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
