What are volcanoes?

A volcano is :
  1. An opening in the Earth's crust from which lava, ash, and hot gases flow or are ejected during an eruption.
  2. A usually cone-shaped mountain formed by the materials issuing from such an opening.

Volcanoes are surficial manifestations of thermal processes occurring inside the Earth. They are basically vents out of which solid, liquid or gaseous material is spewed.
Solid or semi-solid pyroclastic material includes blocks, bombs, lapilli and ash. Liquid material is usually viscous molten rock, or lava. Superheated gases that are ejected can mix with suspended particles to form a fast-moving "glowing cloud" or nuée ardente.

Types of Volcanic Structures

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.

A volcano is an opening in the crust of the Earth through which molten rock and gases from the interior of the Earth reach the surface. The definition of volcano can include the mountain like structures that may be created from the material ejected at the volcano. Volcanoes can be created by the melting of rock at divergent and convergent plate margins, and from decompression melting at hot spots in the mantle, such as the one under the Hawaiian Islands.
A volcano is an opening, or rupture, in a planet's surface or crust, which allows hot, molten rock, ash, and gases to escape from below the surface. Volcanic activity involving the extrusion of rock tends to form mountains or features like mountains over a period of time. Volcanoes are generally found where tectonic plates are diverging or converging. A mid-oceanic ridge, for example the Mid-Atlantic Ridge, has examples of volcanoes caused by "divergent tectonic plates" pulling apart; the Pacific Ring of Fire has examples of volcanoes caused by "convergent tectonic plates" coming together. By contrast, volcanoes are usually not created where two tectonic plates slide past one another. Volcanoes can also form where there is stretching and thinning of the Earth's crust (called "non-hotspot intraplate volcanism"), such as in the African Rift Valley, the Wells Gray-Clearwater volcanic field and the Rio Grande Rift in North America and the European Rhine Graben with its Eifel volcanoes. Volcanoes can be caused by "mantle plumes". These so-called "hotspots" , for example at Hawaii, can occur far from plate boundaries. Hotspot volcanoes are also found elsewhere in the solar system, especially on rocky planets and moons.