An island is a landform that is surrounded on all sides by water. A variety of geological processes can create islands, such as volcanic eruptions, the growth of coral reefs and erosion.
Most islands are formed by the cooling lava that results from volcanic eruptions. This lava forms a rocky, above-water surface. Volcanic islands can be found in oceans, seas, lakes and rivers.
Tectonic plates can also cause islands to form. These plates are constantly pushed and pulled by convection currents in Earth's mantle, causing phenomena such as earthquakes and continental drift. Occasionally, a plate will subduct (sink) underneath another plate. As the plate sinks, it causes a deep trench to develop along its edge. The lighter continental plate may ride over the subducting plate. This can lead to a chain of volcanoes called an island arc, such as those in the Pacific Ocean.
Other times, a continental plate may break apart from the rest of the continent and become an island. This is the case with Greenland and Madagascar. Continental islands can also be created by the erosion of a link of land that once connected the island to the mainland. The famous tidal island of Mont Saint-Michel in France is a good example.
Finally, islands can be formed when a continent shifts over a hot spot. This is the process that formed Hawaii, for example. Hawaii currently has two active volcanoes, Mauna Loa and Kilauea, that sit over a hot spot. As the Earth's tectonic plates continue to move, it's likely that another island will form in Hawaii's place.