Glossary - Fundamentals of the Physical Environment - page 724

Aquitard A rock mass which retards but does not arrest
underground water transfer.
Arch A rock mass spanning a gap weathered or eroded
through its core.
Arctic-alpine Said of the flora or geomorphology of
high-altitude and high-latitude regions, showing common
adaptation to, or reliance on, cold climates and cryospheric
processes; they vary according to differences in daylight
regime and general slope conditions between the two
zones. See also cryonival and cryophyte .
Arctic tundra The distinctive treeless plant community
or broader geoecological environment of the Arctic basin
polewards of the timberline .
Area effect That part of island biogeography theory
which relates the number of plant colonizers to the size
of the island being colonized.
Arête A narrow, precipitous and frost-shattered
mountain ridge forming the remnant divide between two
glacial cirques.
Armouring (1) In fluvial geomorphology, a concentra-
tion of coarse, sorted material on the river bed which
protects finer material beneath from erosion, or (2) a
crust formed on soil surfaces by stones and chemical
cement which restricts infiltration in the soils of arid and
semi-arid regions.
Ash Unconsolidated fine-grained pyroclastic material,
less than 2 mm in diameter, ejected into the atmosphere
by volcanic eruption.
Ash fall The atmospheric fall-out of volcanic ash from
the plume of an explosive eruption.
Ash-fall tuff A lithified volcanic rock formed by ash
which has fallen out of a volcanic cloud.
Ash-flow Volcanic ash suspended in hot gas and capable
of long-distance gravity flow over a land or sea surface as
an incandescent cloud, pyroclastic flow or nuée ardente ;it
forms an ash-flow tuff or ignimbrite on cooling.
Asthenosphere The ductile outer layer of Earth's mantle
immediately underlying the lithosphere , capable of solid-
state creep which is essential to crustal plate motion.
Atmometer An instrument used to measure water
evaporated from porous surfaces. Atmometers are inex-
pensive but they are sensitive to wind speed and are not
a good indicator of evaporation from an open-water
surface.
Atmosphere (1) Earth's envelope of gases, representing
the lightest, volatile products of geological and biological
fractionation retained by gravity. (2) A unit of pressure;
one atmosphere will support a column of

Aulacogen A continental rift, frequently found at plate
triple junctions, which has failed to develop full sea-floor
spreading; it may form a major topographic depression
guiding river basin development.
Aurora australis Light produced in the upper atmos-
phere of the southern hemisphere by the interaction of the
solar wind and the magnetosphere. The gases emit visible
radiation which causes the sky to glow like a neon light.
Aurora borealis Similar phenomenon to the Aurora
australis but in the northern hemisphere.
Autecology The study of the ecology of individual
species, in contrast to the study of whole communities.
Autocatalysis The positive feedbacks from ice sheet
growth which reinforce icehouse conditions, including the
extension of surfaces with high albedo , cold-air drainage ,
falling sea level and seaward extension of grounding lines .
Autogamy A plant adaptation permitting self-
pollination and therefore not dependent on insect or
other pollinators.
Autogenic successions Plant successions driven by
internal processes and internally induced habitat changes.
Autotrophs Plants and micro-organisms capable of
synthesizing organic compounds from inorganic materials
by either photosynthesis or oxidation reactions.
Available nutrients That proportion of the total nutrient
content of soils which plants can absorb and utilize.
Available water The amount of water in a soil available
for plant growth after excess water has drained away under
the influence of gravity.
Average surface lowering A rate of denudation
extrapolated from sediment yields to give the average
thickness of a land surface layer removed per unit area per
unit time.
Avulsion An abrupt rerouting of stream flow into a new
or abandoned channel due to aggradation of the flood
plain.
Azonal Soils still in a raw, immature state (young soils).
B-subduction

See Wadati-Benioff subduction or Benioff

subduction .
Back arc A basin created by crustal extension or
stretching on the further side of a volcanic arc from a
B-subduction zone, floored by oceanic crust and usually
flooded by the sea.
Backshore The upper part of a beach , lying between the
ordinary high-tide mark and the coastline , which is wave-
swept only at exceptionally high tides and may be a source
of landward aeolian sand transport.
Backwash mercury A seaward return pulse of water from

measuring 760 mm in height at sea level.
Attrition The mutual wear and tear of particles in
turbulent contact with each other during transport.

breaking waves.
Bajada A continuous alluvial apron in an arid environ-
ment, composed of a coalescence of Piedmont fans .