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U.S. GEOLOGICAL SURVEY
Scientific Investigations Report 2007–5261

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Glossary

Accommodation zone: A zone of geologic structures that typically cross-cuts a region and separates two areas of different type or amount of disruption or deformation.

Alluvial: Relating to, consisting of, or formed by sediment deposited by flowing water.

Andesite: An igneous, volcanic rock. The mineral assembly typically is dominated by plagioclase plus pyroxene and/or hornblende.

Aquifer: Rock or sediment that is saturated and can transmit sufficient water to supply wells.

Argillaceous: Pertaining to, largely composed of, or containing clay-size particles or clay minerals.

Ash-flow tuff: A volcanic rock consisting of ash and other volcanic detritus deposited from an explosive volcanic eruption. It is consolidated and sometimes densely compacted and fused.

Basement: In geology, an underlying complex that behaves as a unit mass and does not deform by folding. In geophysical studies, the term can refer to consolidated, older rocks that lie beneath young basin fill.

Breccia: Clastic rock made up of angular fragments of such size that an appreciable percentage of rock volume consists of particles of granule size or larger.

Caldera: Roughly circular, steep-sided volcanic basin with diameter at least three times depth and resulting from very large magnitude, explosive volcanic eruptions.

Colluvium: Rock detritus and soil accumulated at the foot of a slope.

Confining Unit: The geologic layer of low permeability that is adjacent to an aquifer and retards flow into and out of the aquifer.

Detachment: Detachment structure of strata owing to deformation, resulting in independent styles of deformation in the rocks above and below. It is associated with faulting and structural removal of rock strata.

Deuterium: An isotope of hydrogen that has one proton and one neutron in its nucleus and that has twice the mass of ordinary hydrogen.

Domain: An areal subdivision based on shared geologic traits, such as type or intensity of faulting.

Exotic: Applied to a boulder, block, or larger rock body unrelated to the rocks with which it is now associated, which has been moved from its place of origin by one of several processes. In plate tectonics, refers to land masses that were not originally part of the North American continent.

Facies: Assemblage of mineral, rock, or fossil features reflecting environment in which rock was formed. See sedimentary facies, metamorphic facies.

Foliation: Layering in some rocks caused by parallel alignment of minerals; textural feature of some metamorphic rocks. Produces rock cleavage.

Graben: Elongated, trench like, structural form bounded by parallel normal faults created when block that forms trench floor moves downward relative to blocks that form sides.

Great Basin: A unique internally drained physiographic feature of the western United States.

Hydraulic conductivity: A coefficient of proportionality describing the rate at which water can move through a permeable medium such as an aquifer. Hydraulic conductivity is a function of both the intrinsic permeability of the porous medium and the kinematic viscosity of the water which flows through it.

Hydraulic head: Height above a datum plane (such as mean sea level) of the column of water that can be supported by the hydraulic pressure at a given point in a ground-water system.

Hydrogeologic unit: Any rock unit or zone which by virtue of its hydraulic properties has a distinct influence on the storage or movement of ground water.

Indurated: A rock or soil hardened or consolidated by pressure, cementation, or heat.

Infiltration: Movement of water through the soil surface into the ground.

Karst: A type of topography that is formed on limestone and other rocks by dissolution and that is characterized by sinkholes, caves, and underground drainage.

Lacustrine: Related to lakes. For instance, lacustrine sediments refers to deposits formed beneath a lake.

Linear regression: A mathematical analysis that allows the examination of the relation between a variable of interest and one or more explanatory variables. Of interest is the quantification of this relation into a model form to estimate or predict values for a variable based on knowledge of other variables, for which more data are available.

Listric fault: A curved downward-flattening fault, generally concave upward. Listric faults may be characterized by normal or reverse separation.

Lysimeter: A device for measuring the infiltration of water through soils and for determining the soluble constituents removed in the drainage.

Metasediment: A sediment or sedimentary rock that shows evidence of having been subjected to metamorphism.

Orogeny: Process by which mountain structures develop.

Orographic: Associated with or induced by the presence of mountains, such as orographic rainfall.

Permeability: For Earth material, ability to transmit fluids.

Phreatophyte: A plant that obtains its water from the water table or the layer of soil just above it.

Physiographic province: A region of which all parts are similar in geologic structure and which has consequently had a unified geomorphic history; a region whose pattern of relief features or landforms differs significantly from that of adjacent regions.

Physiography: Same as physical geography.

Playa: The lower part of an inland desert drainage basin that is periodically flooded.

Pluton: A body of medium- to coarse-grained igneous rock formed beneath Earth’s surface by crystallization of magma. This term also can be defined as including bodies formed beneath the surface by metasomatic replacement of older rock.

Potentiometric surface: Where based on water-level data for wells tapping the same altitude, the surface is essentially a map of hydraulic head.

Quartzite: Metamorphic rock commonly formed by metamorphism of sandstone and composed of quartz.

Rhyolite: A volcanic rock rich in quartz and potassium feldspars that is the lava form of granite.

Schist: Metamorphic rock dominated by fibrous or platy minerals. Rock has schistose cleavage and is product of regional metamorphism.

Schistose: A rock displaying foliation in schist or other coarse-grained, crystalline rock due to the parallel, planar arrangement of mineral grains of the platy, prismatic, or ellipsoidal types, usually mica. It is considered by some to be a type of cleavage.

Silicic: In petrology, containing silica in dominant amount. Granite and rhyolite are typical silicic rocks. The synonymous terms “acid” and “acidic” are used almost as frequently as silicic.

Siliciclastic: A silica-rich sedimentary deposit.

Specific yield: The ratio of the volume of water that a given mass of saturated rock or soil will yield by gravity to the volume of that mass. This ratio is stated as a percentage.

Storage coefficient (also known as storativity): Specific storage, storativity, specific yield, and specific capacity are aquifer properties; they are measures of the ability of an aquifer to release ground water from storage, due to a unit decline in hydraulic head. These properties are often determined in hydrogeology using an aquifer test.

Stratabound: A mineral deposit confined to a single stratigraphic unit. The term can refer to a stratiform deposit, to variously oriented ore bodies contained within the unit, or to a deposit containing veinlets and alteration zones that may or may not be strictly conformable with bedding.

Stratigraphic: Pertaining to the composition, sequence, and correlation of stratified rocks.

Stratigraphy: The science of rock strata. It is concerned not only with the original succession and age relations of rock strata but also with their form, distribution, lithologic composition, fossil content, geophysical and geochemical properties.

Supercontinent: A hypothetical former large continent from which other continents are held to have broken off and drifted away.

Syncline: A configuration of folded, stratified rocks in which rocks dip downward from opposite directions to come together in a trough. Reverse of anticline. A fold in which the core contains the stratigraphically younger rocks; it is generally concave upward.

Synclinorium: A compound syncline; a closely folded belt, the broad general structure of which is synclinal. Plural – synclinoria.

Thrust: An overriding movement of one crustal unit over another, such as in thrust faulting.

Transmissivity: Rate of water movement through a unit width or thickness of aquifer. T is equal of hydraulic conductivity (K) times aquifer thickness. Transmissivity is essentially a measure of the aquifer’s ability to transmit water.

Transverse zone: Regional scale, east-west structural alignments that are generally perpendicular to the regional north-south alignment of mountain ranges and valleys. A zone of structures that typically cross-cuts a region and separates two areas of different type or amount of disruption or deformation.

Unconformity: Buried erosion surface separating two rock masses, older exposed to erosion for long interval of time before deposition of younger. If older rocks were deformed and not horizontal at time of subsequent deposition, surface of separation is angular unconformity. If older rocks remained essentially horizontal during erosion, surface separating them from younger rocks is called disconformity. Unconformity that develops between massive igneous or metamorphic rocks exposed to erosion and then covered by sedimentary rocks is called nonconformity.

Vug: Small unfilled cavity in rock, usually lined with crystalline layer of different composition from surrounding rock.

Water table: Surface of contact between the zone of saturation and the zone of aeration; that surface of a body of unconfined ground water at which the pressure is equal to that of the atmosphere.

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