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date: 23 June 2021

p. 122. Ancient reefs and islandslocked

p. 122. Ancient reefs and islandslocked

  • Charles Sheppard

Abstract

Reefs in deep geological time have been built by a succession of different kinds of life: plant, bacterial, and animal. Stromatolites and bryozoans were major reef-builders that persist today in minor or non-reef-building forms, sponges built entire reefs and are still important reef components, while several groups of major reef-builders flourished for a while and then became extinct: archaeocyathids which were similar to sponges, and coral-like forms including rugose and tabulate corals. Today’s reef-builders, cnidarian corals, appeared well after the great Permian-Triassic extinction event. All of these groups deposited vast quantities of limestone rock on which they live, often visible today as low mountain ranges. Reefs grow to the surface but not beyond, but upon them sand and sediments may build up, forming an island that attracts plants, then birds and other terrestrial forms of life. The sediments become cemented with the aid of rainwater too, and ‘low islands’ develop. Many islands also show their old, central volcanoes, resulting in the vast array of different combinations of coral island type. Today, however, there is a coral reef crisis due to overexploitation of a reef’s rich resources, from pollution of several kinds, and climate change.

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