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vacuum	Space that contains very little matter in a given volume (e.g., a region in which air has been removed).
valley	A trough with sloping walls, cut into the land by a stream.
valley glacier	A river of ice that flows down a mountain valley.
Van Allen radiation belts	Belts of solar wind particles and cosmic rays that surround the Earth, trapped by Earth’s magnetic field.
varve	A pair of thin layers of glacial lake-bed sediment, one consisting of silt brought in during the spring floods, and the other of clay deposited during the winter when the lake’s surface freezes over and the water is still.
vascular plant	A plant with woody tissue and seeds and veins for transporting water and food.
vein	A seam of minerals that forms when dissolved ions carried by water solutions precipitate in cracks.
vein deposit	A hydrothermal deposit in which the ore minerals occur in veins that fill cracks in preexisting rocks.
velocity-versus-depth curve	A graph that shows the variation in the velocity of seismic waves with increasing depth in the Earth.
ventifact (faceted rock)	A desert rock whose surface has been faceted by the wind.
vesicles	Open holes in igneous rock formed by the preservation of bubbles in magma as the magma cools into solid rock.
viscosity	The resistance of material to flow.
volatiles	Elements or compounds such as H<sub>2</sub>O and CO<sub>2</sub> that evaporate easily and can exist in gaseous forms at the Earth’s surface.
volatility	A specification of the ease with which a material evaporates.
volcanic arc	A curving chain of active volcanoes formed adjacent to a convergent plate boundary.
volcanic ash	Tiny glass shards formed when a fine spray of exploded lava freezes instantly upon contact with the atmosphere.
volcanic bomb	A large piece of pyroclastic debris thrown into the atmosphere during a volcanic eruption.
volcanic gas	Elements or compounds that bubble out of magma or lava in gaseous form.
volcanic island arc	The volcanic island chain that forms on the edge of the overriding plate where one oceanic plate subducts beneath another oceanic plate.
volcanic-danger-assessment map	A map delineating areas that lie in the path of potential lava flows, lahars, debris flows, or pyroclastic flows of an active volcano.
volcano	(1) A vent from which melt from inside the Earth spews out onto the planet’s surface; (2) a mountain formed by the accumulation of extrusive volcanic rock.
V-shaped valley	A valley whose cross-sectional shape resembles the shape of a V; the valley probably has a river running down the point of the V.