Continental Islands were once connected to a continent.

Scientists believe that millions of years ago there was only one large continent. Eventually, slow movements of the earth's crust broke the giant continent into several pieces that began to drift apart. When the breakup occurred, some large chunks of land split off along the lines of separation. These fragments of land became islands. Greenland and Madagascar are examples of continental islands that formed in this way.

Other continental islands formed because of changes in sea level. At the peak of the most recent ice age, about 18,000 years ago, ice covered large parts of the earth. Water was locked in glaciers, and the sea level was much lower than it is today. As the glaciers began to melt, the sea level rose. The ocean flooded many low-lying areas, creating islands such as the British Isles, which were once part of the mainland of Europe.

Right: Fair Isle, off the coast of Scotland, was isolated from the rest of Europe when glaciers melted.

Continental islands may also form through the weathering and erosion of a link of land that once connected the island to the mainland. Ocean waves or water from a river may cut across the link and wash it away. The island of Trinidad, for example, was separated from South America when the Orinoco River cut through its link to the mainland.

Oceanic Islands are formed by eruptions of volcanoes on the ocean floor. As the volcanoes erupt, they build up layers of lava that may eventually break the water's surface. When the tops of the volcanoes appear above the water, they form islands such as those of Hawaii.

Left: The oceanic island of Kauai, Hawaii, was built up from the seafloor by volcanic eruptions.

Coral Islands are low islands formed in warm waters by tiny sea animals called corals. Corals build up hard external skeletons, and over time these skeletons form huge reefs of limestone. Some coral reefs may grow up from a plateau on the seafloor until they break the water's surface, forming low islands. The Bahama Islands formed in this way.

Another kind of coral island is the atoll, which is a coral reef that begins by growing in a ring around the sides of a volcanic island. As the volcanic island slowly subsides, or sinks, into the ocean floor, the reef continues to grow. Later, parts of the circular reef may be uplifted and rise above the water's surface as coral islands or islets. Most of the islands in the South Pacific are coral atolls, as are many of the islands in the Indonesia Archipelago.

Right: The world's largest archipelago, Indonesia, consists of some 13,000 separate islands, including this densely populated one.

Barrier Islands are narrow and lie parallel to coastlines. Made up of sediment, these islands are separated from the shore by a lagoon or a sound. They are called barrier islands because they have sand dunes that act as barriers between the ocean and the mainland. The dunes protect the coast from being directly battered by storm waves and winds.

Some barrier islands form when ocean currents pile up sand in sandbars parallel to coastlines. Eventually the sandbars may rise above the water as islands. The islands off the Atlantic coast of southern Florida were formed in this way.

Left: Sandy Hook stretches out from the New Jersey coast. People have built walls along the beach to catch and hold drifting sand.

Other barrier islands formed during the most recent ice age. As glaciers melted, the sea level rose around lines of coastal sand dunes, creating low, sandy islands. Such islands are common along the southeastern coast of the United States.

Some barrier islands were formed of materials deposited by the ice age glaciers. When the glaciers melted, they left piles of the rock, soil, and gravel they had carried. As flooding occurred along coasts after the glaciers melted, these piles were surrounded by water. Long Island, in New York, and Nantucket, off the coast of Massachusetts, are both formed of glacial moraines.

Questions or comments about this page?

Exploring Your World: The Adventure of Geography. Washington, D.C.:National Geographic Society, 1993.

Greenland
Glaciers

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Geography. Anthropology. Recreation.--Physical Geography.--Islands.

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