Glossary of Geological Terms

(Based on GA Guide to Southern Cyprus No 50, by Trevor Greensmith, 1994)

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

   ---------- A ----------  

Acid:   Term applied to igneous rocks with greater than 65 per cent silica content.

Actinolite: One of the amphibole group of minerals. Because its molecular structure is made up of long double chains of silicon atoms, actinolite forms long, thin crystals, sometimes even fibers. Thus actinolite is sometimes classified as asbestos. Actinolite forms from the olivine in many igneous rocks under conditions of high pressure and fairly low temperature (greenschist facies).

Aggregate: Any of several hard, inert materials, such as sand, gravel or crushed stone, used for mixing with a cementing or bituminous material to form concrete, mortar or plaster.

Allochthonous: A large block of rock which has been moved from its original site of formation, usually by low angle thrust faulting.

Alluvium: Deposit laid down from rivers.

Amphibolite:   A metamorphic rock composed essentially of amphibole and plagioclase feldspar.

Amphibolite facies: A group of metamorphic rocks formed under moderately high pressure and temperature.

Amygdale: A gas cavity in igneous rocks infilled by secondary minerals, such as calcite and zeolites.

Andesite: An intermediate, fine grained igneous rock occurring as lava flows and containing plagioclase feldspar and hornblende.

Anhedral: Having no well formed crystal faces or cross-section shape. Anhedral crystal growth occurs in a competitive environment with no free space for the formation of crystal faces.

Aphanitic: Texture of an igneous rock in which the crystals are so fine that individual minerals cannot be distinguished with the naked eye. Aphanitic rocks are extrusive rocks that cooled so quickly that crystal growth was inhibited.

Aplite: The name given to intrusive rock in which quartz and feldspar are the dominant minerals. Aplites are usually very fine-grained, white, grey or pinkish, and their constituents are visible only with the help of a magnifying lens.

Archean: The Archean (also spelled Archaean; formerly Archaeozoic) is a geologic eon before the Proterozoic Eon, before 2.5 Ga (billion years, or 2,500 Ma) ago. The Archean era is generally agreed to have started at 3.8 billion years ago —at the end of the Hadean Eon, but this boundary is informal. The name comes from the ancient Greek Αρχή (Arkhē), meaning “beginning, origin”.

Asbestos: A commercial term applied to a group of silicate minerals that readily separate into thin fibres that are flexible, heat resistant and chemically inert.

Asthenosphere: The layer or shell of the earth below the lithosphere, which is weak and in which isostatic adjustments take place, magmas may be generated and seismic waves are strongly attenuated. The asthenosphere begins about 100 km below the surface and extends to a depth of about 350 km.

Autochthonous: A sediment or rock that can be found at its site of formation or deposition, as opposed to an allochthon.

  ---------- B ----------

Back-arc:  the area behind a volcanic arc.   (See HERE for more)

Banding: The structure of rocks having near-parallel layers of different texture, colour or mineralogy.

Basalt: A fine grained, dark coloured volcanic rock usually occurring as lava flows and formed of plagioclase feldspar, pyroxene and iron ore.  The silica percentage is usually less than 52 per cent.

Bastite: An altered form of enstatite or bronzite, also known as schiller spar. A member of the pyroxene group of minerals.

Beach rock: Recent sand or pebble deposit laid down in the intertidal zone and cemented by secondary calcium carbonate, especially in warm water seas.

Bentonite: A soft, plastic, light-coloured rock composed essentially of clay minerals of the montorillonite group plus colloidal silica, and produced by chemical alteration of glassy igneous material. A clay rock formed by the alteration of volcanic ash and rich in smectite clay minerals.

Bioturbation: The churning of sediment by the activities of burrowing, boring and grazing organisms.

Bivalve: A mollusc in which the two valves are commonly roughly equal in size, shape and symmetry.

Boninite: A glassy high Mg basic to intermediate lava, consisting of phenocrysts of pyroxenes and olivine in a crystallite-rich glassy matrix.. A mafic extrusive rock high in both magnesium and silica, formed in fore-arc environments, typically during the early stages of subduction.

Breccia: A coarse grained rock formed of angular fragments. Can be deposited as a sediment or is present in fault zones.

  ---------- C ----------

Calcarenite: A limestone consisting of predominantly (more than 50%) calcite particles of sand size (consolidated calcareous sand).

Calcrete (Caliche): A near-surface deposit cemented by secondary calcium carbonate precipitated from evaporating groundwater. The process is known as calcretisation. Havara and kafkalla are local names.

Chert: A dense, commonly white to grey siliceous rock, usually occuring as nodules and thin seams.

Chilled margin: The border of an igneous body generally finer grained than the main mass due to more rapid cooling.

Chromite: A brownish-black to iron-black mineral of the spinel group (Fe,Mg)(Cr,Al)2O4.

Chrysotile: A highly fibrous variety of serpentine and the most important type of asbestos.

Clastic: Pertaining to a rock or sediment composed principally of broken fragments that are derived from pre-existing rocks or minerals and that have been transported some distance from their places of origin.

Clay: A very fine grained clastic sedimentary rock in which minute clay minerals usually are a prominent constituent.  More lithified equivalents are called shales or mudrocks.

Columnar structure: A polygonal pattern of near-vertical cooling joints in basaltic rocks.

Concretion: A nodular concentration of secondary minerals precipitated within a pre-existing sediment, such as ironstone and chert nodules.

Conglomerate: A coarse grained clastic sedimentary rock formed of rounded and sub-rounded fragments.

Convolute bedding: A contortion of bedding planes and laminae in semi­lithified sediments.

Cretaceous: A geological period having a duration of approximately 70 million years, from 135 to 65 million years.

Cross-stratification: A general term for layers deposited at an angle to the true bedding surfaces.

Crystallisation: The process whereby crystals separate from a fluid, a viscous or solid state.

  ---------- D ----------

Dacite: A quartz-bearing andesite type of lava flow, intermediate between andesite and rhyolite.

Debris flow: A gravitational flow of sediment and fluid in which large fragments are suspended in a finer grained matrix.

Diabase: A medium-grained igneous rock equivalent to basalt, but intrusive as opposed to to extrusive.  An altered dolerite in British usage.

Diapiric: A structural shape of a rock on Earth that looks either circular or elliptical in outline. This feature originates from the lateral forces of tectonic movement.

Diorite: Dark coarse-grained igneous rock, of feldspar and ferromagnesian silicates.

Dolerite: A medium grained basic igneous rock composed of plagioclase feldspar, pyroxene and iron-ore.

Dunite: An essentially mono-mineralic rock composed of olivine, derived from mantle peridotite.

Dyke: A sheet-like igneous body, usually with a near-vertical attitude.  Dykes closely associated in time and space are known as a swarm.

  ---------- E ----------

Epiclastic: A term used for weathered and eroded volcanic materials.

Epidote: A complex green silicate containing calcium, aluminium and iron found in calcium-bearing rocks which have been metamorphosed.

Euhedral: Crystals that are well-formed with sharp, recognisable faces. Usually formed early in the crystallization of a magma when there is no competition for space.

Eukaryote: An organism whose cells contain complex structures enclosed within membranes. The presence of a nucleus gives eukaryotes their name, which comes from the Greek ευ (eu, "good") and κάρυον (karyon, "nut" or "kernel").

Evaporite: A chemical sediment composed primarily of minerals produced from a saline solution as a result of extensive or total evaporation of the solvent (e.g. gypsum).

Extrusive rocks: Igneous rocks formed by eruption of lava onto the Earth’s surface and thus rapidly cooled.

  ---------- F ----------

Facies: The total characteristics of a rock from which can be inferred the conditions of formation.

Fanglomerate: A poorly sorted conglomerate or breccia deposited in the form of an alluvial fan.

Fault: A fracture or fracture zone along which there has been displacement of the sides relative to one another parallel to the fracture.

Fault breccia: A breccia formed along a fault plane.

Feeder: A dyke or pipe of igneous rock which served as a feeding channel for larger intrusive or extrusive igneous masses.

Flame structure: Curved plume of fine grained sediment fingering into an overlying coarser bed and caused by loading.

Foliation: A compositional layering in metamorphic and igneous rocks, commonly caused by a preferred orientation of minerals.

Foraminifera: Single celled and free-floating marine micro-organisms with a skeleton (test) commonly formed of calcium carbonate.

Forearc:  A forearc is the region between an oceanic trench and the associated volcanic arc.  As such, forearc regions are found at convergent margins. (See HERE for more)

Formation: An assemblage of rocks with a characteristic lithology which allows it to be mapped.

Fracture cleavage: A series of closely spaced parallel joints produced in deformed rocks by earth pressures.

  ---------- G ----------

Gabbro: A coarse grained basic igneous rock consisting essentially of plagioclase feldspar and augite.

Gastropod: A snail-like one-valved mollusc in which the shells are commonly conical.

Gossan: A Cornish word for the oxidised outcrop commonly overlying iron-rich ore bodies.

Graded bedding: Shown by beds which are comparatively coarse grained at the base and which change upwards into finer grained material.

Granite: A coarse-grained, light-coloured, hard igneous rock consisting chiefly of quartz, orthoclase or microcline, and mica. Not found in Cyprus.

Greenschist facies: Mildly metamorphosed basic igneous rocks rich in chlorite and serpentine.

Gypsum: Hydrated calcium sulphate, white but sometimes tinted by impurities, found in beds as an evaporite. Used in the manufacture of plaster of Paris, cement, paint, school chalk, glass, and fertilizer. [from Greek gupsos, chalk, plaster, cement]

  ---------- H ----------

Harzburgite: An ultrabasic igneous rock derived from lherzolite or peridotite mantle magma, rich in orthopyroxenes and olivine, both of which usually show secondary alteration.  Commonly one of the core rocks of an ophiolite.

Hemipelagic: Hemipelagic sediments are those deposited slowly, but not as slowly as pelagic sediments. The sediments deposited on continental shelves and rises, frequently referred to as hemipelagic sediments, ordinarily accumulate too rapidly to react chemically with seawater. In most cases, individual grains thus retain characteristics imparted to them in the area where they formed.

Hyaloclastite:              A glassy volcanic tuff or ash produced by rapid cooling and fragmentation of lava erupted into water.

  ---------- I ----------

Idiomorphic: Minerals occurring naturally in the form of well-developed crystals displaying their intrinsic habit. Often formed early in the rock cooling.

Igneous: Rocks or minerals that solidified from molten or partly molten material, i.e. from magma.

Imbricate: literally 'overlapping'. In sedimentology 'imbrication' refers to a primary depositional fabric consisting of a preferred orientation of clasts such that they overlap one another in a consistent fashion, rather like a run of toppled dominoes.  Imbrication is observed in conglomerates and some volcaniclastic deposits.

Indurate: To grow hard; harden; to become firmly fixed or established.

Inlier: An outcrop of rocks totally surrounded by younger rocks.

Inosilicate:  Inosilicates (from Greek ις is [genitive: ινος inos], fibre), or chain silicates, have interlocking chains of silicate tetrahedra with either SiO₃, 1:3 ratio, for single chains or Si₄O₁₁, 4:11 ratio, for double chains.  Single chain inosilicates include the Pyroxene group, double chain inosilicates include the Amphibole group.

 ---------- J ----------

Jurassic: The Jurassic is a geologic period and system that extends from 201.3± 0.6 Ma (million years ago) to 145± 4 Ma; from the end of the Triassic to the beginning of the Cretaceous. The Jurassic constitutes the middle period of the Mesozoic Era, also known as the Age of Reptiles. The start of the period is marked by the major Triassic–Jurassic extinction event. Two other extinction events occurred during the period: the Late Piensbachian/Early Toarcian event in the Early Jurassic, and the Late Tithonian event at the end; however, neither event ranks among the 'Big Five' mass extinctions. The Jurassic is named after the Jura Mountains within the European Alps, where limestone strata from the period were first identified.
   By the beginning of the Jurassic, the supercontinent Pangaea had begun rifting into two landmasses, Laurasia to the north and Gondwana to the south. This created more coastlines and shifted the continental climate from dry to humid, and many of the arid deserts of the Triassic were replaced by lush rainforests. Mean atmospheric CO2 content was ca. 1950 ppm (7 times pre-industrial level) while mean surface temperature was ca. 16.5 °C (3 °C above modern level).

  ---------- K ----------

Kimberlite: Kimberlite pipes are the most important source of mined diamonds today. The consensus on kimberlites is that they are formed deep within the mantle. Formation occurs at depths between 150 and 450 kilometres (93 and 280 mi), potentially from anomalously enriched exotic mantle compositions, and are erupted rapidly and violently, often with considerable carbon dioxide and other volatile components. It is this depth of melting and generation which makes kimberlites prone to hosting diamond xenocrysts.  Petrogenesis study of kimberlite has the potential to provide information about the composition of the deep mantle.

Komatiite: A rare type of ultramafic mantle-derived volcanic rock, mainly restricted to rocks of Archaean age. Komatiites have low silicon, potassium and aluminium, and high to extremely high magnesium content. Komatiite was named for its type locality along the Komati River in South Africa.

Kyanite: Kyanite is a member of the aluminosilicate series, which also includes the polymorph andalusite and the polymorph sillimanite. Kyanite is strongly anisotropic, in that its hardness varies depending on its crystallographic direction. In kyanite, this anisotropism can be considered an identifying characteristic.   Kyanite in metamorphic rocks generally indicates pressures higher than four kilobars.

  ---------- L ----------

Lherzolite: An ultra-basic, igneous rock or magma similar to peridotite and thought to constitute primary mantle material.

Limestone: A sedimentary rock composed dominantly of organic and inorganic calcite (calcium carbonate).

Lithification: The process of hardening, induration and compaction of sediments leading to the formation of solid rocks.

Load cast: Sedimentary structure where rounded downwards-protrusions of coarse material have sunk vertically into finer material below.

Leucocratic: An igneous rock in which the quantity of light-coloured minerals (feldspars, quartz, and feldspathoids) is greater than normal.

  ---------- M ----------

Magma: The molten fluid generated within the earth and from which igneous rocks are derived.

Mantle: The zone of the geosphere situated at a depth of about 35km below the crust and extending inwards to the core, about 2900km distant.  It is generally assumed to be ultrabasic in composition (peridotite).  Harzburgite is a modified variety of peridotite.

Maquis: Shrubland biome, common in the Mediterranean region, typically consisting of densely growing evergreen shrubs such as holm oak, tree heath, strawberry tree, sage, juniper, buckthorn, spurge olive and myrtle.  In many places it is due to destruction of forest cover, mainly by frequent burning that prevents young trees from maturing.  Otherwise it tends to grow in arid, rocky areas where only drought-resistant plants are able to prosper.

Marble: Metamorphosed limestone.

Marl: A soft and friable sediment formed of a mixture of calcite and clay.

Massive: A term commonly used to describe sedimentary beds which are uniform in texture from top to bottom.  They need not necessarily be thick beds.

Melanocratic: Dark coloured, containing a large proportion of Ferro-magnesian minerals (eg. hornblende, magnetite).

Melange: A mixed sedimentary deposit of variably sized blocks of mixed lithology embedded in a fragmental matrix of finer-grained material, which may be sheared.  Often caused initially by submarine sliding but may then be affected by powerful earth stresses (see olistostrome).

Member: Part of a formation, recognised by some individual lithological peculiarity.

Mercalli scale: An arbitrary scale of earthquake intensity graduated from I to XII.

Mesozoic: The era of geologic time from about 225 to about 65 million years ago.

Metamorphic: Any rock derived from pre-existing rocks by mineralogical, chemical and/or structural changes, essentially in the solid state, in response to marked changes in temperature, pressure and chemical environment, generally at depth in the Earth’s crust.

Migmatite: Migmatite is a rock that is a mixture of metamorphic rock and igneous rock.  It is created when a metamorphic rock such as gneiss partially melts, and then that melt recrystallizes into an igneous rock, creating a mixture of the unmelted metamorphic part with the recrystallized igneous part.  They can also be known as diatexite.  Commonly, migmatites occur within extremely deformed rocks that represent the base of eroded mountain chains, typically within Precambrian cratonic blocks.

Mineralisation: The impregnation of rock with minerals, some of which might be of economic importance.

Mohs Scale: A standard of ten minerals by which the hardness of a mineral may be rated. The scale runs from the softest to the hardest and is numbered one to ten: talc; gypsum; calcite; fluorite, apatite; orthoclase; quartz; topaz; corundum; and diamond.

Montomorillonite: A group of clay minerals.

  ---------- N ----------

Norite: Norite is a mafic intrusive igneous rock composed largely of the calcium-rich plagioclase labradorite and hypersthene with olivine.  Norite is essentially indistinguishable from gabbro without thin section study under the petrographic microscope.  The principal difference between norite and gabbro, however, is the type of pyroxene of which it is composed; norite is predominately composed of orthopyroxenes, largely high magnesian enstatites, whereas the principal pyroxenes in gabbro are clinopyroxenes, generally medially iron-rich augites.

  ---------- O ----------

Obduction: The process of displacement of a plate so that it moves into a position above an adjacent plate.

Oceanic crust: The type of Earth’s crust (outer layer), which underlies the ocean basins. It is about 5-10 km thick and has a density of about 3 g/cm3. 

Ochre: An iron oxide- and manganese oxide-rich pigment commonly formed during the weathering of pyritic ore-bodies.

Oikocryst:   The host crystal in a poikilitic textured rock containing small inclusions of another mineral that it enveloped as it grew.

Olistolith: An exotic block of rock brought into place by slipping and sliding.

Olistostrome: A sedimentary formation, commonly submarine, made up of a chaotic assemblage of blocks of mixed lithology set in a clay matrix.

Ophiolite: An assemblage of ultrabasic and basic igneous rocks, commonly generated along the constructive margins between spreading plates and which has escaped subduction and been stranded on continental crust.

Ophitic texture: A variety of poikilitic texture where no crystallographic relation exists between the two minerals (enclosing and enclosed).

  ---------- P ----------

Palinspastic: In structural geology section restoration or palinspastic restoration is a technique used to progressively undeform a geological section in an attempt to validate the interpretation used to build the section.  It is also used to provide insights into the geometry of earlier stages of the geological development of an area.  A section that can be successfully undeformed to a geologically reasonable geometry, without change in area, is known as a ‘balanced section’.

                        Comparably a palinspastic map is a map view of geological features, often also including present-day coastlines to aid the reader in recognising the area, representing the state before deformation.

Pegmatite: A coarse grained facies of any plutonic igneous rock.

Pelagic: Of, relating to, or living in open oceans or seas rather than waters adjacent to land or inland waters.  From the Ancient Greek: πέλαγος (pélagos) "open sea". The water of the ocean which is an environment for sediment accumulation.

Peridotite: A coarse grained, ultrabasic rock composed mainly of olivine.  Feldspar is absent.  Regarded with lherzolite as being a primary constituent of the upper mantle.  Wehrlite and harzburgite are both forms of peridotite.

Phaneritic: Matrix grains in a rock are large enough to be distinguished with the unaided eye, as opposed to aphanitic (which is too small to see with the naked eye).

Phenocryst: a relatively large and usually conspicuous crystal distinctly larger than the grains of the rock groundmass of an igneous rock. Such rocks that have a distinct difference in the size of the crystals are described as porphyritic. Phenocrysts often have euhedral forms, either due to early growth within a magma, or by post-emplacement recrystallization. In metamorphic rocks, crystals similar to phenocrysts are called porphyroblasts. Phenocrysts are more often found in the lighter (higher silica) igneous rocks such as felsites and andesites, although they occur throughout the igneous spectrum including in the ultramafics. The largest crystals found in some pegmatites are often phenocrysts being significantly larger than the other minerals.

Phlogopite: Phlogopite is the magnesium endmember of the biotite solid solution series, with the chemical formula KMg3AlSi3O10(F,OH)2. Iron substitutes for magnesium in variable amounts leading to the more common biotite with higher iron content. For physical and optical identification, it shares most of the characteristic properties of biotite.

Picrite-basalt: An olivine-enriched basalt, containing phenocrysts of olivine and augite.

Pillow lava: Lava formed during an underwater eruption, characterized by pillow-shaped masses usually with chilled margins.

Plagiogranite: Also called Trondhjemite. A leucocratic, orthoclase feldspar-free, coarse granitoid rock, rich in plagioclase feldspar (oligoclase). Plagiogranite dikes commonly form part of the sheeted dike complex of an ophiolite.

Plate: Relatively thin, being of the order of 100km thick, internally rigid blocks of continental or oceanic material forming the earth’s crust.  They are driven continuously across an underlying weaker asthenosphere probably by mantle convection currents.

Plate tectonics: A unifying theory for explaining the evolution of the Earth’s surface, based on the slow movement of relatively thin, internally rigid lithospheric plates over the asthenosphere, causing seismic and tectonic activity along these boundaries.

Pleistocene: A geological epoch that began 2 million years ago and lasted until 10,000 years ago.

Pliocene: A geological epoch that began 5.1 million years ago and lasted until 2 million years ago.

Plutonic: Term applied to igneous rocks formed at considerable depth by crystallisation (solidification) of magma and/or by chemical alteration.

Poikilitic: Crystals, typically phenocrysts, in an igneous rock which contain small grains of other minerals.

Poikioblast: A larger crystal that contains smaller crystals of other minerals. A texture of metamorphic rocks.

Porphyritic texture: A texture of igneous rocks in which relatively large crystals (phenocrysts) are set within a finer grained matrix.

Porphyroblast: A large mineral crystal in a metamorphic rock which has grown within the finer grained groundmass. Porphyroblasts are commonly euhedral crystals, but can also be partly to completely irregular in shape. The most common porphyroblasts in metapelites (metamorphosed mudstones and siltstones) are garnets and staurolites, which stand out in well foliated metapelites (such as schists) against the platy mica matrix. Porphyroblasts are often confused with porphyroclasts, which can also be large outstanding crystals, but which are older than the matrix of the rock.

Pumice: A light coloured variety of highly porous, acid intermediate igneous lava.

Pyrite ore body: A continuous, well-defined mass of pyrite ore. Its content is economically feasible for extraction.

Pyroxene: A group of crystalline silicate minerals common in igneous and metamorphic rocks and containing two metallic oxides, as of magnesium, iron, calcium, sodium, or aluminium.

Pyroxenite: A coarse grained igneous composed mainly of pyroxene.

  ---------- Q ----------

Quartz: Quartz is the second most abundant mineral in the Earth's continental crust, after feldspar. It is made up of a continuous framework of SiO4  silicon–oxygen tetrahedra, with each oxygen being shared between two tetrahedra, giving an overall formula SiO2.

  ---------- R ----------

Radiolaria: Free-floating, marine siliceous micro-organisms with a porous skeleton that easily disintegrates on death.

Radiolarian chert: A hard and brittle siliceous rock composed mainly of the disintegrated skeletons of radiolaria.

Raised Beach: Also known as a marine terrace.  A deposit of sand, gravel and shell material, originally laid down at the edge of the sea, but now elevated as a consequence of a relative fall in sea-level.

Reef limestone: A limestone formed of organisms capable of building up a rigid structure from the sea floor. Such as corals, sponges and of sediment-binding organic constituents, such as calcareous algae.

Rheology:  The study of the flow of matter, primarily in the liquid state, but also as ‘soft solids’ or solids under conditions in which they respond with plastic flow rather than deforming elastically in response to an applied force.  It applies to substances which have a complex microstructure, such as muds, sludges, suspensions, polymers and other glass formers (e.g., silicates), as well as many foods and additives, bodily fluids (e.g., blood) and other biological materials or other materials which belong to the class of soft matter.

Richter scale: A numerical scale of earthquake magnitude, devised in 1935 by C.F. Richter, running from 1 to 10.

Rift faulting: Faulting along parallel lines, which produces steep-sided flat bottomed valleys, known as rift valleys or graben.

River terrace: A valley side representing the former floor, left behind as a river cuts downwards, usually as a result of its rejuvenation.

Rhyolite: A usually light-colored, fine-grained extrusive igneous rock that is compositionally similar to granite, consisting of quartz, feldspars, and mica or amphibole. It often includes flow lines formed during the extrusion.

  ---------- S ----------

Salt deposits: Mineral salts, such as gypsum and halite, deposited through the evaporation of salt lakes or lagoons.

Sandstone: A clastic medium grained sedimentary rock usually consisting of abundant quartz intermingled with a range of other constituents, such as feldspars, micas, fossil remains and cementing agents.

Sanukitoid:  Sanukitoids are a variety of high-Mg granitoid found in convergent margin settings.  The term “sanukitoid” was originally used to define a variety of Archean plutonic rock, but now also includes younger rocks with similar geochemical characteristics.  They are called “sanukitoid” because of their similarity in bulk chemical composition to high-magnesium andesite from the Setouchi Peninsula of Japan, known as “sanukites” or “setouchites”.  Sanukite rocks are an andesite characterized by orthopyroxene as the mafic mineral, andesine as the plagioclase, and a glassy groundmass.

Schist: A metamophic rock, the foliated structure of which is caused by the prevalence of flaky or elongated minerals.

Sea-floor spreading: The symmetrical displacement of adjacent plates by the addition of crustal material in the zone (or ridge) between them.  Successive additions cause the older material to move sideways in a direction at right angles to the zone at a rate of 1 - 10 cm per year.

Sedimentary: Rocks that formed by deposition and consolidation of sediment.

Selenite: A variety of gypsum with thin, colourless and transparent flakes.

Serpentine: A hydrous magnesium silicate group of minerals and dominant in the rock serpentinite.

Serpentinisation: Serpentinisation is the product of hydration of the harzburgite (rock type), which produces a group of minerals of the serpentinite that have the same chemical composition with the parent rock (harzburgite), but occupy a larger volume and have a much lower specific gravity.  Serpentinisation occurs from hydrothermal fluids that circulate in the parent rock along fractures.

Serpentinite: A rock formed by the hydrothermal alteration (serpentinization) of ultrabasic rocks, such as peridotite, pyroxenite, harzburgite and dunite.

Shard: An angular fragment of volcanic glass, commonly found in glassy tuffs and ashes (hyaloclastites).

Sheet flow: The movement of a dense sediment-water mix down bottom slopes.

Slump deposits: A bed of sediment internally deformed by sliding.

Smectite: A group of clay minerals with swelling properties, more especially the variety montmorillonite.

Stockwork: A mass of rock, often roughly cylindrical, which has been highly fractured and along which mineralisation has occurred.

Strike: The direction of a horizontal line drawn at right angles to the dip of the beds.

Strike-slip fault: A fault in which the rocks on each side move virtually horizontally past each other.

Stromatolite: A laminated structure formed by calcareous algae living in shallow waters.

Subduction: A process where ocean floor is destroyed by one lithospheric plate overriding another.  The overriden plate is pushed down into the mantle and subsequently melted.

  ---------- T ----------

Taphonomy: Taphonomy is the study of decaying organisms over time and how they may become fossilized.  The term taphonomy (from the Greek taphos, τάφος meaning burial, and nomos, νόμος meaning law) was introduced to paleontology in 1940 by Russian scientist Ivan Efremov to describe the study of the transition of remains, parts, or products of organisms, from the biosphere, to the lithosphere, i.e. the creation of fossil assemblages.

Tectonic zone: Complex structures (for example, bedding, faulting, folding) and features of the displayed geological units in a region which have resulted from forces or movements coming from different directions.

Terra rossa: A red ferruginous clay found in soil profiles in southern Europe and formed by the in-place weathering of limestones.  When limestone weathers, the clay contained in the rocks is left behind, along with any other non-soluble rock material, such as iron oxide.  Compared to most clay soils, terra rossa has surprisingly good drainage characteristics. This makes it a popular soil type for wine production, eg Coonawarra in Australia.

Terra verde: A synonym for ‘green earth’. In Cyprus it is the mineral celadonite, which is a soft green or grey-green earthy mineral, consisting of a hydrous silicate of iron, magnesium, and potassium, and generally occurring in cavities in basaltic rocks. Generally, any of various naturally occurring silicates used mainly as bases for green basic dyes and pigments.

Tethys: An ocean that occupied the general position of the Alpine-Himalayan orogenic belt; the Mediterranean Sea is a remnant of Tethys.

Tholeiite: Igneous rocks that are similar in composition to basalt, but are richer in silica and iron and poorer in aluminum than basalt is. Tholeiites form especially at mid-ocean ridges and in continental rift areas.

Thrust fault: A low angle plane of dislocation caused by the near-horizontal displacement of rock masses.

Tonalite: An igneous, plutonic (intrusive) rock, of felsic composition, with phaneritic texture. Feldspar is present as plagioclase (typically oligoclase or andesine) with 10% or less alkali feldspar. Quartz is present as more than 20% of the rock. Amphiboles and pyroxenes are common accessory minerals.

Trace fossils: Structures preserved in sediments caused by the activities of burrowing, grazing and boring organisms.

Trachyte: A fine to medium grained rock, often porphyritic, and composed mainly of alkali feldspars, biotite and augite, which are commonly flow-aligned by motion of the magma while cooling.

Transform fault: A major dislocation within the Earth’s oceanic crust with a strike-slip displacement at right-angles to the axes of spreading plates.

Tuff: An indurated pyroclastic rock formed of volcanic ash fragments.

Turbidity current: A cloud of highly mobile suspended sediment which commonly moves with considerable velocity down bottom slopes.  The sedimentary product is known as a turbidite.

  ---------- U ----------

Ultrabasic: A term describing igneous rocks with a low silica content, such as dunite, and composed of an abundance of ferro-magnesian silicates.

Ultramafic: (or Ultra-basic) rocks are igneous and meta-igneous rocks with very low silica content (less than 45%), generally >18% MgO, high FeO, low potassium, and are composed of usually greater than 90% mafic minerals (dark colored, high magnesium and iron content). The Earth’s mantle is composed of ultramafic rocks.

Umber: A naturally occurring brown earth that consists of manganese oxides as well as hydrated ferric oxide, silica, alumina and lime. Associated with basic lava flows.

Unconformity: A structural feature separating two sets of strata and representing, usually, a considerable time break.  Sometimes there is an angular discordance between the sets.

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Vesicle: A small oval or irregular cavity in an igneous rock produced by escaping gas.

Volcaniclastic rocks: Sediments composed of mechanically transported material of volcanic origin.

Volcaniclastic sandstone: A sediment of sand grade formed predominantly of angular material originally ejected from volcanic vents.  Usually contains traces of volcanic glass and pumice.

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Wehrlite: An ultrabasic igneous rock composed of olivines, pyroxenes, hornblende and iron-ores.

Whin:  The Great Whin Sill is a tabular layer of igneous dolerite in County Durham and Northumberland.  Quarrymen of Northern England used the term ‘sill’ to describe a more or less horizontal body of rock.  ‘Whin’ was applied to dark, hard rocks.  As the intrusive igneous origin of the Whin Sill was determined in the 19th century, the term ‘sill’ was adopted by geologists for concordant, horizontal intrusive bodies.

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Zeolite facies: Rocks showing low grade metamorphism and characterised by the presence of zeolites.

Zeolites: Hydrated silicates of calcium and aluminium, sometimes with sodium and aluminium.  They include analcite (a cubic form) and heulandite.

Igneous rocks by composition