D Index Glossary geology Science
D Index Glossary geology Science
Datum
A reference location or elevation which is used as a starting point for subsequent measurements. Sea level is a datum for elevation measurements. Datums can also be arbitrary such as the starting point for stream stage measurements or based upon a physical feature such as the base of a rock unit.
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 avalanche
The sudden, extremely rapid mass movement downward of entire layers of regolith along very steep slopes. Debris avalanches are generally caused by heavy rains.
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.
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.
dendrochronology
A method of absolute 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.
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.
desertification
The process through which a desert takes over a formerly nondesert 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.
detrital sediment
Sediment that is composed of transported solid fragments of preexisting igneous, sedimentary, or metamorphic rocks.
Datum plane
An artificially established, well surveyed horizontal plane against which elevations, depths, tides, etc. are measured (for example mean sea-level).
Declination
At any place on Earth, the angle between the magnetic and rotational poles.
Delta kame
A deposit having the form of a steep, flat topped hill, left at the front of a retreating continental glacier.
Density
The mass per unit volume of a substance, commonly expressed in grams/ cubic centimeter.
- Mass per unit volume, expressed in grams per cubic centimeter. Rock or formation densities are usually measured as either saturated bulk densities or grain densities. For gravity interpretation, the contrasts between rock bulk densities are of primary interest since these contrasts are responsible for the anomalous gravity field.
- Rock bulk densities have been shown to vary as a function of geologic age, lithology and depth of burial. Rock densities typically range from 1.9 g/cm3 to 3.0 g/cm3.
Deposition remnant magnetization
A weak magnetization created in sedimentary rocks by the rotation of magnetic crystals into line with the ambient field during settling.
Diatreme
A volcanic vent filled with breccia by the explosive escape of gases.
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.
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.
Dip
The angle that a rock unit, fault or other rock structure makes with a horizontal plane. Expressed as the angular difference between the horizontal plane and the structure. The angle is measured in a plane perpendicular to the strike of the rock structure.
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.
discordant
Of or being a pluton that lies perpendicular or oblique to the surrounding layers of rock. See also concordant.
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.
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
An uplift that is round or elliptical in map view with beds dipping away in all directions from a central point.The sediments have often eroded away, exposing the rocks that resulted when the molten material cooled.
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.
ductile deformation
See plastic deformation.
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.
Daughter Element
The element produced through the radioactive decay of a parent element.
Deflation
The removal of clay- and silt-size particles from a soil by wind erosion. The term can also be used in reference to the removal by wind of any unconsolidated material.
Delta
A deposit of sediment that forms where a stream enters a standing body of water such as a lake or ocean. The name is derived from the Greek letter "delta" because these deposits typically have a triangular shape in map view.
Dendritic Drainage
A stream drainage pattern that resembles the veins of a leaf in map view. Occurs mainly where the rocks below have a uniform resistance to erosion.
Density Current
A gravity-driven flow of dense water down an underwater slope. The increased density of the water is a result of a temperature difference, increased salinity or suspended sediment load.
Deposition
The settling from suspension of transported sediments. Also, the precipitation of chemical sediments from mineral rich waters.
Desert Pavement
A ground cover of granule-size and larger particles that is typically found in arid areas. This ground cover of coarse particles is a residual deposit - formed when the wind selectively removes the sand-, silt- and clay-sized materials.
Detrital
A word used in reference to sediments or sedimentary rocks that are composed of particles that were transported and deposited by wind, water or ice.
De Facto Segregation
The spatial and social separation of populations that occurs without legal sanction.
De Jure Segregation
The spatial and social separation of populations that occurs as a consequence of legal measures.
Deciduous
Forests in which the trees lose their leaves each year.
degree
A unit of angular measure: A circle is divided into 360 degrees, represented by the symbol o . Degrees are used to divide the roughly spherical shape of the Earth for geographic and cartographic purposes.
Degree Day
Deviation of one degree temperature for one day from an arbitrary standard, usually the long-term average temperature for a place.
Demography
The systematic analysis of population.
Discriminatory Shipping Rates
A transportation charge levied in a manner that is inequitable to some shippers, primarily because of those shippers' location.
Dry Farming
A type of farming practiced in semi-arid or dry grassland areas without irrigation using such approaches as fallowing, maintaining a finely broken surface, and growing drought-tolerant crops.
Diagenesis
All of the changes which happen to a sediment after deposition, excluding weathering and metamorphism. Diagenesis includes compaction, cementation, leaching and replacement.
Diatom
A one celled plant that lives in the shallow waters of lakes, streams or oceans. Many of these secrete a shell or internal parts composed of silica. Diatoms can occur in very large numbers and can make significant contributions to sea-floor or lake sediment.
Diatomite
A light colored, fine-grained siliceous sedimentary rock that forms from a sediment rich in diatom remains.
Diatom Ooze
A seafloor sediment that consists of at least 30% diatom remains.
Differentiated Planet
A planet that has layers composed of elements and minerals of different densities. As an example, Earth is a differentiated planet because it has a metal-rich core, surrounded by a rocky mantle, and covered by a crust of low-density minerals.
Discharge
The volume of water in a flowing stream that passes a given location in a unit of time. Frequently expressed in cubic feet per second or cubic meters per second. Calculated by the formula Q = A x V where Q is the discharge, A is the cross sectional area of the channel and V is the average velocity of the stream.
-the volume of water that passes a given location within a given period of time. Usually expressed in cubic feet per second.
Discontinuity
A surface separating rock layers of differing properties or compositions. (see seismic discontinuity)
Dissolved Load
The dissolved material being carried by a stream. See also load, suspended load, dissolved load.
Divide
A ridge that separates two adjacent drainage basins.
Drainage Basin
The geographic area that contributes runoff to a stream. It can be outlined on a topographic map by tracing the points of highest elevation (usually ridge crests) between two adjacent stream valleys.
Drainage Divide
The boundary between two adjacent drainage basins. Drainage divides are ridge crests (or less obvious locations where slope of the landscape changes direction). Runoff produced on one side of the ridge flows into stream "A" and runoff on the other side of the ridge flows into stream "B".
Drawdown
A lowering of the water table around a producing well. The drawdown at any given location will be the vertical change between the original water table and the level of the water table reduced by pumping.
Drift
A general term for all sedimentary materials deposited directly from the ice or melt water of a glacier.
desalinization
the removal of salts from saline water to provide freshwater. This method is becoming a more popular way of providing freshwater to populations.
drip irrigation
a common irrigation method where pipes or tubes filled with water slowly drip onto crops. Drip irrigation is a low-pressure method of irrigation and less water is lost to evaporation than high-pressure spray irrigation .
Dune
A mound or ridge of wind-blown sand. Typically found in deserts and inland from a beach. Many dunes are moved by the wind. Earthflow
A detached mass of soil that moves downslope over a curved failure surface under the influence of gravity. An earthflow is more complex than a slump; it has a higher moisture content and the moving mass of soil has some internal movement or "flow". Rates of movement are typically a few inches per year but faster rates can occur.