Glossary

This glossary aims to provide simple explanations of the geological terms used in Chapter 1 and in the 'Introduction' and 'Conclusions' sections of site descriptions. It also includes many of the more important terms encountered in other sections of the volume. The explanations are not intended to be comprehensive definitions, but concentrate instead on the way in which the terms are used in this volume. Bold typeface indicates a further glossary entry.

Chronostratigraphical names not listed in the glossary are given in (Figure 1.2) (Chapter 1). For the names of minerals and non-igneous rock-types, the reader is referred to standard textbooks. The names of most common crystalline igneous rocks are better explained by means of classification diagrams (Figure G1), (Figure G2), (Figure G3), (Figure G4), (Figure G5), (Figure G6), all simplified after Le Maitre (2002) to include only rock names encountered in this volume). Names of igneous or igneous-related rocks that do not fit easily into these classification diagrams are included in the glossary, as are the names of most fragmental volcanic rocks, which require extended explanations commonly involving their mode of formation.

The classification and nomenclature of crystalline igneous rocks used in this volume follow the recommendations of the International Union of Geological Sciences (IUGS) Subcommission on the Systematics of Igneous Rocks (Le Maitre, 2002). Slight modifications follow the classification scheme of the British Geological Survey (BGS) (Gillespie and Styles, 1999), in which an attempt is made to distinguish 'root names' (i.e. largely those which figure on the main classification diagrams) from variants, mostly indicated by mineral qualifiers as prefixes to the root names. This is achieved through a strict use of hyphens:

• Compound root names, usually involving an essential mineral, are hyphenated (e.g. quartz-syenite, olivine-gabbro).
• Mineral qualifiers are hyphenated together (e.g. biotite-hypersthene andesite).
• Mineral qualifiers are not hyphenated to the root name, whether compound or not (e.g. biotite-hornblende trachyte, biotite quartz-trachyte, fayalite-augite nepheline-syenite).

Fragmental volcanic rocks are also classified and named according to the IUGS scheme, with minor modifications from the BGS scheme. Two points should be -noted in particular: the term 'volcaniclastic' is applied to all fragmental rocks that occur in a volcanic setting, including both rocks that have been fragmented by volcanic processes (i.e. pyroclastic rocks) and sedimentary rocks that comprise reworked fragments of volcanic rocks. The terms 'volcanogenic' and 'epiclastic', which are commonly used elsewhere in an inconsistent and confusing manner, are not used in the BGS scheme or in this volume.

The Carboniferous and Permian basic lavas of Scotland are almost invariably porphyritic to varying degrees. This feature was utilized by MacGregor (1928) to devise a nomenclature based in part on phenocryst size (microporphyritic or macroporphyritic) and in part on the phenocryst assemblages (ol + cpx, ol + cpx + pl, pl ± ol). They were assigned type locality names (e.g. 'Craiglockhart type for macroporphyritic ol + cpx-phyric basalts, and 'Hillhouse' type for micro-porphyritic ol + cpx-phyric basalts). This scheme was widely used on Scottish maps for many years as a convenient 'shorthand' way of representing the wide range of petrographical types that are distinctive in the field and hence form mappable units. It also enabled lavas such as the feldspar-phyric 'Markle' and 'Jedburgh' types, which commonly range in composition from basalt to hawaiite, to be assigned a name without the need for an analysis. The scheme has now fallen into disuse in favour of less parochial descriptions, but the 'MacGregor' names are given in some places in this volume in addition, especially where they enable comparison with existing literature (Table G1). Other local names for distinctive rock-types and obsolete names are explained where they occur in the main text.

Aa: lava with a rough, clinkery surface, broken into angular blocks.

Acid: descriptive of light-coloured igneous rocks relatively enriched in silica.

Aeolian: descriptive of sediments or landforms formed under the action of the wind.

Age: a geological time unit (cf. chronostratigraphy), usually taken to be the smallest standard division of geological time, of shorter duration than an epoch.

Agglomerate: a pyroclastic rock with predominantly rounded clasts greater than 64 mm in diameter.

Alkali basalt: a type of basalt, parental to a suite of silica-saturated to -undersaturated igneous rocks characterized by high alkalis relative to silica, formed dominantly in extensional within-plate settings (see (Figure G5) and (Figure G6).

Alkaline: descriptive of igneous rocks that contain more sodium and/or potassium than is required to form feldspar and hence contain, or have the potential to contain (i.e. in the norm), other alkali-bearing minerals such as feldspathoids, alkali pyroxenes and alkali amphiboles.

Amygdale: a gas bubble cavity in an igneous rock that has been infilled later with minerals.

Andesite: a fine-grained, mafic, intermediate igneous rock (see (Figure G1) and (Figure G5).

Aphyric (or non-porphyritic): a textural term, applied to igneous rocks that lack relatively large, conspicuous crystals (phenocrysts) compared with the grain size of the ground-mass.

Aplitic: descriptive of relatively finer-grained areas, typically veins, within an igneous rock (contrast with pegmatitic).

Assimilation: the addition of solid material such as country rock to a magma, changing its composition.

Asthenosphere: a weak layer within the Earth's mantle and immediately below the lithosphere.

Back-arc basin: the region adjacent to a subduction-related volcanic arc, on the opposite side of the arc from the trench and subducting plate. Stresses in the back-arc region are typically extensional.

Basalt: a fine-grained, mafic, basic igneous rock (see (Figure G1), (Figure G5) and (Figure G6)).

Basanite: a fine-grained, mafic, basic or ultrabasic igneous rock with the mineralogy of a basalt but with the addition of a feldspathoid mineral (see (Figure G1), (Figure G5) and (Figure G6)).

Basement: the oldest rocks recognized in a given area; an assemblage of metamorphic and/or igneous rocks that underlies all the sedimentary formations.

Basic: descriptive of an igneous rock relatively rich in the 'bases' of early chemistry (MgO, FeO, CaO, Fe₂O₃); silica (SiO₂) is relatively low (nominally 45–52%).

Basin (i.e. sedimentary basin): a region of prolonged subsidence of the Earth's surface, typically formed either by stretching of the lithosphere under extensional forces, or by flexural subsidence.

Bed: in lithostratigraphical terms, a subdivision of either a member or a formation; the smallest unit within the scheme of formal lithostratigraphical classification. Also used informally to indicate a stratum within a sedimentary rock succession.

Bedding: a feature of sedimentary rocks, in which planar or near-planar surfaces known as 'bedding planes' indicate successive depositional surfaces formed as the sediments were laid down.

Benmoreite: a fine-grained, mafic, intermediate igneous rock, belonging to the alkali basalt suite (see ((Figure G1), (Figure G5) and (Figure G6)).

Bentonite: a light coloured rock, mainly composed of clay minerals and colloidal silica, produced by devitrification and chemical alteration of glassy fine ash (see also tonstein).

Biostratigraphy: the stratigraphical subdivision and correlation of sedimentary rocks based on their fossil content.

Blastomylonite: an extremely sheared (mylonitic) rock in which some recrystalliza-tion and growth of new minerals has taken place during deformation.

Block: a pyroclastic rock fragment, more than 64 mm in diameter, with an angular to sub-angular shape, which indicates that it was formed by the breaking of solid rock.

Bole: a fine, earthy, compact red-brown clay formed by tropical weathering and leaching of a lava surface (see laterite).

Bomb: a pyroclastic rock fragment, more than 64 mm in diameter, which has a 'streamlined' shape or surface indicating that it was erupted as a mass of molten or partially molten lava that then cooled during flight.

Breccia: a rock composed of angular broken fragments greater than 2 mm in diameter; can be pyroclastic, sedimentary or fault-related.

Brockram: a term used in Cumbria for a sedimentary breccia of Permian age; commonly red or purple.

Caldera: a circular, basin-shaped depression, usually many times greater than the size of any individual volcanic vent, caused by collapse of the roof of an underlying magma chamber following an eruption; also refers to the underlying volcanic structure.

Caledonian Orogeny: a major period of orogenesis that took place during the Palaeozoic Era, associated with the closure of the ancient Iapetus Ocean that was situated between Scotland and the rest of present-day Britain.

Camptonite: an alkaline variety of lamprophyre, in which the phenocrysts are various combinations of olivine and titanium-bearing amphibole, augite and biotite. The ground-mass consists of the same minerals (except olivine) together with plagioclase, and possibly subordinate alkali feldspar and/or feldspathoids.

Carbonatite: a magmatic carbonate rock.

Carboniferous Period: a geological time division (period; cf chronostratigraphy), ranging from 354 to 290 million years ago. It precedes the Permian Period.

Chilled margin: that part of an igneous rock adjacent to a contact with an older rock, where the magma has been cooled rapidly (chilled), forming a zone of fine-grained rock.

Chronostratigraphy: the correlation and subdivision of rock units on the basis of relative age — a hierarchy of sequential units to which the layers of sedimentary rocks are allocated, through the study and interpretation of their stratigraphy. The hierarchy of principal chronostratigraphical units is erathem, system, series and stage, which are related, respectively, to the geological time units of era, period, epoch and age.

Cinder cone: a small volcanic cone built almost entirely of loose volcanic fragments, ash and pumice (cinder/scoria).

Clast: a fragment in a pyroclastic or sedimentary rock.

Cleavage: a plane of incipient parting in a rock, produced by the alignment of platy crystals, such as mica, in response to confining pressure during deformation.

Columnar jointing: the division of an igneous rock body into columns by cracks (joints) produced through thermal contraction on cooling. The columns form perpendicular to the cooling surface.

Comagmatic: a term applied to igneous rocks that are considered to have been derived from the same parent magma, or at least from the same source region, at the same time and under identical physical and chemical conditions.

Complex: used herein to refer to a large-scale, spatially related assemblage of igneous rock units possibly, but not necessarily, with complicated igneous and/or tectonic relationships and of various ages and diverse origins.

Composite: used to refer to an igneous intrusion or lava flow that has formed from two or more different pulses of magma, each pulse differing slightly- from the others in mineralogy, texture and/or chemistry.

Concretion: a hard, compact mass, usually rounded, in a sedimentary rock, formed by precipitation of a cementing mineral around a nucleus during or after deposition.

Conglomerate: a sedimentary rock, a significant proportion of which is composed of rounded pebbles and boulders, greater than 2 mm in diameter.

Country rock: rock that has been intruded by an igneous rock.

Crust: the outermost layer or shell of the Earth, above the Moho and mantle. It consists of two parts: a basic layer, which forms the oceanic crust and underlies the continents at depth; and a layer of dominantly acid rocks, which forms the thickest, upper part of the continental crust.

Cryptocrystalline: very finely crystalline, such that individual crystals can only be distinguished under very powerful magnification (e.g. electron microscope).

Crystal fractionation: see fractional crystallization.

Cumulate: an igneous rock formed by crystals that precipitated early from a magma and accumulated due to gravitational settling, current activity or other magmatic processes, without modification by later crystallization.

Depleted mantle: mantle that has been depleted in incompatible elements, through partial melting.

Deuteric: descriptive of the reactions between primary minerals and the water-rich fluids that separate from the same body of magma at a late stage in its cooling history.

Devitrification: the conversion of glass, e.g. in the interstices of a volcanic or hypabyssal rock, to cryptocrystalline or crystalline material.

Diagenesis: the process of mineral growth and/or recrystallization leading to lithification of unconsolidated sediment to form rock.

Diapir: a dome-shaped body of magma or mobile rock that has risen through country rocks due to its lower density and/or greater plasticity.

Diatreme: a breccia-filled volcanic pipe formed by a gaseous explosion.

Dinantian sub-System: a chronostratigraphical division; equivalent to the Lower Carboniferous in Europe, dated at 354–327 Ma. It precedes the Namurian Series, and comprises the Tournaisian and Visean series.

Diorite: a coarse-grained, mafic, intermediate igneous rock (see Figure G2).

Distal: far from the source.

Dolerite: used herein as a synonym of micro-gabbro (see Figure G2).

Dyke: a tabular body of igneous rock, originally intruded as a vertical or steeply inclined sheet. Dyke-swarm: a collection of dykes.

Effusive: descriptive of an eruption as lava rather than as pyroclasts.

Enclave: an inclusion (xenolith) within an igneous rock, usually of some other igneous rock that may or may not be related.

En échelon: descriptive of a series of linear features, such as dykes or faults, which follow roughly the same trend but are 'stepped'.

Enriched mantle: mantle that has been enriched in incompatible elements, through the introduction of partial melts and metasomatism.

Epoch: a geological time unit (cf. chronostratigraphy), of shorter duration than a period and itself divisible into ages.

Equigranular: a texture in which all the crystals are approximately the same size.

Era: a major geological time unit (cf. chronostratigraphy), which is divisible into periods.

Euhedral: descriptive of a mineral grain, such as a phenocryst, with well-formed crystal faces.

Exsolution: the process whereby an initially homogeneous mineral separates into two distinct and commonly intergrown crystalline phases on cooling without a change in the bulk composition.

Extrusive: descriptive of igneous rocks that have been extruded onto the Earth's surface, rather than being intruded beneath the surface (intrusive).

Facies: the characteristic features of a rock unit, including rock-type, mineralogy, texture and structure, which together reflect a particular sedimentary, igneous or metamorphic environment and/or process.

Fault: a fracture in the Earth's crust across which the rocks have been displaced relative to each other.

Felsic: descriptive of light-coloured minerals (feldspar/feldspathoid and silica); or of an igneous rock containing substantial proportions of these minerals; the opposite of mafic

Felsite: a field term for glassy and fine-grained felsic igneous rocks.

Fissure eruption: a volcanic eruption where lava wells up through fissures in the Earth's crust. Often involves very fluid basic lavas, and can spread over very large areas.

Flexural subsidence: the downward bending of the lithosphere due to loading, for example by the weight of sediment (as in a foreland basin) or by tectonic overthrusting.

Flood basalt: a widespread sheet or layer of basaltic lava erupted from a fissure-type eruption.

Fluidization: mobilization resulting from passage of a fluid (usually a gas) through a granular solid.

Fluvial: referring to a river environment.

Foidite: a general term for fine-grained igneous rocks in which the felsic minerals include more than 60% feldspathoids.

Foidolite: a general term for coarse-grained igneous rocks in which the felsic minerals include more than 60% feldspathoids.

Foliation: the planar arrangement of components within a rock.

Foreland basin: a sedimentary basin developed by depression of a convergent continental margin due to the weight of sediment accumulating in front of the orogenic belt.

Formation: a lithostratigraphical unit, hierarchically higher than 'member' and lower than 'group'. A named 'Formation' represents an assemblage of strata that have a common characteristic useful for mapping.

Fractional crystallization: the process in which the early formed crystals in a magma are removed or otherwise prevented from equilibrating with the residual liquid, which consequently becomes progressively more evolved in composition (i.e. more fractionated).

Gabbro: a coarse-grained, mafic, basic igneous rock (see (Figure G2) and (Figure G3)).

GCR: Geological Conservation Review, in which nationally important geological and geomorphological sites were assessed and selected with a view to their long-term conservation as SSSIs.

Glomeroporphyritic: a porphyritic rock containing clusters of phenocrysts.

Gneiss: a coarse-grained, inhomogenous rock, common in relatively high-grade metamorphic terranes, characterized by banding or layering.

Gneissose: a metamorphic texture, which has a 'stripy' appearance due to the segregation of the component minerals into compositionally distinct lenses.

Graben: an elongate, down-faulted crustal block, commonly with a marked topographic expression.

Granite: a coarse-grained, felsic, acid igneous rock (see Figure G2).

Granoblastic: a metamorphic texture in which recrystallization has formed essentially equi-dimensional crystals.

Granulite tacks: the temperature and pressure conditions typical of high-temperature and moderate- to high-pressure regional metamorphism.

Group: a lithostratigraphical unit consisting of one or more formations, important for local and regional lithostratigraphical correlation.

Hanging wall: the upper side of an inclined fault or other dislocation.

Harzburgite: a peridotite consisting mainly of olivine and orthopyroxene (see (Figure G4)).

Hawaiian eruption: a type of eruption characterized by the flow of basaltic lava from an eruptive centre or centres without appreciable explosive activity.

Hawaiite: a fine-grained, mafic, intermediate igneous rock, belonging to the alkali basalt suite (see (Figure G1), (Figure G5) and (Figure G6)).

Hornfels: a well-baked, hard, splintery rock resulting from thermal (contact) metamorphism.

Hyaloclastite: a pyroclastic rock composed of angular fragments of glass, formed when magma is rapidly quenched and shattered on entering water.

Hybridization: the intermixing of two or more magmas, which crystallize as a single rock, commonly having a heterogeneous texture and complex mineralogy.

Hydroclastic: descriptive of fragmentation of magma or hot rock by its interaction with water (see also hydrovolcanic and phreatomagmatic).

Hydromagmatic: descriptive of processes driven by the interaction of magma with water.

Hydrothermal alteration: changes in mineralogy and chemistry in rocks resulting from the reaction of hot water with pre-existing minerals (cf. metasomatism).

Hydrovolcanic: descriptive of volcanic processes driven by the interaction of magma with water.

Hypabyssal: descriptive of an igneous intrusion, or its rock, emplaced at a depth intermediate between plutonic and volcanic.

Incompatible elements: trace elements that are not readily accepted into the crystal structure of common rock-forming minerals during the crystallization of magma and hence are concentrated preferentially into the remaining liquid. They are also concentrated in the first liquids produced during partial melting.

Intermediate: descriptive of igneous rock that is transitional in chemical composition between acid and basic (see Figure G5).

Intrusive: descriptive of igneous rocks that have been intruded into older rocks beneath the Earth's surface, rather than being extruded onto the surface (extrusive).

Joint: a fracture in a rock across which there has been no noticeable displacement. Common types of joints in igneous rocks are cooling joints, formed through thermal contraction as the magma cools.

Juvenile: descriptive of volcanic fragments that have been derived directly from magma.

Laccolith: an igneous intrusion, roughly circular in plan and concordant with the structure of the country rock; it generally has a flat floor, a shallow domed roof and a dyke-like feeder beneath its thickest point.

Lamprophyre: the name used for a distinctive group of largely hypabyssal rocks characterized by abundant phenocrysts of mafic minerals, with felsic minerals confined to the groundmass.

Lapilli-tuff: a pyroclastic rock in which 25–75% of the clasts are between 2 and 64 mm in diameter (lapilli), and are set in a finer-grained matrix.

Lava: molten rock at the Earth's surface (contrast with magma).

Lava tube: a hollow space beneath the solidified surface of a lava, formed by the draining out of molten lava after the crust had formed.

Laterite: a red subsoil, rich in hydrous oxides of iron and/or aluminium and commonly with kaolinite and silica that develops as a residual product of weathering in tropical and subtropical climates.

Leucocratic: descriptive of light-coloured igneous rocks containing few mafic minerals.

Lherzolite: a peridotite consisting mainly of olivine, clinopyroxene and orthopyroxene (see Figure G4).

Lithosphere: the outer layer of the solid Earth, including the crust and upper part of the mantle, which forms tectonic plates above the asthenosphere.

Lithostratigraphy: the stratigraphical subdivision and correlation of rocks based on their lithology. Units are named according to their perceived rank in a formal hierarchy, namely supergroup, group, formation, member and bed.

Maar: a broad, low-rimmed volcanic crater formed by collapse within a shallow cone produced by phreatic or Phreatomagmatic eruptions. Generally composed of less juvenile material than a tuff-ring. Commonly contains a lake, also termed a maar.

Mafic: descriptive of dark-coloured minerals, rich in magnesium and/or iron (Fe), or an igneous rock containing substantial proportions of these minerals, mainly amphibole, pyroxene or olivine; the opposite of felsic.

Magma: molten rock beneath the Earth's surface.

Magma chamber: a large body of magma that has accumulated within the Earth's crust or upper mantle.

Mantle: part of the interior of the Earth, beneath the crust and above the core.

Mass-flow: the transport, down slope under the force of gravity, of large, coherent masses of sediment, tephra or rock; commonly assisted by the incorporation of water, ice or air.

Megacryst: any crystal in a crystalline rock that is very much larger than the surrounding groundmass.

Mélange: a chaotic rock unit, characterized by a lack of internal continuity of contacts between component blocks and including fragments of a wide range of composition and size.

Melanocratic: descriptive of dark-coloured igneous rocks rich in mafic minerals.

Member: a lithostratigraphical unit, hierarchically higher than 'bed' and lower than 'formation'.

Mesocratic: descriptive of igneous rocks intermediate between leucocratic and melanocratic in colour.

Mesostasis: the groundmass in an igneous rock.

Meta: prefix added to any rock name to indicate a metamorphosed variety, e.g. metabasalt is a metamorphosed basalt.

Metaluminous: degree of alumina-saturation in igneous rocks in which the molecular proportion of A1₂0₃ is greater than that of Na₂O + K₂O, but less than that of Na₂O + K₂O + CaO.

Metasomatism: a process involving fluids that introduce or remove chemical constituents from rock thus changing its chemical and mineralogical composition without melting.

Mid-ocean ridge: a continuous median mountain range within the oceans along which new oceanic crust is generated by volcanic activity.

Mid-ocean ridge basalt (MORB): a type of tholeiitic basalt, generated at mid-ocean ridges. A worldwide, voluminous basalt type widely used as a fundamental standard for comparative geochemistry.

Miospore: a fossil spore or pollen grain that is less than 200 microns in diameter.

Moho (=Mohorovicic Discontinuity): the boundary surface within the Earth below which there is an abrupt increase in seismic velocity; marks the base of the crust above the underlying mantle. Geophysical and petrological criteria define slightly different positions for the boundary:

Monchiquite: an alkaline variety of lamprophyre, similar to camptonite except that the groundmass is feldspar-free, being composed almost entirely of glass and feldspathoids.

Mugearite: a fine-grained, mafic, intermediate igneous rock, belonging to the alkali basalt suite (see (Figure G1), (Figure G5) and (Figure G6).

Namurian Series: a chronostratigraphical division; the lowermost series of the Silesian (Upper Carboniferous) sub-System in Europe, dated at 327–315 Ma. It follows the Visean Series and precedes the Westphalian Series.

Neck: the feeder 'pipe' of an ancient volcano, which has been infilled with collapsed material from the surface vent and commonly intruded by further magma to form a plug, after the cessation of eruption. Exposed due to subsequent erosion.

Norm: a recalculation of the chemical composition of an igneous rock to obtain a theoretical mineralogical ('normative') composition; useful for classification purposes and for comparison with experimental studies of magma crystallization.

Ocean island basalt (OIB): a compositionally diverse type of basalt, ranging from tholeiitic to alkali basalt, characteristic of within-plate oceanic settings.

Orogenesis: crustal thickening following the collision of tectonic plates and resulting from magmatism, folding, thrusting and accretion, leading to regional uplift and mountain building.

Pahoehoe: basalt lava with a smooth, ropy surface.

Palaeosol: an ancient or 'fossilized' soil.

Palaeozoic Era: a geological time division (era; cf. chronostratigraphy ),ranging from 545 to 248 million years ago. The Carboniferous and Permian periods occur at the end of this era.

Partial melting: the incomplete melting of a rock to produce a magma that differs in composition from the parent rock.

Pegmatitic: textural description of an area within an igneous rock that is notably more coarsely crystalline and commonly forming veins and dykes (contrast with aplitic).

Pelean: a volcanic eruption characterized by gaseous ash clouds associated with the growth and collapse of volcanic domes.

Peperite: a breccia characterized by isolated blocks and lobes of igneous rock, commonly chilled and mixed with fluidized host sediment; typically present at the margins of high-level sills intruded into water-bearing sediment.

Peralkaline: the degree of alumina-saturation in igneous rocks in which the molecular proportion of M₂0₃ is less than that of Na₂O + K₂O.

Peraluminous: the degree of alumina-saturation in igneous rocks in which the molecular proportion of A1₂0₃ is greater than that of Na₂O + K₂O.

Peridotite: a coarse-grained, ultramafic, ultra-basic igneous rock consisting predominantly of olivine with varying amounts of orthopyroxene and clinopyroxene (see (Figure G3) and (Figure G4)).

Period: a geological time unit (cf. chronostratigraphy), of shorter duration than an era and itself divisible into epochs.

Permian Period: a geological time division (period; cf. chronostratigraphy), ranging from about 290 until 248 million years ago. It follows the Carboniferous Period and precedes the Triassic Period.

Petrogenesis: the origin and evolution of rocks. Petrography: the study of the mineralogy, texture and systematic classification of rocks, especially under the microscope.

Petrology: the study of the origin, occurrence, structure and history of rocks; includes petrography and petrogenesis.

Phenocryst: a crystal in an igneous rock that is larger than those of the groundmass, usually having crystallized at an earlier stage.

Phonolite: a fine-grained, felsic, silica-under-saturated igneous rock containing significant amounts of feldspathoid minerals (see (Figure G1) and (Figure G5)).

Phreatic: descriptive of a volcanic eruption or explosion of steam, not involving juvenile material, that is caused by the expansion of groundwater due to an underlying igneous heat source.

Phreatomagmatic: descriptive of explosive volcanic activity caused by the contact of magma with large volumes of water, producing intensely fine ash and abundant steam.

Phreatoplinian: a rare type of explosive volcanic eruption and its deposits produced by phreatomagmatic processes (contrast with plinian)

-phyric: as in 'plagioclase-phyric', a porphyritic rock containing phenocrysts of plagioclase.

Picrite: a term originally used to describe a variety of dolerite or basalt extremely rich in olivine and pyroxene. Now defined chemically as a group name for rocks with SiO₂ < 47%, total alkalis < 2% and MgO > 18%.

Pillow lava: subaqueously erupted lava, usually basaltic in composition, comprising an accumulation of smooth pillow shapes and lava tubes produced by rapid chilling.

Playa: a flat plain on the coast or at the centre of an inland drainage basin found in arid areas.

Plinian: a type of explosive volcanic eruption and its deposits; magma is fragmented through the release of magmatic gas and released at high velocity to form an eruption column that extends high into the Earth's atmosphere.

Plug: the solidified remains of a cylindrical intrusion of magma, commonly intrusive into, or associated with, a volcanic neck.

Pluton: an intrusion of igneous rock, emplaced at depth in the Earth's crust.

Plutonic: descriptive of igneous rocks formed at depth in the Earth's crust.

Poikilitic: a textural term for an igneous rock in which small crystals of one mineral are enclosed within a larger crystal of another mineral.

Porphyritic: a textural term for an igneous rock in which larger crystals (phenocrysts) are set in a finer-grained or glassy groundmass.

Porphyroblast: a large, well-formed crystal that grew in situ during metamorphic recrystal lization and typically encloses finer-grained crystals that formed earlier.

Porphyry: a field term for an igneous rock that contains phenocrysts within a fine-grained groundmass of indeterminate composition; usually preceded by a mineral qualifier indicating the type of phenocryst present, e.g. feldspar porphyry.

Protolith: the source rock from which an igneous rock was formed, most commonly by melting.

Proximal: near to the source.

Pseudomorph: a replacement product, composed either of a single mineral or an assemblage of minerals, that retains the distinctive overall shape of the parent crystal.

Pumice: light-coloured pyroclast of generally acid, highly vesicular, glass foam.

Pyroclast: a fragment (clast) ejected from a volcano; the terms ash, lapilli, and block or bomb are used to describe pyroclasts that are respectively less than 2 mm, 2–64 mm and more than 64 mm in diameter.

Pyroclastic: descriptive of unconsolidated deposits (tephra) and rocks that form directly by explosive ejection from a volcano.

Pyroclastic breccia: a rock comprising predominantly angular pyroclasts with an average size greater than 64 mm in diameter.

Pyroclastic fall deposit: tephra deposited by fall-out from a volcanic eruption cloud.

Pyroclastic flow: a volcanic avalanche; a hot density current comprising pyroclasts and gases, erupted as a consequence of the explosive disintegration of magma and/or hot rock; also describes the deposit from this eruption.

Pyroclastic surge: similar to a pyroclastic flow but turbulent and less dense.

Pyroxenite: a coarse-grained, ultramafic igneous rock consisting predominantly of orthopyroxene and clinopyroxene, with lesser amounts of olivine in varying proportions (see (Figure G3) and (Figure G4)).

Radiometric age: the age in years calculated from the decay of radioactive elements.

Red beds: a collective term applied to continental sedimentary successions that are predominantly red in colour owing to the presence of iron oxides and hydroxides formed in a highly oxidizing environment.

Restite: the material remaining after partial melting.

Rhyolite: a fine-grained, felsic, acid igneous rock (see (Figure G1), (Figure G5) and (Figure G6)).

RIGS: Regionally Important Geological/geomorphological Sites.

Rifting: the thinning, and rupture along faults, of the lithosphere under extensional stress, commonly accompanied by upwelling of hot material from the asthenosphere below.

Scoriaceous: descriptive of lavas that are very highly vesiculated, giving them a 'clinkery' appearance.

Seismic profile: a cross-section of the lithosphere, constructed by creating artificial earthquakes using explosives or other vibrating devices, and then recording the vibrations that pass through the Earth to an array of instruments at a range of distances along a predetermined line.

Series: a chronostratigraphical unit; it comprises all the rocks formed during an epoch and can be divided into stages.

Serpentinization: hydrothermal alteration of ultramafic rocks in which the mafic minerals are replaced by a range of hydrous secondary minerals, collectively known as 'serpentine'.

Shearing: the deformation of a rock body by the sliding of one part relative to another part, in a direction parallel to their plane of contact.

Sheet flood: a broad expanse of rapidly moving water and debris, not confined to a channel and usually of short duration due to rapid run-off in an arid area.

Silesian sub-System: a chronostratigraphical division; equivalent to the Upper Carboniferous in Europe, dated at 327–290 Ma. It is preceded by the Dinantian sub-System, and comprises the Namurian, Westphalian and Stephanian series.

Silica-saturation: a measure of the amount of silica available to form the major mineral components of an igneous rock, usually calculated from the norm. Silica-oversaturated rocks may contain free silica as quartz; silica-under-saturated rocks may contain feldspathoids in addition to feldspars.

Silicic: alternative term to acid.

Sill: a tabular body of igneous rock, originally intruded as a sub-horizontal sheet and generally concordant with the bedding or foliation in the country rocks.

Slickensides: linear grooves and ridges formed on a fault surface as rocks move against each other along the fault plane.

Spheroidal weathering: a type of weathering commonly found in mafic and ultramafic rocks; also known as 'onion-skin' weathering, because it leads to the formation of a flaky, weathered zone around a central, unweathered, spherical core.

Spherulite: a spherical mass of acicular crystals, commonly feldspar, radiating from a central point; commonly found in glassy silicic volcanic rocks as a result of devitrification.

Stage: a chronostratigraphical unit; it comprises all the rocks formed during an age, and is usually taken to be the smallest standard unit.

Stephanian Series: a chronostratigraphical division; the uppermost series of the Silesian (Upper Carboniferous) sub-System in Europe, dated at 303–290 Ma. It follows the Westphalian Series and precedes the Permian Period.

Stock: a small, discordant igneous intrusion, usually sub-cylindrical and with steep margins.

Stoping: the emplacement of magma by detaching pieces of country rock which either sink through or are assimilated by the magma.

Stratovolcano: a cone-shaped volcano with a layered internal structure.

Strike-slip: see transcurrent.

Strombolian: type of volcanic eruption and its deposits characterized by continuous small explosive 'fountains' of fluid basaltic lava from a central crater.

Subduction: the process of one lithospheric plate descending beneath another during plate convergence.

Syenite: a coarse-grained, felsic, intermediate igneous rock (see (Figure G2)).

System: a chronostratigraphical unit; it comprises all the rocks formed during a period, and can be divided into series.

Tectonic: referring to the movements and deformation of the crust on a large scale.

Tectonic inversion: a change in the relative elevation of a block of crust (e.g. a basin becomes a basement high or vice versa), brought about by the reversal of movement direction along structures such as faults, due to a change in regional forces.

Tephra: an unconsolidated accumulation of pyroclasts.

Terrane: a fault-bound body of oceanic or continental crust having a geological history that is distinct from that of contiguous bodies.

Thermal subsidence: subsidence caused by the sinking of the lithosphere as it thickens and cools through heat conduction to the surface, after the end of a period of extension and active rifting.

Tholeiitic: descriptive of a suite of silica-oversaturated igneous rocks, characterized chemically by strong iron enrichment relative to magnesium during the early stages of evolution of the magma; formed in extensional within-plate settings, at constructive plate margins and in island arcs.

Tonstein: a term used, especially in central Europe, for thin beds of kaolin-rich clay in coal-bearing strata, possibly of volcaniclastic origin (see bentonite).

Tournaisian Series: a chronostratigraphical division; the lowermost series of the Dinantian (Lower Carboniferous) sub-System in Europe, dated at 354–342 Ma. It follows the Devonian Period and precedes the Visean Series.

Trachyte: a fine-grained, felsic, intermediate igneous rock (see (Figure G1), (Figure G5) and (Figure G6)).

Transcurrent (or strike-slip): a large-scale, steeply dipping fault or shear, along which the movement is predominantly horizontal. Movement may be either left-lateral (sinistral) or right-lateral (dextral).

Transgression: used herein to refer to the point where a sill 'steps up' or 'steps down', in changing from one stratigraphical horizon to another.

Transitional basalt: a basalt that is intermediate between an alkali basalt and a tholeiite.

Transpression: crustal shortening as a result of oblique compression across a transcurrent fault or shear zone.

Transtension: crustal extension as a result of oblique tension across a transcurrent fault or shear zone leading to localized rifts or basins.

Trap topography: descriptive of the typical terrain found on predominantly basaltic lava fields, in which layers of resistant lava form cliffs or steep slopes, separated by flat or gently sloping ledges representing more easily weathered material. The ledges form either on the tops and bottoms of flows, or on interflow sedimentary and pyroclastic rocks.

Tuff: a rock comprising pyroclasts with an average grain size less than 2 mm.

Tuff-breccia: a pyroclastic rock in which between 25 and 75% of the pyroclasts are greater than 64 mm in diameter.

Tuffisite: an intrusive tuff, which can be formed by the mechanical breakdown of rocks close to a rock fracture, due to the passage of volcanic gasses.

Tuffite: a tuffaceous sedimentary rock, i.e. one that contains a significant proportion (2575%) of pyroclastic fragments in addition to sedimentary clasts.

Tuff-ring: a shallow cone of pyroclastic deposits formed by phreatic or phreato-magmatic eruptions. Generally composed of a higher proportion of juvenile materials than a maar volcano and lacking the broad collapse crater.

Turbidite: a clastic rock formed through deposition from subaqueous sediment-laden density currents (turbidity currents) that move swiftly down slope under the influence of gravity.

Ultrabasic: descriptive of an igneous rock with a silica content less than that of basic rocks (less than 45% SiO₂).

Ultramafic: descriptive of an igneous rock in which dark-coloured minerals (amphibole, pyroxene, olivine) comprise more than 90% of the rock.

Variscan Orogeny: a period of orogenesis that occurred during the Carboniferous Period and affected rocks in south-west England, south Wales and southern Ireland.

Vein: a term frequently used for an intrusion of igneous rock, commonly irregular, which is narrower than a dyke or a sill, i.e. a few cm wide or less. This contrasts with general usage, e.g. for mineral veins that can be of any width.

Vent: that part of a volcano at which the lava and/or pyroclastic rocks are erupted onto the surface.

Vesicle: a gas bubble cavity, usually in a lava or shallow intrusion.

Visean Series: a chronostratigraphical division; the uppermost series of the Dinantian (Lower Carboniferous) sub-System in Europe, dated at 342–327 Ma. It follows the Tournaisian Series and precedes the Namurian Series.

Vitroclastic: descriptive of a pyroclastic rock characterized by fragments of glass.

Volcaniclastic: generally applied to a clastic rock containing mainly material derived from volcanic activity, but without regard for its origin or environment of deposition (includes pyroclastic rocks and sedimentary rocks containing volcanic debris).

Volcanotectonic fault: fault along which the displacement occurred through sub-surface movement of magma or during its eruption.

Vug: a cavity in a rock, with a lining of crystalline minerals.

Websterite: a pyroxenite consisting mainly of orthopyroxene and clinopyroxene with little olivine (see Figure G4).

Wehrlite: a peridotite consisting mainly of olivine and clinopyroxene (see Figure G4).

Welded tuff: a glass-rich pyroclastic rock in which the grains have been welded together because of heat and volatiles retained by the particles and the weight of the overlying material (not synonymous with ignimbrite though many ancient ignimbrites are welded).

Westphalian Series: a chronostratigraphical division; the middle series of the Silesian (Upper Carboniferous) sub-System in Europe, dated at 315–303 Ma. It follows the Namurian Series and precedes the Stephanian Series.

Xenocryst: a crystal, like a phenocryst, but which is foreign to the igneous rock in which it is found.

Xenolith: a rock fragment that is foreign to the igneous rock in which it is found.

Zeolites: a group of hydrous alumino-silicate minerals formed at relatively low temperatures, particularly during the later stages of cooling of volcanic rocks.

References