D
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dark energy
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An as yet undiscovered form of energy that we cannot see.
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dark matter
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Matter in the universe that doesn’t emit light.
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daughter product
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The product of the radioactive decay of a parent isotope.
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day
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The time it takes for a planet to rotate once on its axis.
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decomposer
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An organism that breaks down the tissues of a dead organism into its various components, including nutrients, that can be used by other organisms.
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deflation
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Wind removes finer grains of silt and clay, causing the ground surface to subside.
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deforestation
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Cutting down and/or burning trees in a forested area.
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deformation
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Strain. The change of shape that a rock undergoes whe it has been altered by stresses.
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delta
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A triangular-shaped deposit of sediments that forms where a river meets standing water.
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density
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The amount of matter in a certain amount of space; mass divided by volume.
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dependent variable
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The variable in an experiment that is being measured as the independent variable is changed.
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desert
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Areas receiving very little precipitation; plants are scarce but well adapted.
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desert pavement
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Rocky, pebbled surface created as finer silts and clays are removed by wind.
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desert varnish
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Dark mineral coating that forms on exposed rock surfaces as windborne clays are deposited.
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destructive forces
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Forces that modify or destroy landforms. Agents of erosion include water, wind, ice, and gravity.
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dew point
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The temperature at which air is saturated with water vapor; when it has 100% humidity.
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differentiation
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The separation of planetary materials by density to create distinctly different layers.
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dip
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The angle of a fault relative to horizontal.
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dip-slip fault
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A fault in which the dip of the fault plain is inclined relative to the horizontal.
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direction
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The location of something relative to something else.
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dissolved load
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The elements carried in solution by a stream.
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divergent plate boundary
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A location where two lithospheric plates spread apart.
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divide
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A ridge that separates one water basin from another.
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dome
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A circular anticline; oldest rocks are in the center and youngest are on the outside.
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Doppler Effect
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A change in the frequency of a wave relative to an observer moving in relationship to the source of the wave.
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dormant volcano
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A volcano that is not currently erupting, but that has erupted in the recent past.
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downwelling
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Sinking water because of higher density.
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drip irrigation
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Pipes and tubes that deliver small amounts of water directly to the soil at the roots of plants.
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drought
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A long period of lower than normal rainfall for a particular region.
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dwarf galaxy
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A small galaxy containing a few million to a few billion stars.
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dwarf planet
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A planet-like object that has not cleared its orbit of other objects.