovered collectively withinside the tropical oceans, associated with every different in this sort of manner that they may be interpreted as representing the revolutionary levels postulated with the aid of using the subsidence idea. Stronger direct proof for subsidence has come from geologic drilling of atolls (first at Enewetak atoll in 1952), which found out the presence of volcanic rock approximately 1,four hundred metres (4,six hundred feet) beneath the present day reef pinnacle. Changes in sea stage complicate the subsidence model. These were especially common over the past 2,000,000 years or greater and basically end result from cycles of glaciation. See additionally coral reef.