This glossary of geology is a list of definitions of terms and concepts relevant to geology, its sub-disciplines, and related fields. For other terms related to the Earth sciences, see Glossary of geography terms (disambiguation).
A
abiotic Non-living chemical or physical component of the environment affecting living organisms and the functioning of ecosystems.
abyssal plain A flat or very gently sloping area on the floor of a deep ocean basin.
absolute dating The process of determining a specific date (in years or some other unit of time) for an archaeological, geological or paleontological site or artifact.
accident A sudden discontinuity of ground, such as a fault of great thickness, bed or lentil of unstable ground.
accretion A process by which material is added to a tectonic plate or landmass.
achnelith A small, glassy volcanic bomb, sphere, dumbbell, or droplet-shaped stone resulting from very liquid magma.
acid rock The groups ultrabasic, basic, intermediate and acid constitute a series with progressively increasing SiO2 content.
aftershock A small earthquake that follows a main shock.
agglomerate An indurated rock built of large angular rock fragments embedded in an ashy matrix and resulting from explosive volcanic activity. Occurs typically in volcanic vents.
aggregate A mass consisting of rock or mineral fragments.
albite The end member of the plagioclase group of minerals, ideally consisting of silicates of sodium and aluminium, but commonly containing small quantities of potash and lime in addition. Compare barbierite.
alkaline A highly basic substance that dissolves in water.
alkaline rock A type of rock characterized by a high content of Na2O and K2O relative to the other oxides. They occur throughout the range from ultrabasic to acid, but have their strongest expression in the acid-intermediate part of the range.
allochthon A fossil, sediment, or rock that was formed elsewhere and later transported into the location where it is presently found, usually by low angle thrust faulting. An object of this type is referred to as allochthonous. Contrast autochthon.
alluvial fan A fan-shaped deposit formed where a fast flowing stream flattens, slows, and spreads typically at the exit of a canyon onto a flatter plain.
alluvium Soil or sediments deposited by a river or other running water.
amber Fossilized resin or tree sap that is appreciated for its vivid colour, usually reddish-orange to gold or yellow.
amphibole An important group of dark-coloured, rock-forming silicate minerals, including hornblende, the commonest.
amphibolite A crystalline, coarse-grained rock, containing amphibole as an essential constituent, together with feldspar and frequently garnet. Like hornblende schist, amphibolite is formed by regional metamorphism of basic igneous rocks, but is not foliated.
amygdaloidal Amygdules or amygdales form when the gas bubbles or vesicles in volcanic lava (or other extrusive igneous rocks) are infilled with a secondary mineral such as calcite, quartz, chlorite or one of the zeolites. Rocks containing amygdules can be described as amygdaloidal.
anatexis Melting of pre-existing rock. Compare metatexis, diatexis, and syntexis.
andalusite One of several crystalline forms of aluminium silicate; a characteristic product of the contact metamorphism of argillaceous rocks.
andesite Fine-grained igneous rock of intermediate composition. Up to half of the rock is plagioclase feldspar with the rest being ferromagnesian minerals.
angular unconformity An unconformity in which younger strata overlie an erosion surface of tilted or folded layered rock.
anorthite Also called Indianite. A mineral from the lime-rich end of the plagioclase group of minerals. Anorthites are usually silicates of calcium and aluminium occurring in some basic igneous rocks, typically those produced by the contact metamorphism of impure calcareous sediments.
anticline An arched fold in which the layers usually dip away from the fold axis. Contrast syncline.
aphanic Having the texture of carbonate sedimentary rocks characterized by individual crystals or clastic grains less than 0.01 mm in diameter.
aphanitic Said of the texture of igneous rock in which the crystalline components are not distinguishable by the naked eye. Both microcrystalline and cryptocrystalline textures are included.
aplogranite A light-coloured rock of granitic texture consisting mainly of alkali feldspar and quartz, with subordinate biotite; muscovite may be present.
aquifer A body of saturated rock or sediment through which water can move readily.
aragonite sea Contains aragonite and high-magnesium calcite as the primary inorganic carbonate precipitates.
Archean Eon The oldest eon of the Earth's history.
archipelago A chain or cluster of islands.
arenaceous Sediments consisting essentially of sand grains; that is, of quartz and rock fragments down to 0.005mm in size. Conglomerates, sandstones, grits and siltstones fall into this category. Particle size 2mm to 1/16mm.
arenite 1. A general term for any consolidated sedimentary rock composed of sand-sized fragments. 2. “Clean” sandstone, well-sorted, less than 10% argillaceous matrix. Opposite to Wacke.
arenitic Pertaining to, having the quality of, or resembling sandstone.
arkose An arenaceous sedimentary rock. Like sandstone in its general character but containing feldspar to at least 10%. Formed by the disintegration of the acid igneous rocks and gneisses.
argillaceous Sedimentary rocks of the clay grade, i.e. composed of minute mineral fragments and crystals less than 0.005 mm in diameter, as well as large amounts of colloidal material. Apart from finely divided detrital matter, they consist of the so-called clay minerals, such as montmorillonite, kaolinite, gibbsite and diaspore. Siltstones, mudstones, shales, clays, etc. may all be referred to as argillaceous.
ash Fragments less than 2 mm (0.079 in) in diameter of pulverized rock, minerals and volcanic glass, created during volcanic eruptions.
asphalt A sticky, black and highly viscous liquid or semi-solid that is present in most crude petroleums and in some natural deposits.
assembled gem Also called a composite gem.
asthenosphere A region of the Earth's outer shell beneath the lithosphere. The asthenosphere is of indeterminate thickness and behaves plastically.
augite A complex aluminous silicate of calcium, iron and magnesium, crystallising in the monoclinic system, and occurring in many igneous rocks, particularly those of basic composition. It is an essential component of basalt, dolerite and gabbro.
aureole A zone surrounding an igneous intrusion in which country rock shows effects of contact metamorphism.
autochthon A fossil, sediment, or rock that was formed or produced in the location where it is now found. The term is widely applied to a coal or peat that originated at the place where the plants comprising it grew and decayed and to rocks that have not been displaced by overthrust faulting. An object of this type is referred to as autochthonous. Contrast allochthon.
B
bajada A series of coalescing alluvial fans along a mountain front.
banded iron formation A distinctive type of rock often found in primordial sedimentary rocks.
basalt A fine-grained, mafic igneous rock composed predominantly of ferromagnesian minerals and with lesser amounts of calcium-rich plagioclase feldspar.
basement rock The thick foundation of ancient, and oldest metamorphic and igneous rock that forms the crust of continents, often in the form of granite.
basic rock Igneous rock with low silica content (<54%). The groups ultrabasic, basic, intermediate, and acid constitute a series with progressively increasing SiO2 content.
basin A landform scooped out by water erosion.
Basin and Range Province A particular topography covering much of the southwestern United States and northwestern Mexico that is typified by elongate north-south trending arid valleys bounded by mountain ranges which also bound adjacent valleys.
batholith A large discordant pluton with an outcropping area greater than 100 square kilometres (39 sq mi).
bedrock Native consolidated rock underlying the loose rock or soil surface of the Earth.
Before Present (BP)
bentonite Swelling clay minerals of the smectite group with many industrial applications (drilling mud, expansive backfill materials…). The name bentonite is inherited from Fort Benton in Wyoming where large swelling clay deposits exist. See also smectite and montmorillonite.
bioerosion The erosion of hard ocean substrates by living organisms through various biological mechanisms.
biogenic Of biological origin – Produced by the present or past activity of living organisms, or by a biological process.
bioherm Landform of organic sedimentary rock enclosed or surrounded by rock of different origin.
biostratigraphy A branch of stratigraphy which focuses on correlating and assigning relative ages of rock strata by using the fossil assemblages contained within them.
biostratinomy The study of the processes that take place after an organism dies but before its final burial.
biostrome A distinctly bedded or broadly lenticular sedimentary rock landform.
biotic Of biological origin – Produced by the present or past activity of living organisms, or by a biological process.
biotite A form of black mica widely distributed in igneous rocks (particularly in granites) as lustrous black crystals, with a singularly perfect cleavage. In composition it is a complex silicate, chiefly of iron and magnesium, together with potassium and hydroxyl.
bioturbation The displacement and mixing of sediment particles by benthic fauna (animals) or flora (plants).
blueschist A rock that forms by the metamorphism of basalt or rocks of similar composition at high pressures and low temperatures, approximately corresponding to a depth of to and a temperature of to.
boudin A structure formed by extension, in which a rigid tabular body such as a bed of sandstone is stretched and deformed amidst less competent beds. See also boudinage.
boulder
Bowen's reaction series The sequence in which minerals crystallize from a cooling basaltic magma.
brackish Water with a salinity higher than freshwater but lower than seawater.
breadcrust bomb A rounded, smooth-surfaced volcanic bomb with a cracked surface resembling a cracked crust of bread, hence the name.
breccia A coarse-grained clastic rock consisting largely of angular fragments of existing rocks.
buckling A failure mode of a rock subjected to high compressive stresses, where the actual compressive stress at the point of failure is less than the ultimate compressive stresses that the material is capable of withstanding. Typically, folding is thought to occur by simple buckling of a planar surface and its confining volume. The volume change is accommodated by layer parallel shortening the volume, which grows in thickness.
C
calcareous Formed from or containing a high proportion of calcium carbonate in the form of calcite or aragonite, used of a sediment, sedimentary rock, or soil type.
calcite A mineral that is the crystalline form of calcium carbonate (CaCO3), showing trigonal symmetry and a great variety of mineral habits. It is one of the commonest of minerals in association with both igneous and sedimentary rocks.
calcite sea A body of water in which low-magnesium calcite is the primary inorganic marine calcium carbonate precipitate.
calcrete 1. A conglomerate of surficial sand and gravel cemented by calcium carbonate precipitated from solution. 2. A calcareous duricrust.
caldera A volcanic feature formed by the collapse of land following a volcanic eruption.
Cambrian The earliest geologic period of the Paleozoic Era, lasting from 541.0 ± 1.0 to 485.4 ± 1.9 million years ago and succeeded by the Ordovician.
carbon film A type of fossil or preservation.
carbonate A salt or ester of carbonic acid.
carbonate hardgrounds Surfaces of synsedimentarily-cemented carbonate layers that have been exposed on the seafloor.
casting The process which occurs when a liquid fills a cavity and then solidifies. If the cavity originated from the decomposition of dead organisms, casting may result in the formation of fossils.
Cenozoic Era The most recent of the geological eras, which followed the Mesozoic Era.
chalk A soft, white, porous sedimentary rock, a form of limestone composed of calcite coccolith plates.
chert A fine-grained, silica-rich, microcrystalline, cryptocrystalline or microfibrous sedimentary rock that sometimes contains small fossils.
chlorite Any of a set of allied non-swelling 2:1 (TOT) clay minerals which may be regarded as hydrated silicates of aluminium, iron, and magnesium. Their non-accessible interlayer is filled by a brucite bridge (Mg(OH)2, or sometimes Fe(OH)2). They crystallise in the monoclinic system and are green in colour. They occur as alteration products of such minerals as biotite and hornblende, and also in schistose rocks.
clast Any individual constituent grain or fragment of a sediment or rock produced by mechanical weathering of a rock mass.
clastic rocks Mechanically redeposited remains of eroded older rocks; i.e. rocks formed of fragments, or clasts, of pre-existing rocks.
clay
cleavage The tendency of a rock to break along preferred planes of weakness, caused by the development of a planar fabric as a result of deformation.
cobble
coccolith An individual plate of calcium carbonate formed by coccolithophores which are arranged around them in a so-called coccosphere.
coccolithophore Also called a coccolithophorid. A type of microfossil of single-celled algae, protists and phytoplankton belonging to the division of haptophytes. These fossils are distinguished by special calcium carbonate plates called coccoliths.
compaction The process by which a newly deposited sediment decreases its porosity and progressively expels its original pore water due to the effects of loading. This forms part of the process of lithification.
compression The process by which rocks shorten or decrease in volume when exposed to certain forces.
conchoidal A type of fracture that results in smoothly curved surface faces.
concretion A volume of sedimentary rock in which a mineral cement fills the porosity (i.e. the spaces between the sediment grains). Not to be confused with nodule.
conglomerate Any type of rock consisting of individual stones that have become cemented together.
contact metamorphism Metamorphism due to the local heating of rocks by the intrusion of magma nearby.
continental crust The layer of granitic, sedimentary, and metamorphic rocks which form the parts of the Earth's crust that comprise the continents, and the areas of shallow seabed close to their shores (known as continental shelves).
continental margin Zone of the ocean floor, separating the thin oceanic crust from thicker continental crust.
continental shelf Extended perimeter of a continent and its associated coastal plain, which is covered, during interglacial periods (such as the current epoch), by gulfs, and relatively shallow seas known as shelf seas.
convergent boundary The boundary between two tectonic plates that are moving toward each other. Contrast divergent boundary.
copal A type of resin produced by plant or tree secretions, particularly identified with the forms of aromatic tree resins used by the cultures of pre-Columbian Mesoamerica as a ceremonially burned incense, as well as for a number of other purposes.
coprolite A fossilized specimen of human or animal dung.
cordierite A silicate of aluminium, iron and magnesium with water, which crystallises in the orthorhombic system and occurs mainly in metamorphic rocks.
core The innermost layer(s) of a planet, referring especially to the Earth's core.
corestone An ellipsoidal or broadly rectangular joint block of granite formed by subsurface weathering in the same manner as a tor but entirely separated from bedrock.
country rock The rock native to an area, as opposed to rock that formed elsewhere and was later transported to the area.
craton An old and stable part of the continental crust that has survived the merging and splitting of continents and supercontinents for at least 500 million years.
cross-bedding An inclined sedimentary structure in a horizontal unit of rock. Such tilted structures indicate the type of depositional environment, not post-depositional deformation.
crude oil A liquid mixture of naturally occurring hydrocarbons.
crust The outermost solid layer of a planet or moon, referring especially to the Earth's crust.
cryptocrystalline
crystal
crystal habit
crystallinity
D
dacite An igneous, volcanic rock with a high iron content. It is an extrusive rock of the same general composition as andesite, but a less calcic feldspar. Synonymous with quartz andesite.
daughter product Any distinct isotope produced by the radioactive decay of an atomic nucleus.
delta A landform where the mouth of a river flows into an ocean, sea, desert, estuary, lake or another river.
degradation The lowering of a fluvial surface, such as a stream bed or floodplain, through erosional processes.
dendrite A crystal that develops with a typical multi-branching tree-like form.
denudation The lowering of the earth's surface through chemical and physical weathering.
deposition The geological process by which material is added to a landform or landmass.
detachment fault A major fault in a mountain belt above which rocks have been intensely folded or faulted.
diagenesis The process of chemical, physical, or biological change undergone by a sediment after its initial deposition and during and after its lithification, exclusive of surface alteration (weathering) and metamorphism.
diamictite A comprehensive non-generic term for a non-sorted or poorly sorted non-calcareous terrigenous sedimentary rock that contains a wide range of particle sizes such as rock with sand or larger particles in a muddy matrix.
diapir A type of intrusion in which a more mobile and ductilely deformable material is forced into brittle overlying rocks; a dome or anticlinal fold of the overlaying rocks which has been ruptured by the squeezing out of the plastic core material.
diatomite A soft, chalk-like sedimentary rock that is easily crumbled into a fine white to off-white powder.
diopside A monoclinic pyroxene, ideally consisting of silicate of calcium and magnesium, but commonly containing a variable content of FeSi2O6 in addition, and then strictly known as ferriferous diopside.
diorite A grey to dark grey intermediate intrusive igneous rock composed principally of plagioclase feldspar (typically andesine), biotite, hornblende, and/or pyroxene.
dike Also spelled dyke. A type of sheet intrusion referring to any geologic body that cuts discordantly across. A form of minor intrusion injected into the crust during its subjection to tension, the dyke being thin with parallel sides, and maintaining a constant direction in some cases for long distances. Some are more resistant to weathering than the surrounding rock and stand up like walls, while others weather faster and form long narrow depressions.
dip slope A geological formation often created by erosion of tilted strata.
disconformity A surface that represents missing rock strata but beds above and below that surface are parallel to one another.
divergent boundary The boundary separating two tectonic plates that are moving away from each other. Contrast convergent boundary.
dolomite A sedimentary carbonate rock and mineral, both composed of crystalline calcium magnesium carbonate CaMg(CO3)2.
dolerite 1. A basic igneous rock of medium grain size, occurring as minor intrusions or in the central parts of thick lava flows. 2. A dark-coloured, basic, igneous rock, composed essentially of pyroxene and a triclinic feldspar with magnetic iron. Considered by some authors to be equivalent to a coarse-grained basalt. 3. A dark, crystalline, igneous rock, chiefly pyroxene with labradorite. 4. Coarse-grained basalt. 5. Diabase. 6. Any dark igneous rock composed chiefly of silicates of iron and magnesium with some feldspar.
dome A geological formation consisting of symmetrical anticlines that intersect each other at their respective apices.
drill core A drill specifically designed to remove a cylinder of material, much like a hole saw.
drumlin An elongated, whale-shaped hill formed by glacial action.
dunite An ultrabasic plutonic rock in which the mafic material is almost entirely olivine, with accessory chromite almost always present. Feldspar mainly plagioclase. See also peridotite.
duricrust A general term for hard crust existing as a layer in or on the surface of the upper horizons of a soil in semi-arid climates. Duricrust is formed by the accumulation of solid minerals deposited by water moving upwards by capillary action and evaporating in the dry season. Compare hardpan.
E
earthquake
eclogite A generally coarse- to medium-grained pyroxene in which are set red garnets. The colour is pistachio green when fresh, but mottled with red when weathered.
Eemian transgression The portion of the Late Pleistocene spanning the period between 120 Ka and 8m before present.
eonThe largest unit of geologic time.
epicenter The point on the Earth's surface that is directly above the hypocenter or focus at which an earthquake or underground explosion originates.
epidiorite Any of a set of altered gabbroic and doleritic rocks in which the original pyroxene has been replaced by fibrous amphibole. The rock may be regarded as a first step in the conversion by dynamothermal metamorphism of a basic igneous rock into a green schist.
epirogenetic The simultaneous rising and falling movements of continents, maintaining isostasy.
epoch A division of the standard geologic time scale subordinate to periods. An example is the Pleistocene Epoch of the Quaternary Period.
erosion The displacement of solids (sediment, soil, rock and other particles) usually by the agents of currents such as wind, water, or ice by downward or down-slope movement in response to gravity or by living organisms (in the case of bioerosion).
erratic A piece of rock that deviates from the size and type of rock native to the area in which it rests. Erratics usually occur as stones ranging in size from pebbles to large boulders which were transported by glacial ice, which upon melting left them stranded far from their original source. The name "erratic" is based on the errant location of these boulders.
escarpment A transition zone between different physiogeographic provinces that involves an elevation differential, characterized by a cliff or steep slope.
esker A long, winding ridge of stratified sand and gravel, examples of which occur in glaciated and formerly glaciated regions of Europe and North America. Eskers are frequently several miles in length and, because of their peculiar uniform shape, somewhat resemble railroad embankments.
estuary A semi-enclosed coastal body of water with one or more rivers or streams flowing into it, and with a free connection to the open sea.
eugeosyncline A geosyncline in which volcanism is associated with clastic sedimentation; the volcanic part of an orthosyncline located away from the craton.
euhedral Bounded by the crystal faces peculiar to the species, used of minerals. Synonymous with idiomorphic.
eustatic movements Changes of sea level, constant over wide areas, due to alterations in the volume of the oceans resulting from the formation or melting of ice caps.
evaporite Any of a diverse set of water-soluble mineral sediments that result from the evaporation of bodies of surface water.
exfoliation The stripping of concentric rock slabs from the outer surface of a rock mass.
extension Strain involving an increase in length. Extension can cause thinning and faulting.
extrusive A mode of igneous volcanic rock formation in which hot magma from inside the Earth flows out (extrudes) onto the surface as lava or explodes violently into the atmosphere to fall back as pyroclastics or tuff.
F
facies The sum of the lithological and faunal characters of a sediment is its facies. Lithological facies involves composition, grain size, texture, colour, as well as such mass characteristics as current bedding, nature of stratification, ripple marks, etc. Similarly, metamorphic facies involves the degree of crystallisation and the mineral assemblage in a group of metamorphic rocks.
fanning Rock deformation related to shear stress.
fault A discrete planar rock fracture which shows evidence of a displacement (the throw of the fault). A fault is a discrete surface.
fault zone The zone where exist different discrete fault planes.
feldspar Any of a set of the most common minerals in the Earth's crust. All feldspars contain silicon, aluminium, and oxygen and may contain potassium, calcium and sodium.
fels A massive metamorphic rock lacking schistosity or foliation.
felsic Silicate minerals, magmas, and rocks which are enriched in the lighter elements such as silicon, oxygen, aluminium, sodium, and potassium. Light minerals (quartz and feldspar) greater than 60% - acid. [Granite (Rhyolite), Adamellite (Rhyo-dacite), Granodiorite (Dacite)]. The term is a mnemonic adjective for igneous rocks having light-coloured minerals in their mode, from "feldspar" and "silica". Contrast mafic.
ferricrete A conglomerate consisting of surficial sand and gravel cemented into a hard mass by iron oxide derived from oxidation of percolating solution of iron salts. A ferruginous duricrust.
ferromagnesian mineral Any iron/magnesium-bearing mineral, such as augite, hornblende, olivine, or biotite.
fission track dating A method that uses tracks that are visible under the microscope to date minerals.
Flandrian transgression
fold A stack of originally flat and planar surfaces, such as sedimentary strata, which have become bent or curved as a result of plastic (i.e. permanent) deformation.
foliation The parallel alignment of textural and structural features of a rock.
fossil Any mineralized or otherwise preserved remains or traces (such as footprints) of animals, plants, or other once-living organisms.
fossiliferous Bearing or being composed of fossils in rocks or strata.
fossilization
fracture Any crack or discontinuity. In its geological definition, it is only used when no displacement can be distinguished.
fumarole A vent in the Earth's surface from which hot gases and vapors are emitted.
G
Ga A non-standard but widely used abbreviation for one billion (1,000,000,000) years, using the metric prefix G (for "Giga") to indicate a quantity of one billion. When not otherwise qualified, it usually indicates 1,000,000,000 years Before Present (or 1,000,000,000 years ago).
gabbro A dark, coarse-grained, intrusive igneous rock chemically equivalent to basalt.
garnet
gastrolith A rock which is or was once held inside the digestive tract of a living animal.
gemology Also spelled gemmology. The branch of geology and mineralogy that studies natural and artificial gemstones.
gemstone Also called a gem, fine gem, jewel, precious stone, or semi-precious stone.
geogenic Of geologic origin – Resulting from naturally occurring geological processes.
geological map A special-purpose map made to show geological features.
geological time scale Also geologic time scale.
geology
geosyncline A mobile down-warping of the Earth's crust, either elongate or basin-like, measured in scores of kilometres, which is subsiding as sedimentary and volcanic rocks accumulate to thicknesses of thousands of metres.
glass Amorphous (non-crystalline) hard and brittle solid such as soda-lime glass.
glauconite A green-coloured, hydrated silicate mineral of potassium and iron that forms on submerged banks. Its occurrence in sands and sandstones is considered an indication of accumulation under marine conditions.
gneiss A coarse-grained, pale-coloured gneissose rock, containing abundant feldspar with quartz, mica, hornblende, and garnet.
Gondwanaland The southern part of the supercontinent of Pangaea which eventually separated to form present-day South America, Africa, India, Australia and Antarctica.
graben A depressed block of the Earth's crust bordered by parallel faults.
granite A coarse-grained, often porphyritic, intrusive, felsic, igneous rock containing megascopic quartz, averaging 25%, much feldspar (orthoclase, microcline, sodic plagioclase) and mica or other coloured minerals. Rhyolite is the volcanic equivalent.
granitoid Any granite-like rock, including granodiorite, diorite, monzonite, and granite itself, among others.
granoblastic An arrangement of mineral grains in a rock of metamorphic origin similar to that of a normal granite, but produced by recrystallisation in the solid and not by crystallisation from a molten condition.
granodiorite An intrusive, felsic, igneous rock similar to granite but containing more plagioclase than potassium feldspar. Dacite is the volcanic equivalent.
granulometry Statistical distribution of the size of individual sediment grains, or lithified particles in clastic rocks.
gravel
graywacke Also spelled greywacke. A variety of sandstone generally characterized by its hardness, dark colour, and poorly sorted, angular grains of quartz, feldspar, and small rock fragments (lithic fragments) set in a compact, clay-fine matrix.
greenstone An omnibus term lacking precision and applied indiscriminately to basic and intermediate igneous rocks of Lower Paleozoic age in which much chlorite has been produced at the expense of the original coloured minerals, staining the rocks green.
grus Freshly eroded, angular grains of quartz and feldspar derived from a granitoid.
H
habit See crystal habit.
half-life The time it takes for a given amount of a radioactive isotope to be reduced by one-half.
hardpan
hemicrystalline Having the properties of certain rocks of igneous origin which contain some interstitial glass in addition to crystalline minerals. Contrast holocrystalline.
hinge The zone of maximum curvature of a fold.
hinge line A line joining the points of maximum curvature along the hinge of a fold.
holocrystalline Having the properties of those igneous rocks in which all of the components are crystalline; glass is absent. Contrast hemicrystalline. hornblende An important rock-forming mineral of complex composition, essentially a silicate of calcium, magnesium and iron, with smaller amounts of potash, soda and hydroxyl. Hornblende crystallises in the monoclinic system and occurs as black crystals or grains in many different types of igneous and metamorphic rocks, including hornblende-granite, syenite, diorite, andesite, hornblende-schist, and amphibole.
hornfels A hard, compact, fine-textured contact-altered argillaceous rock that breaks into splintery fragments.
horst A raised fault block bounded by normal faults.
hot spring A natural spring resulting from the emergence of geothermally heated groundwater from beneath the Earth's crust.
hydrothermal Pertaining to the actions or products of heated water.
hydrothermal vent A fissure in a planet's surface from which geothermally heated water emerges.
hypersaline Having a saltiness or dissolved salt content greater than that of seawater.
I
ichnology The study of traces of organismal behavior.
idiomorphic Bounded by the crystal faces peculiar to the species, used of minerals. Synonymous with euhedral.
igneous rock A type of rock formed by solidification of cooled magma (molten rock), with or without crystallization, either below the surface as intrusive (plutonic) rocks or on the surface as extrusive (volcanic) rocks.
ignimbrite Fine-grained to aphanitic, buff to dark brown compact rock with parallel streaks or lenticles of black glass, produced by violently explosive volcanoes.
ilmenite An oxide of iron and titanium, crystallising in the trigonal system; a widespread accessory mineral in igneous rocks, especially those of basic composition.
illite Non-swelling clay mineral with a three layers 2:1 (TOT) structure. Its interlayers contain no water and anhydrous K+ cations responsible for their total collapse. Water and cations diffusion is not possible in these collapsed interlayers. The name "illite" is inherited from the state of Illinois where natural non-swelling clay deposits exist.
indurated Made hard (by heat or compaction).
interbeddedbeds (layers) of rock lying between or alternating with beds of a different kind of rock.
intrusionbody of igneous rock that has crystallized from molten magma below the surface of the Earth.
island arc A chain of volcanic islands or mountains formed by plate tectonics as an oceanic tectonic plate subducts under another tectonic plate and produces magma.
isomorphic Two crystals that have similar shapes and sizes, usually through the angles.
isotope different forms of an element each having different atomic mass (mass number).
J
joint A discrete discontinuity surface without evidence of displacement. See also diaclase or bedding.
Jurassic A major unit of the geologic timescale that extended from about 199.6 ± 0.6 Ma (million years ago) to 145.4 ± 4.0 Ma, between the end of the Triassic and the beginning of the Cretaceous.
K
Ka A non-standard but widely used abbreviation for one thousand (1,000) years, using the metric prefix K (for "Kilo") to indicate a quantity of one thousand. When not otherwise qualified, it usually indicates 1,000 years Before Present (or 1,000 years ago).
kame An irregularly shaped hill or mound composed of sand, gravel and till that accumulates in a depression on a retreating glacier and is then deposited on the land surface with further melting of the glacier.
kaolinite A finely crystalline form of hydrated aluminium silicate occurring as minute monoclinic flaky crystals with a perfect basal cleavage, resulting mainly from the alteration of feldspars under conditions of hydrothermal or pneumatolytic metamorphism.
karst A distinct type of landscape shaped by the dissolution of a layer or layers of soluble bedrock, usually carbonate rock such as limestone or dolomite. Karst topography is usually characterised by closed depressions or sinkholes, caves and underground drainage.
kettle A fluvioglacial landform occurring as the result of blocks of ice calving from the front of a receding glacier and becoming partially to wholly buried by glacial outwash.
kink A tight curl, twist, or bend in a rock band. See also folding and buckling.
kink band An asymmetric, linear zone of deformation characterised by a tight curled, twisted, or bended rock band. Kink bands may also occur as conjugated sets.
kyanite A silicate of aluminium which crystallises in the triclinic system. It usually occurs as long-bladed crystals, blue in colour, in metamorphic rocks.
L
lacuna A time-stratigraphic unit representing the gap in the stratigraphic record. Specifically the missing interval at an unconformity, representing the interpreted space-time value of both hiatus (period of non-deposition), and degradation vacuity (period of erosion).
lamprophyre Igneous rocks usually occurring as dykes intimately related to larger intrusive bodies; characterised by abnormally high contents of coloured silicates, such as biotite, hornblende and augite, and a correspondingly small amount of feldspar, some being feldspar-free.
large igneous province Massive volcanic formation resulting from flood basalt eruptions.
lava Molten rock expelled by a volcano during an eruption.
leucocratic A term used to denote a light colour in igneous rocks, due to a high content of felsic minerals and a correspondingly small amount of dark, heavy silicates.
limestone A sedimentary rock composed largely of the mineral calcite (calcium carbonate: CaCO3).
liquefaction Soil liquefaction describes the behavior of soils that, when loaded, suddenly suffer a transition from a solid state to a liquefied state, or having the consistency of a heavy liquid.
lithic fragment Also simply called a lithic. A sand-sized grain that is made up of smaller than sand-sized grains, e.g. a shale fragment or basalt fragment in a sandstone.
lithification The process by which sediments compact under pressure, expel connate fluids, and gradually become solid rock. lithology A description of the physical characteristics of a rock unit visible at outcrop, in hand or core samples or with low magnification microscopy, such as colour, texture, grain size, or composition.
lithosphere The rigid, outermost rocky shell of a terrestrial planet or natural satellite. The Earth's lithosphere is composed of the crust and the portion of the upper mantle that behaves elastically on time scales of up to thousands of years or more.
lithotype Specific types of rock classified according to the standards of lithology.
loess A fine, silty, pale yellow or buff-coloured, windblown (eolian) type of unconsolidated deposit.
lustre Also spelled luster.
M
Ma A non-standard but widely used abbreviation for one million (1,000,000) years, using the metric prefix M (for "Mega") to indicate a quantity of one million. When not otherwise qualified, it usually indicates 1,000,000 years Before Present (or 1,000,000 years ago).
mafic A silicate mineral or rock that is rich in magnesium and iron. A mnemonic term for the ferromagnesian and other non-felsic minerals actually present in an igneous rock rich in dark (ferromagnesian) minerals (greater than 60% by volume). Basic [alkali gabbro (alkali basalt), syeno-gabbro (trachybasalt), gabbro (basalt and dolerite)].
magma Molten rock that sometimes forms beneath the surface of the Earth (or other terrestrial planets) and often collects in a magma chamber.
magnetite An oxide of iron which crystallises in the cubic system. It is attracted by a magnet but does not attract iron itself.
Malmesbury group An 830 to 980 million-year-old basal group of the Western Cape comprising at least eight distinct formations, including the Tygerberg, Piketberg, Porterville, Berg river, Klipplaat, Moorreesburg, Franschhoek, and Bridgetown formations.
mantle The highly viscous layer of molten rock situated directly beneath the Earth's crust and above the outer core.
marble A fine to coarse-grained granoblastic calcium carbonate that effervesces in dilute hydrochloric acid. Often banded with various colours and sometimes veined.
marine terrace A narrow, flat area often seen at the base of a sea cliff caused by the action of the waves.
marl A calcium carbonate or lime-rich mud or mudstone which contains variable amounts of clays and aragonite.
massive A description applied to a homogeneous rock which lacks internal structure or layers.
mélange Large-scale breccia formed in the accretionary wedge above a subduction zone.
melanocratic A term applied to rocks which are abnormally rich in dark and heavy ferro-magnesium minerals.
mesocratic A term applied to igneous rocks which in respect of their content of dark silicates are intermediate between those of leucocratic and melanocratic type, and contain 30–60% of dark heavy minerals.
Mesozoic The era of geological time including the Triassic, Jurassic, and Cretaceous ages.
metamorphic rock
metamorphism The solid-state recrystallisation of pre-existing rocks due to changes in heat, pressure, or water-rock interactions, i.e. without melting.
metapelite A metamorphosed pelite (fine grain, clayey) rock.
metasilicate A salt of the non-existing metasilicic acid H2SiO3 based on an analogy with the carbonate system.
metatexis Low-grade anatexis: partial or differential melting of rock components with a low melting point.
mica Any of a set of minerals which crystallise in the monoclinic system. They have similar chemical compositions and highly perfect basal cleavage.
micropaleontology A