andesite: A dark-colored, fine-grained extrusive rock that, when porphyritic, contains phenocrysts composed primarily of zoned sodic plagioclase (esp. andesine) and one or more of the mafic minerals (e.g. biotite, hornblende, pyroxene), with a groundmass composed generally of the same minerals as the phenocrysts; the extrusive equivalent of diorite. Andesite grades into latite with increasing alkali feldspar content, and into dacite with more alkali feldspar and quartz. It was named by Bush in 1826 from the Andes Mountains.
ashfall: 1. A rain of airborne volcanic ash falling froman eruption cloud. 2. A deposit of volcanic ash resulting from such a fall and lying on the ground surface.
ashflow: A density current, generally a highly heated mixture of volcanic gases and ash, traveling down the flanks of a volcano or along the surface of the ground; it is produced by the explosive emission of gas-charged ash from a fissure or group of fissures. The solid materials contained in a typical ash flow are unsorted and ordinarily include pumice, scoria, and blocks in addition to ash.
caldera: A large basin-shaped volcanic depression, more or less circular, the diameter of which is many times greater than that of the included vent or vents, irrespective of steepness of the walls or form of the floor.
crater: 1. A basinlike, rimmed structure at the top or on the flanks of a volcanic cone; it is formed by explosion or collapse. 2. A saucer-shaped pit or depression on the earth's surface, resulting from impact or explosion.
dacite: A fine-grained extrusive rock with the same general composition as andesite but having a lass calcic plagioclase and more quartz; according to many, it is the extrusive equivalent of granodiorite.
fumarole: A hole or vent from which volcanic fumes or vapors issue.
ignimbrite: The rock formed by the widespread deposition and consolidation of ash flows and nuees ardentes. The term includes welded tuff and nonwelded but recrystallized ash flows.
plug dome: A volcanic dome characterized by by an upheaved, consolidated mass filling the conduit.
pumice: A light-colored cellular glassy rock commonly having the composition of rhyolite. It is often sufficiently buoyant to float on water and is economically useful as a lightweight aggregate and as an abrasive.
rhyolite: A group of extrusive igneous rocks, typically porphyritic and commonly exhibiting flow texture, with phenocrysts of quartz and alkali feldspar in a glassy to cryptocrystalline groundmass; also, any rock in that group; the extrusive equivalent of granite.
silicic: Said of a silica-rich igneous rock or magma. Although there is no firm agreement among petrologists, the amount of silica is usually said to constitute at least 65 percent of the rock. In addition to the combined silica in feldspars, silicic rocks generally contain free silica in the form of quartz. Granite and rhyolite are typical silicic rocks.
