Geoscience Reference
In-Depth Information
Terrane
A crustal fragment with a coherent lithological
and structural identity and geological history, quite
distinct from its neighbours.
Terrestrial sediment
Sediments eroded from and
deposited on a land surface.
Terrigenous sediments
Sediments eroded from a land
surface and deposited at continental margins.
Thalassostatic
Said of river terraces cut by incision
whilst sea level is low or falling and
aggradation
whilst sea
level is high or rising.
Thalweg
An imaginary line connecting the lowest points
along a stream bed or valley floor.
Thermal bulge
A section of crust elevated over a mantle
convection current by thermal expansion.
Thermal energy
The energy of a substance which is
stored in the form of
sensible heat
and/or
latent heat
.
Thermal welt
Crustal thickening over a rising
mantle
plume
caused by the intrusion of
magma
(see also
thermal
bulge
).
Thermocline
A zone of marked change in temperature
with ocean depth.
Thermogenesis
The production of heat.
Thermohaline circulation
A global, density-driven ocean
circulation system controlled by differences in
temperature and salinity (see also
Global Ocean Conveyor
).
Thermokarst
Chaotic topography of hollows and earth
slides caused by the melting of the underlying
permafrost
in
periglacial
regions.
Throughfall
Net precipitation at the ground after passing
through a vegetation canopy.
Throughflow
The shallow subsurface transmission of
water through soil, developing lateral movement as the
onward infiltration rate is reduced and emerging as
saturated overland flow
towards valley floors; sometimes
also used to identify the portion of stream
discharge
attributable to such transmission.
Thrusting
The action of overriding of one geological
unit by another caused by low-angled shear.
Tidal bulge
The rise in ocean water surface, caused by
the gravitational attraction of the moon and sun, which
moves around the global ocean, following their motion
relative to Earth.
Tidal current
The horizontal ebb and flow of semi-
diurnal and diurnal tides around the coastline.
Tidal flat
An extensive, low-lying surface occupying an
intertidal zone
and commonly covered in sand, mud or salt
marsh.
Tidal pass
A natural breach through a coastal barrier or
barrier island
through which tides flood and drain a
landward lagoon.
Tidal wave
The semi-diurnal rise and fall of the ocean
surface as the
tidal bulge
sweeps around the global ocean.
Tide
The regular horizontal and vertical motion of the
ocean surface in response to the gravitational attraction
of sun and moon, most noticeable at the coastline, where
its effects are usually amplified.
Tidewater glacier
A glacier which terminates in the sea,
into which
glaciomarine
environment it discharges
sediment, meltwater and
icebergs
.
Till
A coarse, generally unsorted and unstratified
sediment deposited by glacier ice; its bimodal character,
with large clasts and a fine-grained matrix - described in
the now defunct term 'boulder clay' - reflects the
indiscriminate power of glaciers.
Timberline
The upper altitudinal or latitudinal limit
beyond which trees cannot normally grow; locally,
microclimate may sustain pockets of trees beyond the
regional timberline.
Time of rise
The time elapsed from the point at which
discharge increases in response to a precipitation event
and the point of
peak discharge
.
Tolerance model
The model of succession in which the
modification of a habitat by an established species has
little effect on other species, as changes in the composition
of a community are controlled by the life cycle of the
plants.
Tombolo
A sand or gravel bar connecting an island with
another land mass.
Top carnivore
The carnivore which occupies the highest
position in a food web or
energy pyramid
.
Topoclimate
A local mesoclimate extending up to 250
m above a land surface, in which regional climate is
modified by topographic and slope factors such as aspect,
shade, exposure, etc.
Topographic climax
Climax vegetation maintained by
topographic conditions (exposure, soil hydrology or
aspect).
Toppling failure
A rockfall involving a column of rock
(or cohesive soil) whose centre of gravity overhangs a
pivot point; the column rotates outwards at its top before
overturning.
Tor
A residual rock pinnacle or pile on an elevated site,
best developed in massive crystalline rocks, and exposed
by the
weathering
and
mass wasting
of surrounding rock
mass.
Trace element
An essential nutrient for plant growth but
needed only in very small quantities.
Trailing edge
The receding
passive margin
of continental
crust with a distal
orogen
or proximal rift escarpment,
wide continental plain and shelf which influence coastal
development (cf.
leading edge
).
