The appearance or quality of light reflected from the surface of a mineral.

A group of rock-forming minerals, including Augite. They are characterized by dark color and two directions of cleavage at 90 degrees. They are common in dark-colored igneous rocks.

The process by which most igneous rocks melt. Since individual minerals have different melting points, most igneous rocks melt over a temperature range of a few hundred degrees. If the liquid is squeezed out after some melting has occurred, a melt with a higher silica content results.

The formation and growth of a crystalline solid from a liquid or gas.

A tabular-shaped intrusive igneous feature that cuts through the surrounding rock.

Laccolith

Nuée ardente

Vesicular ejecta that is the product of basaltic magma.

• Beds
• Parallel layers of sedimentary rock

Cementation

A type of lithification in which precipitates from water percolate through the sediment, filling open spaces and joining particles into a solid mass.

Compaction

A type of lithification in which the weight of overlying material compresses more deeply buried sediment.

Diagenesis

A collective term for all the chemical, physical, and biological changes that take place after sediments are deposited and during and after lithification.

Lithification

The process, generally cementation and/or compaction, of converting sediments to solid rock.

Sedimentary rock

A rock composed of the particles, precipitates, or organic debris resulting from the erosion, transportation and deposition of the products of weathering.

Strata

Parallel layers of sedimentary rock.

Angularity

A measure of the degree to which a sedimentary clast’s edges and corners are rounded.

Graded bed

A sediment layer characterized by a decrease in sediment size from bottom to top.

Rounding

The reduction in the angularity of a sedimentary clast’s edges and corners, usually by abrasion experienced during transportation.

Sorting

The degree of similarity in particle size in sediment or sedimentary rock.

Biochemical

A type of chemical sediment that forms when material dissolved in water is precipitated by water-dwelling organisms.

Breccia

A poorly sorted detrital sedimentary rock composed of large angular clasts.

Chemical sedimentary rock

Sedimentary rock consisting of material that was precipitated from water by either inorganic or organic means.

Chert

A chemical sedimentary rock composed of microcrystalline quartz.

Detrital sedimentary rock

Rocks that form from the accumulation of materials that originate and are transported as solid particles derived from both mechanical and chemical weathering.

Dolostone

A chemical sedimentary rock composed of the mineral dolomite.

Evaporite

A sedimentary rock formed of material deposited from solution by evaporation of the water.

Fossil

The remains or traces of organisms preserved from the geologic past.

Limestone

A chemical sedimentary rock composed of the mineral limestone.

Nonclastic texture

A term for the texture of sedimentary rocks in which the minerals form a pattern of interlocking crystals.

Organic sedimentary rock

Sedimentary rock composed of organic carbon from the remains of plants that died and accumulated on the floor of a swamp.

Sandstone

A detrital sedimentary rock composed of sand-sized particles.

Shale

A detrital sedimentary rock composed of silt and clay-sized particles

Alluvial fan

A fan-shaped deposit of sediment formed when a stream’s slope is abruptly reduced.

Asymmetrical ripple mark

A wave of sand formed by a directional water or wind current, so that the wave is tipped in the direction of the current.

Crossbedding

Structure in which relatively thin layers are inclined at an angle to the main bedding.

Deep marine environment

Deposition environment that occupies the portion of the seafloor that lies between the continental margin and the oceanic ridge system.

Delta environment

Depositional environment associated with dunes, or hills/ridges of wind-deposited sand.

Dune environment

Depositional environment associated with dunes, or hills/ridges of wind-deposited sand.

Environment of deposition

A geographic setting where sediment accumulates under a particular set of geologic processes and environmental conditions

Evaporite

A sedimentary rock formed of material deposited from solution by evaporation of the water.

Facies

A portion of a rock unit that possesses a distinctive set of characteristics that distinguishes it from other parts of the same unit.

Fossil

The remains or traces of organisms preserved from the geologic past.

Glacial environment

Depositional environments associated with glaciers and glaciation.

Graded bed

A sediment layer characterized by a decrease in sediment size from bottom to top.

Mudcrack

A feature in some sedimentary rocks that forms when wet mud dries out, shrinks, and cracks.

Salt flat

A white crust on the ground produced when water evaporates and leaves its dissolved materials behind.

Sedimentary environment

A geographic setting where sediment accumulates under a particular set of geologic processes and environmental conditions.

Shallow marine environment

System of depositional environments found between the shoreline and the continental rise.

Symmetrical ripple mark

A wave of sand formed by oscillation or back and forth movement of water or wind, so that sides slope equally away from the peak.

Turbidity current

A downslope movement of water in an erratic fashion often characterized by swirling, whirlpool-like eddies.

Confining pressure

An equal, all-sided pressure.

Differential stress

Forces that are unequal in different directions.

Index mineral

A mineral that is a good indicator of the metamorphic environment in which it formed.

Metamorphic facies

A group of associated minerals that are used to establish the pressures and temperatures at which rocks undergo metamorphism.

Metamorphic grade

The degree to which a parent rock changes during metamorphism.

Metamorphism

The changes in mineral composition and texture of a rock subjected to high temperatures and pressures within Earth.

Metasomatism

A metamorphic change in composition by the interaction with fluids around a magma body.

Foliation

A term for linear arrangement of textural features often exhibited by metamorphic rocks.

Gneiss

A high-grade foliated metamorphic rock with a banded appearance.

Gneissic texture

A texture of metamorphic rocks in which dark and light silicate minerals are separated, giving the rock a banded appearance.

Marble

An unfoliated metamorphic rock composed of calcite.

Nonfoliated texture

A texture of metamorphic rocks in which there is no linear arrangement of textural features.

Parent rock

The rock from which a metamorphic rock formed.

Phyllite

a foliate metamorphic rock that is made up mainly of very fine-grained mica.

Quartzite

An unfoliated metamorphic rock composed of quartz.

Rock cleavage

The tendency of rocks to split along parallel, closely spaced surfaces.

Schist

A medium to coarse grained foliated metamorphic rock in which platy minerals are visible.

Schistosity

A type of foliation characterized by parallel arrangement of platy minerals.

Slate

A low-grade foliated metamorphic rock that is fine-grained and composed of undiscernible platy minerals.

Slaty cleavage

The type of foliation in which there is a parallel arrangement of fine-grained metamorphic minerals.

Aureole

A zone or halo of contact metamorphism found in the country rock surrounding an igneous intrusion

Burial metamorphism

Low-grade metamorphism that occurs in the lowest layers of very thick accumulations of sedimentary strata.

Contact metamorphism

Changes in rock caused by the heat from a nearby magma body.

Hydrothermal metamorphism

Chemical alterations that occur as hot, ion-rich water circulates through fractures in rock.

Hydrothermal solution

The hot, watery solution that escapes from a mass of magma during the latter stages of crystallization.

Impact metamorphism

Metamorphism that occurs when meteorites strike Earth’s surface.

Metasomatism

A metamorphic change in composition by the interaction with fluids around a magma body.

Regional metamorphism

Metamorphism associated with large-scale mountain building.

Shock metamorphism

Metamorphism caused when high-speed projectiles (meteorites) impact Earth.

Subduction Zone Metamorphism

High-pressure, low-temperature metamorphism that occurs where sediments are carried to great depths by a subducting plate.

Thermal metamorphism

Changes in rock caused by the heat from a nearby magma body.

Biomass

Organic material that is renewable energy derived from trees, crops and water.

Fossil fuel

General term for any hydrocarbon that may be used as fuel, including coal, oil, natural gas, bitumen from tar sands, and shale oil.

Gas Hydrates

Compact chemical structures made of water and natural gas (usually methane) that occur in permafrost and under the ocean floor at depths greater than 1720 feet.

Geothermal energy

Natural steam used for power generation.

Hydroelectric power

Electricity generated by falling water that is used to drive turbines.

Nonrenewable resource

A mineral resource that forms or accumulates over such long time spans that it must be considered as fixed in total quantity.

Nuclear fission

The splitting of atomic nuclei into smaller nuclei, causing neutrons to be emitted and heat energy to be released.

Renewable resource

A resource that is virtually inexhaustible or that can be replenished over relatively short time spans.

Cap rock

An impermeable layer that constitutes an oil trap by preventing mobile oil and gas from escaping at the surface.

Oil trap

A set of impermeable layers and/or geologic structures that result in the accumulation of oil and gas.

Reservoir rock

The porous, permeable portion of an oil trap that yields oil and gas.

Disseminated deposit

Any economic mineral deposit in which the desired mineral occurs as scattered particles in the rock but in sufficient quantity to make the deposit an ore.

Hydrothermal solution

The hot, watery solution that escapes from a mass of magma during the latter stages of crystallization.

Mineral resource

All discovered and undiscovered deposits of a useful mineral that can be extracted now or at some time in the future.

Nonmetallic mineral resource

A mineral resource that is not a fuel or processed for the metals it contains.

Ore

Usually a useful metallic (and certain non-metallic) mineral that can be mined at a profit.

Pegmatite

A very coarse-grained igneous rock commonly found as dike associated with a large mass of plutonic rock that has smaller crystals.

Placer

Deposit formed when heavy minerals are mechanically concentrated by currents, most commonly streams and waves.

Reserve

Already identified deposits from which minerals can be extracted profitably.

Secondary enrichment

The concentration of minor amounts of metals that are scattered through unweathered rock into economically valuable concentrations by weathering processes.

Vein deposit

A mineral filling a fracture or fault in a host rock.

Exfoliation dome

Large dome-shaped structure, usually composed of granite, formed by sheeting.

Frost wedging

The mechanical breakup of rock caused by the expansion of freezing water in cracks and crevices.

Mechanical weathering

The physical disintegration of rock, resulting in smaller fragments.

Regolith

The layer of rock and mineral fragments that nearly everywhere covers Earth’s land surface.

Salt crystal growth

salt crystallization (haloclasty) causes disintegration of rocks when saline solutions seep into cracks and joints in rock

Sheeting

A mechanical weathering process characterized by the splitting off of slablike sheets of rock.

Acid mine drainage

Mine drainage water that has been rendered acidic by the chemical alteration of pyrite exposed by the mining process.

Chemical weathering

The processes by which the internal structure of a mineral is altered by the removal and/or addition of elements.

Dissolution

A common form of chemical weathering, it is the process of dissolving into a homogeneous solution, as when an acidic solution dissolves limestone.

Hydrolysis

A chemical weathering process in which minerals are altered by chemically reacting with water and acids.

Joints

Fractures in rock along which there has been no movement.

Oxidation

The removal of one or more electrons from an atom or ion.

Spheroidal weathering

Any weathering process that tends to produce a spherical shape from an initially blocky shape.

Differential weathering

The variation in the rate and degree of weathering caused by such factors as mineral makeup, degree of jointing, and climate

Eluviation

The washing out of fine soil components from the A horizon by downward-percolating water.

Horizon

A layer in a soil profile.

Humus

Organic matter in soil produced by the decomposition of plants and animals.

Leaching

The depletion of soluble materials from the upper soil by downward-percolating water.

Parent material

The material upon which a soil develops.

Regolith

The layer of rock and mineral fragments that nearly everywhere covers Earth’s land surface.

Soil

A combination of mineral and organic matter, water, and air; that portion of the regolith that supports plant growth.

Soil profile

A vertical section through a soil, showing its succession of horizons and the underlying parent material.

Soil taxonomy

A soil classification system consisting of six hierarchical categories based on observable soil characteristics.

Angle of repose

The steepest angle at which loose material remains stationary without sliding downslope.

Mass wasting

The downslope movement of rock, regolith, and soil under the direct influence of gravity.

Creep

The slow downhill movement of soil and regolith.

Debris flow

A flow of soil and regolith containing a large amount of water.

Debris slide

The rapid slide of a mass of rock downslope along planes of weakness.

Earthflow

The downslope movement of water-saturated, clay-rich sediment.

Fall

A type of movement common to mass-wasting processes that refers to the freefalling of detached individual pieces of any size.

Flow

A type of movement common to mass-wasting processes in which water-saturated material moves downslope as a viscous fluid.

Lahar

Debris flows on the slopes of volcanoes that result when unstable layers of ash and debris become saturated and flow downslope, usually following stream channels.

Mudflow

A flow of soil and regolith containing a large amount of water.

Permafrost

Any permanently frozen subsoil.

Rock avalanche

The very rapid downslope movement of rock and debris; often aided by a layer of air trapped beneath the debris.

Rockslide

The rapid slide of a mass of rock downslope along planes of weakness.

Slide

A movement common to mass-wasting processes in which the material moving downslope remains fairly coherent and moves along a well-defined surface.

Slump

The downward slipping of a mass of rock or unconsolidated material moving as a unit along a curved surface.

Solifluction

Slow, downslope flow of water-saturated materials common to permafrost areas.

Talus slope

An accumulation of rock debris at the base of a cliff.

Angular unconformity

An unconformity in which the older strata dip at an angle different from that of the younger beds.

Cenozoic era

A time span on the geologic time scale beginning about 65.5 million years ago, following the Mesozoic era.

Conformable layers

Rock layers that were deposited or emplaced parallel to each other.

Correlation

Establishing the equivalence of rocks of similar age in different areas.

Cross-cutting

A principle of relative dating, in which a rock or fault is younger than any rock or fault through which it cuts.

Disconformity

A type of unconformity in which the beds above and below are parallel.

Epoch

A unit of the geologic time scale that is a subdivision of a period.

Era

A major division of the geologic time scale.

Fossil

The remains or traces of organisms preserved from the geologic past.

Fossil succession

Fossil organisms succeed one another in a definite and determinable order, and any time period can be recognized by its fossil content.

Geologic time scale

The division of Earth history into blocks of time – eons, eras, periods, and epochs.

Inclusions

Pieces of one rock unit contained within another.

Index fossil

A fossil that is associated with a particular span of geologic time.

Lateral Continuity

A principle which states that sedimentary beds originate as continuous layers that extend in all directions until they grade into a different type of sediment or thin out at the edge of a sedimentary basin.

Mesozoic era

A time span on the geologic time scale between the Paleozoic and Cenozoic eras (between 238 and 65.5 million years).

Nonconformity

An unconformity in which older metamorphic or igneous rocks are overlain by younger sedimentary strata.

Original horizontality

Layers of sediment that are generally deposited in a horizontal or nearly horizontal position.

Paleozoic era

A time span on the geologic time scale between the Precambrian and Mesozoic eras (between 542 and 251 million years).

Period

A basic unit of the geologic time scale that is a subdivision of an era.

Precambrian

All geologic time prior to the Phanerozoic eon.

Relative dating

Rocks and structures are placed in their proper chronological sequence or order.

Superposition

In any undeformed sequence of sedimentary rocks, each bed is older than the one above and younger than the one below.

Unconformity

A surface that represents a break in the rock record, caused by erosion and nondeposition.

Half-life

The time required for one-half of the atoms of a radioactive substance to decay.

Isotope

Varieties of the same element that have different mass numbers; their nuclei contain the same number of protons but different numbers of neutrons.

Radioactivity

The spontaneous decay of certain unstable atomic nuclei.

Radiocarbon dating

The radioactive isotope of carbon is produced continuously in the atmosphere and used in dating events from the very recent geologic past. Also known as carbon-14 dating.