Islands & Coral Reefs

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

  • Continental islands
  • 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.

continental islands

  • 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.
Volcanic Islands
  • Formed where topmost part of cone of volcano rise from the ocean bed above the sea level
  • Lava accumulates to enormous thickness until it finally protrudes above the ocean surface
  • It may rise to a great height above the ocean surface for e.g. Islands of Hawaii
Coral Islands
  • Unlike the volcanic islands, the coral islands are very much lower & emerge just above the water surface.
  • These islands, built up by the coral animals of various species
  • Are found both near the the shores of mainland & in the midst of oceans

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

Types of Coral Reefs

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

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.

Fringing reef ,Barrier Reefs

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.



  • 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.


Coral Reefs Origin Theory

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.


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.
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