A volcano is a place where molten rock seeps or erupts from beneath the crust. Most volcanoes develop in the oceans, but some occur on land. When the molten rock, which is called lava, erupts with force, large, cone-shaped mountains form, which are also called volcanoes.
There are four types of volcanoes. The most common type develops under water in places where pieces of the earth's crust are moving apart. The moving plates of crust cause the lower layers of rock to melt, and then the molten rock flows through the fissure as lava. The lava cools, forming pillow-like formations on the ocean's floor.
The second type also occurs underwater when a piece of the earth's crust slides underneath another piece. The sliding layers force water into the deeper layers of the earth and result in molten rock erupting from the pressure. This type of eruption forms arc volcanoes that often build up into islands.
The third type of volcano arises where plates of the earth's crust do not meet. Scientists are not sure what causes this type of volcano. One theory is that pressure deep in the earth's mantle creates plumes of molten rock that eventually erupt.
The last type of volcano is the flood volcano. Flood volcanoes occur when large volumes of lava seep out of the earth onto land masses, flooding large areas. This type of volcano has not developed in recent history.