V

VALLEY (STREAM): The entire area between the top of the slopes on either side of a stream.

VALLEY GLACIER: A glacier that is smaller than a continental glacier or an icecap and that flows mainly along well-defined valleys in mountainous regions.

VARVE: A thin pair of sedimentary layers grading upward from coarse to fine and light to dark, found in a glacial lake and representing one year's deposition.

VEIN: A deposit of foreign minerals within a rock fracture or joint.

VENTIFACT: A rock that exhibits the effects of sandblasting or "snowblasting" on its surfaces, which become flat with sharp edges in between.

VISCOSITY: A measure of a liquid's resistance to flow.

VOLCANIC ASH: A volcanic sediment of rock fragments, usually glass, less than 2 mm in diameter, that is formed when escaping gases force out a fine spray of magma.

VOLCANIC ASH-FLOW DEPOSIT: A layer of volcanic ash and debris deposited during a pyroclastic flow.

VOLCANIC BRECCIA: A pyroclastic rock in which all fragments are more than 2 mm in diameter.

VOLCANIC DOME: A rounded accumulation around a volcanic vent of congealed lava too viscous to flow away quickly; hence usually rhyolite lava.

VOLCANIC TUFF: A consolidated rock composed of pyroclastic rock fragments and fine volcanic ash welded together by their own heat.

VOLCANISM: The processes that form volcanoes; the progress of magma as it rises up through the crust, emerges onto the surface as lava, and solidifies into volcanic rocks and landforms.

VOLCANO: A hill or mountain that forms from the accumulation of matter that erupts at the surface.