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Darcy’s law	A mathematical equation stating that a volume of water, passing through a specified area of material at a given time, depends on the material’s permeability and hydraulic gradient.
daughter isotope	The decay product of radioactive decay.
day	The time it takes for the Earth to spin once on its axis.
debris avalanche	An avalanche in which the falling debris consists of rock fragments and dust.
debris flow	A downslope movement of mud mixed with larger rock fragments.
debris slide	A sudden downslope movement of material consisting only of regolith.
decompression melting	The kind of melting that occurs when hot mantle rock rises to shallower depths in the Earth so that pressure decreases while the temperature remains unchanged.
deep current	An ocean current at a depth greater than 100 m.
deep-focus earthquake	An earthquake that occurs at a depth between 300 and 670 km; below 670 km, earthquakes do not happen.
deflation	The process of lowering the land surface by wind abrasion.
deformation	A change in the shape, position, or orientation of a material, by bending, breaking, or flowing.
dehydration	Loss of water.
delta	A wedge of sediment formed at a river mouth when the running water of the stream enters standing water, the current slows, the stream loses competence, and sediment settles out.
delta plain	The low, swampy land on the surface of a delta.
delta-plain flood	A flood in which water submerges a delta plain.
dendritic network	A drainage network whose interconnecting streams resemble the pattern of branches connecting to a deciduous tree.
dendrochronologist	A scientist who analyzes tree rings to determine the geologic age of features.
density	Mass per unit volume.
denudation	The removal of rock and regolith from the Earth’s surface.
deposition	The process by which sediment settles out of a transporting medium.
depositional landform	A landform resulting from the deposition of sediment where the medium carrying the sediment evaporates, slows down, or melts.
desert	A region so arid that it contains no permanent streams except for those that bring water in from elsewhere, and has very sparse vegetation cover.
desert pavement	A mosaic-like stone surface forming the ground in a desert.
desert varnish	A dark, rusty-brown coating of iron oxide and magnesium oxide that accumulates on the surface of the rock.
desertification	The process of transforming nondesert areas into desert.
detachment fault	A nearly horizontal fault at the base of a fault system.
detritus	The chunks and smaller grains of rock broken off outcrops by physical weathering.
dewpoint temperature	The temperature at which air becomes saturated so that dew can form.
differential stress	A condition causing a material to experience a push or pull in one direction of a greater magnitude than the push or pull in another direction; in some cases, differential stress can result in shearing.
differential weathering	What happens when different rocks in an outcrop undergo weathering at different rates.
diffraction	The splitting of light into many tiny beams that interfere with one another.
dike	A tabular (wall-shaped) intrusion of rock that cuts across the layering of country rock.
dimension stone	An intact block of granite or marble to be used for architectural purposes.
dip slope	A hill slope underlain by bedding parallel to the slope.
dipole	A magnetic field with a north and south pole, like that of a bar magnet.
dipole field (for Earth)	The part of the Earth’s magnetic field, caused by the flow of liquid iron alloy in the outer core, that can be represented by an imaginary bar magnet with a north and south pole.
dip-slip fault	A fault in which sliding occurs up or down the slope of the fault.
disappearing stream	A stream that intersects a crack or sinkhole leading to an underground cavern, so that the water disappears into the subsurface and becomes an underground stream.
discharge	The volume of water in a conduit or channel passing a point in one second.
discharge area	A location where groundwater flows back up to the surface, and may emerge at springs.
disconformity	An unconformity parallel to the two sedimentary sequences it separates.
displacement (or offset)	The amount of movement or slip across a fault plane.
disseminated deposit	A hydrothermal ore deposit in which ore minerals are dispersed throughout a body of rock.
dissolution	A process during which materials dissolve in water.
dissolved load	Ions dissolved in a stream’s water.
distillation column	A vertical pipe in which crude oil is separated into several components.
distributaries	The fan of small streams formed where a river spreads out over its delta.
divergence zone	A place where sinking air separates into two flows that move in opposite directions.
divergent plate boundary	A boundary at which two lithosphere plates move apart from each other; they are marked by mid-ocean ridges.
diversification	The development of many different species.
DNA (deoxyribonucleic acid)	The complex molecule, shaped like a double helix, containing the code that guides the growth and development of an organism.
doldrums	A belt with very slow winds along the equator.
dome	Folded or arched layers with the shape of an overturned bowl.
Doppler effect	The phenomenon in which the frequency of wave energy appears to change when a moving source of wave energy passes an observer.
dormant volcano	A volcano that has not erupted for hundreds to thousands of years but does have the potential to erupt again in the future.
downcutting	The process in which water flowing through a channel cuts into the substrate and deepens the channel relative to its surroundings.
downdraft	Downward-moving air.
downgoing plate (or slab)	A lithosphere plate that has been subducted at a convergent margin.
downslope force	The component of the force of gravity acting in the downslope direction.
downslope movement	The tumbling or sliding of rock and sediment from higher elevations to lower ones.
downwelling zone	A place where near-surface water sinks.
drag fold	A fold that develops in layers of rock adjacent to a fault during or just before slip.
drainage divide	A highland or ridge that separates one watershed from another.
drainage network (or basin)	An array of interconnecting streams that together drain an area.
drawdown	The phenomenon in which the water table around a well drops because the users are pumping water out of the well faster than it flows in from the surrounding aquifer.
drilling mud	A slurry of water mixed with clay that oil drillers use to cool a drill bit and flush rock cuttings up and out of the hole.
dripstone	Limestone (travertine in a cave) formed by the precipitation of calcium carbonate out of groundwater.
drop stone	A rock that drops to the sea floor once the iceberg that was carrying the rock melts.
drumlin	A streamlined, elongate hill formed when a glacier overrides glacial till.
dry wash	The channel of an ephemeral stream when empty of water.
dry well	(1) A well that does not supply water because the well has been drilled into an aquitard or into rock that lies above the water table; (2) a well that does not yield oil, even though it has been drilled into an anticipated reservoir.
dry-bottom  glacier	A glacier so cold that its base remains frozen to the substrate.
ductile (plastic) deformation	The bending and flowing of a material (without cracking and breaking) subjected to stress.
dune	A pile of sand generally formed by deposition from the wind.
dust storm	An event in which strong winds hit unvegetated land, strip off the topsoil, and send it skyward to form rolling dark clouds that block out the Sun.
dynamic metamorphism	Metamorphism that occurs as a consequence of shearing alone, with no change in temperature or pressure.
dynamo	A power plant generator in which water or wind power spins an electrical conductor around a permanent magnet.
dynamothermal metamorphism	Metamorphism that involves heat, pressure, and shearing.