Island

Island (O.E. ieg = isle, + land1), in physical geography, a term generally definable as a piece of land surrounded by water. Islands may be divided into two main classes, continental and oceanic. The former are such as would result from the submergence of a coastal range, or a coastal highland, until the mountain bases were cut off from the mainland while their summits remained above water. The island may have been formed by the sea cutting through the landward end of a peninsula, or by the eating back of a bay or estuary until a portion of the mainland is detached and becomes surrounded by water. In all cases where the continental islands occur, they are connected with the mainland by a continental shelf, and their structure is essentially that of the mainland. The islands off the west coast of Scotland and the Isles of Man and Wight have this relation to Britain, while Britain and Ireland have a similar relation to the continent of Europe. The north-east coast of Australia furnishes similar examples, but in addition to these in that locality there are true oceanic islands near the mainland, formed by the growth of the Great Barrier coral reef. Oceanic islands are due to various causes. It is a question whether the numberless islands of the Malay Archipelago should be regarded as continental or oceanic, but there is no doubt that the South Sea islands scattered over a portion of the Pacific belong to the oceanic group. The ocean floor is by no means a level plain, but rises and falls in mounds, eminences and basins towards the surface. When this configuration is emphasized in any particular oceanic area, so that a peak rises above the surface, an oceanic island is produced. Submarine volcanic activity may also raise material above sea-level, or the buckling of the ocean-bed by earth movements may have a similar result. Coral islands (see Atoll) are oceanic islands, and are frequently clustered upon plateaux where the sea is of no great depth, or appear singly as the crown of some isolated peak that rises from deep water.

Island life contains many features of peculiar interest. The sea forms a barrier to some forms of life but acts as a carrier to other colonizing forms that frequently develop new features in their isolated surroundings where the struggle for existence is greater or less than before. When a sea barrier has existed for a very long time there is a marked difference between the fauna and flora even of adjacent islands. In Bali and Borneo, for example, the flora and fauna are Asiatic, while in Lombok and Celebes they are Australian, though the Bali Straits are very narrow. In Java and Sumatra, though belonging to the same group, there are marked developments of bird life, the peacock being found in Java and the Argus pheasant in Sumatra, having become too specialized to migrate. The Cocos, Keeling Islands and Christmas Island in the Indian Ocean have been colonized by few animal forms, chiefly sea-birds and insects, while they are clothed with abundant vegetation, the seeds of which have been carried by currents and by other means, but the variety of plants is by no means so great as on the mainland. Island life, therefore, is a sure indication of the origin of the island, which may be one of the remnants of a shattered or dissected continent, or may have arisen independently from the sea and become afterwards colonized by drift.