Geoscience Reference
In-Depth Information
Glossary
A-subduction
See
Ampferer subduction
.
Abandoned meander core
Ground encircled by the
course of a former river meander but now isolated by cut-
off across the meander neck by subsequent channel
straightening; the core may be a substantial hill in an
incised valley.
Ablation till
A supraglacial coarse-grained sediment or
till, accumulating as the subjacent ice melts and drains
away and finally let down on to the exhumed subglacial
surface.
Abnormal soils
A class of soils in Dokuchaiev's classifi-
cation denoting young or azonal soils, e.g. peat, alluvial
soils, raw sands.
Abrasion
Mechanical wear and tear brought about by
the movement of harder rock fragments, ice pellets or
organic debris against softer rocks or rock fragments.
Absolute zero
The temperature at which atoms and
molecules possess the minimum amount of energy and
no thermal motion. It corresponds to -273·15° on the
Celsius scale.
Absorption
Active layer
Layer of soil between the surface and the
permafrost
table in periglacial regions which freezes in
autumn and melts in spring.
Actual evapotranspiration
The amount of moisture
evaporated from the ground surface and transpired by
plants into the atmosphere.
Adaptive radiation
Formation of many species from a
single common ancestor, so that the new species can
occupy the varying habitats and resources available.
Adret
A mountain slope whose orientation maximizes
the receipt of sunlight.
Adsorption
The accumulation of ions at the surfaces of
clay minerals and humic colloids in soils.
Aeolian
Said of the processes, Earth materials and
landforms involving the role of the wind.
Aerenchyma
Air-filled spaces in the roots and stems of
hydrophytic plants.
Aggradation
A rise in ground level caused by the
accumulation of sediments.
Aggregates
Soil structural units of various shapes,
composed of mineral and organic material; formed by
natural processes, and having a range of stabilities.
Aiguille
A steep, frost-shattered rock pinnacle.
Air capacity
The percentage of soil volume occupied by
air spaces or pores.
Aklé
A wavy- or cuspate-edged transverse dune with the
points of each cusp pointing downwind.
Albedo
An index of the reflecting power of a surface. It
is usually used of
short-wave radiation
. Light-coloured
surfaces such as ice have a high albedo.
Alkali
Said of a substance capable of liberating hydroxide
ions in water, measured by a
pH
of more than 7·0, and
The conversion of radiation to another form
of energy.
Abyssal plain
The profound, almost level, largest single
component of the deep ocean, lying 4-6 km deep between
mid-ocean ridges and trenches.
Accretionary prism
A wedge-shaped rock mass of sedi-
ment and ophiolite transferred during subduction from
descending oceanic plate to the adjacent continental plate.
Acid
A substance capable of liberating hydrogen ions
in water, measured by a
pH
of less than 7·0; acids have
corrosive properties and are important agents of
rock
weathering
.
