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.
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