ASBOG FG PG key terms

A ranking of ten reference minerals from talc (1) to diamond (10) that expresses resistance to scratching; geologists use it to estimate the relative hardness of an unknown mineral specimen in the field.

The tendency of a mineral to break along specific planes of weak atomic bonding, producing flat, reflective surfaces; the number of planes and the angles between them are key identification criteria.

The condition in which the lithosphere floats in gravitational balance on the denser asthenosphere, so thick or low-density crust stands higher than thin or dense crust, and loads are compensated at depth.

An ordered sequence describing the temperatures at which common silicate minerals crystallize from cooling magma, from olivine and calcium-rich plagioclase first to quartz and potassium feldspar last.

Incomplete melting of rock, usually at depth, that produces a silica-rich liquid and leaves a refractory solid residue behind; it is the dominant process that generates magma in the crust and mantle.

Separation of early-formed crystals from a cooling magma, which shifts the remaining liquid toward more silica-rich compositions; it is a principal cause of magmatic differentiation within an intrusion.

The rate at which temperature rises with depth in the Earth, commonly about 20 to 30 degrees Celsius per kilometer in continental crust; it governs metamorphic grade and the depth of hydrocarbon generation.

The ratio of an element's concentration in a mineral to its concentration in the coexisting melt or fluid; it determines whether an element behaves compatibly or incompatibly during melting and crystallization.

A planar fabric in metamorphic rock produced by aligned platy minerals, compositional layering, or preferred crystal orientation; it develops perpendicular to the direction of maximum compressive stress.

The principle that a vertical succession of sedimentary facies records the lateral migration of neighboring depositional environments, so facies found side by side can also stack one above another.

A body of sedimentary rock distinguished by its lithology, sedimentary structures, and fossil content, reflecting a particular depositional environment or a restricted set of environmental conditions.

A surface marking a gap in the rock record where erosion or non-deposition removed time; angular unconformities, disconformities, and nonconformities are the recognized varieties.

A fossil species with a short stratigraphic range and broad geographic distribution, used to correlate rock units between regions and to assign relative ages.

The low-temperature physical, chemical, and biological changes that alter sediment after deposition and before metamorphism, including compaction, cementation, dissolution, and recrystallization.

Inclined layers nested within a sedimentary bed, formed as ripples or dunes migrate; the dip direction of the foreset layers indicates the trend of the ancient current or wind.

Two measurements that define the orientation of a planar geologic feature: strike is the compass direction of the horizontal line on the plane, and dip is the angle of steepest descent.

The rock mass lying above a dipping fault surface; its relative movement distinguishes normal faults, where it drops, from reverse faults, where it rises.

A fold whose two limbs dip away from a central axis, with the oldest exposed rocks in the core; it commonly forms a structural trap that concentrates oil and gas.

The earthquake mechanism in which strain accumulates elastically in rock across a locked fault and is suddenly released as slip, radiating seismic waves.

A low-angle or nearly horizontal detachment fault along which an upper rock mass slides over a footwall, typically lubricated by a weak layer such as salt or shale.

The lowest elevation to which a stream can erode, ultimately sea level but locally a lake, reservoir, or resistant ledge; a change in base level drives incision or deposition along the channel.

The steepest slope angle at which loose granular material stays stable, typically about 30 to 35 degrees for dry sand; it controls the profile of talus slopes and dunes.

A landscape developed by dissolution of soluble rock such as limestone or gypsum, characterized by sinkholes, caves, disappearing streams, and poorly developed surface drainage.

The fraction of a rock or sediment volume that consists of void space; it controls how much water, oil, or gas a material can store.

A measure of how readily a fluid moves through a porous medium under a given hydraulic gradient, reflecting both the permeability of the medium and the density and viscosity of the fluid.

The volume of water that drains by gravity from a unit volume of saturated aquifer material; it approximates the effective porosity available to pumping wells in unconfined aquifers.

An imaginary surface representing the height to which water would rise in a well completed in a confined aquifer; it expresses hydraulic head and the direction of groundwater flow.

A roughly conical downward distortion of the water table or potentiometric surface around a pumping well, created by withdrawal and shaped by aquifer properties and pumping rate.

Water-content thresholds, including the liquid, plastic, and shrinkage limits, that define the consistency states of fine-grained soil and are used to classify soils and predict engineering behavior.

Loss of shear strength in saturated loose sandy soil when earthquake shaking raises pore-water pressure until the material temporarily behaves as a fluid, causing settlement and foundation failure.

The ratio of resisting forces or available strength to driving forces or applied stress on a slope, foundation, or structure; values near one indicate marginal stability.

A measure of rock mass quality calculated as the percentage of drill core recovered in intact pieces longer than 10 centimeters, widely used in rock mass classification for foundations and tunnels.

The minimum concentration of a commodity in rock that makes mining a block economic; it separates ore from waste and depends on metal prices, operating costs, and recovery.

A fine-grained, organic-rich sedimentary rock, typically shale or limestone, that generated hydrocarbons when buried and heated to maturity.

A porous and permeable rock, such as sandstone or fractured limestone, that stores and transmits oil, gas, or water in quantities that can be produced economically.

The temperature interval, roughly 60 to 150 degrees Celsius or about 2 to 4 kilometers depth, in which kerogen in source rock matures into liquid hydrocarbons; hotter conditions yield gas.