D

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