volcanoes

1. An opening in the earth's crust through which molten lava, ash, and gases are ejected.

2. A similar opening on the surface of another planet.

A mountain formed by the materials ejected from a volcano.

An opening in the Earth's crust from which lava, ash, and hot gases flow or are ejected during an eruption.

A usually cone-shaped mountain formed by the materials issuing from such an opening. Volcanoes are usually associated with plate boundaries but can also occur within the interior areas of a tectonic plate. Their shape is directly related to the type of magma that flows from them—the more viscous the magma, the steeper the sides of the volcano. ◇ A volcano composed of gently sloping sheets of basaltic lava from successive volcanic eruptions is called a shield volcano. The lava flows associated with shield volcanos, such as Mauna Loa, on Hawaii, are very fluid. ◇ A volcano composed of steep, alternating layers of lava and pyroclastic materials, including ash, is called a stratovolcano. Stratovolcanos are associated with relatively viscous lava and with explosive eruptions. They are the most common form of large continental volcanos. Mount Vesuvius, Mount Fuji, and Mount St. Helens are stratovolcanos. Also called composite volcano. See more at hot spot, island arc, tectonic boundary, volcanic arc.