Environmental Engineering Reference
In-Depth Information
Armouring
(1) In fluvial geomorphology, a concentration 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 solidstate creep which is essential to crustal
plate
motion.
Atmometer
An instrument used to measure water evaporated from porous surfaces.
Atmometers are inexpensive 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 mercury measuring 760 mm in height at sea
level.
Attrition
The mutual wear and tear of particles in turbulent contact with each other
during transport.
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 atmosphere 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.