The Outer Banks, along the southeastern coast of the United States, are this type of barrier island. As glaciers melted, the sea level rose around coastal sand dunes, creating low-lying, sandy islands. Other barrier islands formed during the most recent ice age. The same currents that formed these barrier islands can also destroy or erode them. Aits, or islands in rivers, form in this way. Eventually the sandbars rise above the water as islands. Some barrier islands form when ocean currents pile up sand on sandbars parallel to coastlines. They protect the coast from being directly battered by storm waves and winds. They are called barrier islands because they act as barriers between the ocean and the mainland. Barrier islands are separated from shore by a lagoon or a sound. Barrier islands can also be coral islands, made from billions of tiny coral exoskeletons. Some are a part of the continental shelf (continental islands) and made of sediment-sand, silt, and gravel. Barrier islands (3) are narrow and lie parallel to coastlines. The famous island of Mont Saint-Michel, France is an example of a tidal island. Tidal islands (2) are a type of continental island where land connecting the island to the mainland has not completely eroded, but is underwater at high tide. Continental islands may form through the weathering and erosion of a link of land that once connected an island to the mainland. Zealandia is a microcontinent off Australia that is almost completely underwater-except for the island nation of New Zealand. These are called microcontinents or continental crustal fragments. Some large continental islands are broken off the main continental shelf, but still associated with the continent. The ocean flooded many low-lying areas, creating islands such as the British Isles, which were once part of mainland Europe. As glaciers began to melt, the sea level rose.
Water was locked in glaciers, and the sea level was much lower than it is today. At the peak of the most recent glacial period, about 18,000 years ago, ice covered large parts of the Earth. Other continental islands formed because of changes in sea level. Greenland and Madagascar are these type of continental islands. When the breakup occurred, some large chunks of land split. Eventually, slow movements of the Earth’s crust broke apart Pangaea into several pieces that began to drift apart. Scientists say that millions of years ago, there was only one large continent. Some formed as Earth’s shifting continents broke apart. Continental islands (1) were once connected to a continent. Island Formation There are six major kinds of islands: continental (1), tidal (2), barrier (3), oceanic (4), coral (5), and artificial (6). Because of isolation, many islands have also been home to some of the world’s most unusual and fascinating wildlife. For centuries, islands have been stopping places for ships. On another island, Manhattan, rise the towering skyscrapers of the financial capital of the world, New York City. Tokyo, one of the world’s largest cities, is on the island of Honshu in Japan. Others are among the most crowded places on Earth. Many islands are little more than barren rock with few plants or animals on them. Other islands, such as the Greek islands known as the Cyclades in the Aegean Sea, are found in closely spaced groups called archipelagoes. Many islands, such as Easter Island in the South Pacific Ocean, are thousands of kilometers from the nearest mainland. Others, such as Tahiti, lie in warm, tropical waters. state of Alaska, are cold and ice-covered all year. Some islands, such as the Aleutian Islands in the U.S. Greenland, for example, covers an area of about 2,166,000 square kilometers (836,000 square miles). Islands in rivers are sometimes called aits or eyots. These tiny islands are often called islets.
Many islands are quite small, covering less than half a hectare (one acre).
They vary greatly in size, climate, and the kinds of organisms that inhabit them. There are countless islands in the ocean, lakes, and rivers around the world. Australia, the smallest continent, is more than three times the size of Greenland, the largest island. Continents are also surrounded by water, but because they are so big, they are not considered islands. An island is a body of land surrounded by water.
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