Islands differ from continents in a way that they are limited in size and surrounded by water. The world’s largest island is the Greenland. Small islands are referred to as islets and a group of islands or islets are called archipelago.
The islands of the world fall into two basic categories – continental islands and oceanic islands. Great Britain and Japan are continental islands. They are fragments that have been separated from a continental mainland by such long term events as changing sea levels and movements of the earth’s crust.
Oceanic islands rise directly from the ocean floor. There are two kinds of oceanic islands – volcanic islands and coral islands. Iceland and Tahiti are the result of volcanic activity. Living organisms can also play a role. Coral forms rim of many volcanic islands and sometimes grows to form islands such as the Bahamas. Newborn volcanic islands are devoid of life and even soil. Seabirds and seals can use them for breeding and resting since they get food from sea. Other animals wait for the arrival of plants the first being the mosses, ferns and other plants that reproduce by spores.
Island life becomes so distinctive that it seems very different compared to the life in continents. Flightlessness is common among birds (kiwi of New Zealand, cormorant of Galapagos) because in the absence of predators they don’t have to make quick escapes. Even some insects do not have wings at all. Giantism is another tendency (Giant tortoise in the Indian Ocean). Plants like sunflower to cacti may grow to tree size.
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