Information Resource: Hundred Thousand Whys (With Adaption)

       Small pieces of lands surrounded by water are called islands. Islands can be divided into four categories according to their formation, namely continental, volcanic, coral and alluvial islands. The continental islands are formed by the separation of part of the land from the mainland due to the uplift of the earth’s crust, the subsidence of the land or the rise of the sea surface and the intrusion of seawater.
       The volcanic islands are formed by the long-lasting eruption of submarine volcanoes and the gradual accumulation of magma, which finally emerged from the water surface. The coral islands are formed by accumulation of the remains of polyps and other shell-covered animals in tropical and subtropical oceans, mainly distributed in the South Pacific and Indian Ocean. There are three types of coral reefs—fringing reefs, barrier reefs and atolls. And the alluvial islands, ordinarily distributed in the outlet of great rivers or on the outer shores of plain coasts, are new lands formed by accumulation of river sediment or ocean currents.