GeoMiner Geology and Mining Glossary

Geology and mining glossary terms

• Ablation — The process by which ice and snow are removed from a glacier or snowfield, primarily through melting, evaporation, sublimation, or calving.
• Accretion — The gradual accumulation of material on a surface, whether by sediment deposition, tectonic plate addition, or crystal growth.
• Adit — A horizontal or nearly horizontal entrance to a mine, driven into the side of a hill or mountain to intersect an ore body or provide access, drainage, or…
• Alluvial — Pertaining to sediment deposited by flowing water, particularly by rivers and streams outside their normal channels during flood events.
• Amorphous — Describing a solid material that lacks the long-range, repeating atomic order characteristic of crystalline substances.
• Andesite — An intermediate volcanic extrusive igneous rock with a silica content of approximately 57–63% by weight, typically gray to dark gray with a fine-grained…
• Anticline — A fold in layered rock in which the strata dip away from the fold axis on both sides, forming an arch-like shape.
• Aphanitic — Describing an igneous rock texture in which individual mineral grains are too small to be distinguished with the unaided eye, typically less than 1 mm in…
• Aquifer — A body of permeable rock, sediment, or soil that stores and transmits groundwater in sufficient quantities to supply wells or springs.
• Arkose — A coarse-grained sandstone containing at least 25% feldspar, typically derived from the rapid erosion of granitic or gneissic source rocks under…
• Assay — A quantitative chemical analysis performed to determine the concentration of a specific element or mineral in a rock, ore, or soil sample.
• Aureole — The zone of thermally metamorphosed rock surrounding an igneous intrusion, also called a contact aureole or metamorphic aureole.
• Basalt — A dark-colored, fine-grained aphanitic mafic volcanic rock with a silica content of approximately 45–52% by weight, composed primarily of plagioclase…
• Batholith — A large body of intrusive igneous rock plutonic rock with an exposed surface area exceeding 100 km², formed by the coalescence of multiple smaller…
• Bedding — The layered arrangement of sedimentary rocks, where each bed or stratum represents a distinct episode of deposition separated from adjacent beds by…
• Benchmark — A surveyed reference point with a precisely established elevation, used as a datum for topographic mapping, construction, and geophysical measurements.
• Biogenic — Produced by or derived from living organisms. In geology, biogenic materials include skeletal remains shells, bones, coral , organic matter peat, coal…
• Biotite — A common dark-colored black to dark brown sheet silicate mineral of the mica group, with the generalized formula K Mg,Fe ₃ AlSi₃O₁₀ OH ₂.
• Botryoidal — A crystal habit in which a mineral forms smooth, rounded, grape-like masses composed of radiating crystal fibers growing from multiple nucleation points.
• Breccia — A coarse-grained rock composed of angular fragments clasts greater than 2 mm in diameter cemented in a finer-grained matrix.
• Caldera — A large, roughly circular volcanic depression formed by the collapse of the ground surface into a partially emptied magma chamber following a major…
• Cementation — The diagenetic process by which loose sediment grains are bound together by minerals precipitated from pore waters, transforming unconsolidated sediment…
• Chert — A hard, dense, microcrystalline or cryptocrystalline sedimentary rock composed almost entirely of silica SiO₂ , usually in the form of chalcedony and…
• Clast — An individual fragment or grain of rock or mineral produced by the mechanical weathering and erosion of a pre-existing rock.
• Cleavage (mineral) — The tendency of a mineral to break along flat planes of weakness determined by the arrangement of atoms in its crystal structure.
• Cleavage (rock) — A type of secondary foliation in metamorphic rocks formed by the parallel alignment of platy minerals micas, chlorite or by the dissolution and removal…
• Columnar jointing — A pattern of regularly spaced, polygonal typically hexagonal fractures that form in cooling lava flows and shallow intrusions as the rock contracts…
• Conchoidal fracture — A type of fracture that produces smooth, curved surfaces resembling the interior of a shell, typically with concentric ridges radiating from the point of…
• Concordant — Describing a geological feature—typically an igneous intrusion or a structural boundary—that is parallel to the bedding or foliation of the surrounding…
• Concretion — A hard, compact mass of mineral matter formed within sedimentary rock by the localized precipitation of cementing material usually calcite, silica…
• Conformable — Describing a stratigraphic succession in which layers were deposited in an unbroken, continuous sequence without significant time gaps unconformities .
• Conglomerate — A coarse-grained clastic sedimentary rock composed predominantly of rounded clasts greater than 2 mm in diameter pebbles, cobbles, or boulders cemented…
• Contact metamorphism — Metamorphism caused by heat transfer from an igneous intrusion into the surrounding country rock, without significant directed pressure.
• Country rock — The pre-existing rock into which an igneous intrusion is emplaced or through which a mineral vein or ore body is hosted.
• Craton — A stable, ancient portion of continental lithosphere that has remained relatively undeformed since the Precambrian, typically older than 1.5 billion…
• Cross-bedding — Internal layering within a sedimentary bed that is inclined at an angle to the main bedding planes, formed by the deposition of sediment on the lee…
• Crust — The outermost solid layer of the Earth, separated from the underlying mantle by the Mohorovičić discontinuity Moho .
• Cryptocrystalline — Describing a material composed of crystals so small that they cannot be resolved under a standard optical microscope, typically less than 1 micrometer.
• Crystal system — One of seven categories into which all crystalline minerals are classified based on the symmetry of their unit cell—the smallest repeating structural…
• Cuprite — A copper oxide mineral Cu₂O with an isometric crystal system, a distinctive deep red color, and an adamantine to submetallic luster.
• Dacite — An intermediate to felsic volcanic rock with a silica content of approximately 63–69% by weight, typically light gray with a porphyritic texture…
• Delta — A landform created by the deposition of sediment at the mouth of a river where it enters a standing body of water such as a lake, sea, or ocean.
• Denudation — The overall lowering of a land surface through the combined processes of weathering, mass wasting, and erosion.
• Diagenesis — All physical, chemical, and biological changes that occur in sediment after deposition and during and after lithification, at temperatures and pressures…
• Dike — A tabular or sheet-like body of igneous rock that cuts across is discordant to the bedding or foliation of the surrounding country rock.
• Diluvium — An obsolete geological term formerly used to describe surficial deposits of gravel, sand, and clay attributed to catastrophic flooding—originally the…
• Dip — The angle between a geological surface bedding plane, fault, foliation, or vein and the horizontal, measured in the direction of steepest descent.
• Discordant — Describing a geological feature that cuts across the bedding, foliation, or other pre-existing structure of the surrounding rock.
• Dolomite — Both a mineral CaMg CO₃ ₂ and a sedimentary rock dolostone composed primarily of that mineral. The mineral dolomite is a calcium-magnesium carbonate with…
• Drumlin — A streamlined, elongated hill of glacial till unsorted glacial sediment shaped by the flow of an overriding ice sheet.
• Ductile deformation — Permanent deformation of rock that occurs without fracturing, through mechanisms such as crystal-plastic flow, dislocation creep, diffusion creep, and…
• Effusive eruption — A volcanic eruption characterized by the relatively gentle outpouring of lava onto the surface, as opposed to explosive fragmentation of magma.
• Epidote — A calcium-aluminum-iron silicate mineral Ca₂ Al,Fe³⁺ ₃ SiO₄ ₃ OH with a distinctive pistachio-green to yellowish-green color, monoclinic crystal system…
• Epigenetic — Describing a geological feature or mineral deposit that formed after the host rock was already in place, as opposed to syngenetic formed at the same time…
• Erosion — The physical removal and transport of weathered rock and soil material from one location to another by natural agents including water, wind, ice, and…
• Erratic — A rock or boulder transported by glacial ice and deposited far from its source, typically resting on bedrock or sediment of a different composition.
• Esker — A long, sinuous ridge of stratified sand and gravel deposited by meltwater flowing in tunnels within or beneath a glacier.
• Exfoliation — A weathering process in which concentric sheets or slabs of rock peel away from an exposed rock surface, typically driven by pressure release unloading…
• Extrusive — Describing igneous rock that formed from magma erupted at the Earth's surface as lava or pyroclastic material , as opposed to intrusive rock that…
• Facies — A body of rock with distinctive lithological, biological, or chemical characteristics that reflect specific depositional conditions.
• Fault — A fracture or zone of fractures in the Earth's crust along which there has been measurable displacement of the rocks on either side.
• Feldspar — The most abundant mineral group in the Earth's crust, comprising framework silicates with the general formula K,Na,Ca Al,Si ₄O₈.
• Felsic — Describing igneous rocks or magmas rich in light-colored minerals such as quartz, feldspar, and muscovite, typically with silica content greater than…
• Fluvial — Pertaining to rivers and streams, or to the processes, landforms, and deposits produced by flowing water in channels.
• Fold — A bend or curvature in originally planar geological surfaces bedding, foliation, or veins produced by ductile deformation under compressive or shear…
• Foliation — Any planar structural feature in a metamorphic rock produced by the preferred alignment of minerals, compositional layering, or closely spaced fractures.
• Fossil — Any preserved remains, trace, or impression of an organism that lived in the past, typically older than approximately 10,000 years.
• Friable — Describing a rock or sediment that crumbles easily when handled, due to poor cementation, abundant porosity, or weak grain-to-grain contacts.
• Fumarole — A vent or opening in the Earth's surface from which volcanic gases and steam are emitted, typically found in active volcanic areas, geothermal fields…
• Gabbro — A dark-colored, coarse-grained phaneritic mafic intrusive igneous rock composed primarily of calcium-rich plagioclase feldspar and pyroxene augite , with…
• Gangue — The commercially worthless rock and mineral material that surrounds or is intermixed with valuable ore minerals in a deposit.
• Geode — A roughly spherical, hollow rock cavity lined with inward-growing crystals, typically quartz amethyst, clear quartz, smoky quartz or calcite.
• Geosyncline — A now-obsolete term formerly used to describe large, elongate troughs in the Earth's crust that accumulated thick sequences of sediment, which were later…
• Geothermal gradient — The rate at which temperature increases with depth in the Earth, averaging approximately 25–30°C per kilometer in continental crust.
• Gneiss — A high-grade metamorphic rock characterized by gneissic banding—alternating light and dark compositional layers—formed under high temperature and…
• Gossan — An intensely oxidized, weathered cap of rock overlying a sulfide ore deposit, typically characterized by a porous, cellular texture and vivid red…
• Graben — A downdropped block of crust bounded on both sides by parallel normal faults, forming a linear valley or trough.
• Graded bedding — A sedimentary structure in which grain size decreases systematically from the bottom to the top of a single bed, with the coarsest particles at the base…
• Granite — A coarse-grained phaneritic felsic intrusive igneous rock composed primarily of quartz 20–60% , alkali feldspar, and plagioclase feldspar, with minor…
• Greenschist — A metamorphic rock of low to intermediate grade characterized by the presence of green minerals such as chlorite, epidote, and actinolite, formed under…
• Groundmass — The fine-grained or glassy matrix of a porphyritic igneous rock that surrounds the larger, more prominent phenocrysts.
• Hardness — A mineral's resistance to scratching, quantified by the Mohs hardness scale 1–10 . Hardness is determined by the strength and density of chemical bonds…
• Hornblende — A common dark-colored amphibole mineral with the generalized formula Ca,Na ₂–₃ Mg,Fe,Al ₅ Al,Si ₈O₂₂ OH ₂, found in a wide variety of igneous and…
• Hornfels — A fine-grained, massive, non-foliated metamorphic rock produced by contact metamorphism in the aureole surrounding an igneous intrusion.
• Horst — An upstanding block of crust bounded on both sides by parallel normal faults, forming an elevated ridge or plateau between adjacent grabens.
• Hydrothermal — Pertaining to hot, aqueous water-based fluids and the geological processes, alteration, and mineral deposits they produce.
• Igneous — Describing rocks formed by the solidification of magma or lava. Igneous rocks are classified as intrusive plutonic if they crystallized below the…
• Inclusion — A fragment of foreign material enclosed within a mineral crystal, rock, or gemstone during its formation.
• Index fossil — A fossil taxon that is useful for correlating and dating the age of rock strata in which it is found.
• Intrusive — Describing igneous rock that crystallized from magma below the Earth's surface, enclosed within pre-existing country rock.
• Isomorphism — The phenomenon in which two or more minerals have the same crystal structure but different chemical compositions, with ions of similar size and charge…
• Isostasy — The principle that the Earth's crust floats in gravitational equilibrium on the denser, partially molten asthenosphere, much as icebergs float in water.
• Joint — A fracture in rock along which there has been no measurable displacement, distinguishing it from a fault.
• Jointing pattern — The systematic arrangement of joints fractures without displacement in a rock body, typically forming two or more intersecting sets with regular…
• Karst — A landscape and subsurface drainage system developed in soluble rocks—primarily limestone, dolomite, marble, and gypsum—through chemical dissolution by…
• Kimberlite — A rare, volatile-rich, potassic ultramafic igneous rock that originates from great depth 150 km in the mantle and is emplaced explosively in narrow…
• Laccolith — A concordant intrusive igneous body that has domed up the overlying strata, creating a mushroom-shaped or lens-shaped mass with a flat floor and an…
• Lamination — Very thin bedding layers, typically less than 1 cm thick, formed by subtle changes in grain size, composition, or organic content during sediment…
• Lapilli — Pyroclastic fragments ejected during a volcanic eruption that range in size from 2 to 64 mm in diameter, intermediate between volcanic ash <2 mm and…
• Laterite — A deeply weathered, iron-rich and aluminum-rich residual soil or rock formed by intense chemical weathering under hot, humid tropical to subtropical…
• Lava — Molten rock magma that has erupted onto the Earth's surface through a volcanic vent or fissure. Once on the surface, lava loses dissolved gases and…
• Lava dome — A mound of viscous lava that piles up over and around a volcanic vent, formed when highly viscous felsic to intermediate lava dacite or rhyolite is…
• Lava tube — A natural tunnel formed within a lava flow when the outer surface solidifies and insulates the interior, allowing the still-molten lava to continue…
• Lineation — Any linear structural feature in a rock, including stretched minerals, elongated pebbles, intersection of foliation planes, fold axes, slickenlines on…
• Lithification — The process by which loose, unconsolidated sediment is transformed into solid sedimentary rock, primarily through compaction and cementation.
• Lithology — The physical and compositional characteristics of a rock unit as observed in hand specimen or outcrop, including mineral composition, grain size…
• Lithosphere — The rigid outer shell of the Earth, comprising the crust and the uppermost portion of the mantle above the asthenosphere.
• Loess — Wind-deposited eolian silt, typically buff to yellow-brown in color, consisting predominantly of angular quartz grains 20–50 micrometers in diameter.
• Lopolith — A large, saucer-shaped or funnel-shaped concordant intrusion that sags downward into the underlying country rock.
• Luster — The quality and intensity of light reflected from a mineral's surface, determined by its refractive index, absorption coefficient, and surface texture.
• Mafic — Describing igneous rocks and magmas rich in magnesium and iron, with silica content typically between 45–52%.
• Magma — Molten or partially molten rock beneath the Earth's surface, containing dissolved gases and suspended crystals.
• Mantle — The thick layer of silicate rock between the Earth's crust and outer core, extending from the Moho 5–70 km depth to the core-mantle boundary 2,900 km…
• Marble — A metamorphic rock formed by the recrystallization of limestone or dolostone under conditions of elevated temperature and pressure.
• Mass wasting — The downslope movement of rock, soil, and regolith under the direct influence of gravity, without the primary involvement of a transporting agent such as…
• Matrix — The finer-grained material that fills the spaces between larger clasts, crystals, or grains in a rock.
• Meander — A sinuous, looping curve in the course of a river, formed by the lateral erosion of the outer bank cut bank and deposition on the inner bank point bar of…
• Metamorphism — The process by which existing rocks are transformed into new rock types through changes in mineralogy, texture, and/or chemical composition in response…
• Metasomatism — A metamorphic process involving the chemical alteration of a rock by the introduction and/or removal of chemical components through interaction with…
• Migmatite — A composite rock found in high-grade metamorphic terranes that contains both metamorphic and igneous-looking components, formed when partial melting…
• Mineral — A naturally occurring, inorganic, solid substance with a definite chemical composition or a defined range of compositions and an ordered internal…
• Mohorovičić discontinuity — The boundary between the Earth's crust and the underlying mantle, identified by a sharp increase in seismic wave velocities.
• Moraine — A landform composed of glacially transported and deposited sediment till , formed at the margins or beneath a glacier.
• Mudstone — A fine-grained sedimentary rock composed predominantly of clay-sized particles <0.004 mm and silt-sized particles 0.004–0.0625 mm , with a massive…
• Normal fault — A dip-slip fault in which the hanging wall the block above the fault plane has moved downward relative to the footwall, driven by extensional tensional…
• Nuée ardente — A hot, fast-moving cloud of volcanic gas, ash, and rock fragments that flows down the flank of a volcano during an explosive eruption—essentially…
• Obsidian — A naturally occurring volcanic glass formed by the rapid cooling of felsic silica-rich lava, resulting in an amorphous solid rather than a crystalline…
• Olivine — A group of orthorhombic nesosilicate minerals with the general formula Mg,Fe ₂SiO₄, forming a complete solid solution between the magnesium end member…
• Ophiolite — A sequence of mafic and ultramafic rocks interpreted as a fragment of oceanic crust and upper mantle that has been tectonically emplaced obducted onto…
• Orogeny — A major mountain-building event caused by tectonic forces, typically involving folding, faulting, metamorphism, and igneous activity over millions of…
• Outcrop — An exposure of bedrock or ancient superficial deposits at the Earth's surface, visible and accessible for geological observation.
• Overburden — The material overlying a useful geological deposit, including soil, sediment, and rock that must be removed to access the resource beneath.
• Pahoehoe — A type of basaltic lava flow characterized by a smooth, billowy, or ropy surface texture, formed when relatively fluid lava cools while still flowing…
• Pangaea — The supercontinent that existed during the late Paleozoic and early Mesozoic eras approximately 335–175 million years ago , comprising virtually all of…
• Pegmatite — An exceptionally coarse-grained igneous rock, typically of granitic composition, with crystals generally exceeding 1 cm and sometimes reaching several…
• Peridotite — A coarse-grained ultramafic intrusive igneous rock composed primarily of olivine 40% with subordinate pyroxene and minor amounts of garnet, spinel, or…
• Permafrost — Ground soil or rock that remains at or below 0°C continuously for at least two consecutive years, regardless of whether water is present.
• Phenocryst — A relatively large, conspicuous crystal embedded in a finer-grained groundmass in a porphyritic igneous rock.
• Placer deposit — A concentration of dense, resistant, economically valuable minerals gold, platinum, diamonds, cassiterite, zircon in alluvial, beach, or other…
• Plate tectonics — The unifying theory of Earth science stating that the Earth's outer shell lithosphere is divided into a number of rigid plates that move relative to one…
• Pluton — A body of intrusive igneous rock that has crystallized from magma below the Earth's surface. Plutons are classified by size and shape: batholiths 100 km²…
• Polymorphism — The phenomenon in which a single chemical compound can crystallize in two or more different crystal structures, each representing a distinct mineral…
• Porphyritic — An igneous rock texture characterized by large crystals phenocrysts set in a finer-grained groundmass, indicating a two-stage cooling history.
• Provenance — The source area and origin of sedimentary material, including the type of rocks eroded, the tectonic setting of the source, and the conditions of…
• Pumice — A highly vesicular, low-density volcanic glass formed during explosive eruptions of viscous, gas-rich felsic or intermediate magma.
• Pyroclastic — Pertaining to fragmental material ejected from a volcanic vent during an explosive eruption, including ash <2 mm , lapilli 2–64 mm , and bombs/blocks 64…
• Pyroxene — A group of important rock-forming inosilicate single-chain minerals with the general formula XYZ₂O₆, where X = Ca, Na, Mg, Fe²⁺; Y = Mg, Fe²⁺, Fe³⁺, Al…
• Quartz — A very common rock-forming mineral composed of silicon dioxide SiO₂ in a three-dimensional framework of SiO₄ tetrahedra.
• Quartzite — A hard, non-foliated metamorphic rock composed almost entirely of recrystallized quartz, formed by the metamorphism of quartz-rich sandstone.
• Regolith — The layer of loose, heterogeneous material covering solid bedrock, including soil, weathered rock fragments, alluvium, glacial deposits, volcanic ash…
• Replacement deposit — An ore deposit formed when pre-existing rock is dissolved and simultaneously replaced by ore and gangue minerals precipitated from hydrothermal fluids.
• Reverse fault — A dip-slip fault in which the hanging wall has moved upward relative to the footwall, driven by compressional tectonic stress.
• Rhyolite — A fine-grained aphanitic felsic volcanic rock with a silica content exceeding 69%, the extrusive equivalent of granite.
• Rift — A zone of crustal extension where the lithosphere is being pulled apart, producing normal faulting, thinning of the crust, volcanism, and the formation…
• Ripple marks — Small-scale ridge-and-trough undulations on the surface of a sedimentary bed, formed by the movement of water or wind over loose sediment.
• Sandstone — A clastic sedimentary rock composed predominantly of sand-sized grains 0.0625–2 mm diameter , typically quartz, feldspar, and/or lithic rock fragments…
• Saprolite — Chemically weathered bedrock that has decomposed in place, retaining the original rock texture and structure but with a dramatically altered mineral…
• Schistosity — A type of foliation in medium-grade metamorphic rocks characterized by the visible, parallel alignment of platy and elongate minerals especially micas…
• Scoria — A dark-colored, vesicular volcanic rock formed from mafic to intermediate magma ejected during moderately explosive eruptions.
• Sedimentary rock — Rock formed at or near the Earth's surface by the accumulation and lithification of sediment clastic sedimentary rocks such as sandstone and shale , the…
• Shale — A fine-grained clastic sedimentary rock composed predominantly of clay minerals and silt-sized particles, characterized by fissility—the ability to split…
• Shield — A large area of exposed Precambrian crystalline basement rock forming the stable core of a continent—the surface expression of a craton where the ancient…
• Silicate — The largest and most important group of rock-forming minerals, all built around the SiO₄ tetrahedron as the fundamental structural unit.
• Sill — A tabular body of intrusive igneous rock that is concordant—parallel to the bedding or foliation of the surrounding country rock.
• Sinkhole — A closed depression in the ground surface formed by the dissolution of soluble bedrock limestone, dolomite, gypsum, salt or by the collapse of an…
• Skarn — A calc-silicate metamorphic rock formed by metasomatism at or near the contact between an igneous intrusion and carbonate country rock limestone or…
• Slickenside — A polished, striated surface on a fault plane produced by friction during fault movement. The striations slickenlines on the surface indicate the…
• Slump — A type of mass wasting in which a coherent block of material slides downslope along a curved concave-up failure surface, rotating backward as it moves.
• Sorting — A measure of the range of grain sizes in a sediment or sedimentary rock. Well-sorted sediments have a narrow range of grain sizes e.g., beach sand …
• Specific gravity — The ratio of the density of a mineral to the density of water 1.0 g/cm³ at 4°C , expressed as a dimensionless number.
• Stalactite — A mineral deposit, typically of calcite CaCO₃ , that hangs downward from the ceiling of a cave like an icicle.
• Stope — An underground excavation from which ore has been extracted, leaving a void in the rock. Stoping is the process of removing ore from around a mine's…
• Stratigraphy — The branch of geology concerned with the study of rock layers strata and their arrangement in time and space, including their distribution, composition…
• Streak — The color of a mineral in powdered form, obtained by rubbing the mineral across an unglazed porcelain plate streak plate .
• Strike — The compass direction bearing of a horizontal line on a tilted geological surface bedding plane, fault, foliation, or vein .
• Subduction — The process by which one tectonic plate descends beneath another into the mantle at a convergent plate boundary.
• Supergene enrichment — A process by which the metal content of an ore deposit is increased by the downward migration and reprecipitation of metals dissolved from the oxidation…
• Superposition — The principle that in an undisturbed sedimentary sequence, the oldest layers lie at the bottom and the youngest at the top.
• Syncline — A fold in layered rock in which the strata dip inward toward the fold axis from both sides, forming a trough-like shape.
• Talus — An accumulation of rock debris at the base of a cliff, steep slope, or mountain, formed by rockfall and frost wedging from the rock face above.
• Tectonic — Pertaining to the large-scale structural deformation and movement of the Earth's lithosphere, driven by forces originating within the planet mantle…
• Terrane — A fault-bounded body of rock with a geological history distinct from that of adjacent terranes, typically interpreted as having been transported from…
• Till — Unsorted, unstratified sediment deposited directly by glacial ice without reworking by meltwater. Till consists of a heterogeneous mixture of grain sizes…
• Transform fault — A plate boundary along which two tectonic plates slide horizontally past each other, with no net creation or destruction of lithosphere.
• Tsunami — A series of ocean waves generated by sudden displacement of the sea floor, typically caused by submarine earthquakes, volcanic eruptions, landslides, or…
• Tuff — A rock composed of consolidated volcanic ash and other fine pyroclastic material <2 mm diameter ejected during explosive volcanic eruptions.
• Turbidite — A sedimentary deposit formed by a turbidity current—a dense, sediment-laden underwater flow driven by gravity down a submarine slope.
• Ultramafic — Describing igneous rocks and magmas with very low silica content <45% and very high magnesium and iron content, composed predominantly of olivine and…
• Unconformity — A buried erosion surface or non-depositional surface separating two rock units of significantly different ages, representing a gap in the geological…
• Varve — A pair of distinct sediment layers one light, one dark deposited in a body of still water over the course of a single year, forming an annual cycle.
• Vein — A sheet-like body of mineral matter that fills a fracture or fissure in rock, precipitated from hydrothermal or other mineral-bearing fluids flowing…
• Vesicle — A small cavity in volcanic rock formed by the expansion and trapping of gas bubbles in solidifying lava or pyroclastic material.
• Viscosity — The resistance of a fluid to flow, determined by the strength of intermolecular bonds. In volcanology, magma viscosity is controlled primarily by silica…
• Volcanic arc — A curved chain of volcanoes formed above a subduction zone, positioned on the overriding plate approximately 100–300 km from the trench.
• Water table — The upper surface of the zone of saturation in an unconfined aquifer—the level below which all pores, fractures, and voids in the rock or soil are…
• Weathering — The in-situ breakdown of rock and mineral material at or near the Earth's surface through physical mechanical , chemical, and biological processes.
• Welded tuff — A pyroclastic rock formed when hot volcanic ash, pumice, and glass fragments are deposited at such high temperatures often 500°C that the particles fuse…
• Wentworth scale — A standardized classification system for sedimentary particles based on grain diameter, established by Chester K. Wentworth in 1922.
• Xenocryst — A single crystal found in an igneous rock that is foreign to the magma in which it occurs, having been incorporated from the country rock or from an…
• Xenolith — A fragment of pre-existing rock country rock that has been incorporated into an igneous rock during magma emplacement or eruption.
• Zeolite — A large group of hydrated aluminosilicate minerals characterized by an open, cage-like crystal structure that can reversibly adsorb and release water…
• Zone of saturation — The subsurface zone below the water table where all pore spaces, fractures, and voids in rock or sediment are completely filled with water.