![]() The fringing reefs become barrier reefs since the outer parts maintain itself close to the sea level while the parts on the inside fall off. A fringing coral reef surrounding a volcanic island will develop upwards as the island subsides leading to a formation of a barrier reef or an atoll. The continued eruption leads to the growth of seamount eventually penetrating the water surface with the top of the volcano forming the oceanic island. The volcano erupts piling up lava on the floor of the sea. The word atoll is derived from an Indo-Aryan language which means “an administrative subdivision.” Formation Of An AtollĪn atoll, just like other tropical island types including high volcanic islands and reef islands, is formed from the gradual subsiding of an oceanic volcano. An atoll can also be described as annular reef enclosing a lagoon with no promontories other than reefs. A slow rate of continued erosions that allow the reef to grow outward and upward is necessary for the atoll to persist. The lagoon is formed over the volcanic crater with the higher rim remaining above the water permitting the coral to grow and form a reef. The corals of the atoll sit on top of the rim of an extinct volcano called seamounts which have eroded beneath the waters. Atoll and lagoons form a rim around an island that offers protection to the central island. ![]() An atoll is a ring-shaped coral reef or series of coral rims that encircle a lagoon either partially or wholly. ![]()
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