Mark Rodrigue, age 15, of Coquitlam, British Columbia, Canada, for his question:
HOW ARE ISLANDS FORMED?
Greenland is the world's largest island. It has a land area of 840,004 square miles. Ranking second is the island of New Guinea with Borneo placing third. Rounding out the top 10 list, in order, are Madagascar, Sumatra, Baffin, Great Britain, Honshu, Victoria and Ellesmere. Islands vary greatly in size. The very smallest ones are called islets.
An island is a parcel of land that is smaller than a continent and surrounded by water. You'll find them in every part of the world.
There are four main kinds of islands: continental islands, volcanic islands, coral islands and barrier islands.
An area of land that at one time was connected to a continent is called a continental island. The British Isles are in this grouping since scientists say the land was most likely connected to the mainland of Europe more than 10,000 years ago.
A volcanic island is made of lava built up from the ocean floor by eruptions of oceanic volcanoes. The Aleutian Islands and those of Japan are in this grouping.
Coral islands are low, flat islands that are made up chiefly of coral reef material. Coral reefs are limestone formations composed of small sea animals and plants and their remains. Reefs form and grow in warm, shallow water.
Barrier islands are made of sand, silt and gravel, called sediment, that build up along a shoreline. Streams and rivers wear away their banks and carry sediment to the shallow waters of the seashores. Winds and waves then pile up the sand into a series of parallel ridges and dunes, forming barrier islands.
Hatteras Island in North Carolina and Padre Island in Texas are examples of some of the barrier islands found on the shores of the United States.
Some coastline islands were formed by glacial deposits. The glaciers that once covered much of the Northern Hemisphere piled up a large ridge of rocks, sand, silt and clay in front of them. Parts of this ridge, called a moraine, have been eroded or submerged, but other parts remain, forming islands. Long Island in New York and Nantucket in Massachusetts were parts of the moraine.
Animals and plants that live on an island are cut off from those elsewhere. Because of their isolation, they often evolve or gradually develop into species not found elsewhere.
Many island species of animals and birds would probably die out elsewhere. They survive on islands, however, where there are fewer animals to prey on them and compete with them for food.