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GC Leong: Summary of Island and Coral Reefs | Geography for UPSC CSE PDF Download


  • An island is a piece of land surrounded on all sides by water, may occur individually or in groups & can be classified in two main types
    (i) Continental islands
    (ii) Oceanic islands

Continental Islands

  • Continental islands are simply unsubmerged parts of the continental shelf that are entirely surrounded by water.
  • Many of the larger islands of the world are of the continental type.
  • Formerly, these islands were part of mainland, which got detached from the continent, may be by a shallow lagoon or deep channels, due to subsidence of some part of the land or a rise in sea level, so that lowland links are submerged by the sea.
  • Their former connection with the neighbouring mainland can be traced from the similar physical structure, flora & fauna that exist on both sides of the channel.
  • Greenland, the largest island, is composed of the same materials as the adjacent North American continent, from which it is separated by a shallow and narrow sea.
  • Likewise New Guinea, world’s second largest island, is part of the Australian continental platform and is separated from it only by the very shallow and narrow Torres Strait.
  • Continental islands may appear as individual islands, island groups (archipelagos) or island arcs.
  • (Festoons - archipelagos in shape of loop, marking the continuation of mountain ranges which can be traced on the continent).

Oceanic Islands

  • These islands are normally small & are located in the midst of oceans.
  • They have no connection with the mainland & have flora & fauna unrelated to that of continents.
  • Due to their remoteness from the major trading centres of the world, most of them are sparsely populated.
  • Some of them provide useful stops for airplanes & ocean steamers that ply between continents.
  • Generally, oceanic islands may be classified as volcanic islands or coral islands.

GC Leong: Summary of Island and Coral Reefs | Geography for UPSC CSE


Coral Reefs

  • In tropical seas, various kinds of coral animals & marine organisms such as coral polyps, calcareous algae, shell forming creatures & lime secreting plants live in large colonies.
  • Though they are tiny creatures, their ability to secrete calcium carbonate within their tiny cells has given rise to a peculiar type of marine landform.
  • Coral reefs are generally made of tiny animals called “polyps” that stay fixed in one place and are the main structure of a reef.
  • Polyps have a hard outer skeleton made of calcium (similar to a snail’s shell)
  • When they die, their limy skeletons are cemented into coralline limestone.
  • The reefs grow best in warm, shallow, clear, sunny and moving water.
  • However, they grow very slowly—anywhere from 0.3 cm to 10 cm per year.
  • The reefs we see today have been growing over the past 5 000 to 10 000 years.
    Favourable conditions for Coral Reefs
  • As a rule they thrive well only in warmer tropical seas, with water temp. Not falling below 20*C approx. & does not flourish in cold currents.
  • This explains why coral reefs are generally absent on western coast of the continent.
  • Depth of the water should not exceed 180 feet or 30 fathom, because beyond this range sunlight is too faint for photosynthesis to take place.
  • Water should be saltish & free from sediments. Corals, thus, survive best in moving ocean waters well away from the silty coasts & best developed on the seaward sides of the reef.

Types of Coral Reefs

1. Fringing Reefs

  • The most common type of reef is the fringing reef.
  • This type of reef grows seaward directly from the shore.
  • They form borders along the shoreline and surrounding islands & may be separated from the shore by narrow, shallow lagoons.
  • The reefs may be about a mile wide, lying just above the level of low water & sloping steeply downwards on the seaward side to a depth of 100 feet.

2. Barrier Reefs

  • Barrier reefs are similar to fringing reefs as they also border a shoreline & are parallel the coastline but are separated by much deeper & wider lagoons.
  • Barrier reef may be partially submerged & where it lies above the water level, sand can get accumulated & little vegetation is possible.
  • Barrier reefs have narrow gaps at several places to allow water from the enclosed lagoon to return to the open surface.
  • Such gaps are also useful for ships to enter or leave the lagoon.

3. Atolls

  • When a fringing reef continues to grow upward from a volcanic island that has sunk entirely below sea level, an atoll is formed.
  • Atolls are usually circular or oval in shape, with an open lagoon in the center.

Probable Origin of Coral Reefs

1. Darwin’s Theory

  • Darwin assumed that all coral reefs began as fringing reefs around an Island or topmost portions of extinct volcanoes, which stood above the ocean bed.
  • Over millions of years, the volcano sinks lower into the sea and the sea level rises around the volcano due to subsidized down-warping.
  • The coral grows upwards to keep from getting too far from the sunlight at the sea surface.
  • The outward side of the coral reef grows fastest since ocean currents bring in the plankton that the corals feed on.
  • Water on the landward side of the reef is still & there is less oceanic plankton
  • Hence here the reef was unable to grow fast enough to keep up with the rising sea level & eventually drowned.
  • A lagoon develops between the reef and the land, resulting in the characteristic barrier reef shape.
  • Eventually, when the volcano land completely submerged, only the outer rim of the reefs was seen, forming an atoll.
  • Thus, the atoll marks position of former islands & lagoons it contains are generally shallow.

2. Daly’s Glacial Control theory

  • Daly noticed the close relationship between glaciation & development of coral reefs.
  • To him, Pleistocene glaciation caused the subsidence of sea level.
  • He believed that, during ice age, water was too cold for any coral growth to take place.
  • With the absence of coral barrier, marine erosion was able to attack & lower the lands gradually.
  • When the climate became warmer, the water that was locked up in the ice sheets melted, consequently rising the sea level, submerging these lower islands forming wave platforms.
  • On these wave planed platforms, corals began to grow upward at the rate of a foot in a decade to keep pace with rising water level.
  • Narrow platform supported fringing reefs; broad platforms supported barrier reefs while isolated platforms supported formation of Atolls.
The document GC Leong: Summary of Island and Coral Reefs | Geography for UPSC CSE is a part of the UPSC Course Geography for UPSC CSE.
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FAQs on GC Leong: Summary of Island and Coral Reefs - Geography for UPSC CSE

1. What are coral reefs and why are they important?
Coral reefs are underwater ecosystems made up of coral polyps, which are tiny animals that live in colonies. They are important because they provide habitat for a diverse range of marine species, protect coastlines from erosion, and contribute to the economy through tourism and fisheries.
2. How do coral reefs form?
Coral reefs form through a process called coral bleaching, where coral polyps secrete calcium carbonate to build a hard skeleton. Over time, as new generations of corals grow on top of the old ones, a reef structure is formed. This process can take thousands of years.
3. What are some threats to coral reefs?
Coral reefs are facing numerous threats, including climate change, ocean acidification, pollution, overfishing, and destructive fishing practices. These factors can lead to coral bleaching, habitat destruction, and the decline of marine biodiversity.
4. What is the relationship between coral reefs and islands?
Coral reefs play a crucial role in the formation and maintenance of islands. As coral polyps build their calcium carbonate skeletons, they create a solid foundation that can accumulate sediment and eventually form an island. Additionally, coral reefs protect coastlines from erosion caused by waves and storms.
5. How can we protect coral reefs and their ecosystems?
There are several ways to protect coral reefs and their ecosystems. These include reducing greenhouse gas emissions to mitigate climate change, adopting sustainable fishing practices, minimizing pollution runoff, establishing marine protected areas, and promoting public awareness and education about the importance of coral reefs.
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