Figure 1. A tropical island.
Figure 2. Santa Rosa Island, Florida.
Figure 3. A island arc.
An island is a comparatively small land area entirely surrounded by water. It may be a result of the buildup of the cone of a submarine volcano, erosion by the sea or glaciers of a parts of coastal regions, diastrophism, or other processes.
A barrier island is any long, low island of sand parallel to a shore and permanently separated from it (see Figure 2). A barrier island may be composed of dunes, swamps, and areas of vegetation.
An island arc (see Figure 3) is a curving chain of volcanoes to the convex side of which is a deep ocean trench. The volcanoes rest on the plate of lithosphere that is not moving down into the Earth. Their andesitic lavas may be formed from the partially melted material of the descending plate. Northern Japan and the Aleutian Islands are examples. Island arcs are often associated with earthquake activity.