Geoscience Reference
In-Depth Information
Transform fault
A large-scale fault between or within
crustal plates, with displacement wholly or mostly in the
horizontal plane (see
strike slip
).
Transform margin
A plate margin which coincides with
and is guided by a
transform fault
.
Transgression
The submergence of low-lying coastal
land by rising sea level or land subsidence and
consequential landward shifts in each component of the
littoral zone.
Transitional soils
An early term for intrazonal soils, i.e.
soils whose profiles are controlled by local factors of
geology and topography.
Translation
The movement of a shallow rock or soil
mass along a failure surface or
planar discontinuity
.
Transmission pore
Pores in soil larger than 0ยท05 mm
diameter which allow water to drain away under the force
of gravity.
Transport(ation)
A process or stage in the rock cycle
whereby all forms of rock and organic debris are carried
by moving water, ice or air from their point of origin to
a point of temporary or permanent deposition.
Transported soil
A soil whose parent material is a
transported sediment.
Trench
A narrow, linear and deep depression in the sea
bed caused by the subduction of oceanic crust at a
destructive plate margin.
Trench suction force
The force which draws some of
the upper plate down into a
subduction zone
with the
descending plate.
Trimline
An abrupt line on a valley side separating
weathered, vegetated upper slopes from newly eroded,
unvegetated lower slopes and marking the upper limit of
a contemporary or recent valley glacier.
Triple junction
A three-way rift developed at the
common divergent margins of three adjacent crustal
plates.
Trophic level
The energy or feeding level of a particular
organism in a food chain or food web.
Trophic relationships
The food web or feeding links in
a community or
ecosystem
.
Troposphere
Lowest layer of atmosphere in which all
weather actually takes place. Deeper in tropics than in
polar latitudes.
Trough
A narrow, deep and steep-sided rock-walled
valley typical of intense erosion by a valley or
outlet glacier
.
Tsunami
A seismic ocean surface wave, triggered at sea
by
seismovolcanic activity
, which rises in elevation in
shallow coastal waters and is capable of
Turbidity current
A turbulent, gravity-induced density
current of suspended sediment, usually in a marine or
lacustrine environment,
which eventually forms a
turbidite
.
Turbulent flow
Fluid motion in which individual flow
strands are confused and multidirectional, with eddies
developing within the general forward motion.
Ubac
A mountain slope whose orientation shades it
from the sun.
Ultramafic
Said of igneous rock crystallized at high
temperature and composed mostly of magnesium-iron
(mafic) minerals.
Unconformity
A contact between two rock units
indicative of a break in a continuous sequence of rock
formation;
it may mark an episode of
inactivity,
weathering or erosion.
Uniformitarianism
The principle, or 'law', attributed to
James Hutton (1795), which asserts that former processes
and characteristics of Earth's surface can be determined
from those currently in operation, and which we can
observe; simplified as 'the present is the key to the past'.
Unit hydrograph
The model plot of river discharge over
time generated in a particular catchment by a specified
unit of precipitation.
Upwelling
The rise of cold water to an ocean surface
induced by divergent surface currents.
Vadose water
Water in the unsaturated zone of an
aquifer
.
Valley train
A valley-wide, braided glacial meltwater
stream and sediment system emerging from a glacier
terminus.
Vapour pressure deficit
The extent of the vapour pres-
sure gradient between subsurface soil and a dry atmos-
phere.
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 iso-
morphous 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
.
inflicting
considerable shoreline damage.
Turbidite
The sedimentary unit formed as a
turbidity
current
comes to rest; generally moderately sorted, with a
fining-upwards or graded sedimentary structure.
