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as flow it flows in a direction and manner dictated by the lay of the land and underlying bedrock batholith A mass of rock formed when a large body of magma cools inside the crust caldera The large hole at the top of a volcano formed when the roof of a volcano's magma chamber collapses. cinder cone a steep, cone-shaped hill or small mountain made of volcanic ash, cinders, and bombs piled up around a volcano's opening columnar joint A regularly spaced crack that commonly develops in lava flows forming five or six sided columns composite cone A volcano composed of both lava flows and pyroclastic material. concordant A term used to describe intrusive igneous masses that form parallel to the bedding of the surrounding rock conduit a passage (a pipe or tunnel) through which water or electric wires can pass continental volcanic arc mountains formed by igneous activity associated with the subduction of oceanic lithosphere beneath a continent crater a bowl-shaped area that forms around a volcano's central opening dike a slab of volcanic rock formed when magma forces itself across rock layers discordant not in agreement or harmony eruption columns jets of hot ash-laden gases that evolve into bouyant plumes that extend thousands of meters into the atmosphere fissure a long narrow depression in a surface fissure eruption the eruption of magma out of a crack in the lithosphere, rather than from a single pipe or vent flood basalt flow of basaltic lava that issue from numerous cracks or fissures and commonly cover extensive areas to thicknesses of hundreds of meters fumarole A volcanic vent emitting only gas and steam hot spot an area where magma from deep within the mantle melts through the crust above it intraplate volcanism igneous activity that occurs within a tectonic plate away from plate boundaries intrusions magma that has pushed into cracks in existing rocks island arc A string of islands formed by the volcanoes along a deep ocean trench laccolith when magma pushes up the earth's surface and cools forming a dome lahar an avalanche of volcanic water and mud down the slopes of a volcano lava tube tunnel in hardened lava that acts as a horizontal conduit for lava flowing from a volcanic vent. lava tubes allow fluid lavas to advance great distances mantle plume A buoyant mass of hot rock rising through Earth's mantle. As it nears the surface of Earth, some of the plume melts and erupts at the surface forming a "hot spot." massive consisting of great mass nuee ardente The most dangerous pyroclastic flow, incandescent volcano debris float up and then fall down in avalanche form, can move at 125 miles per hour, suspended by gases pahoehoe flow a lava flow with a smooth-to-ropy surface parasitic cone a volcanic cone that forms on the flank of a larger volcano pipe a long tube through which magma moves from the magma chamber to Earth's surface pluton large mass of intrusive igneous rock believed to have solidified deep within the earth pyroclastic materials the volcanic rock ejected during an eruption including ash, bombs, and blocks scoria cone a rather small volcano built primarily of pyroclastics ejected from a single vent with hardened lava shield volcano A low, flat, gently sloping volcano built from many flows of fluid, low-viscosity basaltic lava sill (geology) a flat (usually horizontal) mass of igneous rock between two layers of older sedimentary rock stocks ... stratovolcano A concave-shaped volcano containing alternating layers of lava flows and beds of pyroclasts. tabular Describing a feature such as an igneous pluton having two dimensions that are much longer than the third vent a fissure in the earth's crust (or in the surface of some other planet) through which molten lava and gases erupt viscosity resistance of a liquid to sheer forces (and hence to flow) volatiles gaseous components of magma dissolved in the melt. They will readily vaporize at surface pressures volcanic island arc a chain of volcanic islands generally located a few hundred kilometers from a trench where there is active subduction of one oceanic plate beneath another volcanic neck solid igneous core of a volcano left behind after the softer cone has been eroded volcano a fissure in the earth's crust (or in the surface of some other planet) through which molten lava and gases erupt