Volcanoes may erupt without warning, after centuries of dormancy.
Although volcanoes vary in shape and level of activity, all share the same basic components. Volcanoes start out small and increase their mass through multiple eruptions that add lava, or molten rock, to their surface areas. Over time, these eruptions result in volcanic mountains. If a volcano is very active, it can grow rapidly and result in a steep summit.
Inside the Volcano
Volcanoes may form when fissures, or cracks in the earth, create openings through which hot, molten rock, or magma, escapes. Magma contains hot gasses, rock and other materials. The magma chamber that holds this liquid rock may lie deep beneath the earth.
When pressure builds, magma enters a conduit, or natural pipeline within the fissure. It rises through the conduit and erupts through a vent at the surface of the volcano. Once exposed, magma becomes lava that eventually cools and reforms into solid rock.
The cone at the summit, or top of the volcano, consists of hardened lava from previous eruptions. It may harbor a caldera, a large depression that forms when upper rock layers of the cone collapse, or a crater, which occurs when rocks explode upward from below.
Dikes and Sills
When magma pushes upward through conduits that stop short of the surface, it intrudes into the rock layers that lie between the magma chamber and the main vent. There, it forms dikes or sills. While the magma in a sill runs parallel to volcanic rock layers, dikes cut through these layers at their weakest points and form new fissures. After the sills and dikes cool, they harden to create a new rock layer.
Magma and Lava
Magma comes in different forms and contains various elements that determine the type of eruption that will occur. Mafic magma has a high magnesium and iron content as well as low viscosity. Felsic magma contains large amounts of silicon and is very acidic and viscous. Intermediate magma, as the name implies, lies between the two extremes in terms of acidity and viscosity. Felsic magma tends to produce cataclysmic explosions.
Lava cannot flow if the magma that produced it is too viscous. Instead, it piles up into lava domes surrounding the vent. Although these domes do not usually form craters, they may create parasitic domes on the flanks, or sides, of volcanoes.
Other Parts of a Volcano
Other components include the flank vents, or secondary vents, on a volcano's flank. These may contain conduits where lava flows.
Ash clouds consist of fine particles of ash, chemicals and dust. They plume upward from the volcano and enter the atmosphere, often traveling hundreds or even thousands of miles.
Lahars, or mudflows, consist of rock debris and water that moves down the slope of a volcano. Destructive lahars may occur whenever massive eruptions take place.
Tags: rock layers, Felsic magma, magma chamber, molten rock, upward from
