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. Generally speaking, all islands may be grouped under the following types.
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. In the course of time, modification by men and by 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 Islands: These lie just outside the continent, very much associated with the characteristic features of the mainland of which they were once part. Some of the outstanding examples are New foundland, separated from the mainland by the Strait of Belle Isles; Madagascar, by the Mozambique Channel; Sri Lanka 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 e.g. the British Isles, the Balearic Islands of the Mediterranean and also those of the Aegean Sea.
- Festoons or Island arcs: The islands form an archipelago in the shape of a loop around the edge of the mainland, marking the continuation of mountain ranges which can be traced on the continent e.g. the East Indies, the Aleutian Islands, Ryukyu Islands, Kurile Islands and other island arcs of the Pacific coasts.
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 kilometres 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 useful stops for aeroplanes and ocean steamers that ply between continents across vast stretches of water.
Generally speaking, oceanic islands fall into one of the following groups.
1) Volcanic Islands: Many of the islands in the oceans are in fact the topmost parts 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 Mauna Loa in Hawaii, which is 4169 m (13 680 feet) above sea-level.Tracing downwards, Mauna Loa is found to have been built up from the ocean floor at a depth of 5490m (18 000 feet) below 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 Islands and the South Sea islands. In the Atlantic there are the Azores, Ascension, St. Helena, Madeira, Canary Islands, Fernando Po, Annobon, Principe and Sao Tome. Those of the Indian Ocean are Mauritius and Reunion. In the Antarctic Ocean there are the South Sandwich Islands, Bouvet Island and many others.
2) Coral islands: Unlike the volcanic islands, 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.