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D


daughter isotope
An isotope that forms from the radioactive decay of a parent isotope. A daughter isotope may or may not be of the same element as its parent. If the daughter isotope is radioactive, it will eventually become the parent isotope of a new daughter isotope. The last daughter isotope to form from this process will be stable and nonradioactive.
debris flow
1. The rapid, downward mass movement of particles coarser than sand, often including boulders one meter or more in diameter, at a rate ranging from 2 to 40 kilometers per hour. Debris flows occur along fairly steep slopes. 2. The material that descends in such a flow. deflation The process by which wind erodes bedrock by picking up and transporting loose rock particles.
degradation
The process by which a stream's gradient becomes less steep, due to the erosion of sediment from the stream bed. Such erosion generally follows a sharp reduction in the amount of sediment entering the stream.
delta
An alluvial fan having its apex at the mouth of a stream.
dendrochronology
A method of numerical dating that uses the number of tree rings found in a cross section of a tree trunk or branch to determine the age of the tree.
desert
A region with an average annual rainfall of 10 inches or less and sparse vegetation, typically having thin, dry, and crumbly soil. A desert has an aridity index greater than 4.0.
desertification
The process through which a desert takes over a formerly non-desert area. When a region begins to undergo desertification, the new conditions typically include a significantly lowered water table, a reduced supply of surface water, increased salinity in natural waters and soils, progressive destruction of native vegetation, and an accelerated rate of erosion.
desert pavement
A closely packed layer of rock fragments concentrated in a layer along the Earth's surface by the deflation of finer particles.
desert varnish
A thin, shiny red-brown or black layer, principally composed of iron manganese oxides, that coats the surfaces of many exposed desert rocks.
detrital sediment
Sediment that is composed of transported solid fragments of preexisting igneous, sedimentary, or metamorphic rocks.
diagenesis
The set of processes that cause physical and chemical changes in sediment after it has been deposited and buried under another layer of sediment. Diagenesis may culminate in lithification.
dike
A discordant pluton that is substantially wider than it is thick. Dikes are often steeply inclined or nearly vertical. See also sill. dilatancy The expansion of a rock's volume caused by stress and deformation.
diorite
Any of a group of dark, phaneritic, intrusive rocks that are the plutonic equivalents of andesite.
dip
The angle formed by the inclined plane of a geological structure and the horizontal plane of the Earth's surface.
dip-slip fault
A fault in which two sections of rock have moved apart vertically, parallel to the dip of the fault plane.
directed pressure
Force exerted on a rock along one plane, flattening the rock in that plane and lengthening it in the perpendicular plane.
disappearing stream
A surface stream that drains rapidly and completely into a sinkhole.
displaced terrane
A fault-bounded body of rock - sometimes thousands of square kilometers in area - that originated elsewhere geographically and has then moved, perhaps long distances, by plate motion.
dissolution
A form of chemical weathering in which water molecules, sometimes in combination with acid or another compound in the environment, attract and remove oppositely charged ions or ion groups from a mineral or rock.
dissolved load
A body of sediment carried by a stream in the form of ions that have dissolved in the water.
distributary
One of a network of small streams carrying water and sediment from a trunk stream into an ocean.
divergence
The process by which two lithospheric plates separated by rifting move farther apart, with soft mantle rock rising between them and forming new oceanic lithosphere. See also convergence.
dolostone
A sedimentary rock composed primarily of dolomite, a mineral made up of calcium, magnesium, carbon, and oxygen. Dolostone is thought to form when magnesium ions replace some of the calcium ions in limestone, to which dolostone is similar in both appearance and chemical structure.
dome
A round or oval bulge on the Earth's surface, containing the oldest section of rock in its raised, central part. See also basin. drainage basin The area from which water flows into a stream. Also called a watershed.
drainage divide
An area of raised, dry land separating two adjacent drainage basins.
drainage pattern
The arrangement in which a stream erodes the channels of its network of tributaries.
drumlin
A long, spoon-shaped hill that develops when pressure from an overriding glacier reshapes a moraine. Drumlins range in height from 5 to 50 meters and in length from 400 to 2000 meters. They slope down in the direction of the ice flow.
dune
A usually asymmetrical mound or ridge of sand that has been transported and deposited by wind. Dunes form in both arid and humid climates.
dynamothermal metamorphism
A form of regional metamorphism that acts on rocks caught between two converging plates and is initially caused by directed pressure from the plates, which causes some of the rocks to rise and others to sink, sometimes by tens of kilometers. The rocks that fall then experience further dynamothermal metamorphism, this time caused by heat from the Earth's interior and lithostatic pressure from overlying rocks.

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