D

DACITE: Volcanic equivalent of granodiorite.

DEBRIS AVALANCHE: A fast downhill mass movement of soil and rock.

DEBRIS FLOW: A fluid mass movement of rock fragments supported by a muddy matrix. Debris flows differ from earthflows in that they generally contain coarser material and move faster than earthflows.

DEBRIS SLIDE: A mass movement of rock material and soil largely as one or more units along planes of weakness at the base of or within the rock material.

DEFLATION: The removal of clay and dust from dry soil by strong winds that gradually scoop out shallow depressions in the ground.

DELTA: A body of sediment deposited in an ocean or lake at the mouth of a stream.

DENDRITIC DRAINAGE: A stream system that branches irregularly, resembling a branching tree.

DENSITY: The mass per unit volume of a substance, commonly expressed in grams per cubic centimeter.

DEPOSITIONAL REMANENT MAGNETIZATION: A weak magnetization created in sedimentary rocks by the rotation of magnetic crystals into line with the ambient field during settling.

DESERT PAVEMENT: A residual deposit left when continued deflation removes the fine grains of a soil and leaves a surface covered with close-packed cobbles.

DESERT VARNISH: A dark coating commonly found on the surface of rock in the desert. It consists of clays, iron oxides, and magnesium oxides produced during weathering.

DIAGENESIS: The chemical and physical changes undergone by buried sediments during lithification and compaction into sedimentary rock.

DIATREME: A volcanic vent filled with volcanic breccia by the explosive escape of gases.

DIKE: A tabular igneous intrusion that cuts across structures of surrounding rock. (See also Discordant intrusion.)

DIORITE: A plutonic rock with composition intermediate between granite and gabbro; the intrusive equivalent of andesite.

DIP: The maximum angle by which a stratum or other planar feature deviates from the horizontal. The angle is measured in a plane perpendicular to the strike.

DISCHARGE: The rate of water movement through a stream, measured in units of volume per unit time; the exit of groundwater to the surface (opposite of recharge).

DISCONTINUOUS REACTION SERIES: A reaction series in which the end members have different crystal structures (are distinct mineral phases). (Compare Continuous reaction series.)

DISCORDANT INTRUSION: An intrusive igneous rock that has contacts with the country rock cutting across bedding or foliation planes. (Compare Concordant intrusion.)

DISTRIBUTARY: A smaller branch of a large stream that receives water from the main channel; the opposite of a tributary.

DIVERGENT PLATE BOUNDARY: A boundary at which Earth's plates move apart and new lithosphere is created; the site of mid-ocean ridges, shallow-focus earthquakes, and volcanism.

DIVIDE: A ridge of high ground separating two drainage basins emptied by different streams.

DOLOSTONE: A sedimentary rock composed primarily of dolomite, a carbonate mineral with the general formula CaMg(CO₃)₂.

DOME (TECTONIC): A round or elliptical anticlinal upwarp of strata in which strata dip away in all directions from the high point. (See also Volcanic dome.)

DRAAS: Extremely large (1 km or more long and over 100 m high) composite of sand dunes found in deserts.

DRAINAGE BASIN: A region of land surrounded by divides and crossed by streams that funnel all its water into the network of streams draining the area, usuually to converge eventually to one river or lake.

DRAINAGE NETWORK: The pattern of tributaries, large and small, of a stream system.

DRIFT (GLACIAL): A collective term for all the rock, sand, and clay that are deposited by a glacier either as till or as outwash.

DRUMLIN: A smooth, streamlined hill composed of till and, in many cases, bedrock.

DRY WASH: An intermittent streambed in a desert canyon that carries water only briefly after a rain; called "wadi" in the Near East.

DUCTILE MATERIAL: A material that can undergo considerable change in shape by plastic deformation before rupture occurs; the opposite of a brittle material.

DUNE: An elongated mound of sand formed by wind or water.