Glossary - Fundamentals of the Physical Environment - page 917

Vegetation formations Units of natural vegetation occupying distinct geographical
regions of large size, and possessing a uniform physiognomy.
Velocity The rate of movement of an object or material, measured typically for rapid
environmental processes in metres per second or in slow processes in millimetres per
year.
Ventifact A loose stone shaped by wind abrasion on an arid land surface.
Vermiculite An expanding 2:1 clay mineral with isomorphous substitution in both the
silica and the alumina sheets, which makes it very reactive.
Vesicular An igneous rock texture characterized by the empty vesicles or voids vacated
by gas bubbles in the original magma .
Viscous drag A coupling of rigid lithosphere and partially melted moving asthenosphere,
once thought to be the principal mechanism of continental drift .
Vivipary Plant reproduction by the formation of bulblets on the parent.
Volcanic arc An arcuate line of explosive, andesitic island volcanoes erupted through
oceanic crust on the landward side of a B-subduction zone as a result of the hydration
and partial melt of downgoing crust; the arc may eventually migrate and weld on to
adjacent continental crust.
Volcanic breccia A pyroclastic rock formed of consolidated, angular volcanic rock
rubble.
Wadati - Benioff subduction The full title of what is more commonly known as Benioff
or B-subduction .
Water balance The volumetric input, storage and output of water which constitutes the
water budget of a drainage basin per unit time, normally a calendar year.
Water - holding capacity The amount of water held in soil after free drainage under the
influence of gravity.
Water - layer weathering Rock weathering in the intertidal zone by those processes of
slaking , hydration and salt weathering which are enhanced by regular cyclic hydration
and drying.
Watershed The delimiting boundary of a drainage basin , normally at the land surface
but taking into account any lateral underground transfers determined by geological
conditions; alternatively, the drainage basin delimited by such a boundary.
Wave An oscillatory rise and fall in a water surface, marking the horizontal transmission
of wind-driven energy through the orbital motion of water particles.
Wave base The water depth at which the orbital motion of surface waves ceases.
Wave period The time taken for consecutive wave crests to pass a fixed point.
Wave train A regular procession of water surface waves characterized by their
wavelength and wave period .
Wave - cut notch A notch or indentation cut into the base of a cliff by wave action.
Wavelength The distance between crests or troughs of adjacent waves .
Weathering The progressive alteration and eventual chemical or mechanical
disintegration of rock mass at the land surface; exposure to a different thermal and
moisture environment from that in which it formed renders it unstable and susceptible
to weathering.
Weathering rind An outer crust of discoloured and texturally altered rock representing
the initial stage in rock weathering.