Geoscience Reference
In-Depth Information
GlossAry
A
A horizon
The soil horizon that is enriched with
humus. 281
Absorption
The assimilation and conversion of solar
radiation into another form of energy by a medium such as
water vapor. In this process, the temperature of the absorbing
medium is raised. 71
Acid rain
The precipitation by rain, fog, or snow of strong
mineral acids, primarily sulfur dioxide and nitrogen oxides,
that originated in factories. 379
Actual evapotranspiration (actual ET)
The quantity of
water actually removed from a given land area by evaporation
and transpiration. 203
Adiabatic processes
Changes in temperature that occur due
to variations in air pressure. 149
Adret slope
The slope that faces the Sun most
directly. 252
Advection
The horizontal transfer of air. 108
Advection fog
Fog that develops when warm air flows over
cooler air. 153
Aerial photographs
Photographs taken of the Earth's
surface from the air. 32
Aggradation
The progressive accumulation of sediment
along or within a stream. 423
Air mass
A large body of air in the lower atmosphere that
has distinct temperature and humidity characteristics. 170
Air pressure
The force that air molecules exert on a surface
due to their weight. 106
Albedo
The reflectivity of features on the Earth's surface or
in the atmosphere. 73
Albic horizon
A diagnostic horizon of podzolization from
which clay and free iron oxides have been removed, resulting
in a light-colored E horizon. 290
Alfisols
Soils generally found in seasonal midlatitude
regions that formed through podzolization and have an
alkaline argillic horizon. 289
Alluvial fan
A fan-shaped landform of low relief that forms
where a stream flows out of an area of high relief into a broad,
open plane where the gradient is less and deposition thus
occurs. 484
Alluvial terrace
A level, step-like landform that forms when
a stream erodes its bed so that a horizontal surface is raised
relative to the channel. 433
Alluvium
Sediment deposited by a stream. 433
Alpine glacier
A glacier in mountainous regions that flows
down preexisting valleys.
Andisols
Soils formed in parent material that is at least 50%
volcanic ash. 299
Angle of incidence
The angle at which the Sun strikes Earth
at any given place and time. 74
Anthropocene
A proposed new epoch in the geological
timescale based on the recognition that humans are now the
foremost agent of change on the Earth's surface.
328
Anthropogenic
Caused by humans. 226
Anticlinal valley
A topographic valley that occurs along the
axis of a structural anticline. 345
Anticline
A convex fold in rock in which rock layers are bent
upward into an arch. 342
Anticyclones
High-pressure systems. 108
Aphelion
The point of the Earth's orbit where the distance
between the Earth and Sun is greatest (~152 million km or
94.5 million mi). 47
Aquiclude
An impermeable body of rock that may contain wa-
ter but does not allow transmission of water through it. 393
Aquifer
A geological formation that contains a suitable
amount of water to be accessed for human use. 395
Arable land
Land that has the high potential to be cultivated
for crop production. 548
Arête
A sharp ridge that forms between two glacial
cirques. 459
Argillic horizon
A B horizon that is enriched in eluviated
clay. 283
Aridisols
Mineral soils that form in arid environments and
thus are poorly developed. 295
Arroyo
A deep, steep-sided gully that is cut into
alluvium. 484
Artesian well
A well in which water from a confined aquifer
rises to the surface through natural pressure. 396
Artificial levee
An engineered structure along a river that
effectively raises the height of the river bank and thus confines
flood discharge. 439
Asthenosphere
The layer of very soft rock that occurs in the
upper part of the upper mantle. This region is about 40 km to 250
km (25 mi to 105 mi) below the surface of Earth. The soft char-
acter of this rock allows isostatic adjustments to occur. 308
Atmosphere
The gaseous shell that surrounds Earth. 7
Autotrophs
Organisms that synthesize their own food using
heat or light as the source of energy. 236
Avalanche
A large mass of snow or rock that suddenly slides
down a mountainside. 386
Axis
The line around which the Earth rotates, extending
through the poles.
452
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