(taken from Footsteps Through Time… with contributions from Bob Mathews and Raymond Roberts of NRW)

Acadian Orogeny: the early to mid-Devonian phase of mountain building, which produced a range of mountains extending from the NE Appalachians of N. America through western Europe to Poland; traditionally ascribed to a late phase of the Caledonian Orogeny and involving closure of the Iapetus Ocean and collision between the continents of Avalonia and Laurentia, the orogeny is now believed to be related to closure of the Rheic Ocean to the south of Britain. In the UK we usually call the Devonian-Carboniferous orogeny the Hercynian.

Acid: igneous rocks that are rich in silica (SiO2 greater than about 60% by weight); acid igneous rocks include granites and rhyolites.

Accretion: a process by which material is added to a tectonic plate.

Accretionary Wedge (prism): a zone of deformed sediment made up of thrust slices scraped off a subducting oceanic plate and added beneath the overriding plate. These occur at oceanic-oceanic and oceanic-continental convergent (destructive) plate margins. (e.g. mélange, Cemaes Bay and Llanddwyn Island).

Agglomerate: an indurated rock built of large angular rock fragments embedded in an ashy matrix and resulting from explosive volcanic activity. Occurs typically in volcanic vents.

Alkaline basalts: apart from the essential minerals of pyroxene and plagioclase they usually contain nepheline and olivine.

Alpine Orogeny: a phase of mountain building caused by the collision of the African and European continental plates probably commencing in late Cretaceous times.

Alluvium: sediment transported and deposited by running fresh water, for example in rivers and streams.

Alluvial fan: a deposit of alluvium deposited at a point along a river where there is a decrease in gradient and an associated reduction in sediment transport capacity; the resultant landform is commonly fan-shaped and broadens in the downstream direction.

Amphibole: a group of usually black or green silicate minerals that includes hornblende.

Amphibolite: generally a dark, medium- or coarse-grained metamorphic rock mainly composed of amphibole and plagioclase feldspar.

Anticline: an arch-shaped fold in which the oldest rocks occur in the core of the fold (cf. syncline).

Archaean: one of the three divisions of the Precambrian ranging from about 4,000- 2,500 Ma.

Arenaceous: sediments consisting essentially of sand grains, that is, of quartz and rock fragments down to 0.005mm in size. Conglomerates, sandstones, grits and siltstones fall into this category, particle size 2mm to 16mm.

Argillaceous: sedimentary rocks of the clay grade, namely composed of minute mineral fragments and crystals less than 0.005mm in diameter, also much colloidal material. Apart from finely divided detrital matter, they consist of the so-called clay minerals, such as montmorillonite, kaolinite, gibbsite and diaspore. Siltstones, mudstones, shales, clays etc.

Armorican: a mountain-building period also known as Variscan and Hercynian. Their folds trend NE to SW whereas the Variscan trend SW to NE.

Ash: fragments less than 2 mm (0.079 inches) in diameter of pulverized rock, minerals and volcanic glass, created during volcanic eruptions.

Arthropods: for example, trilobites that are characterized by segmented bodies, jointed appendages and an exoskeleton made of chitin.

Aureole: zone surrounding igneous intrusion in which country rock shows effects of contact metamorphism.

Asthenosphere: layer of the Earth immediately below the lithosphere; this layer is hotter and weaker than the lithosphere above and is capable of plastic flow when stress is applied. The top of the asthenosphere is indicated when a temperature of 1300o C is reached

Atoll: an irregular sub-circular, coral-algal reef surrounding a central lagoon in oceanic waters, generally at the top of a seamount.

Arkose: an arenaceous sedimentary rock. Like sandstone in its general character, but containing feldspar to at least 10%. It is formed by the disintegration of the acid igneous rocks and gneisses.

Ash-flow: a volcanic avalanche, comprising hot volcanic fragments (e.g. ash, pumice, bombs) in a gas that moves at great speed in response to gravity. Also known as a pyroclastic flow, the resulting deposits can be many tens of metres thick.

Augen: eye-shaped bodies of feldspar, quartz and garnet found commonly in gneisses and schists.

Autobrecciated lava: a viscous lava flow whose largely solidified crust has been fragmented by the continued movement of molten magma in the interior of the flow.

Avalonia: named after the Avalon Peninsula, Newfoundland, a small early Palaeozoic crustal plate consisting of parts of the maritime states of North America, England, Wales, southeast Ireland and part of western Europe, which split from Gondwana early in the Ordovician, moved northwards and finally collided first with Baltica in the late Ordovician and with Laurentia during mid-Silurian times following closure of the Iapetus Ocean.

Baltica (Baltoscandia): a continent comprising much of NW Europe, which amalgamated with Avalonia and Laurentia to form the continent of Laurussia in Silurian to early Devonian times.

Baltic Shield: large shield-shaped cratons of generally Precambrian rocks, often of granite and highly metamorphosed rocks (e.g. the Baltic or Canadian shields).

Basalt: a dark, fine-grained basic igneous rock, containing calcium-rich plagioclase feldspar and pyroxene, often with olivine or quartz, nepheline and iron-titanium oxide, and formed by partial melting of the Earth’s upper mantle.  

Base level: the lower limit at which erosion can occur; sea level is the regional-scale base level for continents, but on a local scale may be the junction between a main river and its tributary streams.

Basic: describes quartz-free igneous rocks that are relatively rich in iron, magnesium and calcium, the silica (SiO2) content being relatively low (nominally 45-55% by weight); includes such rocks as gabbro and basalt.

Basin: a depression in the surface of the Earth, in which sediments accumulate.

Benioff Zone: an inclined zone of earthquake foci in the upper part of a subducting oceanic plate at a convergent (destructive) plate margin. These zones are sometimes named as Benioff-Wadati zones as Benioff and Wadati discovered them independently.

Bentonite: a light-coloured clay-rich rock formed by the breakdown and alteration of volcanic ash.

Biostratigraphy: the subdivision and correlation of rock sequences using fossils.

Bioturbation: sediments stirred by organisms.

Black smoker: a plume of hot fluid containing finely dispersed sulphide particles issuing from a vent on an oceanic ridge; sulphide minerals produced may include pyrite, marcasite, chalopyrite and sphalerite.

Blueschist: a rock containing blue glaucophane amphibole produced by metamorphism of basic rocks such as gabbro or basalt under low temperature and high pressure conditions which are typically developed within a subduction zone at a depth of around 35km.

Boudinage: original rigid sedimentary layers that are stretched during folding and break into pieces, surrounded by more plastic layers of the intervening mud-rich meta-sediments. They have the appearance of a string of sausages (boudins).

Boulder clay: often used as a synonym for till, but specifically referring to a typically unsorted and unstratified glacial deposit consisting of boulders of various sizes within a clay-dominated matrix.

Braided rivers: rivers that divide into numerous channels, separate and rejoin, occurring under conditions of huge bed load and variable discharge.

Breccia: a rock consisting of more than 30% poorly sorted, angular fragments (clasts) greater than 2mm in size (cf. conglomerate).

Calc-alkaline: term used to define the suite of igneous rocks comprising the extrusive association of basalt-andesite-dacite-rhyolite and the intrusive association gabbro-diorite-granodiorite-granite, all of which are characteristic of subduction zone environments.

Calcite sea: one in which low-magnesium calcite is the primary inorganic marine calcium carbonate precipitate.

Calcrete: a breccia or conglomerate cemented together with calcium-rich material formed in soils in semi-arid conditions.

Caldera: a circular, basin-shaped depression, usually many times greater than the size of any individual volcanic vent, caused by the collapse of the roof of an underlying magma chamber following an eruption. Subsequent injection of more magma can lead to doming of the caldera floor creating what is known as a resurgent caldera.

Caledonian Orogeny: a Lower Palaeozoic phase of mountain building, affecting NW Europe and NE North America, caused by collision between the ancient continents of Laurentia, Baltica and Avalonia.

Caradoc age: a series in the Upper Ordovician Period.

Chert: a very fine-grained, microcrystalline variety of quartz.

Cirque: a horseshoe-shaped, steep-walled valley head formed by glacial erosion; also known as a cwm or corrie.

Clast: any sized fragment of a rock produced by mechanical weathering of a rock mass.

Clastic rocks: mechanically redeposited remains of eroded older rocks. Rocks formed of fragments, or clasts, of pre-existing rocks.

Cleavage: a plane along which a rock will split, produced by the alignment of platy minerals such as mica in response to confining pressure during low-grade metamorphism; particularly well developed in slate.

Composite volcano: a volcano constructed of alternating layers of ash, lava and material eroded from the upper flanks; Mt Fuji in Japan is a fine example of this type of volcano.

Concretion: an irregular, often flattened, spherical- to ellipsoid-shaped body commonly composed of carbonate- or iron-bearing minerals.

Conglomerate: a sedimentary deposit containing more than 30% of rounded rock fragments (clasts) larger than 2mm in diameter (cf. breccia).

Conodonts: smallamber-coloured phosphatic, tooth-shaped fossils found in rocks from the Cambrian to Triassic periods.

Contact metamorphism: metamorphism due to the local heating of rocks by the intrusion of magma nearby.

Contemperaneous: happening at the same time or period.

Continental crust: layer of granitic, sedimentary, and metamorphic rocks which form the continents and the areas of shallow seabed close to their shores, known as continental shelves.

Continental drift: an early 20th Century hypothesis formulated by Alfred Wegener that describes the movement of continents over the surface of the Earth through geological time, and later vindicated by the development of plate tectonic theory.

 Continental margin: zone of the ocean floor that separates the thin oceanic crust from thick continental crust.

Continental shelf: extended perimeter of each continent and associated coastal plain, which is covered during interglacial periods such as the current epoch by relatively shallow seas (known as shelf seas) and gulfs.

Convergent boundary: boundary between two plates that are moving toward each other.

Convection Currents: heat generated by the breakdown of radioactive minerals in the mantle is redistributed by currents that rise at mid-ocean ridges and descend at the ocean trenches. Convection currents were long thought to be responsible for driving plate motion but this is still the subject of intense debate (see slab pull).

Conservative Margin: a plate margin where two plates slide past each other along a transform fault. The margin is characterised by shallow-focus earthquakes but no volcanic activity. The San Andreas Fault Zone that separates the North American Plate from the Pacific Plate is an example; the Great Glen Fault in Scotland is a fossil transform fault. In this case both plates are moving in a NE direction but the Pacific Plate is moving faster than the North American Plate.

Convergent (Destructive) Oceanic-Oceanic Margin: a plate margin where two oceanic plates collide and one is subducted to produce a volcanic island arc-oceanic trench system. The margin is characterised by andesitic volcanism and shallow, intermediate and deep-focus earthquakes. An example is the Caribbean Islands (Montserrat) where the South American Plate is subducted westwards down the Puerto Rico Trench.

Convergent (Destructive) Oceanic: Continental Margin: a plate margin where an oceanic plate is subducted beneath a continental plate to produce an ocean-trench-mountain belt system. The continental margin is characterised by andesitic and rhyolitic volcanism and shallow, intermediate and deep-focus earthquakes. As the oceanic plate is subducted, an accretionary wedge is formed. An example is where the Nazca Plate is subducted eastwards below the South American Plate along the Peru-Chile Trench. The Andes fold mountain chain is formed to the east of the trench.

Convergent (Destructive) Continental-Continental Margin: a plate margin where an ocean closes and two continental plates collide. As continental plates are too buoyant to be subducted they deform and thicken on collision to form a mountain belt with the continental crust thickening to twice the normal average. These margins are characterised by shallow-focus earthquakes, folding, faulting and regional metamorphism. In addition the base of the continental crust may melt to generate granitic magmas, which rise and solidify as plutons. An example is the Himalayas, formed by the collision of India with SE Asia.

Core: innermost layer(s) of a planet.

Craton: the stable interior of a continental plate, unaffected by tectonic deformation since the Precambrian.

Crinoid: an echinoderm, “sea lily” animal, composed of calcite plates with flexible arms for food collecting and attached to the seabed.

Cross-bedding: cross-lamination formed by migrating particles in ripples or dunes and in sandbars in a river.

Crustaceans: an arthropod with antennae (e.g. crabs, lobsters, shrimps).

Crustal shortening: The process that occurs in a fold mountain belt where the crust is laterally shortened by compression caused by plate collision. This reduction in length of the crust is accommodated by faulting, where one block is thrust over another, and/or by folding of the layers.

Cyanobacteria: a group of bacteria which contain chlorophyll and are capable of photosynthesis; they were probably the first oxygen-producing life forms and were responsible for creating the oxygen in the Earth’s atmosphere starting at least 3,500 Ma.

Diachronous: formations where boundaries cut across time frames.

Diagenesis: chemical, physical, or biological change undergone by a sediment after its initial deposition and during and after its lithification, exclusive of surface alteration (weathering) and metamorphism.

Diamictite: a comprehensive non-generic term for a non-sorted or poorly sorted non-calcareous terrigenous sedimentary rock that contains a wide range of particle sizes such as rock with sand or larger particles in a muddy matrix.

Divergent (Constructive) Margin: A plate margin where two oceanic plates are moving away from each other. Divergent margins are responsible for the formation of ocean basins, which start out as rift valleys (e.g. the African Rift). As the plates move apart, pressure on the underlying mantle is reduced and it partially melts to form basalt magma. New oceanic crust is formed as the ridge then moves away in both directions, cools and subsides. These margins are characterised by shallow-focus earthquakes and basaltic volcanism.

Dolerite: A medium-grained intrusive igneous rock, similar in composition to basalt, but with a slower rate of cooling. It is typically associated with intrusive igneous bodies such as dykes and sills.

Drift mine: a horizontal underground tunnel following a vein or bed of rock.

Drumlin: an elongate, rounded hill of boulder clay shaped and streamlined by later ice flowing over it; commonly found in a large group known as a ‘drumlin field’ or ‘drumlin swarm’.

Ductile: a response to stress where there is plastic deformation.

Dynamo-metamorphism: metamorphism caused by intense, localised stresses that tend to break up a rock to form a mylonite and sometimes reducing it to a fine powder.

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

Echinoderm: marine creatures made of numerous calcite plates (e.g. echinoids).

Ephemeral: intermittent, for example, a lake that appears in wet weather when the water table is high and disappears in dry weather when the water table is lower.

Erratic: a stone transported by a glacier and deposited far from its point of origin.

Esker: an elongate ridge of layered gravel and sand probably formed by streams flowing either beneath or on top of a glacier.

Estuary: a partly enclosed coastal body of water where seawater mixes with freshwater draining from the surrounding land.

Eukaryotic cell: a cell containing a nucleus, chromosomes and other internal structures, and which characterizes all life forms apart from bacteria and cyanobacteria (cf. prokaryotic cell).

Evaporites: a rock made up of one or more minerals formed by precipitation from salt-rich brines.

Exhalative deposit: a chemical deposit, principally formed on the seafloor, produced mainly from the outpouring of hydrothermal solutions such as black smokers.

Facies: the sum total of all features (e.g. lithological, palaeontological etc.) that reflect the specific conditions under which a particular rock was formed or deposited.

Fault: a fracture in rock across which there has been an observable amount of displacement.

Feldspar: an important group of rock-forming minerals that are essential components of many igneous rocks; two series exist, the ‘alkali feldspars’ with end members albite (Na-rich) and orthoclase (K-rich) and the ‘plagioclase feldspars’ with end members albite and anorthite (Ca-rich).

Floodplain: a flat area of land, bordering a river, which is periodically flooded when the river overflows its banks.

Folding: The process where rocks are deformed by compressional forces and associated with a shortening of the crust. The rocks show plastic deformation and flow to form anticlines (arch-shaped) and synclines (trough-shaped, like kitchen sinks). Associated with convergent (destructive) plate margins.

Fumarole: a volcanic vent from which volcanic gases escape, as distinct from those erupting magma.

Ga: billions of years ago.

Gabbro:  a coarse-grained basic igneous rock, generally forming large intrusions, that is composed mostly of Ca-rich plagioclase feldspar and pyroxene, sometimes with olivine.

Gaskiers Glaciation: a period of glaciation occurring at the conclusion of Precambrian times, the final glaciation of ‘Snowball Earth” around 560 Ma.

Glaucophane: a metamorphic, blue fibrous mineral formed under low-temperature, high-pressure conditions at around 35 km depth.

Glaciofluvial: pertaining to glacier meltwater streams.

Gneiss: a term applied to fairly coarse-grained, banded rocks produced by high-temperature and high-pressure metamorphism of either an igneous (orthogneiss) or sedimentary (paragneiss) parent rock.

Gondwana: the southern supercontinent, comprising Africa, India, Australasia, South America and Antarctica, which probably formed about 2,000 Ma and began to fragment about 180 Ma.

Graben: an elongate, down-faulted block of the Earth’s crust, commonly with a marked topographical expression.

Granite: a pale-coloured, fine- to coarse-grained igneous rock commonly occurring as large intrusions but also found in veins; consists of between 20-40% quartz and at least two-thirds of the feldspars are alkali feldspar.

Granodiorite: a coarse-grained igneous rock composed of more than 20% quartz and feldspar of which plagioclase makes up at least two-thirds of the total feldspar.

Granophyre: fine- to medium-grained granite displaying small-scale intergrowth of quartz and alkali feldspar caused by the simultaneous crystallization of the two minerals.   

Graptolite: a class of extinct colonial animals, probably of the phylum Hemichordata, composed of a chitinous exoskeleton with individual animals inhabiting tubes (thecae) which overlap in single or double rows (stipes) along branches which develop from a small conical cup (sicula). The range of the class is from the mid-Cambrian to mid-Devonian.

Hanging wall: a term originally used by Cornish miners to describe the surface of an inclined fault plane that has a rock unit lying above it.

Head: a superficial deposit formed through solifluction under periglacial conditions.

Hemipelagic: these are very fine, often in suspension, particles near the bottom of deep sea, consisting of oozes or red clay frequently associated with manganese nodules.

Hercynian: a mountain-building period also known as the Variscan that occurred during Upper Palaeozoic times.

Holocene: the most recent epoch of geological time during the Quaternary Period.

Hornfels: fine- or medium-grained rock, commonly displaying relict sedimentary or tectonic structures, produced by thermal metamorphism and developed beyond the margins of an igneous intrusion.

Hot Spot: an area of abnormally intense active volcanism thought to be underlain by a mantle plume. Many hot spots, for example Hawaii, are located in the middle of a lithospheric plate whilst others such as Iceland are located on divergent (constructive) plate margins.

Hydration: the incorporation of water into a compound (rock) that causes disintegration.

Hydrothermal solution: a hot fluid carrying a high concentration of dissolved solids, responsible for many types of mineral and ore deposit.

Iapetus Ocean: a former ‘proto-Atlantic’ ocean that separated the continental plates of Laurentia, Baltica and Avalonia (which contained Wales) during early Palaeozoic times. The British Isles were split, with the northern part connected to Laurentia and the southern part forming the eastern region of Avalonia, until the ocean floor was consumed in a subduction zone during Ordovician-Silurian times.

Idocrase (vesuvianite): a brown or olive-green mineral that is found in metamorphosed impure limestones.

Igneous rock: one of the three main groups of rock type (sedimentary, metamorphic and igneous) and used to describe a rock that has solidified from molten or partially molten material (magma).

Ignimbrite:  from the Latin terms ignis, meaning fire, and nimbus meaning cloud; equivalent to an ash-flow tuff, a poorly sorted pyroclastic rock, comprising mainly pumice and ash, sometimes with broken phenocrysts and fragmented vent-wall material; may wholly or partly consist of welded tuff.

Incompetent: layers of rock not able to withstand the pressures of folding so tending to flow or become crushed.

Inlier: an area of older rocks completely surrounded by younger rocks, and resulting from faulting or folding followed by erosion.

Interglacial: a period of relatively warm temperatures between glacial phases, during which ice-sheets and glaciers retreated and sea level rose.

Intermediate rock: an igneous rock whose geochemical composition lies between that of basic and acid rocks, for example an andesite.

Intrusion: a body of igneous rock that has been introduced into pre-existing rocks, commonly along structural weaknesses such as bedding planes or a fault.

Island arc: a series of volcanoes that lie on the continental side of an ocean trench at which there is a subduction zone.

Isostatic (rebound): the rebound of land that has been depressed under great weight of overlying rocks or ice, for example, following a period of glaciation when the ice melts. (e.g. raised beaches of Red Wharf Bay).

Isotopic dating: the technique of determining the age of rocks or fossils from the relative proportions of a radioactive parent element and its decay product(s).

Joint: a fracture across which any displacement is too small to be visible to the unaided eye. They are commonly produced by cooling (e.g. of an igneous rock), desiccation (of a sedimentary rock) and tectonic activity. 

Juxtaposes: positioned side by side.

Ka: thousands of years ago.

Kame: an irregular, undulating mound of bedded gravels and sands, deposited unevenly along the front of a stationary or melting ice sheet, or in large crevasses.

Kame terrace: a terrace between a hillside and a glacier formed by the action of glacial meltwater.

Laurasia: the name given to the large landmass of the northern hemisphere that consisted of North America, Greenland, Europe and Asia all joined as a single continent. 

Laurentia: the major North American continental plate in Lower Palaeozoic times (named after the St Lawrence River), comprising mainly the ancient Precambrian core of the Canadian Shield and Greenland together with Scotland and northwest Ireland.

Laurussia: the ‘Old Red Sandstone Continent’ formed by the amalgamation of Laurentia, Baltica and the Avalonia microcontinent in Silurian-Devonian times.

Lignite: a soft, earthy, brown-black coal consisting of fine-grained organic material (humus) together with wood and plant remains.

Limestone: a sedimentary rock type mainly composed of calcite and/or dolomite and which is mainly of organic or chemical origin.

Lithifaction: a process resulting in the formation of a rock from loose sediment or plant and animal remains.

Lithosphere: the upper layer of the solid Earth comprising all oceanic and continental crust as well as the uppermost mantle.

Ma: millions of years ago.

Mafic: a rock rich in magnesium or iron-rich minerals (basic).

Magma: a hot molten fluid, or melt, formed by the total or partial melting of solid crustal or mantle rocks and generally containing suspended crystals and dissolved gases.

Magnetic stripes: linear magnetic stripes, resembling a bar code, run parallel to mid-ocean ridges. The stripes reflect repeated magnetic reversal of the Earth’s magnetic field. The magnetic anomalies are also symmetrical either side of the mid-ocean ridges. The discovery of magnetic anomalies was crucial evidence for seafloor spreading.

Mantle: the zone lying between the Earth’s crust and core, with a thickness of about 2,300 km.

Mantle Plume: hot buoyant rock rising through the mantle from the core-mantle boundary. Thought by some geologists to rise beneath hot spots causing doming up of the crust. Cylindrical in shape, it seems to be fixed in position and have a radius of around 150 km.

Marinoan Glaciation: a Precambrian ice age in late Precambrian times during ‘Snowball Earth”.

Mélange: a large body of rock comprising chaotically arranged rock fragments of varied sizes contained within a fine-grained matrix. It is formed from sediments and rocks on the oceanic crust that are underplated at a destructive plate margin (Llanddwyn and Cemaes Bay).

Metamorphism: the process by which rocks are altered through the application of heat, pressure and fluids.

Metabasite: a metamorphic rock in which the original igneous texture is still visible.

Meltwater channel: a channel resulting from melting ice or snow that flows from beneath an ice sheet or glacier.

Meta-sandstones: a metamorphic rock in which the original sedimentary sandstone texture is still visible.

Meta-gabbro: a metamorphic rock in which the original igneous gabbro texture is still visible.

Meta-pelites: a metamorphic rock in which the original sedimentary mudstone texture is still visible.

Micrite: a semi-opaque microcrystalline texture forming the matrix of chemically precipitated limestone

Mid-Ocean Ridge: the junction between two oceanic plates along a divergent (constructive) plate margin. The ridge comprises a sub-marine mountain chain of basaltic volcanoes and is up to 1.5 km higher than the adjacent abyssal plain. The central part of the ridge has a rift valley running through it and the ridge is offset sideways by transform faults (e.g. the Mid Atlantic Ridge).

Mohorovici discontinuity (Moho): the boundary that separates the Earth’s crust from the mantle where there is a change in velocity.

Monian Supergroup: the original name for Anglesey’s late Precambrian rocks.

Moraine: an accumulation of rock debris (till, boulder clay) that has been transported and deposited by ice. Transport may occur on the surface of the ice (producing lateral and medial moraines), within the ice (englacial) and beneath the ice (subglacial). The debris is deposited when the ice melts and by subglacial streams.

Mountain Building: the process by which fold-mountain belts are formed. These occur at convergent (destructive) plate margins. Another term for a fold-mountain-building period is an orogeny and the subsequent product is an Orogenic Belt. Evidence of past mountain building can be seen in the UK with the Caledonian and Variscan fold-mountain belts.

Mylonite: a fine-grained rock, formed along a fault or shear zone, in which the constituent crystals have been mechanically fractured, broken-up and recrystallised, a process known as cataclasis.

Namurian: a period of time during the Carboniferous when parts of Britain were covered by great river deltas. It occurred after the Lower Carboniferous limestone was deposited and before the Westphalian Coal Measures formed. (Malltraeth).

Neogene: the Miocene and Pliocene epochs of the Tertiary grouped together.

Nuée ardente: a mobile, turbulent, hot cloud of air, volcanic gases and fine-grained tephra created by volcanic activity, and most typical of a Peléean eruption.

Neritic zone: the shallow-water, near-shore marine zone extending from the low-tide level to a depth of 200 m.

Nodule: a spherical or oval-shaped concretion.

Nunatak: an isolated rock peak protruding through an ice sheet.

Obduction: the process whereby ocean crust (and even upper mantle) are scraped off the descending ocean plate at a convergent plate boundary and thrust onto the adjoining plate. Often associated with an accretionary wedge (prism).

Oceanic Crust: the crust that forms the ocean basins. It is basaltic in composition, and comprises an upper layer of pillow lavas, a middle zone of vertical dolerite dykes and a lower layer of gabbro. The average thickness is 7 km and it is formed at mid-ocean ridges and subducted at ocean trenches. The oldest oceanic crust is less than 200 Ma.

Ocean Trench: an elongate depression of the ocean floor, which runs parallel to a volcanic island arc or mountain belt. Oceanic trenches are the deepest part of the oceans and can be up to 11 km. They are locations where oceanic lithosphere is being subducted back into the asthenosphere. An example is the Peru-Chile Trench, which runs parallel to the west coast of South America.

Olistostrome: a sub-marine gravity slide.

Oncoides: round biogenic pseudo-oolites.

Ophiolite: a fragment of oceanic crust and mantle thrust onto a continental margin during ocean closure and at a convergent (destructive) plate margin. Examples in the UK include the Lizard Ophiolite in Cornwall (Variscan Orogeny), the Ballantrae Ophiolite in Scotland (Caledonian Orogeny) and Caer Sais, Anglesey.

OPS – Ocean Plate Stratigraphy: the normal sequence of deposition on the ocean floor between constructive and destructive plate margins.

Orogenic belt:  a fold-mountain belt such as the Andes or Himalayas.

Orogeny: a period of mountain building, generally through the collision of tectonic plates, which leads to the formation of mountain chains.

Ostracoderms: a small bivalve crustacean of the sub class, Arthropoda.

Outwash plain: an extensive accumulation of rock debris deposited from glacial meltwater; the debris is very coarse close to the ice, becoming finer farther away, and its surface is typically cut by braided streams. Also known as a ‘sandur’, which is an Icelandic term.

Palaeontology: the study of ancient life.

Pangaea: meaning ‘all Earth’ and referring to a supercontinent that formed through the collision of Laurussia and Gondwana during late Carboniferous-early Permian times and only began to fragment towards the end of the Triassic Period.

Passive Margin: an ocean-continent boundary that is not an active plate margin, namely there are no earthquakes at the margin. The east and west sides of the Atlantic are good examples at present. In the future, as the Atlantic continues to widen due to seafloor spreading in the centre, the ocean-continent boundaries will eventually become subduction zones with new convergent (destructive) margins being formed.

Palaeogene: the Palaeocene, Eocene, and Oligocene epochs of the Tertiary when grouped together.

Palaeomagnetism: the study of the fossil magnetism locked in rocks which record the inclination and direction of the Earth’s magnetic field at the time of their formation. Such data are used to determine the past arrangement of the continents over time and support the theories of continental drift and seafloor spreading.

Palaeosol: a fossil soil usually buried under more recent deposits.

Pelagic: a term used in marine ecology to describe organisms that inhabit open water.

Peléean eruption: a volcanic eruption characterized by gaseous ash clouds associated with the growth and collapse of volcanic domes; named after Mt Pelée on the island of Martinique in the West Indies.

Pelite (pelitic): very fine-grained rock composed of clay or mud particles and called meta-pelite if weakly metamorphosed.

Peridotite: coarse-grained, dark-coloured igneous rock consisting mainly of olivine and pyroxene, which is believed to be the major constituent of the mantle.

Periglacial: a wide range of cold, non-glacial climatic and geomorphological conditions and processes.

Permafrost: permanently frozen rock, soil and sediment where temperatures below 0°C have persisted for two consecutive winters and the intervening summer; forms some 26% of the Earth’s surface and is up to 1,400 m thick in Siberia.

Petrography: the systematic description of rocks both in hand specimen and in thin sections for analysis under a microscope.

Phanerozoic: derived from the Greek and meaning ‘visible life’, the interval of geological time comprising the Palaeozoic, Mesozoic and Cenozoic eras and marked by the accumulation of sediments containing the remains of animals with mineralized skeletons.

Phenocryst: a crystal in an igneous rock that is larger than those in the surrounding groundmass or matrix, usually having crystallized at an earlier stage and reflecting slow cooling and growth.

Photosynthesis: the process whereby green plants manufacture carbohydrates from carbon dioxide and water using light energy from sunlight.

Phyllite: a regional metamorphic rock derived from argillaceous sedimentary rock.

Picrite: a basic volcanic rock made of olivine and pyroxene and little plagioclase feldspar.

Pillow lava: piles of elongate basalt lava pods, resembling a stacked series of stone pillows, providing good evidence for the sub-marine eruption of the lava.

Pingo: an ice-cored, dome-shaped hill developed in an area of permafrost and probably formed by the local freezing of water that has migrated from nearby upland areas; when the ice core melts, the resultant landform typically comprises elevated, round ramparts that often enclose a peat-filled basin.

Placoderms: extinct fishes with armour-like plates and articulating jaws.

Playa lake: a desert basin where water evaporates away quickly leaving dried lake beds.

Pliny eruption: named after Pliny the elder who died at Pompeii in AD 79 during the eruption of Vesuvius, a type of explosive volcanic eruption in which magma is fragmented by the release of gas to form a column of ejecta that extends high into the Earth’s atmosphere.

Pluton: a medium-sized igneous intrusion of up to 100 km² with a generally oval or circular shape in plan and steep or near-vertical sides. The granite intrusions in South West England such as Bodmin Moor, Carnmenellis and Dartmoor are good examples. Most are associated with convergent (destructive) plate margins

Porcellanous limestone: silica-rich rock with the appearance of unglazed porcelain.

Porphyritic: rocks that contain large and often well-formed crystals (phenocrysts) set within a finer-grained matrix.

Precambrian: an informal term for the longest interval of geological time, ranging from the formation of the Earth’s crust, approximately 4.6 Ga, to the beginning of the Cambrian Period, 543 Ma. The Precambrian is divided into the Hadean or Priscoan (4,600-4,000 Ma), the Archaean (4,000-2,500 Ma) and the Proterozoic (2,500-542 Ma).

Pridoli: the top series of the Silurian.

Prokaryotic cell: a primitive cell in which there is no nucleus, chromosomes and other internal structures (cf. eukaryotic cell); represented at the present-day only by bacteria and cyanobacteria which are the sole groups of the Kingdom Monera.

Proterozoic: the most recent (2,500-542 Ma) of the three divisions of Precambrian time.

Psammite (psammitic): a meta-sedimentary rock with an original composition of more than 80% quartz and feldspar grains (sand).

Pumice: light-coloured pyroclast comprising generally acid, highly vesicular (bubbly and gas-rich), glass foam.

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

Quaternary: the second series of the Cenozoic Era.

Raised beach: a former beach that is now stranded at higher levels above the present shoreline as a result of either a fall in sea level or Earth movements.

Recumbent nappe: large arch-shaped fold lying on its side.

Regolith: the superficial layer of loose, unconsolidated material that overlies bedrock over much of the land surface, and can include such material as saprolite, alluvium and volcanic ash.

Regression: a period when land was uplifted from the sea or the sea retreated from the land caused by a change in sea level.

Rheic Ocean:  a southern ocean that formed in the wake of the Iapetus Ocean after its demise.

Rejuvenation: a marked increase in the rate of erosion that occurs when a landmass is uplifted, for example through mountain building, by the melting of ice sheets, or by a fall in sea level; rivers and streams respond by cutting down into their beds and forming such features as terraces.

Rhyolite: an acidic, quartz-rich lava emitted from a granite-filled magma chamber beneath a continental land mass.

Rodinia: the Precambrian supercontinent that existed between 1.1 Ga and 750 Ma. It broke up in Neoproterozoic times and later formed the basis of all continents we see today.

Roche moutonnée: a mound-like glacial landform of rock, comprising a smoothed, gently inclined and streamlined up-glacier surface and a steeper, shattered down-glacier surface; produced by the action of advancing ice, frost-shattering and the plucking out of blocks.

Sandstone pipes: palaeokarstic solution hollows in limestone at Red Wharf Bay Anglesey, filled with upstanding ‘pipes’ of sandstone and conglomerate.

Saprolite: a fine-grained clay material produced by the deep, in situ weathering of bedrock, in particular crystalline igneous rock and metamorphic rock, under humid, tropical and sub-tropical conditions. The chemically rotted rock lies in situ (e.g. Porth Padrig).

Schist: metamorphic rock characterized by a parallel arrangement of most of the constituent minerals, in particular the platy minerals such as micas, and which is coarse enough to be visible to the naked eye.

Seafloor Spreading: the process by which oceans are formed at divergent (constructive) plate margins. New oceanic crust is formed as two oceanic plates move apart. Radiometric dating and fossil evidence shows that the seafloor becomes progressively older in both directions away from mid-ocean ridges. First proposed by Harry Hess following echo-sounding work to reveal the topography of the ocean basins.

Seamount:  A sub-marine mountain of volcanic origin occurring above hotspots associated with constructive plate boundaries, though not necessarily adjacent.

Seat earth: a thin layer of sediment beneath a coal seam, which contains fossil rootlets and represents the soil in which the vegetation grew.

Shear zone: a zone of deformation, often characterized by the development of mylonites, between two undeformed blocks that have suffered relative displacement.

Sedimentary: unmetamorphosed rocks containing variable-sized particles in layers (beds), for example, sandstone, conglomerate, limestone. They may contain structures found in sedimentary circumstances during or after deposition.

Serpentinite: a metamorphic rock formed by hydrothermal alteration of ultramafic rocks (e.g. peridotites and picrites).

Siderite:  an iron-carbonate rock FeCO2.

Sill: a tabular or sheet-like body of igneous rock originally intruded as a sub-horizontal sheet and emplaced parallel to the bedding of the surrounding rocks.

Slickensides: a rock surface that is polished and striated by the grinding of an adjacent rock mass, along fault planes.

Solifluction: the slow, gravity-driven, downslope movement of water-saturated materials, which occurs, for example, under periglacial conditions.

Snowball Earth:  a period when ice covered the Earth apart from the tops of high mountains between 750 and 580 Ma.

Spheroidal weathering:  a form of chemical weathering that affects jointed bedrock and results in the formation of concentric shells of decayed rock.

Stack:  an isolated pillar of rock that was created by erosion when the sea removed the roof of an arch attaching it to a cliff (e.g. Lleiniog).

Strata:  layers of rock, often in sedimentary rocks.

Stratigraphy: the study of rock sequences preserved in the geological record, in order to reveal the history of the succession of events and life in the past.

Stratotype: a representative standard, chosen at a particular locality, for a stratigraphical unit or the boundary between two stratigraphical units.

Striae:  scratch marks on rock made by ice armed with pebbles.

Strike: is the direction of a level line, where a tilted surface the (dip) expresses itself on the ground. It is more difficult to visualize the strike, but easy to remember because it is always perpendicular to the direction of dip.

Stromatolite: a mound-like structure built up by successive layers of cyanobacteria that trapped sedimentary materials. They represent the earliest known form of life, dating back to the Archaean, but are still forming today in warm shallow seas, for example in Shark Bay, Western Australia (e.g. Cemaes Bay).

Sturtian Glaciation:  the earliest known glaciation that occurred during Snowball Earth around 730 Ma.

Subduction:  the process whereby an oceanic plate is underthrust towards the mantle at a destructive plate margin.

 Subduction Zone: a zone where oceanic lithosphere is recycled back into the mantle. At the surface, the subduction zone generally coincides with the bottom of oceanic trenches. At depth, the subduction zone is marked by earthquakes, some of which are deeper than 400 km.

Sun-cracks: polygonal cracks formed when mud dries out in warm conditions (e.g. Lligwy Bay).

Syncline: a basin-shaped fold in which the youngest rocks occur within the core of the structure (cf. anticline).

Tephra: fragmented pyroclastic material, such as glass shards, pumice, blocks and bombs, produced during a volcanic eruption by the de-gassing and explosive decompression of magma on reaching the surface.

Terrace: a nearly flat landform with a steep edge, formed by a variety of processes including river channel incision. 

Terrane: a small crustal plate or fault-bounded fragment of a larger plate, with distinctive characteristics relative to adjacent areas, and which may have been displaced considerable distances from its original site and added to another plate during tectonic movements.

Thermal metamorphism: metamorphism involving heat only.

Thin section: an extremely thin slice of mineral or rock, mounted on a microscope slide, which is transparent or translucent to light and can be used to study the optical properties of a mineral and aid in its identification.

Thrust: a reverse fault with a low angle of dip.

Till: a collective term for sediments deposited directly from glacier ice. The deposits are classified either by particle size or by the process through which the debris was released. 

Transform Fault: a transform fault marks a conservative plate margin, namely one in which two plates slide past one another and crust is neither created nor destroyed, but is conserved. The San Andreas Fault is a transform fault that separates the Pacific Plate from the North American Plate. Transform faults cut the entire lithosphere and are most common in the oceans, where the segments of a mid-ocean ridge are connected together by active transform faults. The Great Glen Fault in Scotland is a fossil transform fault lying within a continent.

Tournquist’s Sea: an arm of the Iapetus Ocean which separated the Avalonia and Baltica tectonic plates during the Cambrian and Ordovician; the sea was closed when the two plates collided at about the Ordovician-Silurian boundary, approximately 440 Ma.

Tuff: a rock comprising hot ash exploded out from a volcano, usually during the initial stages of eruption, which welds together to form a coherent rock.

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

Unconformity: the surface of contact between two groups of strata which represents a break in the geological record during which there was a combination of no deposition and erosion.

Ultrabasic: rocks with unsaturated silica minerals and its usual minerals being magnesium-rich olivine, pyroxenes and plagioclase feldspars.

Ultramafic: igneous rocks containing more than 90% mafic minerals (magnesium and iron-rich minerals such as olivine, pyroxene, amphiboles and biotite.)

Vacuole: rounded holes in igneous lava rocks that were originally gas bubbles when in their liquid state.

Variscan Orogeny: a phase of mountain building in Europe that climaxed in late Carboniferous-early Permian times as a result of closure of the Rheic Ocean and the collision between Gondwana and Laurussia which united to form the supercontinent of Pangaea.

Volcanic rock: a rock produced by the solidification of lava or pyroclastic material.

Volcanic Island Arc: a chain of volcanoes generally with an arc shape, which runs parallel to an oceanic trench at a convergent (destructive) oceanic-oceanic plate margin. The islands of the Caribbean are a good example where the Atlantic ocean-floor is being subducted westwards below the Caribbean Plate.

Volcaniclastic: descriptive of a sedimentary rock containing volcanic material.

Welded tuff: a volcanic rock made up of explosively fragmented tephra but which retained enough heat on deposition for glass shards and pumice to deform under compaction and adhere together (weld) to produce a lava-like rock.

Westphalian: known alternatively as the Coal Measures, it formed after Carboniferous Namurian time as the deltas became afforested and swamps prevailed. Britain straddled the equator with its hot wet climate and the combination of forest and climate created the coal seams from the enormous quantities of rotted vegetation, interrupted by sandy sediments during periods of higher sea level (e.g. Malltraeth Marsh).

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