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Ablation zone
Area of a glacier that is losing net snow or ice due to melting
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Abrasion
The erosion of a stream channel as sand or gravel is transported down stream
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Absolute humidity
A measure of the actual amount of water vapor in the air at a given time
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Abyssal plain
Large flat regions of the seafloor at depths below 3,000 meters
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Accuracy
The degree to which a set of measured values matches an accepted or known value
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Acid mine drainage
Runoff resulting when water reacts with metallic ores
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Acid rain
Rain with a pH value less than the pH of unpolluted precipitation
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Active continental margins
A margin located adjacent to a plate boundary
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Adiabatic temperature change
Cooling or warming of air caused when air is allowed to expand or is compressed, not because heat is added or subtracted
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Adiabatic lapse rate
Rate of temperature change in the atmosphere due to changing atmospheric pressure
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Aerosols
Tiny solid and liquid particles suspended in the atmosphere
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Aftershock
Sequence of earthquakes that follow a larger event
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A horizon
Top layer of soil defined by the presence of organic matter mixed in with mineral and rock fragments
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Air
A mixture of many discrete gases, of which nitrogen and oxygen are most abundant
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Air density
The mass of gases per volume of air
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Air mass
A large body of air that is characterized by a near-uniform temperature and humidity
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Air pollution
The presence of airborne particles and gases in concentrations that endanger the health and well-being of organisms or disrupt the orderly functioning of the enviornment
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Albedo
The reflectivity of a surface, usually expressed as a fraction of the incident radiation reflected
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Alpine glacier
A mountain glacier
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Altocumulus clouds
A principal cloud type that ccurs at middle of the troposhere
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Altostratus clouds
A high cloud type that appears thin and spread out
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Amphibole
A member of a mineral group in which all members are double-chain silicates
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Andesite
Fine-grained igneous rock of intermediate composition. Up to half of the rock is plagioclase feldspar, with the rest being ferromagnesian minerals.
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Andesitic magma
A magma with intermediate silica content
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Angle of repose
The steepest angle at which loose material remains stationary without sliding downslope
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Anticyclones
A high-pressure center characterized by a clockwise flow of air in the Northern Hemisphere
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Aquifer
A body of rock or regolith with sufficient porosity and permeability to provide water in useful quantities to wells or springs
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Aquitard
Impermeable beds that hinder or prevent groundwater movement
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Archean
A term, meaning ancient, which has been applied to the oldest rocks of the Precambrian
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Artesian aquifer
An aquifer completely filled with pressurized water and separated from the land surface by a relatively impermeable confining bed, such as shale. Also called confined aquifer
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Artificial beach nourishment
Adding sand to a beach by mechanical means
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Ash
Very fine-grained volcanic material erupted from a volcano that commonly forms a cloud
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Asteroid
A small, generally rocky, solid body orbiting the sun and ranging in diameter from a few meters to hundreds of kilometers
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Asteroid belt
Region between Mars and Saturn where majority of solar-orbiting asteroids are found
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Asthenosphere
A weak layer of Earth's mantle below the lithosphere upon which tectonic plates move
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Astronomical unit
The average distance from the sun to Earth
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Atmosphere
The gaseous envelope surrounding a planet or celestial body. One of the four components of the earth system.
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Atmospheric pressure
Pressure exerted by weight of the overlying column of air
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Atom
Smallest possible particle of an element that retains the properties of that element
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Atomic mass number
The total number of neutrons and protons in an atom
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Atomic number
The total number of protons in an atom
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Aurora
A bright display of ever-changing light caused by charged particles interacting with the upper atmosphere in the polar region
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Barrier island
A long, narrow sandy ridge of land that forms parallel to, but separate from, the mainland coast
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Barycenter
A common center of mass around which two objects revolve
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