Physical Geography (Part-11): Islands & Coral Reefs

Islands and Coral Reefs

An island is a piece of land surrounded on all sides by water. It may occur individually or in a group, in open oceans or seas. Smaller ones of only local significance are found even in lakes and rivers. 

All Islands may be grouped under the following types:

1.) Continental Islands: These islands were formerly part of the mainland and are now detached from the continent. They may be separated by a shallow lagoon or a deep channel. Their separation could be due to subsidence of some part of the land or to a rise in sea level so that the lowland links are submerged by the sea. Their former connection with the neighbouring mainland can be traced from the similar physical structure, flora and fauna that exist on both sides of the channel. Over time, modification by men and other natural forces may give rise to different surface features. But even then, the basic structural features will remain the same. Continental islands may appear as:

• Individual Island: These lie just outside the continent, very much associated with the characteristics features of the mainland of which they were once part. Example- Newfoundland, separated from the mainland by the Strait of Belle Isle, Madagascar, by the Mozambique Channel, Ceylon by the Palk Strait, Tasmania by the Bass Strait and Formosa by the Formosa Strait.

• Archipelagoes or Island groups: These comprise groups of islands of varying sizes and shapes. Eg. the British Isles, the Balearic Islands of the Mediterranean and also those of the Aegean Sea.

• Festoons or Island arc: The islands form an archipelago in the shape of a loop around the edge of the mainland, marking the continuation of mountain ranges that can be traced in the continent. Eg. the East Indies, the Aleutian Island, Ryukyu Islands, Kurile Islands and other arcs of the Pacific coasts. 

2.) Oceanic Islands: These islands are normally small and are located in the midst of oceans. They have no connection with the mainland which may be hundreds or thousands of miles away. They have a flora and fauna unrelated to those of the continents. The Galapagos Islands have many unique species of animals. Due to their remoteness from the major trading centres of the world, most of the oceanic islands are very sparsely populated. Some of them provide usefully stops for aeroplanes and ocean streams that ply between continents across vast stretches of water.

Generally, oceanic islands fall into one of the following groups:

• Volcanic Islands: Many of the islands in the oceans are in the fact the topmost part of the cones of volcanoes that rise from the ocean bed. Most of them are extinct, but there are also some active ones. The best known volcanic peak of the Pacific Ocean is Muana Loa in Hawaii, which is 13,680 feet above sea level. Mauna Loa is found to have been built up from the water surface! Other volcanic islands have emerged from the submarine ridges of the oceans. The volcanic islands are scattered in most of the earth's oceans. In the Pacific Ocean, they occur in several groups such as Hawaii, the Galapagos Island and the South Sea islands. In the Atlantic are the Azores, Ascension, St. Helena, Madeira and the Canary Islands. Those of the Indian Ocean are Mauritius and Reunion. In the Antarctic Ocean are the South Sandwich Islands, Bouvet Island and many others.

• Coral Island: The coral islands are very much lower and emerge just above the water surface. These islands, built up by coral animals of various species, are found both near the shores of the mainland and in the midst of oceans. Coral islands include the Marshall Islands, Gilbert and Ellice Islands of the Pacific, Bermuda in the Atlantic and the Laccadives and Maldives of the Indian Ocean.

Coral Reefs:

In tropical seas, many kinds of coral animals and marine organisms such as coral polyps, calcareous algae, shell-forming creatures and lime-secreting plants live in large colonies. Though they are very tiny creatures, their ability to secrete calcium carbonate within tiny cells has given rise to a particular type of marine landform. They exist in numerous species of my forms, colours and shapes. Under favourable conditions, they grow in great profusion just below the water level. Taking coral animals as a whole, the polyps are the most abundant and also the most important. Each polyp resides in a tiny cup of coral and helps to form coral reefs. When they die, their limy skeletons are connected to coastline limestones. There are also non-reef-building species such as the 'precious corals' of the pacific ocean and the 'red coral' of the Mediterranean which may survive in the colder and even the deeper waters. As a rule, they thrive well only in the warmer tropical seas.

The reef-building corals survive best under the following conditions:

• The water temperature must not fall below 68° F (20°C). This virtually limits the areal distribution of coral s to the tropical, and subtropical zones. Again they will not flourish where there are cold currents because of the upwelling of the cold water from the depths that cools the warm surface water. This explains why coral reefs are generally absent on the western coasts of continents. On the other hand of the warming effect of the warm currents eg. the Gulf stream, means that corals are found to the north of the West Indies in the Atlantic Ocean. The pacific and the Indian oceans, however, have the most numerous coral reefs.

• The depth of the water should not exceed 30 fathoms or 180 feet, because beyond this depth sunlight is too faint for photosynthesis to take place. This is essential for the survival of the microscopic algae, on which the coral polyps depend. The shallow water of fewer than 100 feet is ideal. But there should always be plenty of water as polyps cannot survive for too long out of water.

• The water should be saltish and free from sediment. Corals, therefore, survive best in the moving ocean water well away from the silty coasts or muddy mouths of streams. The corals are best developed on the seaward side of the reef, where constantly moving waves, tides and currents maintain an abundant supply of clear, oxygenated water. They also bring an adequate supply of food in the form of microscopic organisms.

Types of Coral Reefs:

There are three (3) main types of coral reefs:

1.) Fringing reefs: A fringing reef is a coralline platform lying close to the shore extending outwards from the mainland. It is sometimes separated from the shore by a shallow lagoon. It is widest when fringing a protruding headland but completely absent when facing the mouth of the stream. The outer edge grows rapidly because of the splashing waves that continuously renew the supply of fresh food. The reefs may be about a mile wide, lying just above the level of low water and sloping steeply downwards on the seaward side to a depth of about 100 feet.

2.) Barrier reef: A barrier reef is separated from the coast by a much wider and deeper channel or lagoon. The reef is partially submerged. where it lies above the water level and sand can accumulate on it, a little vegetation is possible. The barrier reefs have narrow gaps at several places to allow the water from the enclosed lagoon to return to the open ocean. Such gaps are very useful for shipping and provide the only entrances for ships to enter or leave the lagoon. Eg. the Great Barrier reef off the coast of Queensland (Australia). It is 1,200 miles long, separated from the coast by a channel 100 miles wide in places and over 200 feet deep.

3.) Atolls: Atolls are similar to barrier reefs except that they are circular in shape, enclosing a shallow lagoon without any land in the centre. The encircling ring is usually broken in a few places to allow the free flow of water. On the inside of the reefs, sand and limestone debris collect and palm trees like coconuts may grow. Such palm trees thrive well in the brackish water of the lagoon. The nuts fall into the water and are distributed widely by floating from one coral island to another. The calm waters are useful for fishing and canoeing. Some of the large atolls eg. Suvadiva (Maldives), west of Ceylon, have a lagoon over 40 miles across. A number of them provide essential air bases for trans-Pacific aircraft. 

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