Geological Glossary - D

A B C D E F G H I J K L M N O P Q R S T U V W X Y Z
Devonian	A period in the geologic time scale that spans from 408 to 360 million years ago.
A B C D E F G H I J K L M N O P Q R S T U V W X Y Z
dacite	Volcanic rock (or lava) that characteristically is light in color and contains 62 to 69 percent silica and moderate amounts of sodium and potassium.
datum plane	An artificially established, well surveyed horizontal plane against which elevation, depths, tide, etc. are measured (for example mean sea-level).
daughter element	Also daughter product. An element that occurs in a rock as end product of the radioactive decay of another element.
daughter element	An element produced by decay of a radioactive parent element.
debris avalanche	A flow of unsorted masses of rock and other material downslope under the influence of gravity. Water is commonly involved as a catalyst and/or lubricant. For example, a rapid mass movement that included fragmented cold and hot volcanic rock, water, snow, glacial ice, trees and other debris and hot pyroclastic material was associated with the May 18 1980 eruption of Mt. St. Helens. Most of the deposits in the upper valley of the North Fork Toutle River and in the vicinity of Spirit Lake are from the debris avalanche resulting from the eruption.
debris flow	A type of landslide made up of a mixture of water-saturated rock debris and soil with a consistency similar to wet cement. Debris flows move rapidly downslope under the influence of gravity. Sometimes referred to as earth flows or mud flows.
declination	At any place on Earth, the angle between the magnetic and rotational poles.
deep-sea fan	Continental derived sediment located seaward of large rivers or submarine canyon having a cone-shape or fan-shape.
deflation	The removal of clay and dust from dry soil by strong winds.
deformation	General term for folding, faulting, and other processes resulting from shear, compression, and extension of rocks.
delta	A body of sediment deposited in an ocean or lake at the mouth of a stream.
delta	A fan-shaped deposit that form where a stream enters a lake or ocean and drops its load of sediment.
delta kame	A deposit having the form of a steep, flat topped hill, left at the front of a retreating continental glacier.
dendritic crystals	Crystals that grow in a branching form resembling trees.
dendritic drainage	A stream system that branches irregularly and resembles a branching tree in plan.
dendrogeomorphology	Application of tree-ring studies (dendrochronology) to the interpretation of geomorphologic processes. Often supplemented by related botanical observations.
density	The mass per unit volume of a substance commonly expressed in grams per cubic centimeter.
density current	A subaqueous current that flows on the bottom of a sea or lake because entering water is more dense due to temperature or suspended sediment.
deposition	A general term for the accumualtion of sediment by either physical or chemical sedimentation.
depostion remanent magnetization	A weak magnetization created in sedimentary rocks by the rotation of magnetic crystal into line with the ambient field during settling.
desert pavement	A residual deposit produced by continued deflation, which removes the fine grain of a soil and leaves a surface covered with closely packed cobble.
detachment plane	The surface along which a landslide disconnects from its original position.
detrital sediment	A sediment deposited by a physical process.
detritus	Loose rock or mineral material that is dislodged from bedrock by mechanical means and transported from its place of origin.
dew point	The temperature (elevation) where adiabatic cooling results in the initiation of condensation of water vapor into cloud droplets.
di-polar	The arrangement of the hydrogen atom of a water molecule at 105 deg. across the oxygen results in a slight electrical charge to the molecule.
diagenesis	A group 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 of sediment, turning it into solid rock.
diapir	Forceful, upward intrusion of a rock mass into overlying rock. In the case of an igneous diapir, the intruding rock may be magma or a crystal-rich mush, either of which is less dense than the surrounding rock.
diatom	A one celled plant that has a siliceous framework and grows in oceans and lakes.
diatom ooze	A fine muddy sediment consisting of the hard parts of diatom.
diatomite	A siliceous chert-like sediment formed from the hard parts of diatom.
diatreme	A volcanic vent filled with breccia by the explosive escape of gases.
differential erosion	see differential weathering
differential weathering	The process where some rocks are worn away less rapidly than others causing them to stand out in greater relief than the softer rocks in an exposure.
differentiated planet	One that is chemically zoned because heavy materials have sunk to the center and light materials have accumulated in a crust.
differentiation (fractionation)	"Process by which one magma gives rise to another of different composition. Differentiated or fractionated magmas are said to be evolved from a more primitive parent. Processes by which magmas evolve from basaltic to more silicic compositions; includes crystal fractionation, magma mixing, and crustal contamination."
dike	A sheet-like or tabular-shaped igneous intrusion that cuts across the sedimentary layering, metamorphic foliation, or other texture of a pre-existing rock.
dilatancy	Inelastic volume increase caused by the occurrence of small cracks in rock or soil under stress.
dinoflagellates	Single-celled, microscopic algae which are found as fossil from the mid Triassic to the present. Modern dinoflagellates are often responsible for the phosphorescence of the sea and toxic red tide. Fossil dinoflagellates are used to date and correlate rocks from the Triassic to the Quaternary.
diorite	Intrusive igneous rock made of plagioclase feldspar and amphibole and/or pyroxene. Similar to gabbro only not as so dark, and containing less iron and magnesium.
dip	The angle at which a bed, stratum, or vein is inclined from the horizontal, measured perpendicular to the strike and in the vertical plane.
dip-slip fault	"A fault in which the relative displacement is along the direction of dip of the fault plane; the offset is either normal or reverse."
disappearing stream	In karst areas, streams often disappear into the ground usually at a sinkhole.
discharge	The amount of water issuing from a spring or in a stream that passes a specific point in a given period of time.
discharge area	Region in which there is a net loss of water from the groundwater system (discharge) to surface water bodies or by evapotranspiration.
dispersion (wave)	The spreading out of a wave train due to each wavelength traveling with its own velocity.
displacement	Vector defining offset on a fault that may represent slip in a single event or the accumulation of slip over a much longer time.
dissolution	The process of chemical weathering of bedrock in which the combination of water and acid slowly removes mineral compounds from solid bedrock and carries them away in liquid solution. Also called chemical solution.
dissolved load	Part of the total stream load consisting of substances in dissolved form such as Ca2+, HCO-3, H4SiO4.
distributary	One of the branches into which a river divide, typically at a delta.
divergent plate boundary	A boundary in which two tectonic plate move apart.
divide	A ridge of high ground separating two drainage basin emptied by different streams.
dolomite	A magnesium-rich carbonate sedimentary rock. Also, a magnesium-rich carbonate mineral (CaMgCO3)
dormant volcano	A volcano that is presently inactive but which may erupt again. The major volcanic cone of the Cascade Range in Washington, Oregon, and California, for example, are believed to be dormant rather than extinct.
drainage basin	A region of land surrounded by divide and crossed by streams that eventually converge to one river or lake.
drift	A collective term for all te rock, sand, and clay that is transported and deposited by a glacier either as till or as outwash.
drumlin	A smooth, streamlined hill composed of till.
dry wash	An intermittent stream bed in an arroyo or canyon that carries water only briefly after a rain.
dune	An elongated mound of sand formed by wind or water.
durable crust	An outer rind or crust formed on a rock. Durable crusts form when rock chemically reacts with water and possibly atmospheric dust, producing a hard outer surface that resists weathering.
duricrust	Indurated soil crust or hardcap occurring on or near an eroded planation surface. May be indicative of climatic change from tropical to drier conditions with alternating wet and dry seasons. Contains tubular voids acting as avenues of translocation.
dust veil index	A quantitative method developed by H. H. Lamb for comparing the magnitude of volcanic eruption. The formulae use observations either of the depletion of the solar beam, temperature lowering in the middle latitude, or the quantity of solid matter dispersed as dust. The reference dust veil index is 1000, assigned to the Krakatoa 1883 eruption and the index is calculated using all three methods, where the information is available for statistical comparison purposes. Abbreviated D.V.I.
dynamo action	Process whereby motions within an electrcally conducting fluid, interacting with a magnetic field, convert their kinetic energy into magnetic energy, thereby sustaining the magnetic field.