Category: Geology Dictionary

urban geography : The sub-discipline of geography that derives from the study of cities, urban processes, and the built environment.

distance decay : The decrease in cultural or spatial interactions between two places as the distance between them increases. This effect may be noticeable in towns and cities, where certain descriptive characteristics such as pedestrian traffic, building height, and land value tend to decline with greater distance from the city center.

corrie loch : See tarn.

Zircon : A zirconium silicate mineral belonging to the group of nesosilicates. Its corresponding chemical formula is ZrSiO₄.

Zeolite : Microporous, aluminosilicate minerals commonly used as adsorbents.

Ypresian : The oldest age or the lowest stratigraphic stage of the Eocene. It spans the time between ~56 Ma and ~49 Ma (million years ago).

Yellow cake : Also yellowcake or urania.

A kind of uranium concentrate powder obtained from leach solutions in an intermediate step in the enrichment of uranium ores.

Young’s modulus: Also called the tensile modulus.

In solid mechanics, a measure of the stiffness of an isotropic elastic material. It is defined as the ratio of the uniaxial stress over the uniaxial strain in the range of stress in which Hooke’s Law holds.

X-ray fluorescence (XRF) : The emission of characteristic “secondary” (or fluorescent) X-rays from a material that has been excited by bombarding with high-energy X-rays or gamma rays. The phenomenon is widely used for elemental analysis and chemical analysis of minerals.

Xyloid coal : Also called lignite or brown coal.

A soft brown fuel with characteristics that put it somewhere between coal and peat.

Xenotime : A rare earth phosphate mineral whose major component is yttrium orthophosphate (YPO₄).

X-ray diffraction (XRD) : A method of determining the arrangement of atoms within a crystal, in which a beam of X-rays strikes a crystal and diffracts into many specific directions.

Wolframite : An iron manganese tungstate mineral with the chemical formula (Fe,Mn)WO₄.

Xenolith : A rock fragment which becomes enveloped in a larger rock during the latter’s development and hardening. In geology, the term is almost exclusively used to describe inclusions in igneous rock during magma emplacement and eruption.

Vug : A small cavity in a rock filled or lined with crystals or minerals that are different from the host rock.

Wiggle trace : A graph that plots wave amplitudes (recorded by seismic reflection and borehole logging) as a function of time, with the positive peaks shaded in a single dark colour.

Volcanic : Rocks that have crystallised from magma poured out at the surface or introduced at shallow depth. They have cooled relatively rapidly, the grain size of the crystals is small, some part of the melt may solidify as glass, volatiles are lost and anhydrous minerals with high temperatures of crystallisation are present.

Volcanic bomb : Rounded or spindle-shaped rock of mainly basaltic composition ejected during eruptions.

Vitrophyre : See pitchstone.

Vitrinite : A group of macerals that are the most common component of coal.

Vergence : The direction of overturning of asymmetric folds, which matches the direction of thrusting.

Vermiculite : A hydrous silicate mineral that is classified as a phyllosilicate and that expands greatly when heated. Exfoliation occurs when the mineral is heated sufficiently.

Vein : A mineral filling of a fracture or other crack within a rock in a sheet-like or tabular shape.

Varve : An annual layer of sediment or sedimentary rock.

Variscan orogeny : Also called the Hercynian orogeny.

A geologic mountain-building event caused by Late Paleozoic continental collision between Euramerica (Laurussia) and Gondwana to form the supercontinent of Pangaea.

Vacuole : A bubble inclusion within mineral grains (typically monocrystalline quartz), filled with liquid, gas, or both liquid and gas. Vacuoles are randomly distributed in contrast to the oriented bubble trains of Boehm Lamellae.

Ultramafic : Sometimes used interchangeably with ultrabasic.

Almost feldspar-free. Examples of ultramafic rocks include dunite, peridotite, and pyroxenite.

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

Turbidity current : A current of rapidly moving, sediment-laden water moving down a slope through water, or another fluid. The current moves because it has a higher density than the fluid through which it flows.

Tygerberg formation : A component of the PreCambrian Malmesbury group of South Africa

Tundra : A vast, flat, treeless Arctic region of Europe, Asia, and North America in which the subsoil is permanently frozen.

Turbidite : The deposit of a turbidity current.

Triclinic : In the triclinic system, the crystal is described by vectors of unequal length, and none of the three vectors are orthogonal to another.

Tuff : A rock formed of compacted volcanic fragments, some of which can be distinguished by the naked eye. If the fragments are larger than the rock grades into an agglomerate.

Trachyte : A fine-grained igneous rock type of intermediate composition, in most cases with little or no quartz, consisting largely of alkali-feldspars (sanidene or oligooclase) together with a small amount of coloured silicates such as diopside, horneblende, or mica.

Travertine : A terrestrial sedimentary rock formed by the precipitation of carbonate minerals from solution in ground and surface waters and/or geothermally heated hot springs.

Topography : The arrangement of the natural and artificial physical features of an area.

Tor : A large, free-standing residual mass (rock outcrop) that rises abruptly from the surrounding smooth and gentle slopes of a rounded hill summit or ridge crest.

Tillite : A type of sedimentary rock derived from glacial till which has been indurated or lithified by subsequent burial into solid rock.

Till : Also called glacial till.

Unsorted glacial sediment. Glacial drift is a general term for the coarsely graded and extremely heterogeneous sediments of glacial origin. Glacial till is that part of glacial drift which was deposited directly by the glacier.

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.

Tephra : Fragmental material produced by a volcanic eruption regardless of composition, fragment size or emplacement mechanism. Once clasts have fallen to the ground they remain as tephra unless hot enough to fuse together into pyroclastic rock or tuff.

Tenacity : A mineral’s behavior when deformed or broken.

Talus: Also called scree.

A collection of broken rock fragments at the base of crags, mountain cliffs, volcanoes or valley shoulders that has accumulated through periodic rockfall from adjacent cliff faces. Landforms associated with these materials are often called talus deposits.

Table Mountain Group : A group of rock formations within the Cape Supergroup sequence of rocks.

Tektite : Natural glass formed from terrestrial ejecta during a meteorite impact.

syntaxis : An abrupt change in the orientation of a mountain belt or individual fold/thrust structure

Syncline : A geological fold with strata dipping inwards towards the fold axis. Contrast anticline.

Syenite : A coarse-grained igneous rock of intermediate composition, composed essentially of alkali-feldspar to the extent of at least two thirds of the total, with a variable content of mafic materials, of which common hornblende is characteristic.

Surficial : Of or pertaining to the surface.

Stylolite : An irregular discontinuity or non-structural fracture in limestone and other sedimentary rocks. Stylolites result from compaction and pressure solution during diagenesis.

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

Storm ridge : A beach ridge usually located further or higher inland caused by wave action from storms.

Staurolite : A red brown to black, mostly opaque, nesosilicate mineral with a white streak.

Sphene : Also called titanite.

A calcium titanium nesosilicate mineral.

Sorting : Sorting describes the distribution of grain size of sediments, either in unconsolidated deposits or in sedimentary rocks. Very poorly sorted indicates that the sediment sizes are mixed (large variance); whereas well sorted indicates that the sediment sizes are similar (low variance).

Soil liquefaction : The process describing the behavior of soils that, when loaded, suddenly suffer a transition from a solid state to a liquefied state, or which have the consistency of a heavy liquid.

Slump : A form of mass wasting that occurs when a coherent mass of loosely consolidated materials or rock layers moves a short distance down a slope.

Slickenside : A smoothly polished surface caused by frictional movement between rocks along the two sides of a fault. This surface is normally striated in the direction of movement.

Slaty cleavage : The property of splitting easily along regular, closely spaced planes of fissility, produced by pressure in fine-grained rocks, with the cleavage planes lying in the directions of maximum elongation of the mass.

Slate : Very fine-grained sedimentary rock of the clay or silt grade which as a consequence of regional metamorphism has developed a slaty cleavage.

Silt : Granular material of a size somewhere between sand and clay whose mineral origin is quartz and feldspar. Silt may occur as a soil or as suspended sediment (also known as suspended load) in a surface water body. It may also exist as soil deposited at the bottom of a water body.

Silcrete : An indurated soil duricrust formed when surface sand and gravel are cemented by dissolved silica.

Shingle beach : A beach which is armoured with pebbles or small- to medium-sized cobbles (as opposed to fine sand), typically ranging from 2 to 200 millimetres (0.1 to 7.9 in) diameter.

Shield : A large area of exposed Precambrian crystalline igneous and high-grade metamorphic rocks that form tectonically stable areas.

Shear zone : A tabular to sheet-like, planar or curviplanar zone composed of rocks that are more highly strained than rocks adjacent to the zone. See also fault.

Sericite : A white potash-mica, similar to muscovite in chemical composition and general character but occurring as a secondary mineral, often as a decomposition product of orthoclase.

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

Sediment trap: A depression in which sediments substantially accumulate over time.

Scree : See talus.

Schist : A group of medium-grade metamorphic rocks, chiefly notable for the preponderance of lamellar minerals such as micas, chlorite, talc, hornblende, graphite, and others. In French, schist is understood as shale.

Sanidine : A form of potash feldspar identical in composition with orthoclase but physically different, formed under different conditions and occurring in different rock types. It is the high temperature form of orthoclase, into which it inverts at 900 °C (1,650 °F). Occurs in lavas and dyke rocks.

Sandstone : Sand (with grains up to 2 mm in diameter) in which the grains are cemented together by secondary silica or calcite. Maybe loosely cemented and soft or well cemented and hard. Buff to brownish; sometimes reddish, due to presence of iron oxides, or greenish, due to presence of glauconite.

Saccharoidal : Having a texture similar to that of granulated sugar.

Rudite : Any of a set of sedimentary rocks composed of rounded or angular detrital grains, i.e. granules, pebbles, cobbles, and boulders, which are coarser than sand in size.

Rudaceous : Having the composition and characteristics of clastic rocks, i.e. coarse-grained sedimentary rocks, conglomerates, and breccias, with a particle size of less than 2 mm.

Roche moutonnée : An elongated post-glacial rock formation with a smoothed surface on the uphill side and a “plucked” surface on the downhill side.

Rhyolite: An aphanitic, buff to greyish flow-banded rock, often containing spherulites or phenocrysts of quartz and feldspar.

Reverse fault : Also called a thrust fault.

Dip-slip faults can be sub-classified into the types “reverse” and “normal”. A reverse fault occurs when the crust is compressed such that the hanging wall moves upward relative to the footwall. Contrast normal fault.

Retrograde metamorphism : The reconstitution of a rock via revolatisation under decreasing temperatures (and usually pressures), allowing the mineral assemblages formed in prograde metamorphism to revert to those more stable at less extreme conditions.

Reticulite : A basaltic pumice in which the walls of the vesicles have collapsed, leaving a network of fine, interconnecting glass threads. It is the lightest rock known.

Regional metamorphism : Over wide areas resulting from deep burial with consequent rise in temperature and static pressure, usually with the help of folding movements that accompany the formation of mountain ranges.

Quartzite : A compact, hard, very fine-grained white to creamy white rock which breaks into sharp angular fragments. Quartzite is always associated with other metamorphic rocks, while cemented sandstone is always associated with other sedimentary rocks.

Guartzite : A compact, hard, very fine-grained white to creamy white rock which breaks into sharp angular fragments. Quartzite is always associated with other metamorphic rocks, while cemented sandstone is always associated with other sedimentary rocks.

Pyroxenite : A coarse-grained, holocrystalline igneous rock consisting mainly of pyroxenes. It may contain biotite, hornblende, or olivine as accessories.

Pyroxene : Any of a set of mineral species which, although falling into different systems (orthorhombic, monoclinic, and triclinic), are closely related in form and structure. They are metasilicates of calcium, magnesium, and iron with manganese, and less often with sodium, potassium, zirconium, and fluorine.

Pyroclastic : A volcanic fragment, such as a volcanic bomb, breadcrust bomb, or achnelith.

Pyroclastic flow : A fast-moving current of hot gas and rock (collectively known as tephra), which normally hugs the ground and travels downhill or spreads laterally under gravity.

Pumice : A light-coloured, highly vesicular volcanic rock of very low density.

Pseudomatrix : A weaker material (mainly lithic fragments) that becomes crushed and matrix-like in a rock.

Psammite : A general term for a sandstone, most often used to describe a metamorphosed rock unit with a dominantly sandstone protolith.

Protolith : The source rock from which a metamorphic, or in some rare cases a sedimentary, rock was formed. In most cases the appropriate sedimentary term is “provenance” rather than “protolith”, since the material has been transported.

Prograde metamorphism : Mineral changes in rocks under increasing pressure and/or temperature conditions.

Precipitate: A rock or mineral precipitated into solid form from an aqueous solution.

Precambrian : A non-standard geologic time period immediately preceding the Phanerozoic Eon, divided into several eons of the geologic time scale. It spans from the formation of Earth about 4540 Ma (million years ago) to the beginning of the Cambrian Period, about 541.0 ± 1.0 Ma, when macroscopic hard-shelled animals first appeared in abundance.

Porphyry: 1.  A rock that is porphyritic, containing large and small crystals.2.  In mining, a specific deposit containing widely disseminated metals, typically copper.

Porphyroblast : A large mineral crystal in a metamorphic rock which has grown within the finer-grained groundmass.

Pneumatolysis : The destructive after-action of the concentrated volatile constituents of a magma, effected after the consolidation of the main body of the magma.

Plumose structure : A ladder or grid pattern that occurs during jointing that resembles plumes, oriented perpendicular to the stress, hence which usually form parallel to the upper and lower surfaces of the constituent rock unit.

Plutonic : Having crystallised at depth within the Earth’s crust, used of a rock. Plutonic rocks are slow-cooling and coarse-grained and have relatively low temperatures of final consolidation.