Islands

Technically, an island is a body of land completely surrounded by water. Islands are distinguished from a continent by their smaller size. By definition, at least part of an island must remain above water, even at high tide. However, unusual landmasses called tidal islands become peninsulas during low tide.

Types of Islands

Islands range in size from tiny ledges of rock peeping above the surface of the sea to enormous landmasses measuring thousands of square miles in area. They fall into several groups. Continental islands are bodies of land that were formerly joined to continental areas. Island arcs are groups of islands strung out in long, curving chains off the coasts of a continent. Oceanic islands—found far from the shores of any continent—have been built up from the ocean bottom by natural forces, such as volcanoes, or by reef-building animals.

Some islands are formed of beach material separated from the main shoreline by a shallow lagoon. These types of islands, called barrier bars, act as buffers between the shore and powerful ocean waves. A large stretch of barrier bars extends from the southern shore of Long Island, New York, to the Florida Keys.

Glacial deposits also contribute to the formation of islands. As ice sheets move across continental areas, the sheets can carry large amounts of loose, rocky material to the coast. This material is deposited as the ice sheet recedes, and the deposits can form islands in shallow-water areas.

Continental Islands

Continental islands represent the unsubmerged portions of a continental shelf. Some are cut off from the mainland by the erosive force of waves, or are formed when those waves deposit sediment offshore. Others come into being with the sinking of coastal highlands. Only the tops of these highlands now remain above water. Still other islands represent portions of a growing delta at the mouth of a river, formed by sediment carried downstream.

In some cases, nothing but a shallow channel lies between a continent and an adjacent island. The English Channel, for example, is so shallow that Great Britain would rejoin the European mainland if sea levels were to fall roughly 300 feet (90 meters), or if the seabed were to rise by the same amount. A deeper channel of ancient origin lies between Africa and the continental island of Madagascar in the Indian Ocean. It likely represents the drifting apart of Madagascar and Africa on separate tectonic plates.

There are a great many examples of continental islands. Long Island, Block Island, Martha's Vineyard, and Nantucket, all lying off the Atlantic coast, were once attached to North America. The European mainland once included a number of land areas that have since become islands, including Sardinia, Corsica, Novaya Zemlya, Ireland, and Great Britain.

Island Arcs

Island arcs are curving chains of islands that extend from the coasts of continental areas. Among them are some of the world's largest island groups—the Indonesian Archipelago, Japan, the Aleutians, and the West Indies.

Each island arc has an outward-curving side facing the open sea, and an inner, concave side fronting the nearest continent. The outer side is often mountainous and is typically flanked by deep oceanic trenches; the inner side is frequently lined with volcanoes, some of which are devastatingly active.

Island arcs lie over zones of instability in Earth's crust. It is believed that the arcs form when two tectonic plates (separate pieces of Earth's crust) move closer together. This movement causes the ocean crust between the plates to buckle, creating a trench with a bordering arc of volcanoes. As the volcanoes breach the ocean surface, they form the island arc.