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 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)
- 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.
- 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
- 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.
- 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 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.