2. Features of Coral reefs.
- Coral reef and atoll are significant submarine features
- These are formed due to accumulation and compaction of skeleton of lime
secreting organisms known as ‘Coral polyps’.
- Coral reefs are formed by shell over shell accumulation.
- ‘Coral polyp’ cannot live above the water level and hence below it.
- They are generally attached to submarine system
- Coral reefs have diversity of 10,00,000 species, so called rainforests of oceans.
3.
4. Condition for growth of Coral reefs.
1) Found mainly in “Tropical Oceans” (20 – 21 degree C)
2) Coral don’t live deeper in water, and above sea level also.
- They don’t get sunlight and oxygen below 91m, so.
3) Clean and sediment- free water is required to grow polyps.
- Mud will clog the mouth of polyps otherwise.
4) Fresh water is also not good for their growth due to lack of minerals in it.
5) High salinity of water is bad for their growth. Requires 27°/ 00 and 30° / 00
Salinity.
6) Ocean currents and waves bring food for polyps. Inside Lagoons there will be
dead polyps.
7) Polyps need platform to grow below sea level to grow. Up to sea level they will
grow. And after this outward growth will be seen.
5. Human Effect on Coral reefs
- Global warming causes adverse effect on coral reefs.
- Corals cant survive at warm temperature.
- 10% Corals died due to global warming by anthropogenic factors, mainly
industrialisation.
- Coral bleaching begin with rice in 10 C temperature in sea water.
- Pollution of ocean water , excess flux of sediments, wastes are also reasons for the
death of corals.
6. Types of Coral reefs
Submarine Coral reefs classified in two ways
1) On basis of nature, shape, & mode of occurrence.
i. Fringing reefs
ii. Barrier reefs
iii. Atoll
2) On the basis of location
i. Tropical Coral reefs
ii. Marginal belt coral reefs
7.
8. 1) Fringing reefs
- Reefs developed along the continental margins or along the islands are called
fringing reefs.
- Usually attached to the coastal land.
- But some times there will be gap between land and reef, forms lagoons, called
‘boat channel’.
- Reefs are narrow in width but long. Breaks at river drains.
9. 2)Barrier reefs
- The largest coral reefs off the coastal platforms but parallel to them are called
‘barrier reefs.’
- There is extensive but shallow lagoon between coastal land and barrier reef.
- These reefs are continuous.
- Some times there will be tidal inlets
- These reefs require 91m depth for development
- Ex: Great Barrier reef, parallel to the east coast of Australia.
10. 3) Atoll
- A ring of narrow growing corals of horseshoe shape around the island shape
coastal area are called Atolls.
- The depth of lagoons ranges from 240 to 420 feet.
- 3 types of Atolls are there
i. True atoll – Circular reef enclosing shallow lagoon, without island.
ii. Island atoll - Circular reef enclosing shallow lagoon, with island.
iii. Coral island atoll - Island is not present initially, but formed
lateron, due to deposition.
True atoll Island atoll
11. ORIGIN OF CORAL REEF AND ATOLLS.
- All the theories of reef and atoll development are grouped into two large groups.
1) Subsidence Theory.
2) Non Subsidence Theory.
12. 1) Subsidence theory of Darwin.
- All coral reefs develop due to growth of polyps at shallow depth, because at
greater depth they cant survive.
- Fringes, Barrier, Atoll are successive stages of development of coral reefs.
- First, Polyps grow at suitable stage below sea level initially and develop. And
form fringe reef.
- After, land is subject to subsidence due tectonic forces, thus coral reef reach
greater depth, and there they may not survive.
- The growth of polyp is retarded near the coastal land but vigorous at outer edge
of the land.
- Consequently lagoon is formed between coastal land and reef hence Barrier
Reef.
- Further subsidence of land and island is completely submerged under water and
a ring of coral reef in the form of atoll is formed.
- Depth of lagoon does not increases because of sedimentation.
13.
14. 2) Stand still Theory of Murray
- Corals grow upon suitable stable submarine platforms with unchanging sea level.
- Coral Polyps can live up to depth of 30 fathoms (180 feet).
- Sea level and submarine platforms are stable.
- Several submarine platforms, volcanic peaks, islands are present.
- If submarine platforms above the sea level, they are subjected to wave erosion.
- If submarine platforms are below seal level, they are subjected to deposition of
marine sediments.
- After getting suitable depth of 180 feet, coral polyps start growing to form “fringing reefs”.
- Continuous outward growing of polyps form “barrier reefs”.
- Atolls are formed due to outward growth of corals in all directions at the top of
submarine platform.
- Lagoon side of atoll is by dead corals while seaward side has living corals which
continuously grow outward.
- Evaluations:
1) Submarine platforms not possible to find everywhere.
2) Marine erosion and deposition at depth of 30 fathoms are contradictory.
3) Lagoons might be filled with depositions if land is stable.
4) Reefs are found from below 30 fathoms also.
15. 3) Glacial Control theory of Daly.
- Daly propounded this theory in 1915.
- He said Corals formed after Pleistocene Ice age.
- The corals in sea died due to the fall in sea level, and lowering temperature
because of glaciation on continents during Pleistocene age.
- ‘Wave cut platforms’ were formed due to sea waves.
- At the end of ice age the ice melted, resulting into rise of sea level above the
‘ Wave cut platforms’ at coasts.
- The corals, which were able to survive in ice age grown above the ‘ Wave cut
platforms’
- The Fringing reefs formed on narrow wave cut platforms.
- The Barrier reefs formed on broad eroded platforms.
- Atolls were formed around isolated wave eroded island peaks.
Evaluations:
1) According to theory the lagoons should have uniform depth, which is not.
2) Cliffs formed in Pleistocene should be present till today which is seldom found.
16. 4) Concept of W.M.Davis
- W.M. Davis supported ‘Subsidence theory’ with proofs.
- According to him corals grow along subsiding lands.
- The shallowness of lagoons is due to deposition of debris.
- If submarine platforms are stable, the lagoons would have been filled with
debris, and over spilling of water could killed the seaward corals.
17. Coral Bleaching
- Coral bleaching refers to loss of algae from the corals resulting into white colour
which is indicative of death of corals.
- Global warming has been reported as the major factor of coral bleaching.
- The coral bleaching in 1997-98 has recorded as most catastrophic event.
1) Catastrophic Bleaching - Affecting 95 % of corals.
2) Severe Bleaching – Affecting 50-70% of corals.
3) Moderate bleaching – Affecting 20-50 % of corals. Have quick recovery.
4) Insignificant bleaching – no bleaching.
- The bleaching is related to increase in 10 C temperature of the sea.
- 1997-98 Bleaching resulted due to 20 C temperature increase.
- Some times bleaching occurs due to over siltation, due to mass flux of
sediments.
- Over salinity is also a reason for death of the corals.
