A
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abiotic
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Non-living features of an ecosystem include space, nutrients, air, and water.
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abrasion
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A form of mechanical weathering that occurs whenever one rock hits another.
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absolute age
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The actual age of a material in years.
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abyssal plains
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Very flat areas that make up most of the ocean floor.
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acid rain
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Rain that has a pH of less than 5.0.
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active volcano
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A volcano that is currently erupting or is just about to erupt.
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adaptation
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A trait that an organism inherits that helps it survive in its natural environment.
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adaptive radiation
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An explosion in the diversity of species as vacant niches are filled. This often occurs after a mass extinction.
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advection
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Horizontal movement of a fluid or the transport of a substance in the flow.
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air mass
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A large mass of air with the same temperature and humidity characteristics.
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air pressure
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The force of air pressing on a given area.
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albedo
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The amount of light that reflects off a surface; snow and ice have high albedo.
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alkaline
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Also called basic. Substances that have a pH of greater than 7.0.
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alluvial fan
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Curved, fan-shaped, coarse-sediment deposit that forms when a stream meets flat ground.
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alpine (valley) glacier
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A glacier found in a valley in the mountains.
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altitude
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Distance above sea level.
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amber
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Fossilized tree sap.
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amino acid
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Organic molecules that are the building blocks of life.
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amniote egg
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An egg that contains all the nutrients needed for the developing embryo and is protected by a shell.
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amplitude
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The height of a wave from the center to the top of the crest (or the bottom of the trough).
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anticline
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A fold that arches upward; older rocks are in the center and younger rocks are at the outside.
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anticyclone
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Wind system that rotates around a high pressure center.
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aphotic zone
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The zone in the water column deeper than 200 m where sunlight does not penetrate.
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apparent polar wander
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The path on the globe showing where the magnetic pole appeared to move over time.
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aquaculture
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Agriculture of the sea; farmed fish, seafood and aquatic plants.
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aquifer
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A layer of rock, sand, or gravel that holds large amounts of ground water.
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arch
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An erosional landform that is produced when waves erode through a cliff.
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asterism
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A group or cluster of stars that appear close together in the sky.
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asteroid
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Rocky objects larger than a few hundred meters that orbit the Sun.
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asteroid belt
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Region between the orbits of Mars and Jupiter where many asteroids are found.
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astronomer
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Scientists who study the universe beyond Earth.
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atmosphere
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The layer of gases that surrounds a planet.
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atom
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The smallest unit of a chemical element.
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atomic mass
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The number of protons and neutrons in an atom.
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aurora
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A spectacular light display that occurs in the ionosphere near the poles.
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avalanche
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Mass of snow that suddenly moves down a mountain under the influence of gravity.
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axis
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An imaginary line that runs from the North Pole to South Pole, and it includes the center of Earth.