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.

• 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.

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

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.

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.

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.

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.