Island is a body of land smaller than a continent and surrounded by water. Islands lie in oceans, rivers, and lakes throughout the world. They vary greatly in size. Greenland is the largest island in the world. It covers 836,331 square miles (2,166,086 square kilometers). Some islands cover a smaller area than a city block. A small island is called an islet. The distinction between a continent and an island is based on size. Australia is more than three times as large as Greenland. Because of its size, geographers class it as a continent.
Islands make up the entire land area of some countries, including Japan and the Philippines. Millions of people live on such islands. Other islands have no people. Some of these islands, such as Pelican Island in Florida and the Oregon Islands in Oregon, are wildlife refuges.
Throughout history, islands have served as stopping points on migration and trade routes and as stations for refueling and supplying ships. Islands have thus aided the spread of people, animals, and plants from one continent to another.
Some islands were formed hundreds of millions of years ago. But new ones are forming continually. For example, Iceland was formed millions of years ago by an oceanic volcano. In 1963, Surtsey, a new volcanic island, appeared near the coast of Iceland.
A broad expanse of sea with a large number of islands is called an archipelago. In most cases, all the islands of an archipelago are formed by the same process. For example, the Galapagos Islands were formed by volcanoes that built up from the ocean floor.
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Kinds of islands.
There are five main kinds of islands: (1) continental islands, (2) tectonically formed islands, (3) volcanic islands, (4) coral islands, and (5) barrier islands. Each kind is formed in a different way. Some islands are formed by a combination of processes.
Continental islands
are areas of land that were once connected to a continent. Some of these islands were isolated from the mainland as a result of a rise in sea level. For example, the islands of Great Britain and Ireland were connected to the mainland of Europe more than 10,000 years ago, when glaciers covered parts of the Northern Hemisphere. As the glaciers began to melt, they made the sea level rise. As a result, water covered the land that had connected the islands of Great Britain and Ireland to the mainland.
Other continental islands resulted from the erosion of a former link with the mainland. Streams, rivers, and ocean waves may, over thousands of years, wear away land that had connected an island to a continent.
Tectonically formed islands
are created by movements of the huge rigid plates that make up the earth’s rocky outer shell. The plates are in very slow, but constant, motion. When one plate is pushed under another plate, the top plate may scrape off pieces of the bottom plate. Over millions of years, this material piles up to form an island. The Caribbean island of Barbados and Kodiak Island near Alaska were formed this way.
Movements of Earth’s tectonic plates also cause land masses to drift apart. Scientists believe that the present continents are parts of what was once a single large continent. This land mass broke up about 200 million years ago, and ocean basins formed between the fragments. The smaller land fragments thus became tectonically formed islands. Greenland and Madagascar were formed this way. The tectonic plates are still slowly moving, and so new islands will continue to be formed. In several million years, southwestern California will probably have moved to the northwest to form an island off the west coast of North America. See Plate tectonics.
Volcanic islands
consist of lava and ash built up from the ocean floor by eruptions of oceanic volcanoes. These islands include the Hawaiian Islands. Some volcanic islands, such as the Aleutian Islands and those of Japan, form island arcs. Island arcs are narrow, curving chains of volcanic islands that form along the border of deep trenches in the ocean floor. The Lesser Antilles, an island arc in the West Indies, were built up by volcanic activity about 50 million years ago.
Coral islands
are low, flat islands that consist chiefly of coral reef material. Coral reefs are limestone formations composed of tiny sea organisms and their remains. They form and grow in warm, shallow water.
Many coral islands develop from reefs that grow up around volcanic islands. Some volcanic islands sink because of the movement of oceanic plates. Others become submerged following a rise in the sea level. As islands sink or as the sea level rises, the reefs grow upward until only atolls remain. An atoll is a circular reef that surrounds a central body of water called a lagoon. Ocean waves break away parts of the reefs and pile up this material into flat, sandy islands. The Pacific Ocean has many atolls, including Bikini Atoll and Enewetak. See Atoll ; Coral reef .
Barrier islands
consist of sediment (sand, silt, and gravel) that builds up along a shoreline. Streams and rivers wear away their banks and carry sediment to the shallow waters of seashores. Ocean waves and winds pile up the sand into a series of ridges and dunes parallel to the shoreline, forming barrier islands. Many barrier islands lie along the gently sloping Atlantic coast of the United States and the shores of the Gulf of Mexico. They include Hatteras Island in North Carolina and Padre Island in Texas.
Some barrier islands that lie along coastlines are formed by glacial deposits. The glaciers that once covered much of the Northern Hemisphere piled up large ridges of rocks, sand, silt, and clay in front of them in what are now coastal areas. Parts of these ridges, called moraines, have been eroded or submerged, but other parts remain, forming barrier islands. Long Island in New York and Nantucket in Massachusetts were formed this way.
How life develops on islands.
Continental islands originally had a plant and animal community like that of the continent from which they separated. Volcanic, coral, and barrier islands originally have no land animals or plants. These lands become inhabited by birds that fly across the sea and by other animals that swim to the islands. Some animals and insects may be carried to islands on logs or other debris. Plant seeds may float across the sea or be carried by birds or the wind.
The animals and plants that live on an island are cut off from those elsewhere. Because of their isolation, these animals and plants may evolve (gradually develop) into species not found on the mainlands. For example, the giant tortoises of the Galapagos Islands evolved from ancestors that were much smaller. Some sunflower plants that grow on the Galapagos Islands have developed into tall trees.
Many island animals evolve into numerous species, each of which lives in a very particular habitat. For example, the finch, a songbird, has developed into more than 10 species on the Galapagos Islands. Some of these species live chiefly in the trees and feed on insects. Other types of finches live mainly on the ground and eat seeds and fruits.
Many kinds of island animals would probably not survive elsewhere. But on islands, there are fewer animals to prey on them and to compete with them for food.
