Goh Cheng Leong Chapter 11: Islands and Coral Reefs YouTube Lecture Handouts

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Island – land surrounded by water on all sides, may occur individually or in group

Continental Island: formerly part of mainland and now detached – due to land subsidence or rise in sea level

These can occur as:

  • Individual Island – Occur outside continent and associated to mainland like Newfoundland (separated from mainland by Strait of Belle Isle); Madagascar by Mozambique Channel; Ceylon by Palk Strait; Tasmania by Bass Strait and Formosa by Formosa strait

  • Archipelago or Island Group – British Isles, Balearic Island of Mediterranean and Aegean Sea

  • Festoons or Island Arcs: In Loop around edge or mainland – East Indies, Aleutian Island, Ryukyu Is., Kurile Island and island arcs in Pacific Coast

Oceanic Islands: Small and lie in oceans, flora and fauna is unrelated to continents; Galapagos Island have unique species of animals; sparsely populated

  • Volcanic Islands: island as topmost part of cone of volcano – some are extinct. Mauna Loa in Hawaii which is 13,680 ft ASL.

    • Atlantic Ocean: Azores, Ascension, St. Helena, Madeira and Canary Islands

    • Indian Ocean: Mauritius and Reunion

    • Antarctica Ocean: South Sandwich Island, Bouvet Island

  • Coral Islands: These are lower and emerge above water level

    • Pacific Ocean: Marshall Islands, Gilbert and Ellice Islands

    • Atlantic Ocean: Bermuda Island

    • Indian Ocean: Laccadives and Maldives

Coral Reefs

Image of Coral Reefs

Image of Coral Reefs

Image of Coral Reefs

  • Coral Polyps – calcareous algae, shell forming creatures and lime secreting plants live in large colonies – secrete calcium carbonate

  • Polyps are most abundant amongst all coral animals

  • When polyp die, skeleton are cemented into coralline limestone

  • Non reef building species – precious coral of Pacific Ocean & Red coral of Mediterranean Sea

  • Survive best in warmer tropical seas

  • Water temperature must not be below

  • Not flourish in cold currents due to upwelling of cold waters from depth that cools the warm water surface

  • Most corals are found in Pacific and Indian Ocean

  • Depth of water should not exceed 30 fathoms or 180 feet as beyond it sunlight is too faint. Shallow water of less than 100 feet is ideal.

  • Water should be saltish and free from sediments - corals are best developed on seaward side of reef – abundant supply of clear oxygenated water

Types of Coral Reefs

  • Fringing Reefs: Lies close to coast and extend outwards from mainland – separated by shallow lagoon – widest when fringing protruding headland but absent when facing mouth of stream

  • Barrier Reefs: Separated by wider and deeper channel – partially submerged – has narrow gaps at several places to allow water – useful for shipping. Great Barrier Reef off the coast of Queensland, Australia (1200 miles long separated from coast by 100 miles wide in places and over 200 feet deep)

  • Atolls: Circular and enclose shallow lagoon without any land in center – on inside palm and coconut trees might grow (thrive well in brackish waters) – Suvadiva in Maldives, west of Ceylon have 40 miles across lagoons. Also provide airbase for trans-Pacific aircraft

Origin of Coral Reefs

Subsidence Theory by Charles Darwin

  • After his voyage to Pacific Islands in 1842

  • All corals began as fringing around island or extinct volcano. Due to downwrapping of earth, island’s subsided. Growth was vigorous at outer edge and reef widened

  • Submergence of land led to formation of atolls with circular lagoon which is shallow

Glacial Control Theory by R.A. Daly

  • In 1910 during visit to Mauna Kea in Hawaii

  • Developed relation between glaciation and development of coral reefs

  • At Ice Ages peak, water was too cold for coral growth to take place. With absence of coral barrier, marine erosion was able to attack and lower islands. With warmer climate, water that locked up in ice sheets melted.

  • Rise in sea level, lower islands were submerged – here coral started to grow upwards. Atolls were formed.

  • Recent boring evidences favor Daly’s evidence of sea level change and consequent erosion of islands. Deepest boring reveal basaltic rocks which correspond to subsided islands by Darwin.