|Creation and extinction of an oceanic island. First, a volcano emerges from water, forming an island. Second, the island makes a platform which increases the diameter of the island. Third, the shore rocks crumble to form soil. Last, the island disappears leaving a barrier reef of coral, or atoll. Figure source: http://www.panda.org/kids/wildlife/mnisland.htm|
Volcanic islands, such as Hawaii and the Galapagos are composed of oceanic island basalt (OIB) formed over a mantle plume by material erupted from inside the earth. These islands are usually characterized by forming in the middle of a tectonic plate, instead of forming along plate boundaries. The origin of oceanic islands is theorized to be from deep within the mantle, due to their small, but significant levels of some elements, such as, sodium, potassium, and aluminum that is depleted elsewhere (Chernicoff, 1995). Another kind of basalt, called mid-ocean ridge basalt (MORB), is erupted at great depths on the bottom of the ocean floor.