An island is a piece of land surrounded completely by water. Even though the continent of Australia looks like a big island, it is really three times the size of Greenland which is called the largest island. Some people say that originally all land was connected in one mass called Pangaea. They say that several chunks broke off or became disconnected. These became islands. Experts think that Madagascar and Greenland were formed this way. They are called continental islands.
There are six main types of islands. As mentioned above, some continental islands broke off from continents. Other continental islands were formed when the glaciers melted. At first, the sea level rose. Lands were flooded. Then the seas receded leaving patches of land surrounded by water. Continental islands may also be formed due to the erosion of the land connecting it to the mainland.
A tidal island is formed when the land connecting it to the mainland is under water at high tide. The famous island in Northwest France called Mont Saint-Michel is an example of a tidal island. The third type of island is a barrier island. It runs parallel to the coast. The ones which are on the continental shelf are made of sediment (sand, gravel, and silt). Some barrier islands are coral islands. Coral is made from the castoff outer shells of sea creatures. It is very hard.
A lagoon or sound separates the barrier islands from the mainland. These islands protect the coast from high waves and storms. They serve as a barrier. Sometimes barrier islands form when the ocean currents bring more sand to the sandbars along the coast. Eventually, these sandbars rise above the water and become barrier islands. However, the same currents that formed them may also, later on, erode the sand from these islands.
The Outer Banks off the coast of North Carolina were formed after a glacier melted, and the sea level rose, covering over some of the sandbars and creating low-lying sandy islands. Another type of barrier island was formed when the glaciers melted and left piles of rocks and gravel. These are called moraines. They became surrounded by water from the melting of the glacier and thus became islands. Nantucket, in Massachusetts and Long Island in New York are moraines.
Another type of island is the oceanic island. They are also called 'high islands.' They form when a volcano erupts in the ocean and deposits lava which eventually rises above the surface of the water. A seamount is the name of the volcano when it is below the water.
Different types of volcanoes can create islands. One type is formed when one tectonic plate moves under another. The island of Japan lies where four tectonic plates touch. Two of these plates are the lighter Eurasian plate to the west and North American plate to the north. The heavier oceanic plates (Pacific and Philippine) slide under the lighter ones. Japan has many volcanoes.
In addition, when two plates split apart a new island may be formed. The island of Surtsey near Iceland was created in this manner. Another type of oceanic island is made over a hot spot where hot material from below the crust comes up through the crust. A new Hawaiian island is still a seamount. It is called Loihi.
Coral Islands are the fifth type of island. They build up from the external skeletons of shellfish which are made of calcium carbonate. Many colonies of coral can form a reef. The Bahamas are coral islands. The sixth type of island is the artificial island created by man for one of many purposes. Land can be drained from low areas creating an island. Material can be brought in to fill in an area in a body of water.
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