Coral Reefs/Coral Islands PDF

Coral reefs / Coral islands Dr V S Joji Associate Professor, Dept. of Marine Geology and Geophysics, CUSAT & National Recognised Trainer in Direct Trainer Skills, DoPT, Govt. of India...

Coral reefs / Coral islands Dr V S Joji Associate Professor, Dept. of Marine Geology and Geophysics, CUSAT & National Recognised Trainer in Direct Trainer Skills, DoPT, Govt. of India 1 Introduction- I N T R O – EB Key points: Reefs, Growth, Theories Objectives: At the end of the learning event the participants will be able to Explain different types of Coral reef / coral island Explain origin of Coral reefs / coral islands 2  Coral reefs are formed due to accumulation of calcareous remains of marine organisms known as corals upon submarine banks or surrounding the existing islands. Coral islands should be described as reefs, since they generally occur in the form of reefs or ridge like form. The reef building corals are known as coral polyps and are CaCO3 in Composition.  Types of Coral reef / coral island Fringing reefs (the reefs are in touch with an existing island), Barrier reefs (the reefs are at a distance from existing island, separated by a shallow body of water called lagoon eg. Great Barrier Reef, Queensland, NE Australia), and Atoll (no central island, corals occur in an arcuate pattern).  Classification of coral reef (after Darwin) Charles Darwin, Father of Evolution Theory classified coral reefs into different categories on the basis of reef morphology, the size and shape of a reef, and its relation to nearby land (if any). Fringing reefs (shore reef) –the reefs are in touch with an existing island, width is 800 m or more and they face to open sea, Barrier reefs –the reefs are at a distance from existing island, separated by a shallow body of water called lagoon eg. Great barrier reef, Queensland, NE Australia, and Atoll – There is no central island, corals occur in an arcuate pattern.  A typical atoll Kavarati Coral organic Island in the Laccadive Sea  Conditions for the growth of corals and others Areas situated between 30o North and South of Equator, Water must be clear and salty, Opposite to the mouth of river where the diluted seawater receives suspended silt and mud, corals cannot live and few reefs appear, Tropical conditions, Favourable temperature is 23 to 25oC and 18oC is the lowest limit of tolerance, and Average rate of coral growth is 6-45mm per year under all the necessary circumstances. Average is generally considered 14mm / year and depth 180 m bmsl.  Theories on origin of coral reefs A. Subsidence theory of origin of Coral reefs Proposed by Charles Darwin, father of theory of evolution, on February 12, 1809. He proposed the theory after his voyage by HMS Beagle among the Galapagos islands in Pacific Ocean. He initially found a number of islands were fringed with corals due to growth of corals around them under favourable conditions. He described them as Fringing reefs. Partial subsidence of zone of islands, with concomitant seaward growth of fringing reef, gradually brought the barrier reef into being. When enclosed islands were totally submerged under water and the growing reefs therefore gave rise to atolls.  Glacial Control Theory (after Daly) Due to the Pleistocene glaciations, large scale condensation of water and water vapours upon the continents must have depressed the sea level to an appreciable extend. End of glaciations caused rise in temperature, on the earth surface initiated the growth of reefs upon exposed wave cut benches. After Pleistocene rise of temperature followed by the melting of glaciers, sea level began to rise. The growth of coral reefs and rise in sea level kept pace with each other, till the original sea level restored. The fringing reef formed towards the end of glaciations, therefore converted into barrier reef or atoll, depending upon the amount of submergence suffered the then islands, due to the subsequent rise in sea level.  Third concept It suggests no submergence of islands or rise in sea level. The growth of corals around the existing islands gave rise to fringing reef only. Barrier reef and atolls are formed in a similar manner due to growth of coral reefs upon the smaller submarine banks and ridges which exist underwater underneath the surface of oceans and seas. No scientific backing / support  Summary Able to attain the said objectives References Shephard , F P (1967):Submarine Geology. Harper International, 556pp. Coates : Coastal geomorphology Joji V S (2017): An Introduction to Geology With Multiple Choice Questions. IK International, New Delhi, ISBN: 978- 93-85909-20-7 I have a question 11