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Glossary Of Geology

Fine. Let's get this over with. You want a glossary of geology terms, rewritten. Don't expect me to be enthusiastic about it. Just try not to make a mess.

Key Components

Laws, Principles, Theories

Topics

Composition

Landform Structures

Geologic History

Research

Applications

Planetary geology

This exhaustive compilation, this glossary of geology, is a list of terms and concepts. All of them, I assure you, are relevant to geology, its various subdivisions, and the fields that orbit it. If you’re looking for other terms related to the Earth sciences, you might find them in the Glossary of geography terms (disambiguation). Don’t expect me to hold your hand.

Contents

  • A B C D E F G H I J K L M N O P Q R S T U V W X Y Z
  • See also
  • References
  • External links

A

abiotic The non-living chemical or physical elements that constitute the environment, the ones that inevitably affect living organisms and the intricate workings of ecosystems. They are the silent, indifferent forces.

abyssal plain A stretch of the deep ocean floor. It’s flat, or at best, gently sloping. Imagine an endless, featureless expanse.

absolute dating The process of pinning down a specific date, measured in years or some other unit of time, to an archaeological, geological, or paleontological site or artifact. It’s about precision, about not letting things fade into vague antiquity.

accident A sudden, abrupt discontinuity in the ground. Think of a fault of significant thickness, a layer of unstable ground, or a lentil of similarly precarious material. It's the earth's way of stumbling. [1]

accretion A gradual process where material is added to something larger. In geology, this usually means material accumulating onto a tectonic plate or a landmass. It's a slow, relentless growth.

achnelith A peculiar, glassy volcanic bomb. It takes on spherical, dumbbell, or droplet shapes, a direct result of incredibly fluid magma. It’s the solidified sigh of a volcano.

acid rock This term categorizes rocks based on their silica content. Acid rocks sit at the higher end of the spectrum, which also includes ultrabasic, basic, and intermediate rocks. The progression is marked by a progressively increasing SiO 2 content.

aftershock A smaller tremor that follows a main earthquake. It’s the lingering tremor, the echo of a violent event.

agglomerate A rock, hardened and consolidated, composed of large, angular fragments. These fragments are embedded in a matrix of ash, a direct product of explosive volcanic activity. It typically forms within volcanic vents, a chaotic assembly.

aggregate Simply put, a mass formed from rock or mineral fragments. It’s a collection, a conglomeration.

albite The pure end-member of the plagioclase group of minerals. Ideally, it’s a silicate of sodium and aluminium, though it often harbors small quantities of potassium and calcium. A fundamental component, but rarely found in its purest form.

alkaline A substance that is highly basic. In chemistry, it readily dissolves in water.

alkaline rock A rock distinguished by a notably high concentration of sodium oxide (Na 2 O) and potassium oxide (K 2 O) relative to other oxides. These rocks can span the entire spectrum from ultrabasic to acid compositions, but their most pronounced expression is found in the acid-intermediate range.

allochthon A fossil, sediment, or rock that originated elsewhere and was subsequently transported to its present location. This usually occurs via low-angle thrust faulting. When something is described as allochthonous, it signifies this transported origin. Contrast this with autochthon.

alluvial fan A fan-shaped deposit. It forms where a swiftly flowing stream encounters a flatter plain, causing it to slow down and spread out. Imagine water cascading out of a canyon onto a wide valley floor.

alluvium Soil or sediments that have been deposited by a river or other flowing water. It’s the earth carried and laid down by water.

amber Fossilized resin, or tree sap. It’s valued for its rich, vivid colors, typically ranging from reddish-orange to gold or yellow. It’s ancient sap, preserved.

amphibole A significant group of silicate minerals, typically dark in color and commonly found in rocks. Hornblende, one of the most prevalent, belongs to this group. They are essential building blocks of many geological formations.

amphibolite A crystalline rock, coarse-grained and tough, where amphibole is a primary mineral constituent, alongside feldspar and often garnet. Like hornblende schist, it’s formed through regional metamorphism of basic igneous rocks, but it lacks foliation.

amygdaloidal These are amygdales, which are essentially infillings of secondary minerals like calcite, quartz, chlorite, or zeolites within the gas bubbles or vesicles of volcanic lava or other extrusive igneous rocks. Rocks containing these are described as amygdaloidal. It’s the rock’s memory of its fiery birth.

anatexis The process of melting pre-existing rock. It’s a transformation through heat, distinct from metatexis, diatexis, and syntexis.

andalusite One of several crystalline forms of aluminium silicate. It’s a characteristic product of contact metamorphism affecting argillaceous rocks – rocks rich in clay.

andesite A fine-grained igneous rock of intermediate composition. It's characterized by plagioclase feldspar making up about half of the rock, with the remainder being ferromagnesian minerals.

angular unconformity A type of unconformity, which is a break in the geological record. In this case, younger strata lie directly on top of an erosion surface that has cut into tilted or folded older rock layers. It’s a jarring juxtaposition of geological history.

anorthite Also known as Indianite. It’s a mineral from the lime-rich end of the plagioclase group. Anorthites are primarily silicates of calcium and aluminium, found in some basic igneous rocks, often formed by the contact metamorphism of impure calcareous sediments.

anticline An arched fold in rock layers. The layers typically dip away from the central axis of the fold. It’s the opposite of a syncline. Think of a mountain's upward curve.

aphanic This describes the texture of carbonate sedimentary rocks where individual crystals or clastic grains are exceptionally small, measuring less than 0.01 mm in diameter. The texture is fine, almost imperceptible.

aphanitic Refers to the texture of igneous rock where the individual crystalline components are too small to be distinguished with the naked eye. This includes both microcrystalline and cryptocrystalline textures. It’s a texture of fine detail.

aplogranite A light-colored rock with a granitic texture. It's composed mainly of alkali feldspar and quartz, with a smaller amount of biotite. Muscovite may also be present. It’s a simpler, lighter version of granite.

aquifer A body of saturated rock or sediment that readily allows water to move through it. It’s an underground reservoir.

aragonite sea A geological condition where aragonite and high-magnesium calcite are the primary inorganic precipitates of calcium carbonate in marine environments. This contrasts with the calcite seas of later geological periods.

Archean Eon The earliest eon in Earth's history. It’s a vast, ancient chapter of our planet's past.

archipelago A chain or cluster of islands. A scattering of land in the ocean.

arenaceous Sediments composed primarily of sand grains. This means they consist of quartz and rock fragments down to 0.005mm in size. Conglomerates, sandstones, grits, and siltstones all fall into this category. The particle size ranges from 2mm down to 1/16mm. It’s the stuff of beaches and deserts.

arenite

  1. A broad term for any consolidated sedimentary rock made up of sand-sized fragments.
  2. More specifically, a "clean" sandstone, well-sorted, with less than 10% argillaceous matrix. It's the opposite of a wacke.

arenitic Pertaining to, possessing the quality of, or resembling sandstone.

arkose A type of arenaceous sedimentary rock. It's similar to sandstone but contains at least 10% feldspar. It typically forms from the disintegration of acid igneous rocks and gneisses. It’s sandstone with a significant feldspar component.

argillaceous Describes sedimentary rocks composed of fine-grained material, essentially of the clay grade. This means they are made up of minute mineral fragments and crystals less than 0.005 mm in diameter, along with substantial amounts of colloidal material. Besides finely divided detrital matter, they consist of clay minerals like montmorillonite, kaolinite, gibbsite, and diaspore. Siltstones, mudstones, shales, and clays all fall under this description. It’s the realm of fine particles.

ash Fragments, less than 2 mm in diameter, composed of pulverized rock, minerals, and volcanic glass. These are created during volcanic eruptions. It's the fine debris of fire.

asphalt A sticky, black, and highly viscous liquid or semi-solid. It's found in most crude petroleums and in some natural deposits. It’s the thick, dark residue of ancient organic matter.

assembled gem Also known as a composite gem. It's a gemstone made from multiple pieces.

asthenosphere A region within the Earth's outer shell, situated beneath the lithosphere. The asthenosphere is of indeterminate thickness and exhibits plastic behavior, meaning it can deform slowly over time. It's the yielding layer beneath the rigid crust.

augite A complex silicate mineral, a compound of calcium, iron, and magnesium. It crystallizes in the monoclinic system and is a common constituent of many igneous rocks, particularly those of basic composition. It's a key component of basalt, dolerite, and gabbro.

aureole A zone surrounding an igneous intrusion. Within this zone, the surrounding country rock shows the effects of contact metamorphism due to the heat of the magma. It’s the baked fringe of an intrusion.

autochthon A fossil, sediment, or rock that formed or originated in the very location where it is now found. This term is commonly applied to coal or peat that originated from plants that grew and decayed in situ. It also applies to rocks that haven't been displaced by overthrust faulting. An object of this type is described as autochthonous. Contrast this with allochthon.

B

bajada A series of coalescing alluvial fans that form along the base of a mountain range. It’s a broad, sloping apron of sediment.

banded iron formation A distinctive type of sedimentary rock, frequently found in ancient rock strata. It's characterized by alternating bands of iron-rich minerals and silica-rich layers. A signature of early Earth chemistry.

basalt A fine-grained, mafic igneous rock. It's composed primarily of ferromagnesian minerals, with a smaller proportion of calcium-rich plagioclase feldspar. It’s the rock of oceanic crust and vast lava flows.

basement rock The thick, ancient foundation of metamorphic and igneous rock that forms the continental crust. It's often granite and represents the oldest, most stable parts of the continents.

basic rock Igneous rocks with a low silica content, generally less than 54%. The groups ultrabasic, basic, intermediate, and acid represent a series with progressively increasing SiO 2 content.

basin A landform, often scooped out by the action of water erosion. It's an area drained by a river and its tributaries.

Basin and Range Province A distinctive physiographic region characterized by a series of elongate, north-south trending valleys bounded by mountain ranges. This topography covers much of the southwestern United States and northwestern Mexico. It's a landscape shaped by stretching and faulting.

batholith A large, discordant pluton – an igneous intrusion – with an exposed surface area exceeding 100 square kilometers. These are massive, deep-seated intrusions.

bedrock The native, consolidated rock that lies beneath the loose surface layer of soil or rock. It’s the solid foundation.

Before Present (BP) A convention used in dating, where "present" is conventionally set to the year 1950 CE. Dates are expressed as years before this reference point.

bentonite A type of clay mineral belonging to the smectite group, known for its swelling properties. It has numerous industrial applications, including use in drilling mud and as expansive backfill materials. The name originates from deposits found near Fort Benton, Wyoming. See also smectite and montmorillonite.

bioerosion The process by which living organisms erode hard ocean substrates through various biological mechanisms. It's the slow work of life on rock.

biogenic Originating from biological processes. Produced by the present or past activity of living organisms.

bioherm A landform composed of organic sedimentary rock, distinct from the surrounding rock. It's essentially a fossil reef structure.

biostratigraphy A sub-discipline of stratigraphy that focuses on correlating and assigning relative ages to rock strata based on the fossil assemblages they contain. It uses the history of life to date rock layers.

biostratinomy The study of the processes that occur after an organism dies but before it is finally buried. It’s the study of the transition from life to fossil.

biostrome A sedimentary rock landform that is distinctly bedded or broadly lenticular. It's a layered, organic deposit.

biotic Of biological origin. Produced by the present or past activity of living organisms or by a biological process.

biotite A common mica mineral, often appearing as lustrous black crystals in many igneous rocks, particularly granites. Chemically, it's a complex silicate of iron and magnesium, along with potassium and hydroxyl groups.

bioturbation The disturbance and mixing of sediment particles by benthic fauna (animals) or flora (plants). It’s the geological record of life burrowing and moving through sediment.

blueschist A type of metamorphic rock that forms from the metamorphism of basalt or similar rocks under high pressures and low temperatures. These conditions typically correspond to significant depths within the Earth.

boudin A structure formed by extensional forces, where a rigid, tabular body, like a sandstone bed, is stretched and deformed amidst less competent surrounding beds. It resembles a string of sausages. See also boudinage.

boulder A large rock fragment, typically rounded, moved by natural forces.

Bowen's reaction series This describes the sequence in which minerals crystallize from a cooling basaltic magma. It’s a fundamental concept in understanding igneous petrology.

brackish Water with a salinity level higher than freshwater but lower than seawater. It’s a transitional state.

breadcrust bomb A rounded volcanic bomb with a cracked surface that resembles the crust of bread. This texture forms as the bomb cools and contracts.

breccia A coarse-grained clastic rock composed mainly of angular fragments of pre-existing rocks. It’s a fragmented rock, cemented together.

buckling A failure mode in rocks subjected to high compressive stresses. The rock shortens, leading to folding. This is a fundamental process in mountain building.

C

calcareous Formed from or containing a significant proportion of calcium carbonate, usually in the form of calcite or aragonite. This term applies to sediments, sedimentary rocks, or soil types.

calcite The crystalline form of calcium carbonate (CaCO 3 ). It's a very common mineral found in both igneous and sedimentary rocks. It’s the primary mineral in limestone.

calcite sea A body of water where low-magnesium calcite is the primary inorganic precipitate of calcium carbonate. This contrasts with the aragonite seas of earlier periods.

calcrete

  1. A conglomerate formed from surficial sand and gravel cemented by calcium carbonate that has precipitated from solution.
  2. A type of calcareous duricrust, a hardened surface layer.

caldera A large, cauldron-like volcanic depression formed by the collapse of land following a volcanic eruption. It’s the scar left by a massive eruption.

Cambrian The earliest geologic period of the Paleozoic Era. It spans from approximately 541.0 ± 1.0 to 485.4 ± 1.9 million years ago, preceding the Ordovician period. It marks a significant explosion of life.

carbon film A type of fossil preservation where a thin film of carbon is left behind, outlining the shape of the original organism. It's a ghostly imprint.

carbonate A salt or ester of carbonic acid. In geology, it refers to minerals and rocks rich in carbonate ions.

carbonate hardgrounds Surfaces of synsedimentarily-cemented carbonate layers that have been exposed on the seafloor. They represent periods of non-deposition and lithification.

casting A process where a liquid fills a cavity and then solidifies. If the cavity was formed by the decomposition of an organism, this can result in the formation of a fossil cast.

Cenozoic Era The most recent of the geological eras, following the Mesozoic Era. It’s the era of mammals and modern life.

chalk A soft, white, porous sedimentary rock primarily composed of calcite coccolith plates. It’s a fine-grained limestone.

chert A fine-grained, silica-rich, microcrystalline or cryptocrystalline sedimentary rock. It sometimes contains small fossils and is known for its hardness and conchoidal fracture.

chlorite A group of related clay minerals with a 2:1 (TOT) structure that do not swell. They are hydrated silicates of aluminium, iron, and magnesium, often appearing green. They are common alteration products of minerals like biotite and hornblende.

clast Any individual grain or fragment within a sediment or sedimentary rock that was produced by the mechanical weathering of a pre-existing rock. It's a piece of a broken rock.

clastic rocks Rocks formed from the mechanically redeposited remains of eroded older rocks. They are essentially rocks made of fragments, or clasts.

clay Extremely fine-grained sedimentary material, typically composed of clay minerals.

cleavage The tendency of a rock to break along preferred planes of weakness. This is caused by the development of a planar fabric during deformation.

cobble A large stone, typically rounded, between 64 and 256 mm (2.5 to 10.1 inches) in diameter.

coccolith An individual plate of calcium carbonate produced by coccolithophores, which are single-celled algae. These plates often form large fossil deposits.

coccolithophore Also called a coccolithophorid. A type of microfossil belonging to single-celled algae, specifically protists and phytoplankton of the division haptophytes. They are distinguished by their calcium carbonate plates, called coccoliths.

compaction The process by which a newly deposited sediment decreases in volume and expels its pore water due to the weight of overlying sediments. It's a key step in lithification.

compression The process by which rocks shorten or decrease in volume when subjected to certain forces. It’s a squeezing, a reduction.

conchoidal A type of fracture that produces smoothly curved surfaces, like the inside of a shell. Obsidian and quartz often exhibit this fracture.

concretion A localized concentration of mineral cement within a sedimentary rock, filling the pore spaces between grains. It's a hardened lump within the rock, distinct from a nodule.

conglomerate Any rock type composed of individual stones that have become cemented together. It’s a rock made of rounded pebbles and cobbles.

contact metamorphism Metamorphism that occurs due to the localized heating of rocks by the intrusion of nearby magma. It’s the baking of rock by heat.

continental crust The layer of granitic, sedimentary, and metamorphic rocks that forms the continents and the adjacent continental shelves. It's thicker and less dense than oceanic crust.

continental margin The zone of the ocean floor that separates the thinner oceanic crust from the thicker continental crust. It includes the continental shelf, slope, and rise.

continental shelf The extended perimeter of a continent, covered by relatively shallow seas. It's the submerged edge of the continent.

convergent boundary The boundary between two tectonic plates that are moving toward each other. This is where subduction or continental collision occurs. Contrast with divergent boundary.

copal A type of resin produced by plant or tree secretions. It’s often used ceremonially as incense, particularly in Mesoamerican cultures.

coprolite Fossilized fecal material. It’s the preserved excrement of ancient animals or humans.

cordierite A silicate mineral containing aluminium, iron, and magnesium, often found in metamorphic rocks. It crystallizes in the orthorhombic system.

core The innermost layer(s) of a planet, specifically referring to the Earth's core. It's the planet's center.

corestone An ellipsoidal or rectangular joint block of granite formed by subsurface weathering. It's like a rounded boulder still partially embedded in weathered rock.

country rock The native rock of an area, as opposed to rock that has been transported from elsewhere. It’s the original rock surrounding an intrusion or fault.

craton An old, stable part of the continental crust that has remained intact through multiple cycles of continental merging and splitting. These are the ancient cores of continents.

cross-bedding Inclined sedimentary structures within a larger, horizontal layer of rock. These tilted layers indicate the direction of depositional currents and are not a result of post-depositional deformation.

crude oil A liquid mixture of naturally occurring hydrocarbons. It's petroleum in its raw state.

crust The outermost solid layer of a planet or moon, specifically the Earth's crust. It's the thin, rocky shell we live on.

cryptocrystalline A texture where the crystalline components are too small to be resolved even under a microscope. It’s a texture of extreme fineness.

crystal A solid material whose constituent atoms, molecules, or ions are arranged in an orderly, repeating pattern extending in all three spatial dimensions.

crystal habit The characteristic external shape or form of a crystal, determined by the relative development of its faces.

crystallinity The degree to which a material is crystalline.

D

dacite An igneous, volcanic rock with a high iron content. It's compositionally similar to andesite but contains less calcic feldspar. It’s also known as quartz andesite.

daughter product An isotope produced by the radioactive decay of another isotope (the parent product). It’s the result of radioactive decay.

delta A landform created where the mouth of a river flows into a larger body of water, such as an ocean, sea, or lake. It’s characterized by branching channels and sediment deposition.

degradation The lowering of a land surface through erosional processes. In fluvial systems, this means the lowering of a stream bed or floodplain.

dendrite A crystal that grows in a characteristic multi-branching, tree-like form.

denudation The process of lowering the Earth's surface through weathering and erosion. It’s the wearing away of the land.

deposition The geological process by which material is added to a landform or landmass. This can include sediments, volcanic ash, or other substances.

detachment fault A major fault in a mountain belt, above which rocks have been intensely folded or faulted. It signifies significant crustal shortening and deformation.

diagenesis The sum of all physical, chemical, and biological changes that occur in a sediment after its deposition and before metamorphism. It's the process of sediment becoming rock, short of intense heat and pressure.

diamictite A comprehensive, non-generic term for a poorly sorted, non-calcareous, terrigenous sedimentary rock that contains a wide range of particle sizes, often with larger particles embedded in a muddy matrix. It's a jumbled mix of sediment sizes.

diapir An intrusion where more mobile, ductile material is forced upwards into brittle overlying rocks. This creates a dome or anticlinal fold that has been ruptured by the flow of the core material. It's like a geological blister.

diatomite A soft, chalk-like sedimentary rock composed of the fossilized remains of diatoms (microscopic algae). It's easily crumbled into a fine powder and has various industrial uses.

diopside A monoclinic pyroxene mineral, ideally a silicate of calcium and magnesium, but often containing iron. It's a common mineral in many igneous rocks and some metamorphic rocks.

diorite A grey to dark grey, intermediate, intrusive igneous rock. It's composed mainly of plagioclase feldspar (typically andesine), along with biotite, hornblende, and/or pyroxene.

dike Also spelled dyke. A type of sheet intrusion where a geologic body cuts discordantly across existing rock structures. It's a tabular intrusion injected into the crust. Some dikes are more resistant to weathering and stand out as walls, while others form depressions.

dip slope A geological feature often created by the erosion of tilted rock strata, where the slope follows the dip of the rock layers.

disconformity An unconformity where the rock strata above and below the erosional surface are parallel. The missing time is only apparent from the missing rock record.

divergent boundary The boundary between two tectonic plates that are moving away from each other. This is where new crust is formed, like at mid-ocean ridges. Contrast with convergent boundary.

dolomite A sedimentary carbonate rock and mineral, both composed of crystalline calcium magnesium carbonate (CaMg(CO 3 ) 2 ). It's similar to limestone but contains magnesium.

dolerite

  1. A basic igneous rock of medium grain size, found in minor intrusions or the central parts of thick lava flows.
  2. A dark-colored, basic igneous rock composed primarily of pyroxene and a triclinic feldspar. It's often considered equivalent to a coarse-grained basalt.
  3. A dark, crystalline igneous rock, chiefly pyroxene with labradorite.
  4. Coarse-grained basalt.
  5. Also known as diabase.
  6. Any dark igneous rock mainly composed of silicates of iron and magnesium with some feldspar.

dome A geological structure characterized by symmetrical anticlines that intersect at their respective apices. It's an upward bulging of rock layers.

drill core A cylinder of rock or soil removed by a drill, used for geological analysis.

drumlin An elongated, whale-shaped hill formed by glacial action. It's a streamlined landform sculpted by ice.

dunite An ultrabasic plutonic rock composed almost entirely of olivine, with accessory chromite. Feldspar is generally absent. It’s a rock of deep, mantle origin.

duricrust A hard crust layer found on or within the upper horizons of soil, typically in semi-arid climates. It forms from the accumulation of minerals deposited by upward-moving water that evaporates in dry seasons. Compare with hardpan.

E

earthquake A sudden shaking of the ground caused by the passage of seismic waves through Earth's rocks.

eclogite A coarse- to medium-grained metamorphic rock, typically green with red garnets. It forms under high pressure and temperature conditions deep within the Earth.

Eemian transgression The period within the Late Pleistocene spanning from 120,000 to 80,000 years before present, characterized by a significant rise in sea level.

eon The largest unit of geologic time, subdividing Earth's history into vast intervals.

epicenter The point on the Earth's surface directly above the focus (hypocenter) of an earthquake or underground explosion.

epidiorite Altered gabbroic and doleritic rocks where the original pyroxene has been replaced by fibrous amphibole. It represents an early stage of metamorphism of basic igneous rocks into green schists.

epirogenetic Pertaining to movements of the Earth's crust, specifically the simultaneous rising and falling of continents that maintain isostasy.

epoch A subdivision of a geologic period, representing a shorter span of time. The Pleistocene is an example of an epoch within the Quaternary Period.

erosion The displacement of solids – sediment, soil, rock, or other particles – typically by agents like wind, water, or ice. It can also occur through downward or down-slope movement in response to gravity or by living organisms (bioerosion). It's the process of wearing away and transporting material.

erratic A piece of rock that differs in type and size from the native rock of the area where it rests. Erratics are typically transported by glacial ice and left behind when the ice melts. Their presence indicates past glaciation.

escarpment A steep slope or cliff that marks the boundary between two different physiographic provinces, often involving a significant elevation difference.

esker A long, winding ridge of stratified sand and gravel, typically found in glaciated regions. Eskers are formed by meltwater streams flowing within or beneath glaciers.

estuary A semi-enclosed coastal body of water where one or more rivers or streams flow in, with a direct connection to the open sea. It’s a mixing zone of fresh and saltwater.

eugeosyncline A type of geosyncline characterized by associated volcanism alongside clastic sedimentation. It represents a more mobile, volcanic part of an orthosyncline located away from the continental craton.

euhedral Describes a mineral crystal that is perfectly formed, bounded by its own crystal faces. Synonymous with idiomorphic.

eustatic movements Changes in global sea level that affect wide areas. These are caused by alterations in the volume of the oceans, such as from the formation or melting of ice caps.

evaporite Any of a diverse group of water-soluble mineral sediments formed by the evaporation of bodies of surface water. Examples include rock salt and gypsum.

exfoliation The process by which concentric slabs of rock are stripped from the outer surface of a rock mass. This is often caused by pressure release or thermal expansion/contraction.

extension Strain that involves an increase in length. Extension can lead to the thinning and faulting of rock.

extrusive Describes the formation of igneous rocks where molten rock (magma) flows onto the Earth's surface as lava or is ejected as pyroclastics. These rocks cool relatively quickly.

F

facies The sum of the lithological (rock type) and faunal (fossil content) characteristics of a sediment or rock. It reflects the depositional environment. Lithological facies involve composition, grain size, texture, and color, while metamorphic facies relate to mineral assemblages and the degree of recrystallization.

fanning A type of rock deformation related to shear stress.

fault A discrete planar fracture in rock showing evidence of displacement along the plane. It’s a break in the rock where movement has occurred.

fault zone The area where multiple distinct fault planes exist. It signifies a region of significant structural weakness and deformation.

feldspar A group of abundant rock-forming silicate minerals. All feldspars contain silicon, aluminium, and oxygen, and may include potassium, calcium, and sodium. They are crucial components of many igneous rocks.

fels A massive metamorphic rock that lacks schistosity or foliation. It’s a more granular metamorphic rock.

felsic Describes silicate minerals, magmas, and rocks that are rich in lighter elements like silicon, oxygen, aluminium, sodium, and potassium. These rocks are typically light in color, with minerals like quartz and feldspar being dominant (greater than 60%). Examples include granite and rhyolite. Contrast with mafic.

ferricrete A conglomerate formed from surficial sand and gravel cemented by iron oxides. It’s a ferruginous duricrust, a hardpan rich in iron.

ferromagnesian mineral Any mineral containing both iron and magnesium, such as augite, hornblende, olivine, or biotite. These are typically dark-colored minerals.

fission track dating A radiometric dating method that uses microscopic tracks left by the spontaneous fission of uranium-238 atoms in minerals to determine their age.

Flandrian transgression The rise in sea level during the current geological epoch, the Holocene.

fold A stack of originally flat surfaces, such as sedimentary strata, that have been bent or curved due to plastic deformation.

foliation The parallel alignment of textural and structural features within a metamorphic rock, often due to directed pressure. It gives the rock a layered appearance.

fossil Any preserved remains or traces (like footprints) of ancient organisms, whether mineralized or otherwise preserved. It's evidence of past life.

fossiliferous Bearing or composed of fossils. A rock that contains fossils is fossiliferous.

fossilization The process by which the remains or traces of an organism are preserved in the geological record.

fracture Any crack or discontinuity in a rock. In geology, the term is used specifically when no displacement is evident along the break.

fumarole A vent in the Earth's surface from which volcanic gases and vapors are emitted. It’s a sign of volcanic activity.

G

Ga An abbreviation for one billion (1,000,000,000) years. When unqualified, it usually refers to 1,000,000,000 years Before Present (BP).

gabbro A dark, coarse-grained, intrusive igneous rock that is chemically equivalent to basalt. It forms deep within the Earth.

garnet A group of silicate minerals commonly found in metamorphic rocks. They are known for their hardness and often brilliant colors.

gastrolith A rock that was once held inside the digestive tract of an animal, likely used for grinding food. Fossil gastroliths are sometimes found.

gemology Also spelled gemmology. The scientific study of natural and artificial gemstones.

gemstone Also called a gem, fine gem, jewel, precious stone, or semi-precious stone. A piece of mineral crystal which, in cut and polished form, is used to make jewelry or other adornments.

geogenic Originating from geological processes. Resulting from naturally occurring geological phenomena.

geological map A specialized map that depicts geological features, such as rock formations, faults, and mineral deposits.

geological time scale Also geologic time scale. A system of chronological dating that relates geological strata to time. It's Earth's calendar.

geology The scientific study of the Earth, including its composition, structure, physical properties, history, and the processes that shape it.

geosyncline A large, mobile down-warping of the Earth's crust that subsides as thick sequences of sedimentary and volcanic rocks accumulate. These features are often precursors to mountain building.

glass An amorphous (non-crystalline) solid that exhibits a glassy luster. Volcanic glass, like obsidian, is common in geology.

glauconite A green-colored, hydrated silicate mineral of potassium and iron that forms in marine environments. Its presence in sediments often indicates accumulation under marine conditions.

gneiss A coarse-grained metamorphic rock characterized by distinct banding (gneissose texture). It contains abundant feldspar, quartz, mica, and often hornblende or garnet.

Gondwanaland The southern portion of the supercontinent Pangaea, which eventually broke apart to form present-day South America, Africa, India, Australia, and Antarctica.

graben A depressed block of the Earth's crust bounded by parallel normal faults. It's a down-dropped block, often forming valleys.

granite A coarse-grained, intrusive, felsic igneous rock. It is composed mainly of quartz, feldspar (orthoclase and plagioclase), and mica. Rhyolite is its volcanic equivalent. It's the archetypal continental crust rock.

granitoid Any granite-like rock, including granodiorite, diorite, monzonite, and granite itself. It's a broad classification for felsic intrusive rocks.

granoblastic Describes the texture of a metamorphic rock where mineral grains are arranged in a mosaic-like pattern, similar to that of granite, but formed by recrystallization in the solid state rather than from melt.

granodiorite An intrusive, felsic igneous rock similar to granite, but with a higher proportion of plagioclase feldspar compared to potassium feldspar. Dacite is its volcanic equivalent.

granulometry The statistical distribution of the sizes of individual sediment grains or particles in clastic rocks. It's the measure of grain size.

gravel A coarse-grained sedimentary material consisting of rounded or angular fragments of rock and pebbles.

graywacke Also spelled greywacke. A type of sandstone characterized by its dark color, poorly sorted, angular grains of quartz, feldspar, and rock fragments (lithic fragments), set in a compact, clay-fine matrix. It's a dirty, poorly sorted sandstone.

greenstone An informal term often applied to basic and intermediate igneous rocks of Lower Paleozoic age that have been altered, with much chlorite replacing original minerals, giving the rock a green color.

grus Freshly eroded, angular grains of quartz and feldspar derived from granitoid rocks. It's the granular product of granite weathering.

H

habit See crystal habit.

half-life The time required for half of a sample of a radioactive isotope to decay into its daughter products. Crucial for radiometric dating.

hardpan A dense, impermeable layer in a soil profile, often formed by the accumulation and cementation of clay, iron, or silica. It impedes water and root penetration.

hemicrystalline Describes rocks, usually of igneous origin, that contain both crystalline minerals and interstitial glass. Contrast with holocrystalline.

hinge The zone of maximum curvature in a fold.

hinge line A line connecting the points of maximum curvature along the hinge of a fold.

holocrystalline Describes igneous rocks where all components are crystalline, with no interstitial glass. Contrast with hemicrystalline.

hornblende An important rock-forming silicate mineral, typically black or dark green. It's a common constituent of many igneous and metamorphic rocks.

hornfels A hard, compact, fine-grained metamorphic rock formed by contact metamorphism. It typically breaks into splintery fragments.

horst A raised block of the Earth's crust bounded by normal faults. It's an uplifted block, the opposite of a graben.

hot spring A natural spring where geothermally heated groundwater emerges from beneath the Earth's crust.

hydrothermal Pertaining to the actions or products of heated water, often associated with volcanic or geothermal activity.

hydrothermal vent A fissure on a planet's surface from which geothermally heated water emerges. These are significant in oceanography and astrobiology.

hypersaline Characterized by a saltiness or dissolved salt content greater than that of normal seawater.

I

ichnology The study of traces of organismal behavior, such as footprints, burrows, and trails, preserved in the geological record. It’s the study of ancient life’s actions.

idiomorphic Bounded by the crystal faces peculiar to the species; used of minerals. Synonymous with euhedral.

igneous rock Rock formed from the solidification of cooled magma or lava. It can be intrusive (plutonic) or extrusive (volcanic).

ignimbrite A fine-grained to aphanitic, compact volcanic rock with flow banding, produced by violently explosive eruptions. It's a type of tuff formed from pyroclastic flows.

ilmenite An oxide of iron and titanium, found as an accessory mineral in many igneous rocks, especially those of basic composition.

illite A non-swelling clay mineral with a 2:1 (TOT) layer structure. Its interlayers are collapsed and do not allow for significant water or cation diffusion. The name originates from deposits in Illinois.

indurated Made hard, typically by heat or compaction.

interbedded Layers of rock lying between or alternating with beds of a different kind of rock.

intrusion A body of igneous rock that has crystallized from molten magma below the Earth's surface.

island arc A chain of volcanic islands formed by plate tectonics where an oceanic plate subducts under another plate, generating magma.

isomorphic Describes crystals that have similar shapes and sizes, often due to similar atomic arrangements.

isotope Atoms of the same element that have different numbers of neutrons, and therefore different atomic masses. Crucial for radiometric dating.

J

joint A discrete discontinuity surface in rock without evidence of displacement. It's a crack or fracture.

Jurassic A major geologic period spanning from about 199.6 to 145.4 million years ago, between the Triassic and Cretaceous periods. It was the age of dinosaurs.

K

Ka An abbreviation for one thousand (1,000) years. When unqualified, it usually refers to 1,000 years Before Present (BP).

kame An irregularly shaped hill or mound composed of sand, gravel, and till, deposited by meltwater streams on a retreating glacier.

kaolinite A clay mineral, a hydrated aluminium silicate, often formed by the alteration of feldspars under hydrothermal conditions. It's a primary component of kaolin.

karst A distinctive landscape formed by the dissolution of soluble bedrock, typically limestone or dolomite. It's characterized by sinkholes, caves, and underground drainage.

kettle A depression in glacial outwash plains formed when blocks of ice calve from a glacier, become buried by sediment, and then melt.

kink A sharp bend or twist in a rock layer.

kink band An asymmetric zone of deformation characterized by a tight bend or twist.

kyanite A silicate mineral of aluminium, often occurring as long, blue, bladed crystals in metamorphic rocks.

L

lacuna A gap in the stratigraphic record, representing missing time or deposition. It signifies a break in the geological history.

lamprophyre Igneous rocks, often found as dikes, characterized by high contents of colored silicates like biotite and hornblende, and relatively little feldspar.

large igneous province Massive volcanic formations resulting from extensive flood basalt eruptions, often associated with mantle plumes.

lava Molten rock erupted onto the Earth's surface.

leucocratic Describing igneous rocks that are light in color due to a high content of felsic minerals and a low proportion of dark, heavy silicates.

limestone A sedimentary rock composed primarily of calcium carbonate (calcite).

liquefaction The process by which soil or sediment loses its strength and behaves like a liquid, often triggered by seismic shaking.

lithic fragment A sand-sized grain composed of smaller-sized rock fragments. For example, a piece of shale or basalt within a sandstone.

lithification The process by which loose sediments are transformed into solid rock through processes like compaction and cementation.

lithology The description of the physical characteristics of a rock unit, such as color, texture, grain size, and composition. It's the rock's basic description.

lithosphere The rigid, outermost rocky shell of a planet, including the crust and the uppermost part of the mantle. It is broken into tectonic plates.

lithotype Specific types of rock classified according to lithological standards.

loess A fine, silty, windblown deposit, typically pale yellow or buff in color. It's fertile soil formed from dust carried by the wind.

lustre Also spelled luster. The way light reflects off the surface of a mineral.

M

Ma An abbreviation for one million (1,000,000) years. When unqualified, it usually refers to 1,000,000 years Before Present (BP).

mafic Describes silicate minerals or rocks that are rich in magnesium and iron. These rocks are typically dark-colored. Examples include basalt and gabbro.

magma Molten rock found beneath the Earth's surface. When it erupts, it becomes lava.

magnetite An oxide of iron, a strongly magnetic mineral. It crystallizes in the cubic system.

Malmesbury group A sequence of rock formations in the Western Cape of South Africa, dating from 980 to 830 million years ago.

mantle The layer of the Earth situated between the crust and the core. It's composed of dense silicate rock and is largely solid but behaves plastically over geological timescales.

marble A metamorphic rock derived from limestone or dolomite. It's characterized by recrystallized calcite, often with a granular texture and various colors due to impurities.

marine terrace A narrow, flat area often found at the base of a sea cliff, formed by wave action. It represents a former sea level.

marl A calcium carbonate or lime-rich mud or mudstone, containing variable amounts of clay.

massive A description applied to a rock that lacks any internal structure or layering. It's uniform and homogeneous.

mélange A large-scale breccia formed in the accretionary wedge above a subduction zone, consisting of a chaotic mixture of rock fragments.

melanocratic Describing rocks that are abnormally rich in dark, ferro-magnesium minerals.

mesocratic Describing igneous rocks with an intermediate content of dark silicates, between leucocratic and melanocratic rocks (containing 30–60% dark heavy minerals).

Mesozoic A geologic era spanning from approximately 252.2 to 66 million years ago, encompassing the Triassic, Jurassic, and Cretaceous periods. It was the age of reptiles.

metamorphic rock Rock that has been transformed from its original type (igneous, sedimentary, or another metamorphic rock) by heat, pressure, or chemical reactions, without melting.

metamorphism The process of solid-state recrystallization of pre-existing rocks due to changes in heat, pressure, or water-rock interactions. It does not involve melting.

metapelite A metamorphosed pelite – a fine-grained, clay-rich sedimentary rock.

metasilicate A theoretical salt of metasilicic acid (H 2 SiO 3 ), analogous to carbonates.

metatexis Low-grade anatexis, involving the partial or differential melting of rock components with low melting points.

mica A group of silicate minerals characterized by their perfect basal cleavage, allowing them to be easily split into thin sheets. Muscovite (white mica) and biotite (black mica) are common examples.

micropaleontology The branch of paleontology that studies microfossils – fossils too small to be seen with the naked eye.

mid-oceanic ridge An underwater mountain range, typically with a rift valley along its axis, formed by the divergence of tectonic plates. It's where new oceanic crust is generated.

migmatite A composite rock consisting of both igneous or igneous-looking material and metamorphic components, usually distinguishable megascopically. It represents a rock on the verge of melting.

mineral A naturally occurring solid inorganic substance with a definite chemical composition and a specific crystalline structure.

mineralization The hydrothermal deposition of economically important metals, forming ore bodies or lodes.

mineralogy The scientific study of minerals, including their chemical composition, crystal structure, physical properties, and occurrence.

miogeosyncline A type of geosyncline located near the continental craton, characterized by sedimentary deposition without associated volcanism.

Mohs scale of mineral hardness Also simply called the Mohs scale. A relative scale of mineral hardness, ranging from 1 (talc) to 10 (diamond).

molasse A sedimentary facies, partly marine and partly continental or deltaic, consisting of thick sequences of soft, ungraded, cross-bedded, fossiliferous conglomerates, sandstones, shales, and marls. It typically forms in front of rising mountain ranges.

molding The process of shaping pliable material using a mold. In geology, it can refer to the formation of fossil casts and molds.

montmorillonite A swelling clay mineral with a 2:1 (TOT) structure. Its interlayers contain hydrated sodium or calcium cations and water molecules. It's a key component of bentonite.

monocline A fold with a single limb, causing a sudden steepening of the dip in rock layers, which then return to near horizontal on either side.

monoclinic A crystal system characterized by three unequal axes, with two of the interfacial angles being right angles and the third oblique.

moraine An accumulation of unconsolidated debris deposited by a glacier. Moraines can take various forms, such as ridges or mounds.

mullion A specific type of reworked boudin, often found in deformed rock layers.

muscovite The common white mica, a silicate of aluminium and potassium. It crystallizes in the monoclinic system and has a perfect basal cleavage.

N

Namibian age A geological time designation corresponding to the Neoproterozoic Era, roughly 900 to 542 million years ago.

Neogene A geologic period that began 23 million years ago and ended either 2.6 million years ago (with the start of the Quaternary) or continues to the present day, depending on the definition used.

nodule A small, irregularly rounded mass of a mineral or mineral aggregate that has a composition different from the surrounding sediment or rock. Unlike a concretion, it's not formed by precipitation within the pores.

non-clastic Describes sedimentary rocks formed by chemical or organic precipitation, such as limestone or evaporites, rather than by the accumulation of detrital fragments.

normal fault Also called an extensional fault. A type of dip-slip fault where the hanging wall moves downward relative to the footwall, typically caused by tensional forces. Contrast with reverse fault.

O

obsidian A naturally occurring volcanic glass formed as an extrusive igneous rock. It is typically black in color, very hard, brittle, and has a shiny surface.

oligoclase A plagioclase feldspar mineral, an intermediate member between albite and anorthite. It is found primarily in acidic igneous rocks.

olivine A silicate mineral of magnesium and iron, common in basic and ultramafic igneous rocks. It crystallizes in the orthorhombic system.

Ordovician A geologic period of the Paleozoic Era, spanning from about 485.4 to 443.4 million years ago. It followed the Cambrian and preceded the Silurian.

orogenesis The process of mountain building, involving the formation and growth of mountains related to tectonic activity.

orogeny A mountain-building event, encompassing the forces and processes that lead to large-scale deformation of the Earth's lithosphere due to the interaction of tectonic plates.

orthoclase A potassium aluminium silicate mineral, a common feldspar found in granitic and syenitic rocks. It crystallizes in the monoclinic system.

orthogeosyncline A geosyncline situated between continental and oceanic cratons, containing both volcanic and non-volcanic belts.

orthorhombic A crystal system characterized by three mutually perpendicular axes of unequal lengths.

orthosilicate An older classification referring to silicate minerals based on the hypothetical orthosilicic acid (H 4 SiO 4 ), containing isolated [SiO 4 ] 4− tetrahedra.

oxbow lake A crescent-shaped lake formed when a meander loop of a river is cut off from the main channel.

P

pahoehoe A type of basaltic lava flow with a smooth, ropy surface texture, formed by the cooling of highly fluid lava.

Palaeozoic Also spelled Paleozoic. The earliest of the three geologic eras of the Phanerozoic Eon, spanning from about 541 to 252.2 million years ago. It was a time of significant diversification of life.

paleocurrent An indication of the direction of fluid flow (water or wind) at the time of sediment deposition, as inferred from sedimentary structures.

paleontology The scientific study of ancient life through the examination of fossils.

palagonite An alteration product formed when volcanic glass interacts with water. It typically has a yellowish to brownish color.

paralithic A weathered layer of bedrock, lying just beneath the soil horizon.

pegmatite An exceptionally coarse-grained igneous rock, often containing large crystals of feldspar, quartz, and mica. It forms from late-stage, water-rich magma.

pelite A descriptive term for a fine-grained clastic sedimentary rock, typically composed of clay minerals and silt-sized particles.

peridotite A coarse-grained, ultramafic igneous rock composed mainly of olivine, with pyroxene often present. It's a primary rock of the Earth's upper mantle.

petrology The branch of geology that studies the origin, composition, structure, and classification of rocks.

phenoclast A large, conspicuous fragment within a sediment or sedimentary rock.

phenocryst A relatively large crystal embedded within a finer-grained matrix in an igneous rock.

phyllite A low-to-medium grade metamorphic rock intermediate between slate and schist, characterized by a silky sheen due to the fine alignment of mica and chlorite.

phyllonite A rock that looks like phyllite but is formed by the mechanical degradation (mylonization) of coarser rocks like granite or gneiss.

piercing point A feature that has been cut by a fault and displaced. Reconstructing these points helps determine the amount of fault movement.

pillar A vertical, standing, often spire-shaped, natural rock formation.

pitchstone A dark, glassy volcanic rock, a variety of rhyolite or basalt, containing mineral clusters. It resembles pitch in appearance.

plate tectonics The scientific theory describing the large-scale motion of the Earth's lithosphere, which is divided into rigid tectonic plates that move relative to each other.

Pleistocene A geologic epoch lasting from about 2.588 million to 11,700 years ago, characterized by repeated glacial cycles. It's the most recent epoch of the Quaternary Period.

Pliocene A geologic epoch extending from 5.332 million to 2.588 million years ago. It is the second-youngest epoch of the Neogene Period, following the Miocene and preceding the Pleistocene.

pluton A body of intrusive igneous rock that solidified beneath the Earth's surface. Batholiths and dikes are examples of plutons.

plutonic Describing igneous rocks that have crystallized at depth within the Earth's crust. These rocks are typically coarse-grained due to slow cooling.

plumose structure A feather-like pattern found on joint surfaces, oriented perpendicular to the stress that caused the jointing.

pneumatolysis The alteration of rocks by hot, volatile gases emanating from a magma, occurring after the main body of the magma has solidified.

polymictic Composed of many different types of fragments or clasts.

polysynthetic twinning A type of crystal twinning where multiple composition planes are present, resulting in a striated appearance.

pore water Water present in the pore spaces of rocks and soils. Also called interstitial water.

porphyroblast A large mineral crystal within a metamorphic rock that has grown larger than the surrounding fine-grained groundmass.

porphyry

  1. A rock that exhibits a porphyritic texture, containing large crystals (phenocrysts) in a finer-grained groundmass.
  2. In mining, a term for a deposit containing widely disseminated metals, typically copper.

Precambrian A vast span of geologic time preceding the Cambrian Period, encompassing the Hadean, Archean, and Proterozoic Eons. It covers the first ~4 billion years of Earth's history.

precipitate A solid rock or mineral formed from an aqueous solution.

pressure solution A deformation mechanism in rocks where mineral grains dissolve under stress, leading to compaction and the formation of features like stylolites.

prograde metamorphism The process of mineral recrystallization in rocks under increasing pressure and/or temperature conditions, typically associated with burial and tectonic compression.

protolith The original rock from which a metamorphic rock is formed. For sedimentary rocks, the term "provenance" is often used.

psammite A general term for sandstone, often used to describe a metamorphosed rock unit with a sandstone protolith.

pseudomatrix A matrix-like material in a rock formed by the crushing of weaker components, such as lithic fragments.

pumice A light-colored, highly vesicular volcanic rock of very low density. It forms from frothy lava.

pyroclastic flow A fast-moving current of hot gas, ash, and rock fragments (tephra) that flows downslope from a volcano.

pyroclastic Rock fragments ejected during a volcanic eruption, such as volcanic bombs, breadcrust bombs, or ash.

pyroxene A group of important rock-forming silicate minerals, common in mafic and ultramafic igneous rocks. They are metasilicates of various cations.

pyroxenite A coarse-grained, ultramafic igneous rock composed mainly of pyroxenes.

Q

quartz A hard, crystalline mineral composed of silicon and oxygen (silicon dioxide, SiO 2 ). It's one of the most common minerals in the Earth's crust.

quartzite A hard, dense metamorphic rock formed from sandstone. It is composed primarily of quartz and breaks with a granular fracture.

Quaternary The most recent geologic period, following the Neogene. It began 2.588 million years ago and continues to the present, characterized by the rise of humans and recent ice ages.

R

reef knoll A landform composed of a large accumulation of calcareous material formed on an ancient sea floor, such as a fossil reef.

regional metamorphism Metamorphism that occurs over wide areas, typically associated with deep burial and the pressures and temperatures involved in mountain building.

reticulite An extremely lightweight basaltic pumice where the vesicle walls have collapsed, forming a delicate network of glass threads. It's the lightest known rock.

retrograde metamorphism The process of mineral reconstitution in rocks under decreasing temperatures and pressures, allowing mineral assemblages formed during prograde metamorphism to revert to those stable under lower-grade conditions.

reverse fault Also called a thrust fault. A type of dip-slip fault where the hanging wall moves upward relative to the footwall, typically caused by compressional forces. Contrast with normal fault.

rhyolite A fine-grained, extrusive igneous rock, the volcanic equivalent of granite. It is typically light-colored and may contain phenocrysts of quartz and feldspar.

rift A valley formed by the downward displacement of a block of land between two parallel faults. Often associated with tectonic plate boundaries.

roche moutonnée An elongated rock formation sculpted by glacial erosion, with a smooth, abraded upstream surface and a rough, plucked downstream surface.

rock A naturally occurring solid aggregate of one or more minerals or mineraloids.

rolling hills Terrain characterized by undulating low hills. Similar to German "Hügelland".

rudaceous Describing coarse-grained sedimentary rocks composed of fragments larger than sand, such as conglomerates and breccias.

rudite A general term for sedimentary rocks composed of detrital grains coarser than sand, including granules, pebbles, cobbles, and boulders.

S

saccharoidal Having a texture resembling granulated sugar.

Saldanian orogeny A geological event of mountain building that occurred in southern Africa.

sand Granular material composed of finely divided rock and mineral particles, typically ranging in size from 0.0625 to 2 millimeters.

sandstone A sedimentary rock composed primarily of sand-sized grains (0.0625 to 2 mm) that have been cemented together. Its color varies depending on the cementing material and accessory minerals.

sanidine A high-temperature variety of potassium feldspar found in volcanic rocks like lavas and dikes.

schist A medium-grade metamorphic rock characterized by the parallel alignment of platy or elongated minerals, such as mica, chlorite, or hornblende. This alignment creates a foliated texture.

scree See talus.

sediment trap A geological depression where sediments accumulate over time.

sedimentary rock Rock formed from the accumulation and cementation of sediments derived from pre-existing rocks, or from chemical precipitation, or from organic matter.

sequence A series of geological events, processes, or rocks arranged in chronological order.

sericite A fine-grained, white mica, often formed as an alteration product of feldspar. It contributes to the silky texture of some metamorphic rocks.

shale A fine-grained, clastic sedimentary rock composed predominantly of clay minerals and silt-sized particles. It is characterized by its fissility, splitting easily into thin layers.

shear zone A tabular zone of rocks that have undergone intense strain due to shear stress. These zones are often associated with faulting.

shield A large area of exposed Precambrian crystalline igneous and high-grade metamorphic rocks forming the ancient, stable core of a continent.

shingle beach A beach composed of pebbles or cobbles, rather than sand.

silcrete A type of duricrust formed by the cementation of surface sand and gravel by dissolved silica.

silt Sediment particles with sizes ranging between sand and clay (typically 0.004 to 0.0625 mm). It can occur as loose sediment or in consolidated rock.

siltstone An indurated sedimentary rock composed predominantly of silt-sized particles.

slate A very fine-grained, low-grade metamorphic rock derived from shale. It is characterized by slaty cleavage, allowing it to split into thin, flat sheets.

slaty cleavage The property of splitting easily along closely spaced planes, developed in fine-grained metamorphic rocks due to directed pressure.

slickenside A smooth, polished, and often striated surface on a fault plane, caused by friction during fault movement.

slump A form of mass wasting where a coherent mass of rock or sediment moves a short distance down a slope, typically resulting in a rotational failure.

snowball Earth A hypothesis proposing that the Earth experienced a series of global glaciations during the Precambrian, where the entire planet may have been covered in ice.

smectite A group of clay minerals with a 2:1 (TOT) layered structure that can absorb significant amounts of water and cations between the layers, causing them to swell. Montmorillonite and bentonite are examples.

soil liquefaction The phenomenon where saturated soil temporarily loses strength and behaves like a liquid, often during earthquakes.

sorting The degree to which sediment grains in a deposit are of similar size. Well-sorted sediments have a narrow range of grain sizes, while poorly sorted sediments have a wide range.

speleothem A mineral deposit formed in a cave by the slow dripping of water, such as stalactites and stalagmites. They are valuable archives of past environmental conditions.

sphene An older name for the mineral titanite (CaTiSiO 5 ).

staurolite A nesosilicate mineral, typically reddish-brown to black, often forming distinctive intersecting or twinned crystals. It's a common index mineral in metamorphic rocks.

stone A hard, solid, nonmetallic mineral matter of which rock is made, especially as distinguished from soil or sand.

storm ridge A beach ridge formed by wave action during storms, often located further inland than typical beach ridges.

strain A change in the shape or volume of a rock mass in response to stress.

stratigraphy The branch of geology concerned with the study of rock strata (layers), their sequence, composition, origin, and correlation.

stratum A layer of rock, especially one of the parallel layers that make up a rock formation.

stylolite An irregular surface or suture in limestone and other sedimentary rocks, formed by pressure solution during diagenesis. They resemble interlocking teeth.

surficial Pertaining to the surface.

syenite A coarse-grained intrusive igneous rock composed mainly of alkali feldspar, with subordinate amounts of mafic minerals like hornblende. It's chemically similar to granite but lacks significant quartz.

syncline A fold in rock layers where the strata dip inward toward the fold axis. It's the opposite of an anticline.

syntaxis An abrupt change in the orientation of a mountain belt or fold structure.

T

Table Mountain Group A sequence of rock formations within the Cape Supergroup in South Africa, known for its prominent sandstone layers.

talus A collection of broken rock fragments at the base of a cliff or steep slope, accumulated through rockfall. Also called scree.

tectonic plate A massive, rigid slab of rock that makes up part of the Earth's lithosphere. These plates move relative to each other, causing earthquakes, volcanic activity, and mountain building.

tekton A general term for a structural unit or feature within a rock mass.

tektite Natural glass formed from terrestrial ejecta during a meteorite impact. They are often found scattered over large areas.

tempestite Storm deposits found in sedimentary formations, characterized by accumulations of shell debris or other materials moved by strong wave action.

tenacity A mineral's resistance to breaking or deforming.

tephra Fragmental material ejected during a volcanic eruption, including ash, lapilli, and volcanic bombs. Once deposited, it remains tephra unless it fuses into pyroclastic rock.

Tethys Ocean A prehistoric ocean that existed between the continents of Gondwana and Laurasia during the Mesozoic Era, before the opening of the Indian Ocean.

till Unsorted glacial sediment deposited directly by glacial ice. It's a heterogeneous mixture of clay, silt, sand, gravel, and boulders.

tillite A sedimentary rock formed from indurated glacial till. It's evidence of past glaciation.

titanite A calcium titanium nesosilicate mineral (CaTiSiO 5 ), also known as sphene.

topography The arrangement and configuration of the physical features of an area of land.

tor A large, isolated mass of rock that stands abruptly above its surroundings, often found on hilltops. It forms from differential weathering.

trace fossil A fossil representing the activity of an organism, such as a burrow, track, or trail, rather than its body.

trachyte A fine-grained, extrusive igneous rock of intermediate composition, rich in alkali feldspars and containing minor amounts of mafic minerals.

travertine A terrestrial sedimentary rock formed by the precipitation of calcium carbonate from groundwater, often found around hot springs or in caves.

triclinic A crystal system characterized by three unequal axes that are not orthogonal (mutually perpendicular).

tuff A volcanic rock composed of compacted volcanic fragments, primarily ash-sized particles. If larger fragments are present, it grades into agglomerate.

tundra A vast, treeless, and generally flat region characterized by permafrost (permanently frozen subsoil).

turbidite The deposit formed by a turbidity current, typically characterized by graded bedding.

turbidity current A rapid, sediment-laden current flowing underwater due to its higher density. These currents can transport large amounts of sediment down continental slopes.

Tygerberg formation A component of the Precambrian Malmesbury group in South Africa, known for its sedimentary rocks.

U

ultramafic Rocks that are almost entirely composed of mafic minerals (rich in magnesium and iron) and contain very little or no feldspar. Examples include dunite, peridotite, and pyroxenite.

undulating hills See "rolling hills".

Urgonian A shallow-water carbonate facies deposited along the northern margins of the Tethys Ocean during the Barremian and Aptian stages.

V

vacuole A bubble-like inclusion within mineral grains, filled with liquid, gas, or both.

Variscan orogeny Also called the Hercynian orogeny. A mountain-building event during the Late Paleozoic caused by the collision of continental plates, leading to the formation of the supercontinent Pangaea.

varve An annual layer of sediment or sedimentary rock, typically found in lake deposits formed under glacial conditions.

vein A sheet-like or tabular mineral filling of a fracture or crack within a rock.

vermiculite A hydrous silicate mineral that expands significantly when heated. It's classified as a phyllosilicate.

vergence The direction of overturning of asymmetric folds, which typically corresponds to the direction of thrusting.

vesiculated Describes igneous rocks containing numerous vesicles, or small cavities, formed by gas bubbles trapped during solidification.

vitrinite A major component maceral group in coal, derived from woody plant material.

vitrophyre See pitchstone.

volcanic Pertaining to rocks that have crystallized from magma erupted onto the Earth's surface. These rocks cool relatively quickly.

volcanic bomb A mass of molten rock ejected during a volcanic eruption that solidifies in mid-air, often taking on rounded or spindle shapes.

vug A small cavity within a rock, often lined or filled with crystals of a different mineral.

W

wiggle trace A graphical representation of seismic wave amplitudes over time, often used in seismic reflection and borehole logging.

wolframite An iron manganese tungstate mineral, an important ore of tungsten.

X

xenolith A fragment of foreign rock that becomes enclosed within an igneous rock during its formation or emplacement. It’s an inclusion of older rock within younger igneous rock.

xenotime A rare earth phosphate mineral, primarily composed of yttrium orthophosphate (YPO 4 ).

X-ray diffraction (XRD) A technique used to determine the atomic and molecular structure of a crystal, based on the diffraction pattern produced when X-rays strike the crystal.

X-ray fluorescence (XRF) A method used for elemental analysis, where characteristic X-rays are emitted by a material after it has been excited by bombarding X-rays or gamma rays.

xyloid coal Also called lignite or brown coal. A soft, brown fuel intermediate between peat and coal.

Y

Yellowcake A concentrate of uranium oxide, produced during the processing of uranium ore.

Young's modulus A measure of the stiffness of a material, defined as the ratio of uniaxial stress to uniaxial strain in the elastic range. Also called the tensile modulus.

Ypresian The oldest age or stratigraphic stage of the Eocene epoch, spanning from approximately 56 to 49 million years ago.

Z

zeolite A group of microporous aluminosilicate minerals, commonly used as adsorbents and catalysts.

zircon A zirconium silicate mineral (ZrSiO 4 ), known for its hardness and resistance to weathering. It's widely used in geochronology due to its suitability for radiometric dating.

See also