Geoscience Reference
In-Depth Information
possessing caustic properties; it can neutralize hydrogen
ions, with which it reacts to form a salt and water, and is
an important agent in
rock weathering
.
Allogenic successions
Plant successions affected by
material originating from elsewhere (seeds, sediment).
Allopatric speciation
Formation of new species by
geographical isolation of a single species into separated,
discrete populations.
Alluvial fan
A fan-shaped spread of
alluvium
deposited
where a tributary stream loses power on entering a more
gently sloping valley.
Alluvial toeslope
The lowest component of a hill slope
bordering the valley floor, where slope and channel
processes interact.
Alluvium
A general term for unconsolidated, granular
sediments deposited by rivers.
Alp
A high-altitude bench overlooking a glacial trough
or the
alpine meadow
growing there; sometimes used
to describe a glaciated mountain in or resembling the
European Alps.
Alpha diversity
Diversity within a habitat measured by
the number of species of a specified taxonomic group in
a specified area.
Alpine
Said of a rugged, steep and high mountain or
mountainous region resembling the European Alps; the
region is likely to be a young orogen high enough to
support
alpine glaciers
.
Alpine glacier
A temperate, warm-based mountain
glacier characterized by vigorous
mass balance
, high flow
velocities and confinement to rock-walled channels.
Alpine heath
A vegetation community with dominant
dwarf woody shrubs adapted to high-altitude conditions.
Alpine landsystem
A distinctive landsystem with
integrated glacial, cryonival, mountain slope and
ecosystem
elements dominated by cold climate processes
on steep slopes.
Alpine meadow
A vegetation community characterized
by grasses and flowering plants adapted to high-altitude
conditions.
Alteration compounds
Colloids (clays, iron oxides)
formed by the weathering of rock minerals.
Altiplanation
The combined processes of frost shattering
and solifluction which progressively level mountain
summits, leaving residual rock platforms and
blockfields
.
Alumino-silicates
Silicate minerals in which aluminium
substitutes for one or more silicate cation.
Ammonification
The production of ammonia and
ammonium-nitrogen through the decomposition of
organic nitrogen compounds in soil organic matter.
Ampferer subduction or A-subduction
The downward
displacement of continental crust, usually in an inter-
continental collision zone, named after an Alpine
geologist; buoyancy prevents the granitic crust from being
recycled into the mantle but it may melt locally.
Amphidromic point
An ocean hub of zero tidal range,
around which co-tidal lines radiate towards adjacent
coasts, caused by the impact of Earth's rotation on the tidal
bulge.
Amygdaloidal
The texture of an igneous rock created by
the crystallization of secondary minerals in former gas
vesicles or voids.
Anabatic
Said of the inflowing and ascending portion
of a local thermal circulation cell, driven by the warming
phase of diurnal heating in a mountainous region.
Anabranching
Said of the individual river channels
which form a braided channel reach where intervening
bars are large and stable.
Anaerobism, anaerobic
The absence of free oxygen, or
organisms active in the absence of free oxygen.
Anatexis
The melting of adjacent (country) rock by an
igneous intrusion which swells the size of the developing
pluton.
Andesite line
An imaginary line dividing the Pacific
Ocean and rim into two volcanic provinces based on their
magma petrology and volcanic morphology; intraplate,
basaltic shield volcanoes are distinguished from subduc-
tion zone, andesitic stratovolcanoes.
Anion
An atom which has gained one or more negatively
charged electrons and is thus itself negatively charged.
Annual
A plant that completes its life cycle in one
growing season.
Antarctic circumpolar current
The east-flowing, clock-
wise cold surface current which encircles the Antarctic
continent in the absence of land barriers and feeds cold
water into the anticlockwise gyres of adjacent oceans.
Antarctic convergence
The boundary between the
north-flowing cold, dense Antarctic water mass and
less dense south-flowing water masses in the adjacent
oceans.
Anthropogenic
Said of a process or material originating
from human activity.
AOGCM (Atmosphere-Ocean General Circulation Model)
a global climate forecast model which integrates atmos-
pheric and ocean components.
Aphelion
The point on the orbit of Earth when it is
farthest from the sun.
Aquiclude
A rock mass which absorbs underground
water but impedes its onward transfer.
Aquifer
Rocks and sediments capable of storing ground
water.
Aquifuge
An impermeable rock mass which arrests
underground water transfer.
