Tony Esposito, age 14, of Henderson, Nev., for his question:
HOW MANY KINDS OF ISLANDS ARE THERE?
An island is an area of land that is smaller than a continent and is completely surrounded by water. Islands can be found in all parts of the world in oceans, lakes and even rivers. There are four kinds of islands: continental islands, volcanic islands, coral islands and barrier islands.
Continental islands are those that were once connected to a continent. The British Isles, the scientists say, were once connected to the mainland of Europe. Then, more than 10,000 years ago, glaciers started to melt and they caused the level of the sea to rise. In time, water flooded the land that at one time connected Britain to Europe.
Volcanic islands are made of lava built up from the ocean floor. Japan and the Aleutian Islands are volcanic islands that formed along the border of deep trenches in the ocean floor.
Coral islands are made up of limestone formations composed of tiny sea animals and plants. Reefs form and grow in warm, shallow water.
Barrier islands are made up of sand, silt and gravel that builds up along a shoreline. Lots of them can be found along the Southeastern coast of the United States and along the Gulf of Mexico. Two famous barrier islands are Hatteras Island in North Carolina and Padre Island in Texas.
Ocean waves and winds pile up sand into a series of parallel ridges and dunes on many of the barrier islands.
Japan and the Philippines are countries with land areas completely made of islands.
Scientists tell us that the island of Iceland was formed millions of years ago by oceanic volcanoes. An island near the coast of Iceland, called Surtsey, was formed as recently as 1963 by volcanic action. Additional islands could be made at any time in the future.
Some islands were formed by glacial deposits. Northern Hemisphere glaciers piled up a large ridge of rocks, sand and silt 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 are islands that were once parts of moraines, formed as moving glaciers piled up ridges of earth and rock.
Australia isn't called an island because it is large enough to be classified as a continent.
The world's largest island is Greenland, which has an area of 840,000 square miles. Next in order of size are New Guinea, Borneo, Madagascar, Sumatra and Baffin.
Ranking as the world's seventh largest island is Great Britain, with its 88,770 square mile area, and eighth place honors go to the 87,805 square mile Japanese island of Honshu.