Glossary - Fundamentals of the Physical Environment - page 750

is marked by intense crustal deformation and resorption ,
accompanied by seismovolcanic activity.
Submarine canyon A steep-sided submarine valley cut
across the continental shelf and slope , frequently the
continuation of, or originating from, terrestrial valleys and
swept by turbidity currents .
Subsystems A series of smaller systems linked together
by a series of flows of energy and matter.
Succession The process of change in plant communities
which, successively, occupy a given area and culminate in
climax vegetation.
Successional community A community in a succession
which is creating conditions more and more favourable
for a succeeding community.
Succulent A plant with fleshy, water-storing tissues,
characteristic of arid or saline areas.
Suction The force with which water is held in soil, or the
force required to remove water from soil.
Supercontinental cycle The periodic coalescence and
rifting apart of supercontinents, driven by plate tectonics
over c. 500 Ma time scales caused by the inevitability of
plate collisions in one area as a result of sea-floor spreading
in another; also known as the Wilson cycle (after J. T.
Wilson).
Surf zone The turbulent water from breaking waves
which lies between the breaker and swash zones in the
nearshore environment.
Surging glacier A glacier enjoying a transient phase of
very rapid, extending and thinning flow which exceeds the
glacier's mass balance capacity and leads either to the
runaway collapse of the glacier or to a quiescent period
of recovery.
Suspect terrane A crustal fragment or displaced terrane
among a collage of such fragments forming continental
crust, whose external and even internal common
geographical origins are in doubt, or 'suspect', by virtue of
the endless mobility which characterizes plate boundaries.
Suture The linear, convergent boundary at which two
continental plates are welded together and marked by
compression structures and remnants of the ocean which
formerly separated them.
Swash A forward pulse of water released by a breaking
wave after it has broken, capable of moving sand up a
beach.
Swelling clay A clay capable of absorbing very large
quantities of water and greatly increasing in volume which
may contribute to mechanical weathering by creating
tensile stress in a rock mass.
Symbiotic fixation Conversion of atmospheric nitrogen
into organic nitrogen compounds by Rhizobium bacteria
living in nodules on the roots of leguminous plants.

Syndepositional sedimentary structure A structure
formed during the final settlement of a soft sediment,
reflecting, for example, slumping or faulting on contrac-
tion as water is expelled, or flowage of more saturated
components.
Synecology

The ecology of organisms living as a

community.
Syngenesis Minor, non-destructive changes in the
mechanical or chemical properties of rock during its
emplacement, which act as early stages in its lithification .
System A network of variables (objects, phenomena or
components) which are linked by relationships and which
interact to behave as a unified whole.
Systems analysis The study of systems, for example
hydrological systems, atmospheric systems and ecosystems
in physical geography.
Talik

A pocket of unfrozen ground, other than the active

layer, within the boundary of an otherwise frozen

permafrost environment.
Talus An alternative name for scree .
Talus cone A cone of coarse, angular rock debris or
scree accumulated at the base of a rock chute .
Taxon (pl. taxa) Any taxonomic unit in biology, i.e. a
family, a genus, a species or a variety.
Tectonic cycle Cycle of ocean and continental expansion
and contraction, synonymous with a supercontinental or
Wilson cycle .
Temperate glacier A warm-based glacier, at or near
pressure-melting point throughout and generally typical of
alpine valley glaciers or the lower portions of outlet
glaciers .
Temperate stage A period of Earth history marked by
the expansion of mid-latitude temperate belts at the
expense of cold climates and ice sheets. Equivalent to the
term 'interglacial' in the Quaternary period, when they
occur as brief (approx. 10 kyr) interludes between more
prolonged (approx. 100 kyr) cold stages.
Tensile stress A stress tending to stretch, pull apart and
fracture rock mass.
Tephra The collective term for all pyroclastic rock mater-
ial 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.