Glossary of geology#M

{{glossary}}

{{term|abiotic}}

{{defn|Non-living chemical or physical component of the environment affecting living organisms and the functioning of ecosystems.}}

{{term|abyssal plain}}

{{defn|A flat or very gently sloping area on the floor of a deep ocean basin.}}

{{term|absolute dating}}

{{defn|The process of determining a specific date (in years or some other unit of time) for an archaeological, geological or paleontological site or artifact.}}

{{term|accident}}

{{defn|A sudden discontinuity of ground, such as a {{gli|fault}} of great thickness, bed or lentil of unstable ground.{{cite dictionary

| encyclopedia = Dictionary of Civil Engineering: English-French

| publisher = Springer Science & Business Media

| date = 2007-05-08

| orig-year = 2004

| editor-first = Jean-Paul

| editor-last = Kurtz

| series = EngineeringPro

| isbn = 978-0-306-48474-2

| doi = 10.1007/b104633

| entry = geological accident - accident géologique

| quote = A sudden discontinuity of ground such as fault of great thickness, bed or lentil of unstable ground, etc.

| page = 573

| entry-url = https://books.google.com/books?id=Yp8SBwAAQBAJ&pg=PA573

| entry-url-access = limited| title = Dictionary of Civil Engineering

}}}}

{{term|accretion}}

{{defn|A process by which material is added to a {{gli|tectonic plate}} or landmass.}}

{{term|achnelith}}

{{defn|A small, glassy {{gli|volcanic bomb}}, sphere, dumbbell, or droplet-shaped stone resulting from very liquid magma.}}

{{term|acid rock}}

{{defn|The groups {{gli|ultrabasic}}, {{gli|basic}}, intermediate and acid constitute a series with progressively increasing SiO₂ content.}}

{{term|aftershock}}

{{defn|A small earthquake that follows a main shock.}}

{{term|agglomerate}}

{{defn|An {{gli|indurated}} rock built of large angular rock fragments embedded in an ashy matrix and resulting from explosive {{gli|volcanic}} activity. Occurs typically in volcanic vents.}}

{{term|aggregate}}

{{defn|A mass consisting of rock or mineral fragments.}}

{{term|albite}}

{{defn|The end member of the {{gli|plagioclase}} group of minerals, ideally consisting of silicates of sodium and aluminium, but commonly containing small quantities of potash and lime in addition. Compare {{gli|barbierite}}.}}

{{term|alkaline}}

{{defn|A highly basic substance that dissolves in water.}}

{{term|alkaline rock}}

{{defn|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.}}

{{term|allochthon}}

{{defn|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 {{gli|autochthon}}.}}

{{term|alluvial fan}}

{{defn|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.}}

{{term|alluvium}}

{{defn|Soil or sediments deposited by a river or other running water.}}

{{term|amber}}

{{defn|Fossilized resin or tree sap that is appreciated for its vivid colour, usually reddish-orange to gold or yellow.}}

{{term|amphibole}}

{{defn|An important group of dark-coloured, rock-forming silicate minerals, including {{gli|hornblende}}, the commonest.}}

{{term|amphibolite}}

{{defn|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.}}

{{term|amygdaloidal}}

{{defn|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.}}

{{term|anatexis}}

{{defn|Melting of pre-existing rock. Compare {{gli|metatexis}}, {{gli|diatexis}}, and {{gli|syntexis}}.}}

{{term|andalusite}}

{{defn|One of several crystalline forms of aluminium silicate; a characteristic product of the contact metamorphism of argillaceous rocks.}}

{{term|andesite}}

{{defn|Fine-grained igneous rock of intermediate composition. Up to half of the rock is {{gli|plagioclase}} feldspar with the rest being ferromagnesian minerals.}}

{{term|angular unconformity}}

{{defn|An unconformity in which younger strata overlie an erosion surface of tilted or folded layered rock.}}

{{term|anorthite}}

{{ghat|Also called Indianite.}}

{{defn|A mineral from the lime-rich end of the {{gli|plagioclase}} group of minerals. Anorthites are usually {{gli|silicate|silicates}} of calcium and aluminium occurring in some basic {{gli|igneous rock|igneous rocks}}, typically those produced by the contact metamorphism of impure calcareous sediments.}}

{{term|anticline}}

{{defn|An arched fold in which the layers usually dip away from the fold axis. Contrast {{gli|syncline}}.}}

{{term|aphanic}}

{{defn|Having the texture of {{gli|carbonate}} {{gli|sedimentary rocks}} characterized by individual crystals or clastic grains less than 0.01 mm in diameter.}}

{{term|aphanitic}}

{{defn|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.}}

{{term|aplogranite}}

{{defn|A light-coloured rock of granitic texture consisting mainly of alkali feldspar and quartz, with subordinate biotite; {{gli|muscovite}} may be present.}}

{{term|aquifer}}

{{defn|A body of saturated rock or sediment through which water can move readily.}}

{{term|aragonite sea}}

{{defn|Contains {{gli|aragonite}} and high-magnesium calcite as the primary inorganic carbonate precipitates.}}

{{term|Archean Eon}}

{{defn|The oldest eon of the Earth's history.}}

{{term|archipelago}}

{{defn|A chain or cluster of islands.}}

{{term|arenaceous}}

{{defn|Sediments consisting essentially of sand grains; that is, of quartz and rock fragments down to 0.005mm in size. Conglomerates, {{gli|sandstones}}, grits and {{gli|siltstones}} fall into this category. Particle size 2mm to 1/16mm.}}

{{term|arenite}}

{{defn|no=1|A general term for any consolidated {{gli|sedimentary}} rock composed of sand-sized fragments.}}

{{defn|no=2|“Clean” {{gli|sandstone}}, well-sorted, less than 10% {{gli|argillaceous}} matrix. Opposite to Wacke.}}

{{term|arenitic}}

{{defn|Pertaining to, having the quality of, or resembling {{gli|sandstone}}.}}

{{term|arkose}}

{{defn|An {{gli|arenaceous}} {{gli|sedimentary rock}}. Like {{gli|sandstone}} in its general character but containing {{gli|feldspar}} to at least 10%. Formed by the disintegration of the acid igneous rocks and {{gli|gneiss|gneisses}}.}}

{{term|argillaceous}}

{{defn|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. {{gli|siltstone|Siltstones}}, {{gli|mudstone|mudstones}}, {{gli|shale|shales}}, {{gli|clay|clays}}, etc. may all be referred to as argillaceous.}}

{{term|ash}}

{{defn|Fragments less than {{convert|2|mm|in|abbr=on}} in diameter of pulverized rock, minerals and volcanic glass, created during volcanic eruptions.}}

{{term|asphalt}}

{{defn|A sticky, black and highly viscous liquid or semi-solid that is present in most crude petroleums and in some natural deposits.}}

{{term|assembled gem}}

{{ghat|Also called a composite gem.}}

{{defn|}}

{{term|asthenosphere}}

{{defn|A region of the Earth's outer shell beneath the {{gli|lithosphere}}. The asthenosphere is of indeterminate thickness and behaves plastically.}}

{{term|augite}}

{{defn|A complex aluminous silicate of calcium, iron and magnesium, crystallising in the {{gli|monoclinic}} system, and occurring in many igneous rocks, particularly those of basic composition. It is an essential component of {{gli|basalt}}, {{gli|dolerite}} and {{gli|gabbro}}.}}

{{term|aureole}}

{{defn|A zone surrounding an igneous intrusion in which {{gli|country rock}} shows effects of {{gli|contact metamorphism}}.}}

{{term|autochthon}}

{{defn|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 {{gli|allochthon}}.}}

{{glossaryend}}

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{{glossary}}

{{term|bajada}}

{{defn|A series of coalescing alluvial fans along a mountain front.}}

{{term|banded iron formation}}

{{defn|A distinctive type of rock often found in primordial {{gli|sedimentary rocks}}.}}

{{term|basalt}}

{{defn|A fine-grained, mafic igneous rock composed predominantly of ferromagnesian minerals and with lesser amounts of calcium-rich {{gli|plagioclase}} feldspar.}}

{{term|basement rock}}

{{defn|The thick foundation of ancient, and oldest metamorphic and igneous rock that forms the crust of continents, often in the form of granite.}}

{{term|basic rock}}

{{defn|Igneous rock with low silica content (<54%). The groups ultrabasic, basic, intermediate, and acid constitute a series with progressively increasing SiO₂ content.}}

{{term|basin}}

{{defn|A landform scooped out by water erosion.}}

{{term|Basin and Range Province}}

{{defn|A particular {{gli|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.}}

{{term|batholith}}

{{defn|A large discordant {{gli|pluton}} with an outcropping area greater than {{convert|100|sqkm|sqmi}}.}}

{{term|bedrock}}

{{defn|Native consolidated rock underlying the loose rock or soil surface of the Earth.}}

{{term|Before Present (BP)}}

{{defn|}}

{{term|bentonite}}

{{defn|Swelling clay minerals of the {{gli|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 {{gli|smectite}} and {{gli|montmorillonite}}.}}

{{term|bioerosion}}

{{defn|The erosion of hard ocean substrates by living organisms through various biological mechanisms.}}

{{term|biogenic}}

{{defn|Of biological origin – Produced by the present or past activity of living organisms, or by a biological process.}}

{{term|bioherm}}

{{defn|Landform of organic sedimentary rock enclosed or surrounded by rock of different origin.}}

{{term|biostratigraphy}}

{{defn|A branch of stratigraphy which focuses on correlating and assigning relative ages of rock strata by using the fossil assemblages contained within them.}}

{{term|biostratinomy}}

{{defn|The study of the processes that take place after an organism dies but before its final burial.}}

{{term|biostrome}}

{{defn|A distinctly bedded or broadly lenticular sedimentary rock landform.}}

{{term|biotic}}

{{defn|Of biological origin – Produced by the present or past activity of living organisms, or by a biological process.}}

{{term|biotite}}

{{defn|A form of black {{gli|mica}} widely distributed in igneous rocks (particularly in {{gli|granite|granites}}) as lustrous black crystals, with a singularly perfect {{gli|cleavage}}. In composition it is a complex {{gli|silicate}}, chiefly of iron and magnesium, together with potassium and hydroxyl.}}

{{term|bioturbation}}

{{defn|The displacement and mixing of sediment particles by benthic fauna (animals) or flora (plants).}}

{{term|blueschist}}

{{defn|A rock that forms by the metamorphism of {{gli|basalt}} or rocks of similar composition at high pressures and low temperatures, approximately corresponding to a depth of {{gli|15|to|30|km|mi}} and a temperature of {{gli|200|to|500|C|F}}.}}

{{term|boudin}}

{{defn|A structure formed by {{gli|extension}}, in which a rigid tabular body such as a bed of {{gli|sandstone}} is stretched and deformed amidst less competent beds. See also boudinage.}}

{{term|boulder}}{{anchor|boulders}}

{{defn|}}

{{term|Bowen's reaction series}}

{{defn|The sequence in which minerals crystallize from a cooling {{gli|basaltic}} magma.}}

{{term|brackish}}

{{defn|Water with a salinity higher than freshwater but lower than seawater.}}

{{term|breadcrust bomb}}

{{defn|A rounded, smooth-surfaced {{gli|volcanic bomb}} with a cracked surface resembling a cracked crust of bread, hence the name.}}

{{term|breccia}}

{{defn|A coarse-grained clastic rock consisting largely of angular fragments of existing rocks.}}

{{term|buckling}}

{{defn|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.}}

{{glossaryend}}

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{{glossary}}

{{term|calcareous}}

{{defn|Formed from or containing a high proportion of calcium carbonate in the form of {{gli|calcite}} or aragonite, used of a sediment, {{gli|sedimentary rock}}, or soil type.}}

{{term|calcite}}

{{defn|A {{gli|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.}}

{{term|calcite sea}}

{{defn|A body of water in which low-magnesium {{gli|calcite}} is the primary inorganic marine calcium carbonate precipitate.}}

{{term|calcrete}}

{{defn|no=1|A {{gli|conglomerate}} of surficial sand and gravel cemented by calcium carbonate precipitated from solution.}}

{{defn|no=2|A {{gli|calcareous}} {{gli|duricrust}}.}}

{{term|caldera}}

{{defn|A {{gli|volcanic}} feature formed by the collapse of land following a volcanic eruption.}}

{{term|Cambrian}}

{{defn|The earliest geologic {{gli|period}} of the {{gli|Paleozoic|Paleozoic Era}}, lasting from 541.0 ± 1.0 to 485.4 ± 1.9 million years ago and succeeded by the {{gli|Ordovician}}.}}

{{term|carbon film}}

{{defn|A type of {{gli|fossil}} or preservation.}}

{{term|carbonate}}

{{defn|A salt or ester of carbonic acid.}}

{{term|carbonate hardgrounds}}

{{defn|Surfaces of synsedimentarily-cemented carbonate layers that have been exposed on the seafloor.}}

{{term|casting}}

{{defn|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 {{gli|fossil|fossils}}.}}

{{term|Cenozoic Era}}

{{defn|The most recent of the geological eras, which followed the {{gli|Mesozoic Era}}.}}

{{term|chalk}}{{anchor|chalks}}

{{defn|A soft, white, porous {{gli|sedimentary rock}}, a form of {{gli|limestone}} composed of {{gli|calcite}} {{gli|coccolith}} plates.}}

{{term|chert}}

{{defn|A fine-grained, silica-rich, {{gli|microcrystalline}}, {{gli|cryptocrystalline}} or microfibrous {{gli|sedimentary rock}} that sometimes contains small {{gli|fossil|fossils}}.}}

{{term|chlorite}}

{{defn|Any of a set of allied non-swelling 2:1 (TOT) clay {{gli|mineral|minerals}} which may be regarded as hydrated {{gli|silicate|silicates}} of aluminium, iron, and magnesium. Their non-accessible interlayer is filled by a {{gli|brucite}} bridge ({{chem2|Mg(OH)2}}, or sometime {{chem2|Fe(OH)2}}). They crystallise in the {{gli|monoclinic}} system and are green in colour. They occur as alteration products of such minerals as {{gli|biotite}} and {{gli|hornblende}}, and also in schistose rocks.}}

{{term|clast}}

{{defn|Any individual constituent grain or fragment of a sediment or rock produced by mechanical {{gli|weathering}} of a rock mass.}}

{{term|clastic rocks}}

{{defn|Mechanically redeposited remains of eroded older rocks; rocks formed of fragments, or {{gli|clast|clasts}}, of pre-existing rocks.}}

{{term|clay}}{{anchor|clays}}

{{defn|}}

{{term|cleavage}}

{{defn|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.}}

{{term|cobble}}

{{defn|}}

{{term|coccolith}}

{{defn|An individual plate of calcium carbonate formed by {{gli|coccolithophore|coccolithophores}} which are arranged around them in a so-called coccosphere.}}

{{term|coccolithophore}}

{{ghat|Also called a coccolithophorid.}}

{{defn|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 {{gli|coccolith|coccoliths}}.}}

{{term|compaction}}

{{defn|The process by which a newly deposited {{gli|sediment}} decreases its {{gli|porosity}} and progressively expels its original pore water due to the effects of loading. This forms part of the process of {{gli|lithification}}.}}

{{term|compression}}

{{defn|The process by which rocks shorten or decrease in volume when exposed to certain forces.}}

{{term|conchoidal}}

{{defn|A type of {{gli|fracture}} that results in smoothly curved surface faces.}}

{{term|concretion}}

{{defn|A volume of {{gli|sedimentary rock}} in which a mineral cement fills the porosity (i.e. the spaces between the sediment grains). Not to be confused with {{gli|nodule}}.}}

{{term|conglomerate}}

{{defn|Any type of rock consisting of individual stones that have become cemented together.}}

{{term|contact metamorphism}}

{{defn|Metamorphism due to the local heating of rocks by the intrusion of magma nearby.}}

{{term|continental crust}}

{{defn|The layer of granitic, sedimentary, and metamorphic rocks which form the parts of the Earth's {{gli|crust}} that comprise the continents, and the areas of shallow seabed close to their shores (known as {{gli|continental shelf|continental shelves}}).}}

{{term|continental margin}}

{{defn|Zone of the ocean floor, separating the thin {{gli|oceanic crust}} from thicker {{gli|continental crust}}.}}

{{term|continental shelf}}

{{defn|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.}}

{{term|convergent boundary}}

{{defn|The boundary between two {{gli|tectonic plate|tectonic plates}} that are moving toward each other. Contrast {{gli|divergent boundary}}.}}

{{term|copal}}

{{defn|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.}}

{{term|coprolite}}

{{defn|A fossilized specimen of human or animal dung.}}

{{term|cordierite}}

{{defn|A {{gli|silicate}} of aluminium, iron and magnesium with water, which crystallises in the orthorhombic system and occurs mainly in {{gli|metamorphic rock|metamorphic rocks}}.}}

{{term|core}}

{{defn|The innermost layer(s) of a planet, referring especially to the Earth's core.}}

{{term|corestone}}

{{defn|An ellipsoidal or broadly rectangular joint block of {{gli|granite}} formed by subsurface weathering in the same manner as a {{gli|tor}} but entirely separated from {{gli|bedrock}}.}}

{{term|country rock}}

{{defn|The rock native to an area, as opposed to rock that formed elsewhere and was later transported to the area.}}

{{term|craton}}

{{defn|An old and stable part of the {{gli|continental crust}} that has survived the merging and splitting of continents and supercontinents for at least 500 million years.}}

{{term|cross-bedding}}

{{defn|An inclined sedimentary structure in a horizontal unit of rock. Such tilted structures indicate the type of depositional environment, not post-depositional deformation.}}

{{term|crude oil}}

{{defn|A liquid mixture of naturally occurring hydrocarbons.}}

{{term|crust}}

{{defn|The outermost solid layer of a planet or moon, referring especially to the Earth's crust.}}

{{term|cryptocrystalline}}

{{defn|}}

{{term|crystal}}{{anchor|crystals}}

{{defn|}}

{{term|crystal habit}}

{{defn|}}

{{term|crystallinity}}{{anchor|crystalline}}

{{defn|}}

{{glossaryend}}

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{{glossary}}

{{term|dacite}}

{{defn|An igneous, {{gli|volcanic}} rock with a high iron content. It is an extrusive rock of the same general composition as {{gli|andesite}}, but a less calcic feldspar. Synonymous with quartz andesite.}}

{{term|daughter product}}

{{defn|Any distinct isotope produced by the radioactive decay of an atomic nucleus.}}

{{term|delta}}

{{defn|A landform where the mouth of a river flows into an ocean, sea, desert, estuary, lake or another river.}}

{{term|degradation}}

{{defn|The lowering of a fluvial surface, such as a stream bed or floodplain, through erosional processes.}}

{{term|dendrite}}

{{defn|A crystal that develops with a typical multi-branching tree-like form.}}

{{term|Denudation}}

{{defn|The lowering of the earth's surface through chemical and physical weathering.}}

{{term|deposition}}

{{defn|The geological process by which material is added to a landform or landmass.}}

{{term|detachment fault}}

{{defn|A major fault in a mountain belt above which rocks have been intensely folded or faulted.}}

{{term|diagenesis}}

{{defn|The process of chemical, physical, or biological change undergone by a sediment after its initial deposition and during and after its {{gli|lithification}}, exclusive of surface alteration (weathering) and metamorphism.}}

{{term|diamictite}}

{{defn|A comprehensive non-generic term for a non-sorted or poorly sorted non-calcareous terrigenous {{gli|sedimentary rock}} that contains a wide range of particle sizes such as rock with sand or larger particles in a muddy matrix.}}

{{term|diapir}}

{{defn|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.}}

{{term|diatomite}}

{{defn|A soft, {{gli|chalk}}-like {{gli|sedimentary rock}} that is easily crumbled into a fine white to off-white powder.}}

{{term|diopside}}

{{defn|A {{gli|monoclinic}} {{gli|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.}}

{{term|diorite}}

{{defn|A grey to dark grey intermediate intrusive {{gli|igneous rock}} composed principally of {{gli|plagioclase}} {{gli|feldspar}} (typically andesine), {{gli|biotite}}, {{gli|hornblende}}, and/or {{gli|pyroxene}}.}}

{{term|dike}}

{{ghat|Also spelled dyke.}}

{{defn|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.}}

{{term|dip slope}}

{{defn|A geological formation often created by erosion of tilted strata.}}

{{term|disconformity}}

{{defn|A surface that represents missing rock strata but beds above and below that surface are parallel to one another.}}

{{term|divergent boundary}}

{{defn|The boundary separating two {{gli|tectonic plate|tectonic plates}} that are moving away from each other. Contrast {{gli|convergent boundary}}.}}

{{term|dolomite}}

{{defn|A {{gli|sedimentary}} {{gli|carbonate}} rock and mineral, both composed of crystalline calcium magnesium carbonate CaMg(CO₃)₂.}}

{{term|dolerite}}

{{defn|no=1|A basic {{gli|igneous rock}} of medium grain size, occurring as minor intrusions or in the central parts of thick lava flows.}}

{{defn|no=2|A dark-coloured, basic, igneous rock, composed essentially of {{gli|pyroxene}} and a triclinic {{gli|feldspar}} with magnetic iron. Considered by some authors to be equivalent to a coarse-grained {{gli|basalt}}.}}

{{defn|no=3|A dark, crystalline, igneous rock, chiefly pyroxene with labradorite.}}

{{defn|no=4|Coarse-grained basalt.}}

{{defn|no=5|{{gli|diabase|Diabase}}.}}

{{defn|no=6|Any dark igneous rock composed chiefly of {{gli|silicate|silicates}} of iron and magnesium with some feldspar.}}

{{term|dome}}

{{defn|A geological formation consisting of symmetrical anticlines that intersect each other at their respective apices.}}

{{term|drill core}}

{{defn|A drill specifically designed to remove a cylinder of material, much like a hole saw.}}

{{term|drumlin}}

{{defn|An elongated, whale-shaped hill formed by glacial action.}}

{{term|dunite}}

{{defn|An {{gli|ultramafic|ultrabasic}} {{gli|plutonic}} rock in which the mafic material is almost entirely {{gli|olivine}}, with accessory chromite almost always present. Feldspar mainly plagioclase. See also peridotite.}}

{{term|duricrust}}

{{defn|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 {{gli|hardpan}}.}}

{{glossaryend}}

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{{term|earthquake}}

{{defn|}}

{{term|eclogite}}

{{defn|A generally coarse- to medium-grained {{gli|pyroxene}} in which are set red {{gli|garnet|garnets}}. The colour is pistachio green when fresh, but mottled with red when weathered.}}

{{term|Eemian transgression}}

{{defn|The portion of the Late Pleistocene spanning the period between 120 Ka and 8m before present.}}

{{term|eon}}{{defn|The largest unit of geologic time.}}

{{term|epicenter}}

{{defn|The point on the Earth's surface that is directly above the {{gli|hypocenter}} or focus at which an {{gli|earthquake}} or underground explosion originates.}}

{{term|epidiorite}}

{{defn|Any of a set of altered gabbroic and doleritic rocks in which the original pyroxene has been replaced by fibrous {{gli|amphibole}}. The rock may be regarded as a first step in the conversion by dynamothermal metamorphism of a basic igneous rock into a green {{gli|schist}}.}}

{{term|epirogenetic}}

{{defn|The simultaneous rising and falling movements of continents, maintaining isostasy.}}

{{term|epoch}}

{{defn|A division of the standard geologic time scale subordinate to {{gli|period|periods}}. An example is the {{gli|Pleistocene|Pleistocene Epoch}} of the Quaternary Period.}}

{{term|erosion}}

{{defn|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).}}

{{term|erratic}}

{{defn|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.}}

{{term|escarpment}}

{{defn|A transition zone between different physiogeographic provinces that involves an elevation differential, characterized by a cliff or steep slope.}}

{{term|esker}}

{{defn|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.}}

{{term|estuary}}

{{defn|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.}}

{{term|eugeosyncline}}

{{defn|A geosyncline in which volcanism is associated with {{gli|clastic rocks|clastic}} sedimentation; the volcanic part of an orthosyncline located away from the {{gli|craton}}.}}

{{term|euhedral}}

{{defn|Bounded by the crystal faces peculiar to the species, used of minerals. Synonymous with {{gli|idiomorphic}}.}}

{{term|eustatic movements}}

{{defn|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.}}

{{term|evaporite}}

{{defn|Any of a diverse set of water-soluble mineral sediments that result from the evaporation of bodies of surface water.}}

{{term|exfoliation}}

{{defn|The stripping of concentric rock slabs from the outer surface of a rock mass.}}

{{term|extension}}

{{defn|Strain involving an increase in length. Extension can cause thinning and faulting.}}

{{term|extrusive}}

{{defn|A mode of {{gli|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.}}

{{glossaryend}}

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{{glossary}}

{{term|facies}}

{{defn|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.}}

{{term|fanning}}

{{defn|Rock deformation related to shear stress.}}

{{term|fault}}

{{defn|A discrete planar rock {{gli|fracture}} which shows evidence of a displacement (the throw of the fault). A fault is a discrete surface.}}

{{term|fault zone}}

{{defn|The zone where exist different discrete {{gli|fault}} planes.}}

{{term|feldspar}}

{{defn|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.}}

{{term|fels}}

{{defn|A massive metamorphic rock lacking {{gli|schistosity}} or {{gli|foliation}}.}}

{{term|felsic}}

{{defn|{{gli|silicate|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 {{gli|mafic}}.}}

{{term|ferricrete}}

{{defn|A {{gli|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 {{gli|duricrust}}.}}

{{term|ferromagnesian mineral}}

{{defn|Any iron/magnesium-bearing mineral, such as {{gli|augite}}, {{gli|hornblende}}, {{gli|olivine}}, or {{gli|biotite}}.}}

{{term|fission track dating}}

{{defn|A method that uses tracks that are visible under the microscope to date minerals.}}

{{term|Flandrian transgression}}

{{defn|}}

{{term|fold}}

{{defn|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.}}

{{term|foliation}}

{{defn|The parallel alignment of textural and structural features of a rock.}}

{{term|fossil}}

{{defn|Any mineralized or otherwise preserved remains or traces (such as footprints) of animals, plants, or other once-living organisms.}}

{{term|fossiliferous}}

{{defn|Bearing or being composed of {{gli|fossil|fossils}} in rocks or strata.}}

{{term|fossilization}}

{{defn|}}

{{term|fracture}}

{{defn|Any crack or discontinuity. In its geological definition, it is only used when no displacement can be distinguished.}}

{{term|fumarole}}

{{defn|A vent in the Earth's surface from which hot gases and vapors are emitted.}}

{{glossaryend}}

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{{glossary}}

{{term|Ga}}

{{defn|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 {{gli|Before Present}} (or 1,000,000,000 years ago).}}

{{term|gabbro}}

{{defn|A dark, coarse-grained, intrusive igneous rock chemically equivalent to {{gli|basalt}}.}}

{{term|garnet}}

{{defn|}}

{{term|gastrolith}}

{{defn|A rock which is or was once held inside the digestive tract of a living animal.}}

{{term|gemology}}

{{ghat|Also spelled gemmology.}}

{{defn|The branch of geology and {{gli|mineralogy}} that studies natural and artificial {{gli|gemstones}}.}}

{{term|gemstone}}

{{ghat|Also called a gem, fine gem, jewel, precious stone, or semi-precious stone.}}

{{defn|}}

{{term|geogenic}}

{{defn|Of geologic origin – Resulting from naturally occurring geological processes.}}

{{term|geological map}}

{{defn|A special-purpose map made to show geological features.}}

{{term|geological time scale}}

{{ghat|Also geologic time scale.}}

{{defn|}}

{{term|geology}}

{{defn|}}

{{term|geosyncline}}

{{defn|A mobile down-warping of the Earth's {{gli|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.}}

{{term|glass}}

{{defn|Amorphous (non-crystalline) hard and brittle solid such as soda-lime glass.}}

{{term|glauconite}}

{{defn|A green-coloured, hydrated {{gli|silicate}} mineral of potassium and iron that forms on submerged banks. Its occurrence in {{gli|sands}} and {{gli|sandstones}} is considered an indication of accumulation under marine conditions.}}

{{term|gneiss}}

{{defn|A coarse-grained, pale-coloured gneissose rock, containing abundant {{gli|feldspar}} with {{gli|quartz}}, {{gli|mica}}, {{gli|hornblende}}, and {{gli|garnet}}.}}

{{term|Gondwanaland}}

{{defn|The southern part of the supercontinent of Pangaea which eventually separated to form present-day South America, Africa, India, Australia and Antarctica.}}

{{term|graben}}

{{defn|A depressed block of the Earth's crust bordered by parallel {{gli|fault|faults}}.}}

{{term|granite}}

{{defn|A coarse-grained, often porphyritic, intrusive, {{gli|felsic}}, {{gli|igneous rock}} containing megascopic {{gli|quartz}}, averaging 25%, much {{gli|feldspar}} (orthoclase, microcline, sodic plagioclase) and {{gli|mica}} or other coloured minerals. {{gli|rhyolite|Rhyolite}} is the volcanic equivalent.}}

{{term|granitoid}}

{{defn|Any {{gli|granite}}-like rock, including {{gli|granodiorite}}, {{gli|diorite}}, monzonite, and granite itself, among others.}}

{{term|granoblastic}}

{{defn|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.}}

{{term|granodiorite}}

{{defn|An intrusive, felsic, igneous rock similar to {{gli|granite}} but containing more {{gli|plagioclase}} than potassium {{gli|feldspar}}. {{gli|dacite|Dacite}} is the volcanic equivalent.}}

{{term|granulometry}}

{{defn|Statistical distribution of the size of individual sediment grains, or lithified particles in clastic rocks.}}

{{term|gravel}}

{{defn|}}

{{term|graywacke}}

{{ghat|Also spelled greywacke.}}

{{defn|A variety of {{gli|sandstone}} generally characterized by its hardness, dark colour, and poorly sorted, angular grains of {{gli|quartz}}, {{gli|feldspar}}, and small rock fragments (lithic fragments) set in a compact, clay-fine matrix.}}

{{term|greenstone}}

{{defn|An omnibus term lacking precision and applied indiscriminately to basic and intermediate igneous rocks of Lower {{gli|Paleozoic Era|Paleozoic}} age in which much {{gli|chlorite}} has been produced at the expense of the original coloured minerals, staining the rocks green.}}

{{term|grus}}

{{defn|Freshly eroded, angular grains of {{gli|quartz}} and {{gli|feldspar}} derived from a {{gli|granitoid}}.}}

{{glossaryend}}

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{{term|habit}}

{{defn|See {{gli|crystal habit}}.}}

{{term|half-life}}

{{defn|The time it takes for a given amount of a radioactive isotope to be reduced by one-half.}}

{{term|hardpan}}

{{defn|}}

{{term|hemicrystalline}}

{{defn|Having the properties of certain rocks of igneous origin which contain some interstitial {{gli|glass}} in addition to crystalline minerals. Contrast {{gli|holocrystalline}}.}}

{{term|hinge}}

{{defn|The zone of maximum curvature of a {{gli|fold}}.}}

{{term|hinge line}}

{{defn|A line joining the points of maximum curvature along the {{gli|hinge}} of a {{gli|fold}}.}}

{{term|holocrystalline}}

{{defn|Having the properties of those igneous rocks in which all of the components are crystalline; {{gli|glass}} is absent. Contrast {{gli|hemicrystalline}}.}}

{{term|hornblende}}

{{defn|An important rock-forming mineral of complex composition, essentially a {{gli|silicate}} of calcium, magnesium and iron, with smaller amounts of {{gli|potash}}, soda and hydroxyl. Hornblende crystallises in the {{gli|monoclinic}} system and occurs as black crystals or grains in many different types of igneous and metamorphic rocks, including hornblende-granite, syenite, {{gli|diorite}}, {{gli|andesite}}, hornblende-schist, and {{gli|amphibole}}.}}

{{term|hornfels}}

{{defn|A hard, compact, fine-textured contact-altered {{gli|argillaceous}} rock that breaks into splintery fragments.}}

{{term|horst}}

{{defn|A raised fault block bounded by normal faults.}}

{{term|hot spring}}

{{defn|A natural spring resulting from the emergence of geothermally heated groundwater from beneath the Earth's crust.}}

{{term|hydrothermal}}

{{defn|Pertaining to the actions or products of heated water.}}

{{term|hydrothermal vent}}

{{defn|A fissure in a planet's surface from which geothermally heated water emerges.}}

{{term|hypersaline}}

{{defn|Having a saltiness or dissolved salt content greater than that of seawater.}}

{{glossaryend}}

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{{glossary}}

{{term|ichnology}}

{{defn|The study of {{gli|trace fossil|traces}} of organismal behavior.}}

{{term|idiomorphic}}

{{defn|Bounded by the crystal faces peculiar to the species, used of minerals. Synonymous with {{gli|euhedral}}.}}

{{term|igneous rock}}

{{defn|A type of {{gli|rock}} formed by solidification of cooled magma (molten rock), with or without crystallization, either below the surface as intrusive ({{gli|plutonic}}) rocks or on the surface as extrusive ({{gli|volcanic}}) rocks.}}

{{term|ignimbrite}}

{{defn|Fine-grained to aphanitic, buff to dark brown compact rock with parallel streaks or lenticles of black glass, produced by violently explosive volcanoes.}}

{{term|ilmenite}}

{{defn|An oxide of iron and titanium, crystallising in the trigonal system; a widespread accessory mineral in igneous rocks, especially those of basic composition.}}

{{term|illite}}

{{defn|Non-swelling clay mineral with a three layers 2:1 (TOT) structure. Its interlayers contain no water and anhydrous {{chem2|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.}}

{{term|indurated}}

{{defn|Made hard (by heat or compaction).}}

{{term|interbedded}}{{defn|beds (layers) of rock lying between or alternating with beds of a different kind of rock.}}

{{term|intrusion}}{{defn|body of igneous rock that has crystallized from molten magma below the surface of the Earth.}}

{{term|island arc}}

{{defn|A chain of {{gli|volcanic}} islands or mountains formed by plate tectonics as an oceanic tectonic plate subducts under another tectonic plate and produces magma.}}

{{term|isomorphic}}

{{defn|Two crystals that have similar shapes and sizes, usually through the angles.}}

{{term|isotope}}

{{defn|different forms of an element each having different atomic mass (mass number).}}

{{glossaryend}}

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{{glossary}}

{{term|joint}}

{{defn|A discrete discontinuity surface without evidence of displacement. See also {{gli|diaclase}} or {{gli|bed|bedding}}.}}

{{term|Jurassic}}

{{defn|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.}}

{{glossaryend}}

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{{glossary}}

{{term|Ka}}

{{defn|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 {{gli|Before Present}} (or 1,000 years ago).}}

{{term|kame}}

{{defn|An irregularly shaped hill or mound composed of sand, gravel and {{gli|till}} that accumulates in a depression on a retreating glacier and is then deposited on the land surface with further melting of the glacier.}}

{{term|kaolinite}}

{{defn|A finely crystalline form of hydrated aluminium silicate occurring as minute monoclinic flaky crystals with a perfect basal {{gli|cleavage}}, resulting mainly from the alteration of {{gli|feldspar|feldspars}} under conditions of hydrothermal or pneumatolytic metamorphism.}}

{{term|karst}}

{{defn|A distinct type of landscape shaped by the dissolution of a layer or layers of soluble {{gli|bedrock}}, usually {{gli|carbonate}} rock such as {{gli|limestone}} or {{gli|dolomite}}. Karst topography is usually characterised by closed depressions or sinkholes, caves and underground drainage.}}

{{term|kettle}}

{{defn|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.}}

{{term|kink}}

{{defn|A tight curl, twist, or bend in a rock band. See also {{gli|fold|folding}} and {{gli|buckling}}.}}

{{term|kink band}}

{{defn|An asymmetric, linear zone of deformation characterised by a tight curled, twisted, or bended rock band. Kink bands may also occur as conjugated sets.}}

{{term|kyanite}}

{{defn|A {{gli|silicate}} of aluminium which crystallises in the triclinic system. It usually occurs as long-bladed crystals, blue in colour, in {{gli|metamorphic rock|metamorphic rocks}}.}}

{{glossaryend}}

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{{glossary}}

{{term|lacuna}}

{{defn|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).}}

{{term|lamprophyre}}

{{defn|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.}}

{{term|large igneous province}}

{{defn|Massive {{gli|volcano|volcanic}} formation resulting from flood {{gli|basalt}} eruptions.}}

{{term|lava}}

{{defn|Molten rock expelled by a {{gli|volcano}} during an eruption.}}

{{term|leucocratic}}

{{defn|A term used to denote a light colour in {{gli|igneous rock|igneous rocks}}, due to a high content of {{gli|felsic}} minerals and a correspondingly small amount of dark, heavy silicates.}}

{{term|limestone}}{{anchor|limestones}}

{{defn|A {{gli|sedimentary rock}} composed largely of the mineral {{gli|calcite}} (calcium carbonate: CaCO₃).}}

{{term|liquefaction}}

{{defn|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.}}

{{term|lithic fragment}}

{{ghat|Also simply called a lithic.}}

{{defn|A sand-sized grain that is made up of smaller than sand-sized grains, e.g. a {{gli|shale}} fragment or {{gli|basalt}} fragment in a {{gli|sandstone}}.}}

{{term|lithification}}

{{defn|The process by which sediments compact under pressure, expel connate fluids, and gradually become solid {{gli|rock}}.}}

{{term|lithology}}

{{defn|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.}}

{{term|lithosphere}}

{{defn|The rigid, outermost rocky shell of a terrestrial planet or natural satellite. The Earth's lithosphere is composed of the {{gli|crust}} and the portion of the upper {{gli|mantle}} that behaves elastically on time scales of up to thousands of years or more.}}

{{term|lithotype}}

{{defn|Specific types of rock classified according to the standards of {{gli|lithology}}.}}

{{term|loess}}

{{defn|A fine, silty, pale yellow or buff-coloured, windblown (eolian) type of unconsolidated deposit.}}

{{term|lustre}}

{{ghat|Also spelled luster.}}

{{defn|}}

{{glossaryend}}

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{{term|Ma}}

{{defn|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 {{gli|Before Present}} (or 1,000,000 years ago).}}

{{term|mafic}}

{{defn|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), {{gli|gabbro}} ({{gli|basalt}} and {{gli|dolerite}})].}}

{{term|magma}}

{{defn|Molten rock that sometimes forms beneath the surface of the Earth (or other terrestrial planets) and often collects in a magma chamber.}}

{{term|magnetite}}

{{defn|An oxide of iron which crystallises in the cubic system. It is attracted by a magnet but does not attract iron itself.}}

{{term|Malmesbury group}}

{{defn|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.}}

{{term|mantle}}

{{defn|The highly viscous layer of molten rock situated directly beneath the Earth's {{gli|crust}} and above the outer {{gli|core}}.}}

{{term|marble}}

{{defn|A fine to coarse-grained granoblastic calcium carbonate that effervesces in dilute hydrochloric acid. Often banded with various colours and sometimes veined.}}

{{term|marine terrace}}

{{defn|A narrow, flat area often seen at the base of a sea cliff caused by the action of the waves.}}

{{term|marl}}{{anchor|marls}}

{{defn|A calcium carbonate or lime-rich mud or {{gli|mudstone}} which contains variable amounts of {{gli|clays}} and {{gli|aragonite}}.}}

{{term|massive}}

{{defn|A description applied to a homogeneous rock which lacks internal structure or layers.}}

{{term|mélange}}

{{defn|Large-scale {{gli|breccia}} formed in the {{gli|accretionary wedge}} above a subduction zone.}}

{{term|melanocratic}}

{{defn|A term applied to rocks which are abnormally rich in dark and heavy ferro-magnesium minerals.}}

{{term|mesocratic}}

{{defn|A term applied to {{gli|igneous rock|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.}}

{{term|Mesozoic}}

{{defn|The {{gli|era}} of geological time including the {{gli|Triassic}}, {{gli|Jurassic}}, and {{gli|Cretaceous}} ages.}}

{{term|metamorphic rock}}

{{defn|}}

{{term|metamorphism}}

{{defn|The solid-state recrystallisation of pre-existing rocks due to changes in heat, pressure, or water-rock interactions, i.e. without melting.}}

{{term|metapelite}}

{{defn|A metamorphosed pelite (fine grain, clayey) rock.}}

{{term|metasilicate}}

{{defn|A salt of the non-existing metasilicic acid H₂SiO₃ based on an analogy with the carbonate system.}}

{{term|metatexis}}

{{defn|Low-grade {{gli|anatexis}}: partial or differential melting of rock components with a low melting point.}}

{{term|mica}}{{anchor|micas}}

{{defn|Any of a set of minerals which crystallise in the {{gli|monoclinic}} system. They have similar chemical compositions and highly perfect basal {{gli|cleavage}}.}}

{{term|micropaleontology}}

{{defn|A branch of {{gli|paleontology}} which studies microfossils.}}

{{term|mid-oceanic ridge}}

{{defn|An underwater mountain range typically having a valley known as a rift running along its axis, formed by {{gli|plate tectonics}}.}}

{{term|migmatite}}

{{defn|A composite rock composed of igneous or igneous-looking and/or metamorphic materials which are generally distinguishable megascopically.}}

{{term|mineral}}{{anchor|minerals}}

{{defn|}}

{{term|mineralization}}

{{defn|The hydrothermal deposition of economically important metals in the formation of ore bodies or "lodes".}}

{{term|mineralogy}}{{anchor|mineralogical}}

{{defn|}}

{{term|miogeosyncline}}

{{defn|A {{gli|geosyncline}} in which {{gli|volcanism}} is not associated with sedimentation, or the non-volcanic part of the {{gli|orthogeosyncline}} located near the {{gli|craton}}.}}

{{term|Mohs scale of mineral hardness}}

{{ghat|Also simply called the Mohs scale.}}

{{defn|}}

{{term|molasse}}

{{defn|A partly marine, partly continental or deltaic sedimentary {{gli|facies}} consisting of a very thick sequence of soft ungraded {{gli|cross-bedding|cross-bedded}} {{gli|fossiliferous}} {{gli|conglomerates}}, {{gli|sandstones}}, {{gli|shales}}, and {{gli|marls}}.}}

{{term|molding}}

{{defn|The process of manufacturing by shaping pliable raw material using a rigid frame or model called a mold.}}

{{term|montmorillonite}}

{{defn|Swelling clay mineral with a three layer 2:1 (TOT) structure whose interlayers are mainly occupied by hydrated {{chem2|Na+}} and {{chem2|Ca(2+)}} cations and water molecules. The name comes from Montmorillon, France. Montmorillonite and {{gli|smectite}} are two interchangeable synonyms, the first being preferentially used in the US while the second one is more frequent in the literature from UK and Europe. See also {{gli|bentonite}}.}}

{{term|monocline}}

{{defn|A fold with a single limb which produces a sudden steepening of the dip; the rocks, however, soon approximate to horizontal on either side of this flexure.}}

{{term|monoclinic}}

{{defn|Having three crystal axes of unequal lengths, with one intersection oblique and the other two perpendicular.}}

{{term|moraine}}

{{defn|A glacially formed accumulation of unconsolidated debris which can occur in currently glaciated and formerly glaciated regions, such as those areas acted upon by a past ice age.}}

{{term|mullion}}

{{defn|A particular type of reworked {{gli|boudin}}. The term is likely derived from an architectural structure with the same name.}}

{{term|muscovite}}

{{defn|The common or white {{gli|mica}}; for the most part an orthosilicate of aluminium and potassium that crystallises in the {{gli|monoclinic}} system.}}

{{glossaryend}}

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{{glossary}}

{{term|Namibian age}}

{{defn|900 to 542 Ma (Neoproterozoic).}}

{{term|Neogene}}

{{defn|A geologic {{gli|period}} starting 23 million years ago and, depending on definition, either lasting until today or ending 2.6 million years ago with the beginning of the {{gli|Quaternary}}.}}

{{term|Nodule}}

{{defn|Small mass of a mineral with a contrasting composition to the enclosing sediment or rock. Not to be confused with {{gli|concretion}}.}}

{{term|non-clastic}}

{{defn|Having the properties of any chemically and/or organically deposited rock, such as {{gli|limestones}}, {{gli|chalks}}, and {{gli|evaporite}} deposits.}}

{{term|normal fault}}

{{ghat|Also called an extensional fault.}}

{{defn|Dip-slip faults can be sub-classified into the types "reverse" and "normal". A normal fault occurs when the {{gli|crust}} is extended such that the hanging wall moves downward relative to the footwall. Contrast {{gli|reverse fault}}.}}

{{glossaryend}}

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{{glossary}}

{{term|obsidian}}

{{defn|}}

{{term|oligoclase}}

{{defn|One of the {{gli|plagioclase}} feldspars consisting of the albite and anorthite molecules combined in the proportions 9:1 to 7:3. It is found especially in the more acid {{gli|igneous rock|igneous rocks}}.}}

{{term|olivine}}

{{defn|An orthosilicate mineral of iron and magnesium which crystallises in the orthorhombic system and occurs widely in the basic and {{gli|ultramafic}} igneous rocks. It includes olivine-gabbro, olivine–dolerite, olivine-basalt, and peridotites, among others.}}

{{term|Ordovician}}

{{defn|A geologic {{gli|period}} and system, the second of six of the {{gli|Paleozoic Era}}, spanning the time between 485.4 ± 1.9 to 443.4 ± 1.5 million years ago. It follows the {{gli|Cambrian}} and is followed by the {{gli|Silurian}}.}}

{{term|orogenesis}}

{{defn|The formation and growth of mountains related to {{gli|plate tectonics|tectonic}} activity.}}

{{term|orogeny}}

{{defn|Any set of forces and events leading to a large structural deformation of the Earth's {{gli|lithosphere}} due to the engagement of {{gli|tectonic plate|tectonic plates}}. It is the primary mechanism by which mountains are built on continents.}}

{{term|orthoclase}}

{{defn|A silicate of potassium and aluminium which crystallises in the {{gli|monoclinic}} system and occurs as an essential constituent in granitic and syenitic rocks and as an accessory in many other rock types.}}

{{term|orthogeosyncline}}

{{defn|A {{gli|geosyncline}} between continental and oceanic {{gli|craton|cratons}} containing both volcanic and non-volcanic belts.}}

{{term|orthorhombic}}

{{defn|Having a crystal structure with three perpendicular axes all of different lengths.}}

{{term|orthosilicate}}

{{defn|An obsolete classification based on the totally deprotonated tetrahedral {{chem2|SiO4(4-)}} anion of the monomeric orthosilicic acid, {{chem2|H4SiO4, or Si(OH)4}}. Compare with the hypothetical and non-existing planar trigonal {{gli|metasilicate}} {{chem2|SiO3(2-)}} anion imagined from the analogy to the {{gli|carbonate}}, {{chem2|CO3(2-)}}, anion.}}

{{term|oxbow lake}}

{{defn|A crescent-shaped lake found within a floodplain or fluvial terrace created by the cut-off and abandonment of an active meander within a river or stream channel.}}

{{glossaryend}}

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{{glossary}}

{{term|pahoehoe}}

{{defn|A type of vesicular, {{gli|basaltic}} {{gli|lava}} often with a ropy surface texture.}}

{{term|Palaeozoic}}

{{ghat|Also spelled Paleozoic.}}

{{defn|The earliest of the three geologic {{gli|era|eras}} of the {{gli|Phanerozoic Eon}}, spanning the time from roughly 541 to 252.2 million years ago. It is the longest of the Phanerozoic eras and is subdivided into six geologic {{gli|periods}}: the {{gli|Cambrian}}, {{gli|Ordovician}}, {{gli|Silurian}}, {{gli|Devonian}}, {{gli|Carboniferous}}, and {{gli|Permian}}. The Paleozoic Era follows the {{gli|Neoproterozoic|Neoproterozoic Era}} of the {{gli|Proterozoic Eon}}, and is followed by the {{gli|Mesozoic Era}}.}}

{{term|paleocurrent}}

{{defn|An indication of the direction of fluid flow (at the time of deposition) visible in a rock.}}

{{term|paleontology}}{{anchor|paleontological}}

{{defn|}}

{{term|palagonite}}

{{defn|An alteration product from the interaction of water with {{gli|volcanic glass}} of chemical composition similar to {{gli|basalt}} or from the interaction between water and {{gli|basalt}} melt.}}

{{term|paralithic}}

{{defn|A weathered layer of bedrock.}}

{{term|pegmatite}}

{{defn|Exceptionally coarse-grained igneous rock.}}

{{term|pelite}}

{{defn|A descriptive name for a {{gli|clastic rocks|clastic rock}} with a grain size of less than 1/16 mm (originally sand or silt).}}

{{term|peridotite}}

{{defn|An olive green when fresh, medium brown when weathered, saccharoidal intrusive igneous rock composed mainly of {{gli|olivine}}, sometimes with {{gli|pyroxene}}.}}

{{term|petrology}}

{{defn|The branch of geology that studies the origin, composition, distribution, and structure of {{gli|rock|rocks}}.}}

{{term|phenoclast}}

{{defn|A large, conspicuous fragment in sediment or {{gli|sedimentary rock}} composed of various sizes of material.}}

{{term|phenocryst}}

{{defn|A relatively large crystal in an {{gli|igneous rock}}.}}

{{term|phyllite}}{{anchor|phyllites}}

{{defn|Any of a set of {{gli|argillaceous}} rocks in a condition of {{gli|metamorphism}} between {{gli|slate}} and mica-schist.}}

{{term|phyllonite}}

{{defn|A rock that macroscopically resembles {{gli|phyllite}} but that is formed by mechanical degradation (mylonization) of initially coarser rocks (e.g., {{gli|graywacke}}, {{gli|granite}}, or {{gli|gneiss)}}.}}

{{term|piercing point}}

{{defn|A feature that is cut by a fault and moved. Reconstruction of that object can show how much the fault has moved.}}

{{term|pillar}}

{{defn|A vertical, standing, often spire-shaped, natural rock formation.}}

{{term|pitchstone}}

{{defn|A black, opaque {{gli|volcanic glass}} that may contain irregular, whitish clusters of minerals. Resembles pitch in appearance.}}

{{term|plate tectonics}}

{{defn|The set of natural processes and phenomena which result in large-scale movements of portions of the Earth's {{gli|lithosphere}}, which is fragmented into multiple {{gli|tectonic plates}} of various sizes.}}

{{term|Pleistocene}}

{{defn|The geologic {{gli|epoch}} which lasted from about 2,588,000 to 11,700 years ago, spanning the world's recent period of repeated glaciations. The Pleistocene is the first epoch of the {{gli|Quaternary}} Period and the sixth epoch of the {{gli|Cenozoic Era}}.}}

{{term|Pliocene}}

{{defn|The geologic {{gli|period}} that extends from 5.332 million to 2.588[2] million years Before Present. It is the second-youngest epoch of the {{gli|Neogene}} Period in the {{gli|Cenozoic Era}}. The Pliocene follows the {{gli|Miocene}} and is followed by the {{gli|Pleistocene}}.}}

{{term|pluton}}

{{defn|}}

{{term|plutonic}}

{{defn|Having crystallised at depth within the Earth's {{gli|crust}}, used of a rock. Plutonic rocks are slow-cooling and coarse-grained and have relatively low temperatures of final consolidation.}}

{{term|plumose structure}}

{{defn|A ladder or grid pattern that occurs during {{gli|joint|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.}}

{{term|pneumatolysis}}

{{defn|The destructive after-action of the concentrated volatile constituents of a magma, effected after the consolidation of the main body of the magma.}}

{{term|polymictic}}

{{defn|}}

{{term|polysynthetic twinning}}

{{defn|}}

{{term|pore water}}

{{defn|Water present in rock and soil pore spaces. Also called interstitial water.}}

{{term|porphyroblast}}

{{defn|A large {{gli|mineral}} {{gli|crystal}} in a {{gli|metamorphic rock}} which has grown within the finer-grained groundmass.}}

{{term|porphyry}}

{{defn|no=1|A rock that is porphyritic, containing large and small crystals.}}

{{defn|no=2|In mining, a specific deposit containing widely disseminated metals, typically copper.}}

{{term|Precambrian}}

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

{{term|precipitate}}

{{defn|A rock or mineral precipitated into solid form from an aqueous solution.}}

{{term|pressure solution}}

{{defn|Rock deformation mechanism involving minerals dissolution under mechanical stress.}}

{{term|prograde metamorphism}}

{{defn|Mineral changes in rocks under increasing pressure and/or temperature conditions.}}

{{term|protolith}}

{{defn|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.}}

{{term|psammite}}

{{defn|A general term for a {{gli|sandstone}}, most often used to describe a metamorphosed rock unit with a dominantly sandstone {{gli|protolith}}.}}

{{term|pseudomatrix}}

{{defn|A weaker material (mainly {{gli|lithic fragment|lithic fragments}}) that becomes crushed and matrix-like in a rock.}}

{{term|pumice}}

{{defn|A light-coloured, highly vesicular {{gli|volcanic}} rock of very low density.}}

{{term|pyroclastic flow}}

{{defn|A fast-moving current of hot gas and rock (collectively known as {{gli|tephra}}), which normally hugs the ground and travels downhill or spreads laterally under gravity.}}

{{term|pyroclastic}}

{{defn|A {{gli|volcanic}} fragment, such as a {{gli|volcanic bomb}}, {{gli|breadcrust bomb}}, or {{gli|achnelith}}.}}

{{term|pyroxene}}

{{defn|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.}}

{{term|pyroxenite}}

{{defn|A coarse-grained, {{gli|holocrystalline}} igneous rock consisting mainly of {{gli|pyroxene|pyroxenes}}. It may contain {{gli|biotite}}, {{gli|hornblende}}, or {{gli|olivine}} as accessories.}}

{{glossaryend}}

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{{glossary}}

{{term|quartz}}{{anchor|quartzes}}

{{defn|}}

{{term|quartzite}}{{anchor|quartzites}}

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

{{term|Quaternary}}

{{defn|The most recent of the three {{gli|periods}} of the {{gli|Cenozoic Era}} in the standard geologic time scale. It follows the {{gli|Neogene}} Period, spanning 2.588 ± 0.005 million years ago to the present.}}

{{glossaryend}}

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{{glossary}}

{{term|reef knoll}}

{{defn| Landform comprising an immense pile of calcareous material previously accumulated on an ancient sea floor. Reef knolls can be divided into bioherms and biostromes. A {{gli|bioherm}} is a landform of organic sedimentary rock enclosed or surrounded by rock of different origins. A {{gli|biostrome}} is a distinctly bedded or broadly lenticular sedimentary rock landform.}}

{{term|regional metamorphism}}

{{defn|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.}}

{{term|reticulite}}

{{defn|A {{gli|basaltic}} {{gli|pumice}} in which the walls of the vesicles have collapsed, leaving a network of fine, interconnecting glass threads. It is the lightest rock known.}}

{{term|retrograde metamorphism}}

{{defn|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.}}

{{term|reverse fault}}

{{ghat|Also called a thrust fault.}}

{{defn|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 {{gli|normal fault}}.}}

{{term|rhyolite}}

{{defn|An aphanitic, buff to greyish flow-banded rock, often containing spherulites or phenocrysts of quartz and feldspar.}}

{{term|rift}}

{{defn|}}

{{term|roche moutonnée}}

{{defn|An elongated post-glacial rock formation with a smoothed surface on the uphill side and a "plucked" surface on the downhill side.}}

{{term|rock}}{{anchor|rocks}}

{{defn|}}

{{term|rolling hills}}

{{defn|undulating low hill terrain; cf. "Hügelland"}}

{{term|rudaceous}}

{{defn|Having the composition and characteristics of {{gli|clastic rocks}}, i.e. coarse-grained sedimentary rocks, conglomerates, and breccias, with a particle size of less than 2 mm.}}

{{term|rudite}}

{{defn|A generic term for any of a set of {{gli|sedimentary}} rocks composed of rounded or angular detrital grains, i.e. granules, pebbles, cobbles, and boulders, which are coarser than sand in size.}}

{{glossaryend}}

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{{glossary}}

{{term|saccharoidal}}

{{defn|Having a texture similar to that of granulated sugar.}}

{{term|Saldanian orogeny}}

{{defn|}}

{{term|sand}}{{anchor|sands}}

{{defn|}}

{{term|sandstone}}{{anchor|sandstones}}

{{defn|Sand (with grains up to 2 mm in diameter) in which the grains are cemented together by secondary silica or calcite. Sandstone may be loosely cemented and soft or well cemented and hard, and is usually buff to brownish in color, sometimes reddish, due to the presence of iron oxides, or greenish, due to the presence of {{gli|glauconite}}.}}

{{term|sanidine}}

{{defn|A form of potash {{gli|feldspar}} identical in composition with {{gli|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 {{convert|900|C|F}}. Occurs in lavas and dyke rocks.}}

{{term|schist}}{{anchor|schists}}

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

{{term|scree}}

{{defn|See {{gli|talus}}.}}

{{term|sediment trap}}

{{defn|A depression in which sediments substantially accumulate over time.}}

{{term|sedimentary rock}}{{anchor|sedimentary rocks|sedimentary}}

{{defn|}}

{{term|sequence}}

{{defn|A sequence of geological events, processes, or rocks arranged in chronological order.}}

{{term|sericite}}

{{defn|A white potash-mica, similar to {{gli|muscovite}} in chemical composition and general character but occurring as a secondary mineral, often as a decomposition product of {{gli|orthoclase}}.}}

{{term|shale}}{{anchor|shales}}

{{defn|A fine-grained, clastic {{gli|sedimentary rock}} composed of mud that is a mix of flakes of clay minerals and tiny fragments (silt-sized particles) of other minerals, especially {{gli|quartz}} and {{gli|calcite}}.}}

{{term|shear zone}}

{{defn|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.}}

{{term|shield}}

{{defn|A large area of exposed Precambrian crystalline {{gli|igneous}} and high-grade {{gli|metamorphic rock|metamorphic rocks}} that form {{gli|tectonics|tectonically}} stable areas.}}

{{term|shingle beach}}

{{defn|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.}}

{{term|silcrete}}

{{defn|An indurated soil duricrust formed when surface sand and gravel are cemented by dissolved {{gli|silica}}.}}

{{term|silt}}{{anchor|silts}}

{{defn|Granular material of a size somewhere between {{gli|sand}} and {{gli|clay}} whose mineral origin is {{gli|quartz}} and {{gli|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.}}

{{term|siltstone}}{{anchor|siltstones}}

{{defn|Indurated {{gli|silt}} whose {{gli|granulometry}} is somewhere between that of {{gli|sand}} and {{gli|clay}}.}}

{{term|slate}}{{anchor|slates}}

{{defn|Very fine-grained sedimentary rock of the clay or silt grade which as a consequence of regional metamorphism has developed a {{gli|slaty cleavage}}.}}

{{term|slaty cleavage}}

{{defn|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.}}

{{term|slickenside}}

{{defn|A smoothly polished surface caused by frictional movement between rocks along the two sides of a {{gli|fault}}. This surface is normally striated in the direction of movement.}}

{{term|slump}}

{{defn|A form of {{gli|mass wasting}} that occurs when a coherent mass of loosely consolidated materials or rock layers moves a short distance down a slope.}}

{{term|snowball Earth}}

{{defn|Series of global glaciations in the {{gli|Precambrian}}, 600 to 700 million years ago, before the hard-shelled animal life explosion.}}

{{term|smectite}}

{{defn|Swelling {{gli|clay}} minerals with a three-layer 2:1 (TOT) structure whose interlayers are mainly occupied by {{chem2|Na+}} or {{chem2|Ca(2+)}} hydrated cations and water molecules ({{etymology|grc|{{Wikt-lang|grc|σμηκτός}} ({{grc-transl|σμηκτός}})|lubricated}}; {{etymology||{{Wikt-lang|grc|σμηκτρίς}} ({{grc-transl|σμηκτρίς}})|walker's earth, fuller's earth}}; {{lit|rubbing earth; earth that has the property of cleaning}}). See also {{gli|montmorillonite}} and {{gli|bentonite}}. For non-swelling 2:1 clay minerals, see {{gli|illite}} and {{gli|chlorite}}.}}

{{term|soil liquefaction}}

{{defn|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.}}

{{term|sorting}}

{{defn|Sorting describes the distribution of grain size of sediments, either in unconsolidated deposits or in {{gli|sedimentary rock|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).}}

{{term|speleothem}}

{{defn|A geological formation by mineral deposits that accumulate over time in natural caves. Speleothems most commonly form in calcareous caves due to carbonate dissolution reactions. They can take a variety of forms, depending on their depositional history and environment. Their chemical composition, gradual growth, and preservation in caves make them useful paleoclimatic proxies. Includes stalactites and stalagmites.}}

{{term|sphene}}

{{defn|Ancient deprecated name for a calcium titanium {{gli|nesosilicate}} mineral, {{chem2|CaTiSiO5}}.}}

: {{ghat|Since 1982, officially called {{gli|titanite}} by the International Mineralogical Association.}}

{{term|staurolite}}

{{defn|A red brown to black, mostly opaque, nesosilicate mineral with a white streak.}}

{{term|stone}}{{anchor|stones}}

{{defn|}}

{{term|storm ridge}}

{{defn|A beach ridge usually located further or higher inland caused by wave action from storms.}}

{{term|strain}}

{{defn|A change in the volume or shape of a rock mass in response to stress.}}

{{term|stratigraphy}}

{{defn|}}

{{term|stratum}}

{{defn|}}

{{term|stylolite}}

{{defn|An irregular discontinuity or non-structural {{gli|fracture}} in {{gli|limestone}} and other sedimentary rocks. Stylolites result from {{gli|compaction}} and {{gli|pressure solution}} during {{gli|diagenesis}}.}}

{{term|surficial}}

{{defn|Of or pertaining to the surface.}}

{{term|syenite}}

{{defn|A coarse-grained {{gli|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 {{gli|mafic}} materials, of which common {{gli|hornblende}} is characteristic.}}

{{term|syncline}}

{{defn|A geological fold with strata dipping inwards towards the fold axis. Contrast {{gli|anticline}}.}}

{{term|syntaxis}}

{{defn|An abrupt change in the orientation of a mountain belt or individual fold/thrust structure}}

{{glossaryend}}

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{{glossary}}

{{term|Table Mountain Group}}

{{defn|A group of rock formations within the Cape Supergroup sequence of rocks.}}

{{term|talus}}

{{ghat|Also called scree.}}

{{defn|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.}}

{{term|tectonic plate}}{{anchor|tectonic plates}}

{{defn|}}

{{term|tekton}}

{{defn|}}

{{term|tektite}}

{{defn|Natural glass formed from terrestrial ejecta during a meteorite impact.}}

{{term|tempestite}}

{{defn|Storm deposits found in sedimentary formations where shell debris accumulate under the effect of waves and seawater movement.}}

{{term|tenacity}}

{{defn|A mineral's behavior when deformed or broken.}}

{{term|tephra}}

{{defn|Fragmental material produced by a {{gli|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.}}

{{term|Tethys Ocean}}

{{defn|A prehistoric ocean that existed between the continents of Gondwana and Laurasia during the Mesozoic era before the opening of the Indian Ocean.}}

{{term|till}}

{{ghat|Also called glacial till.}}

{{defn|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.}}

{{term|tillite}}

{{defn|A type of {{gli|sedimentary rock}} derived from {{gli|till|glacial till}} which has been indurated or lithified by subsequent burial into solid rock.}}

{{term|titanite}}

{{defn|A calcium titanium {{gli|nesosilicate}} mineral, {{chem2|CaTiSiO5}}. See also {{gli|sphene}}.}}

{{term|topography}}

{{defn|The arrangement of the natural and artificial physical features of an area.}}

{{term|tor}}

{{defn|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.}}

{{term|trace fossil}}

{{defn|}}

{{term|trachyte}}

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

{{term|travertine}}

{{defn|A terrestrial {{gli|sedimentary rock}} formed by the precipitation of carbonate minerals from solution in ground and surface waters and/or geothermally heated hot springs.}}

{{term|triclinic}}

{{defn|In the triclinic system, the crystal is described by vectors of unequal length, and none of the three vectors are orthogonal to another.}}

{{term|tuff}}

{{defn|A rock formed of compacted {{gli|volcanic}} fragments, some of which can be distinguished by the naked eye. If the fragments are larger than the rock grades into an agglomerate.}}

{{term|tundra}}

{{defn|A vast, flat, treeless Arctic region of Europe, Asia, and North America in which the subsoil is permanently frozen.}}

{{term|turbidite}}

{{defn|The deposit of a turbidity current.}}

{{term|turbidity current}}

{{defn|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.}}

{{term|Tygerberg formation}}

{{defn|A component of the PreCambrian Malmesbury group of South Africa}}

{{glossaryend}}

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{{glossary}}

{{term|ultramafic}}

{{ghat|Sometimes used interchangeably with ultrabasic.}}

{{defn|Almost {{gli|feldspar}}-free. Examples of ultramafic rocks include dunite, peridotite, and pyroxenite.}}

{{term|undulating hills}}

{{defn|see "rolling hills"}}

{{term|Urgonian}}

{{defn|A shallow-water carbonate {{gli|facies}} deposited along the northern margins of the {{gli|Tethys Ocean}} during the Barremian and Aptian.}}

{{glossaryend}}

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{{glossary}}

{{term|vacuole}}

{{defn|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.}}

{{term|Variscan orogeny}}

{{ghat|Also called the Hercynian orogeny.}}

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

{{term|varve}}

{{defn|An annual layer of sediment or {{gli|sedimentary rock}}.}}

{{term|vein}}{{anchor|veins}}

{{defn|A {{gli|mineral}} filling of a fracture or other crack within a rock in a sheet-like or tabular shape.}}

{{term|vermiculite}}

{{defn|A hydrous {{gli|silicate}} mineral that is classified as a phyllosilicate and that expands greatly when heated. Exfoliation occurs when the mineral is heated sufficiently.}}

{{term|vergence}}

{{defn|The direction of overturning of asymmetric folds, which matches the direction of thrusting.}}

{{term|vesiculated}}

{{defn|}}

{{term|vitrinite}}

{{defn|A group of {{gli|maceral|macerals}} that are the most common component of coal.}}

{{term|vitrophyre}}

{{defn|See {{gli|pitchstone}}.}}

{{term|volcanic}}

{{defn|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.}}

{{term|volcanic bomb}}

{{defn|Rounded or spindle-shaped rock of mainly {{gli|basaltic}} composition ejected during eruptions.}}

{{term|vug}}

{{defn|A small cavity in a rock filled or lined with {{gli|crystals}} or {{gli|minerals}} that are different from the host rock.}}

{{glossaryend}}

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{{glossary}}

{{term|wiggle trace}}

{{defn|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.}}

{{term|wolframite}}

{{defn|An iron manganese tungstate mineral with the chemical formula (Fe,Mn)WO₄.}}

{{glossaryend}}

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{{term|xenolith}}

{{defn|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 {{gli|igneous rock}} during magma emplacement and eruption.}}

{{term|xenotime}}

{{defn|A rare earth phosphate mineral whose major component is yttrium orthophosphate (YPO₄).}}

{{term|X-ray diffraction (XRD)}}

{{defn|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.}}

{{term|X-ray fluorescence (XRF)}}

{{defn|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.}}

{{term|xyloid coal}}

{{ghat|Also called lignite or brown coal.}}

{{defn|A soft brown fuel with characteristics that put it somewhere between coal and peat.}}

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{{term|Yellowcake}}

{{ghat|Also urania.}}

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

{{term|Young's modulus}}

{{ghat|Also called the tensile modulus.}}

{{defn|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.}}

{{term|Ypresian}}

{{defn|The oldest age or the lowest stratigraphic stage of the {{gli|Eocene}}. It spans the time between ~56 Ma and ~49 Ma (million years ago).}}

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{{term|zeolite}}

{{defn|Microporous, aluminosilicate minerals commonly used as adsorbents.}}

{{term|zircon}}

{{defn|A zirconium {{gli|silicate}} mineral belonging to the group of nesosilicates. Its corresponding chemical formula is ZrSiO₄.}}

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{{Portal|Geology}}

• Outline of geology
• Index of geology articles
• Glossary of geography terms
• Glossary of landforms

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• {{Cite web |title=Schlumberger {{!}} Energy Glossary |author=Schlumberger|work=glossary.slb.com |date=2022 |access-date=25 October 2022 |url= https://glossary.slb.com/ |language=English}}

{{DEFAULTSORT:Glossary Of Geological Terms}}

{{Geology}}

{{Glossaries of science and engineering}}

Category:Glossaries of science

Category:Wikipedia glossaries using description lists