Vadose zone: The region in the ground between the surface and the water table in which pores are not filled with water. Also called the unsaturated zone.

Valence electron: An electron of the outermost shell of an atom; one of those most active in bonding.

Van der Waals bond: A bond much weaker than the ionic or covalent, which bonds atoms by small electrostatic attraction.

Varve: A thin layer of sediment grading upward from coarse to fine and light to dark, found in a lake bed and representing one year's deposition of glacial outwash.

Vector: A mathematical element that has a direction and magnitude, but no fixed position. Examples are force and gravity.

Ventifact: A rock that exhibits the effects of sand-blasting or "snowblasting" on its surfaces, which become fiat with sharp edges in between.

VERB:The part of speech that expresses existence, action, events and states.

Vertical exaggeration: The ratio of the horizontal scale (for example, 100,000: 1) to the vertical scale (for example, 500: 1) in an illustration.

Viscosity: A measure of resistance to flow in a liquid.

Volcanic ash: A volcanic sediment of rock fragments, usually glass, less than 4 millimeters in diameter that is formed when escaping gases force out a fine spray of magma.

Volcanic cone: The deposit of lava and pyroclastic materials that has settled close to the volcano's central vent.

Volcanic pipe: The vertical chamber along which magma and gas ascend to the surface; also, a formation of igneous rock that cooled in a pipe and remains after the erosion of the volcano.

Volcano: Any opening through the crust that has allowed magma to reach the surface, including the deposits immediately surrounding this vent.

V-shaped valley: A valley whose walls have a more-or-less uniform slope from top to bottom, usually formed by stream erosion.