v                              Coral reefs

                                                    Meaning

•      Coral colonies grow continuously in size by budding of polyps & often form extensive masses- Coral Reefs

•      Coral reef is a mound of limestone, the upper surface of which is near the surface of sea & which is formed chiefly of calcium carbonate secreted by coral polyps

•      Principal builders of coral reefs are stony corals (Madreporaria), but other important contributors are hydrocorallines & alcyonarians.

•       Coralline algae & Foraminiferan Protozoa also participate in coral reef formation

•      Reef building corals normally require warm shallow water (above 20°C)

•      Restricted to Indo-Pacific, the Central- Western Pacific & Carribean regions north of Bermuda

•      About 50 species of corals contribute in the formation of reefs along the Florida key sand in West Indies

ü Kinds of coral reefs

•      Classification is based on their formation

1.        Fringing Reefs                 2.Barrier Reef                                                 3.Atoll

1.     Fringing Reef

• Coral reefs lying close to the shores of some volcanic island/ part of some continent
• May be extended to a distance of a quarter mile from shore with most active zone of coral growth facing sea
• Zone is called- Front
• A shallow water channel 50-100m broad, lies between reef edge & shore
• At low tide water of channel recedes at quickly exposing a flat bottom surface called reef flat
• Largely composed of coral sand, mud, dead & living coral colonies & other animals

2.     Barrier reef

• Like fringing reefs but are located some distance away from shore
• Stretch of water separating barrier reef from land may be half a mile to 10 miles/ more in width- Lagoon
• 10-50 fathoms deep & suitable for navigation
• Most notable example is Great Barrier Reef along north-eastern coast of Australia, 2,000 km long & up to 150 km from shore

3.     Atoll

• Also termed a coral island
• Horse-shoe shaped reef encircling a lagoon but not an island
• Lagoon varies from a few to about 90 km
• May be complete/ broken by a number of channels, only few navigable
• Outer end slopes rather steeply into depth of ocean
• Atoll of Bikine is famous for hydrogen & atomic bomb tests, lies in Pacific Ocean
• Formation of coral reefs

Two theories seem to be more convincing:

      1.                    Subsidence theory by Darwin.

      2.                   Glacial control theory by Daly.

1.     Subsidence theory by Darwin

• Darwin gave this theory in 1831
• It suggests that fringing reef was formed first on sloping shore of an island
• Subsidence of sea floor then took place in the regions of reef followed by upward & outward growth of coral

•      Fringing reef then became barrier reef

•      By gradually sinking the island ultimately vanished & the barrier reef became a coral atoll with a central lagoon

•      Later on it acquired growth of vegetation

2.     Glacial control theory by Daly

• Put forward by Daly
• Accounts for lowering of sea level by withdrawal of water for glacial formation
• This resulted in exposing several flat platforms cut off by the action of waves
• When glaciers melted & temperature became favourable, corals began to grow on these on platforms, building higher as ocean level rose
• Most coral reefs grew at a rate of 10-200 mm each year & have formed in a period of 15,000-30,000 years

Þ                       Economic importance of coral reefs

•      Corals are favourable sites for accumulation of petroleum deposits, so important for oil industry

•      Serve as habitats for many plants & animals

    like sponges, molluscs, echinoderms, fishes etc.

•      man also

•      Corallum rubrum is a precious decorative stone in India & China

•       Red & pipe corals are used as medicines in Sri Lanka

•      Coral skeletons are used in preparation of lime, mortar cement, also they help in making nursery ground for commercially important fishes