Glossary - Fundamentals of the Physical Environment - page 914

Tephra The collective term for all pyroclastic rock material thrown into the atmosphere
by volcanic eruption, from fine ash to large rocks.
Tephrochronology A dating tool dependent on the recognition of individual ash falls or
tephra layers from specific volcanic eruptions; it provides either a relative age in a
stratigraphic sequence or an absolute age, by isotopic dating or direct knowledge of
the date of eruption.
Terrace A topographic bench or hillside step, cut in bedrock or formed by sediment
aggradation , at the margins of a river, glacier, lake or sea; it slopes steeply from a
level or gently sloping upper surface.
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 ).
Third - stage community A community occupying the third stage of a succession.
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 semidiurnal 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 land-ward lagoon.