Aa flows: Massive lava flows that grade upward to a chaotic jumble of blocky and clinker lava fragments. Aa flows tend to advance more slowly than pahoehoe flows, where broken lava blocks tumble down the flow front and are overridden by the oncoming mass. (cf. pahoehoe budding).
Accidental pyroclasts: Fragmental debris derived from basement rocks during an explosive eruption. May occur with Juvenile and Cognate pyroclasts.
Accretionary aggregates: The aggregation of fine ash into pellets, a few millimetres in diameter, within turbulent, wet ash columns and plumes derived by explosive phreatic and phreatomagmatic eruptions. Electrostatic charges in the turbulent plume play an important role. Experimental evidence also indicates that cementation by sulphates and other minerals can occur rapidly in the plume – this increases their preservation potential. Pellet cores may contain fine ash, or fine lapilli. Pellets may be completely unstructured, or consist of concentrically layered fine ash. Aggregates with multiple concentric layers constitute the well know accretionary lapilli. They may flatten in impact with the ground. They range from about 5 – 25 mm diameter. There is some evidence they have formed on Mars.
Accretionary lapilli: Accretionary aggregates of fine ash surrounded by multiple, concentric layers (onion like) that form within turbulent, wet ash columns and plumes during explosive phreatic and phreatomagmatic eruptions.
Aerosol: Small droplets of liquid of solid particles suspended in air, mainly by air turbulence. Liquid aerosols commonly have dissolved compounds like sulphuric and hydrochloric acid derived from volcanic eruptions. They are important in Earth’s upper atmosphere because can they reflect incoming solar energy, resulting in cooling, or absorb heat that raises atmospheric temperatures.
Airfall ash/tephra: Volcanic ejecta (ash to block sizes) that falls to the surface from an eruption column. Deposits tend to mantle topography but may be reworking by precipitation runoff. Deposits may be size-sorted because of gravitational settling; individual beds become finer and thinner with distance from source.
Albite twins: Common twinning in plagioclases and potassium feldspars, presented as multiple, parallel lamellae that traverse the entire crystal section. The width of twin segments decreases and the number of lamellae increases in more calcic plagioclases.
Amygdaloids: Vesicles that are filled with mineral precipitates (commonly calcite, zeolite, chlorite). Precipitation occurs after the magma has cooled. cf. Spherulites.
Andesite: An extrusive volcanic rock having composition intermediate between basalt and rhyolite, with 52 and 63 weight percent silica (SiO2). Commonly fine grained with feldspar micro-laths in the groundmass and plagioclase phenocrysts, plus pyroxenes and hornblende. Common constituents of volcanic arcs associated with subduction zones. Named from the Andes where volcanics of this composition are common. The intrusive equivalent is diorite.
Antidunes: Bedforms that develop in Upper Flow Regime, Froude supercritical flow. The corresponding stationary (surface) waves are in-phase with the bedforms. Unlike ripples, the accreting bedform face grows upstream – antidunes migrate upstream in concert with deposition on the stoss face. When flow conditions wane, they become unstable and wash out or surge downstream. Their preservation potential is low.
Ash flow tuff: Another name for pyroclastic density current or pyroclastic surge.
BAF: The acronym for block and ash flows.
Ballistics (volcaniclastic): Blocks and bombs ejected by powerful explosive volcanic eruptions that follow a parabolic trajectory to be deposited as tephra.
Basalt: The most common rock type on Earth, an extrusive, dark brown to black rock with silica contact 45-53%. The upper part of oceanic crust is predominantly basalt, but also occurs in shield volcanoes and volcanic arcs. In lava flows it is more fluid than andesite and rhyolite. Is erupted as flows, fire fountains, and explosive magmatic and phreatomagmatic columns from which pyroclastic density currents are commonly generated. Typically, there is a glassy groundmass with plagioclase micro-laths, common plagioclase phenocrysts at the calcic end of the feldspar spectrum, plus common pyroxenes, amphiboles, and olivine. The intrusive equivalent is gabbro.
Base surge: Synonymous with pyroclastic surge. The term base surge was first used to describe turbulent, bottom hugging flows generated by nuclear test detonations in the late 1940s and early 1950s.
Batholith: A large intrusive body that is >/= 100 km2, usually composed of granite, granodiorite and diorite.
Bedform: Sedimentary structures produced by bedload transport of loose, non-cohesive sediment. Typically manifested as ripple and dune-like structures.
Bedload: Loose or non-cohesive sediment particles (silt, sand, gravel – sizes) at the sediment-water or sediment-air interface, that will move along the bed if fluid flow velocities exceed the threshold velocity. The bedload consists of a traction carpet, and a suspension load.
Block and ash flow: Ground-hugging, concentrated PDCs characterised by a vast range of clast sizes, including blocks having dimensions measured in metres. They are usually derived from collapsing lava domes. They are commonly associated with pyroclastic surges. Deposits consist of a mix of ash and blocks; they are poorly sorted, usually matrix-supported, and poorly- or ungraded. Block angularity is highly variable.
Blocks/bombs (volcaniclastics): Both terms are used as textural descriptions for primary volcaniclastics (regardless of their origins). Clast sizes are 64 mm and coarser.
Boil over: PDCs can be generated by fire fountains that eject large volumes of fragmented lava over a crater rim. PDCs formed in this way are concentrated in lapilli, splatter, and flattened or aerodynamically shaped bombs.
Bombs – ballistics (volcaniclastic): Ejected lava fragments, or bombs (particularly in Hawaiian and Strombolian fire fountains), can be shaped aerodynamically into spindle-like ballistics while being flung through the air. If the lava is still molten when it lands it will spatter and cool in a variety of shapes (e.g. cow-pat, bombs, bread crust bombs).
Bomb sags (Volcaniclastic): Large, ballistic blocks and incandescent fragments of magma ejected during an eruption, may land on earlier deposited tephra causing the beds to sag. The bedding deformation may be accentuated during compaction.
Bowen reaction series: A predictable order of mineral crystallization in a cooling magma, after the early 20th Century geologist Norman Bowen. One of the first minerals to crystallize from magma is olivine (from about 1300o to 1200oC). Feldspar, the most common rock-forming mineral, begins to form below temperatures of about 1000oC, and one of the last to appear, quartz at about 800oC. Bowen’s discovery revolutionised the way we think about the evolution of igneous rocks.
Breadcrust bomb: A volcanic ballistic/bomb that has a cracked surface. The surface layer of the bomb cools rapidly and coincidently the inner molten part expands as gases are exsolved, resulting in brittle failure of the outer crust.
Breccia: Consists predominantly of angular clasts larger than 2mm. Like conglomerates they are poorly sorted, clast-supported frameworks. The degree of clast angularity indicates little or no reworking.
Bubble texture (volcaniclastic): A texture characteristic of volcanic ash presented as highly arcuate apophyses in shard walls, or as complete bubble outlines within shards. They commonly form during explosive eruptions, from the introduction of superheated steam when magma is in contact with water (as in phreatomagmatic eruptions), or from degassing of volatiles within the magma.
Buoyancy: Buoyancy is the result of fluid forces acting on a body immersed in a fluid. If the resultant force is greater than the gravitational force acting on the body (that itself is a function of its density), then the body will rise (positive buoyancy – negative buoyancy is the opposite). Buoyancy plays an important role in many processes – the rise of mantle plumes and magmas, diapirism, density and temperature stratification in the oceans, the support of clasts in sediment gravity flows and pyroclastic flows.
Buoyant plume: A turbulent mix of gas, air and fine particles that is less dense than air. It develops above the main body of a pyroclastic flow or sediment gravity flow by elutriation of particles from the main flow. The plume dissipates as the particles settle gravitationally.
Caldera: A large volcanic collapse basin resulting from withdrawal and eruption of large volumes of magma or explosive pyroclastics. Basin walls are initially steep but may become degraded over time. Many become lakes post-eruption. Calderas are the sites of some of the largest known eruptions (e.g. Yellowstone, Krakatoa, Taupo).
Carlsbad twins: Common twins in plagioclase and some potassium feldspars. It is an penetration twin with a plane that separates two crystal segments.
Chute and pool Chute and pool conditions usually develop at flow velocities higher than those responsible for unstable antidunes. Chute and pool morphology is centred on a hydraulic jump – upstream flow in the chute is supercritical, and immediately downstream flow is subcritical (the pool). Chutes and pools can also migrate upstream which means the hydraulic jump moves in tandem.
Cinder cone: A cone-shaped volcanic edifice formed airfall of fragmental lava in close proximity to the erupting vent. The composition is ash, lapilli, and blocks, including aerodynamically shaped ballistics such as spindle bombs and cowpat bombs. Usually associated with lava fire fountains.
Cognate epiclasts: Also called Accessory Pyroclasts. Pyroclasts derived from earlier-formed and co-magmatic volcanic rocks at the same volcano. Cf. Accidental pyroclasts, Juvenile pyroclasts.
Colonnade jointing: Columnar ‘organ-pipe’ like cooling joints oriented at right angles to magma body margins. In lava flows, colonnades may be tiered, with a larger cross-section columns at the base and smaller columns that intersect the lava surface (and cooled more quickly). Cf. entablature.
Column collapse: Plinian and Vulcanian eruptions produce columns of hot, turbulent mixtures of juvenile fragmentals, gas and air. Gravity-induced collapse of the column produces hot pyroclastic flows and surges. This is the most common mechanism of PDC generation.
Columnar jointing: Regular arrays of joints formed during cooling and contraction of magma. They can occur in lava and hot ignimbrite flows, and intrusive dykes and sills. Cooling begins from the outer surfaces and progresses towards the centre of the magma body where joints are oriented normal to the outer surface. They form as straight to slightly curved columns with 4 to 8 sided polygonal cross-sections. Cf. colonnade, entablature, fracture porosity.
Crater lakes: Water that accumulates in volcanic craters, extinct or active. On active volcanoes, eruption through a crater lake may have a strong phreatic or phreatomagmatic imprint (depending on whether there is new magma) until all the water has been vapourised.
Critical flow: Also called Tranquil flow. The flow conditions for a Froude number of 1 , at some critical flow velocity and flow depth, where any surface wave will remain stationary (it will not move upstream or downstream). Surface waves will usually be in-phase with their bedforms, for example antidunes. See also subcritical and supercritical flows.
Cryptodome: Shallow magma intrusion into a volcanic edifice, causing distortion and deformation of the volcano flanks (e.g., bulging), but with no accompanying eruption.
Cyclic steps Cyclic steps are basically trains of chutes and pools, where supercritical to subcritical transitions occur repeatedly downstream. At each transition there is a hydraulic jump – this is the step in each flow transition. As the hydraulic jumps move upstream they erode sediment that is then deposited on the stoss face immediately downstream. The wavelength of cyclic steps is potentially 100-500 times the water depth, and is significantly greater than that for stationary waves and their associated antidunes.
Dacite: A volcanic rock, more felsic than andesite with Silica content (SiO 2) 63%-69%. Usually in shades of grey. Porphyritic with phenocrysts of plagioclase (andesine, labradorite, oligoclase) and amphibole, some quartz, and less commonly pyroxene and biotite. Groundmass includes felted masses of plagioclase and the same ferromagnesian minerals noted above. It can occur as an extrusive rock in subduction-related settings, and as intrusive dykes or sills. The intrusive equivalent is granodiorite.
Debris flow: A type of sediment gravity flow containing highly variable proportions of mud, sand, and gravel, in which the two primary mechanisms for maintaining clast support are (mud) matrix strength (a function of viscosity) and dispersive pressures caused by clast collisions. Rheologically they behave as (non-Newtonian) plastics or hydroplastics. Unlike turbidites, there is no turbulence, hence normal grading is absent or poorly developed. Some debris flows develop significant internal shear that imparts a crude stratification and/or an alignment of clasts. Terrestrial flows include highly mobile mud flows, and lahars in volcanic terrains. The more mobile types may grade to hyperconcentrated flows
Dense rock equivalent (DRE): A conversion from the volume of fragmental deposits (ash, lapilli, blocks) to an equivalent volume of non-fragmented lava. DRE values are used to compare the magnitude, or total volumes of eruptions.
Diabase: An intrusive rock having the composition of basalt and gabbro, commonly seen in dykes and sills at shallow crustal levels associated with volcanic edifices. It is also called dolerite or microgabbro. It is dark grey to greenish black. Crystal size varies from aphanitic to porphyritic with labradorite feldspar, augite, magnetite, and olivine.
Diatremes: Funnel-shaped pipes (funnel opening upward) containing a chaotic mix of brecciated basement rock formed by focused explosive eruptions, commonly phreatomagmatic; many are associated with maar eruption centres. The eruptions bring deep crustal rock to much shallower levels, including those containing very high pressure minerals like diamonds. Diatremes are exposed by deep erosion of the surface cover.
Dike/dyke (igneous): (Dike = North American; Dyke = English) Sheet-like magma feeders to volcanic eruption sites, that have been forced through and are oriented at a high angle to stratification or layering. Columnar jointing is common and normal to the dike walls. Dyke sheets commonly branch. Cf. igneous Sill.
Diorite: Typically speckled white-grey, medium to coarse grained (phaneritic)intrusive rock, intermediate in composition between granite and gabbro. SiO2 52%-63%, plagioclase, amphibole and pyroxene, rarely quartz. Common source of andesite in volcanic arcs, or as exposed intrusives in unroofed arcs. It is the equivalent of extrusive andesite.
Directed blast: Explosive blasts that are directed laterally transform rapidly to pyroclastic flows. They are commonly generated by flank landslides, larger sector collapse, or lava dome collapse – the last mechanism was responsible for the main PDC during the 1980 eruption of Mt. St. Helens.
Dispersive pressure: An important mechanism of flow support in grain flows, some debris flows, concentrated pyroclastic density currents (e.g., ignimbrites) and block and ash flows, where grain-to-grain collisions transfer momentum that keeps particles dispersed. c.f. other mechanisms like turbulence, matrix strength, and fluidization.
Dolerite: See Diabase.
Dunite: The pure (or nearly so) olivine variety of peridotite is called Dunite. It is usually exposed in obducted oceanic crust along with gabbro and pillow basalts that together compose ophiolite.
Effusive eruption: A non-explosive eruption where magma issues from a vent as a lava flow. The VEI score is zero.
Ejecta: Pyroclastic debris (ash, lapilli, blocks, bombs), fragmented lava and country rock that is ejected during a volcanic eruption.
Elutriation: Removal of fine particles by the upward flow of fluid or gas, through the body of a pyroclastic density current or sediment gravity flow. Elutriation is responsible for the development of a buoyant plume above such flows.
Endogenic lava dome: Domes that expand as viscous magma is intruded into the dome interior (i.e., they inflate from within).
Entablature jointing: Lava cooling joints that form a range of patterns, from irregular accumulations to spectacular radial clusters. Thick lava flows may contain both a lower band of colonnade joints overlain by entablature joints.
Epiclastic: Sedimentary clasts formed from pre-existing rocks; this applies to most siliciclastic rocks and to many redeposited volcaniclastic sediments.
Eruption column: Ejected rock and gas that rises above a vent during explosive eruptions. Column heights range from a few 100 m to more than 40 km in ultraplinian eruptions. Denser, cooler parts of the column can collapse rapidly, depositing blocks and ash on the flanks. In some situations, the collapsing column form pyroclastic density currents. Eruption clouds form downwind of the column.
Eruption cloud: The accumulation of fine ash and gas downwind of an eruption column. Umbrella-like clouds commonly form at the top of the column.
Eruption intensity: The rate at which pyroclastic mass is ejected.
Eruption magnitude: The total volume if rock erupted calculated as the dense-rock equivalent volume. C.f. eruption intensity.
Exogenic lava dome: Domes that grow externally by addition and stacking of lava extrusions.
Feldspar: The most abundant mineral in Earth’s crust. It is present in nearly every kind of igneous rock; it is also a common hydrothermal product. In sedimentary rocks it occurs as a common detrital component, and as an authigenic phase (usually albite). Feldspar is a sheet silicate comprising two main groups: the Alkali feldspar group – most common are potassium-bearing feldspars (K-spar) that forms a solid solution series with Albite; and the Plagioclase group that form a solid solution series from Albite (sodium end member) to Anorthite (Calcium end member). All feldspars have good cleavage. There are several types of twinning, for example the common perthite and Gridiron twinning of K-spars, and albite, carlsbad, pericline twins in plagioclases.
Feldspar laths: Microscopic, needle-like crystals of feldspar, usually plagioclase, in a glassy groundmass. Alignment of laths provides an indication of flow. These textures commonly develop in rapidly cooled basalt lavas.
Felsic: Refers to igneous rock compositions where SiO2 is generally >65%. Felsic: A contraction of ‘feldspar and silica’, that describes in very general terms igneous and metamorphic rocks that are rich in silica and minerals containing lighter alkali metals like sodium and potassium. For igneous rocks this includes rhyolites, dacites, and granites. Cf. Basic igneous rocks.
Fiamme: Lenticular, lozenge-shaped to wispy fragments of glassy and pumiceous tephra that are common in hot pyroclastic flows, and are stretched while in a viscous state. Lengths range up to a few centimetres. The plane of flattening approximates bedding.
Fire fountain: another name for lava fountain, usually seen as periodic or continuous fountains of lava erupted into the air, commonly reaching 200-300 m above the vent. They can also develop as lava curtains along active rifts. The erupted material can accumulate as fragmentals on the volcano slope (or cinder cone) or produce lava flows.
Flame structures: Wispy, flame-like mudstone structures that form during early, differential compaction of sandstone-mudstone interbeds. The flames appear to protrude from the mudstone into the overlying sand.
Flank eruption: An eruption on the side, or flank of a volcanic edifice, rather than the summit crater. It can be effusive or explosive.
Flow banding (volcanic): Banding that is concentric, or intricately folded is common in rhyolite and dacite domes; each band represents a slightly different texture and/or mineral composition and develops during the slow movement of highly viscous magma. Intricate folding can also occur, particularly around magma fragment. This folding is NOT tectonic. Flow banding can also occur in hot pyroclastic ash flows, where temperatures are high enough to render ash fragments fully ductile.
Flow unit: A term that applies equally to sediment gravity flows (like turbidites, debris flows) and pyroclastic density currents. It refers to a stratigraphic unit, bed or layer deposited during a single flow event. Pyroclastic flows like ignimbrites and surges may contain many flow units.
Flux melting: A term derived from welding and glass making. A flux is a substance that lowers the melting point of solids. It applies to magma generation in the mantle where water, derived by dehydration of mica, glaucophane, and serpentinite minerals, lowers melting points by 200°C and more. Flux melting is a critical stage in the formation of partial melts.
Froude number: A dimensionless number that expresses the characteristics of flow, including surface waves and bedforms, as the ratio between gravitational forces and inertial forces:
Fr = V/√g.D
Where V is bulk flow velocity that reflects the dominant effect of gravity on surface flows, and the inertial component is √g.D where g is the gravitational constant, and D is water depth. The denominator represents the speed of a surface wave relative to the bulk flow velocity. Whether the surface wave is faster, slower or the same speed as the bulk flow will depend on its resistance to move, or its inertia.
Fumaroles: Also known as Solfataras. Geothermal gas and steam vents where temperatures are >/= 100°C. The proportion of liquid water is low. They tend to form when the watertable is deep. , Hot springs are more common where watertables. are shallow.
Gabbro: An intrusive rock of mafic composition with SiO2 45%-52%. Typically, dark grey – black. Medium to coarse grained (phaneritic), composed mostly of calcic plagioclase, pyroxene, and some olivine; rarely quartz. It is the plutonic equivalent to basalt – common in oceanic crust – will be exposed if the crust has been obducted.
Geopetal: Textures and fabrics that allow the interpretation of stratigraphic top, or ‘way-up’. This definition would include normal grain size grading in a turbidite. However, there is a tendency these days to restrict the meaning to structures where cements or sediments partially fill a void, such that the top of the fill represents a depositional or precipitation surface. Examples include fossils that have preserved chambers, the interstices between pillow lavas, and cavernous porosity in reef frameworks or caves.
Glowing avalanche: A hot pyroclastic flow that reveals a glowing flow head as it careens down the volcano slope. Also called Nuées ardentes.
Granite: An intrusive, plutonic rock, felsic composition the equivalent of extrusive rhyolites with SiO2 69%-77%. It tends to be light coloured, equigranular with abundant potassium feldspar, sodic plagioclase, and quartz, plus biotite, muscovite, and hornblende. One of the most common plutonic rocks underlying continental crust and island arcs.
Granodiorite: A plutonic rock intermediate between granite and diorite, with SiO2 63-69%. Medium to coarse grained (phaneritic), consisting primarily of quartz, plagioclase (the dominant feldspar, plus lesser K-feldspar, biotite, and amphibole. Common constituent of island arcs; one of the largest granodiorite provinces in the world is the Coast Mountain Belt in British Columbia, the unroofed roots to ancient arcs.
Gridiron twinning: A common diagnostic twin in potassium feldspars twins that belongs exclusively to microcline. It is presented as a cross-hatching of thin albite and perthite twin lamellae. Also called tartan twins.
Guyot: The name given by Harry Hess (1946) to flat-topped seamounts, commonly found at dpeths of 100s of metres. Hess’ original definition was based on seafloor acoustic data collected during World War 2. The truncated morphology of these seamounts is attributed to erosion and wave attack during subaerial exposure followed by isostatic subsidence.
Hawaiian eruption: Effusive eruptions of fluid basaltic lava in lava lakes and associated flank fissures. VEI = 0-1. Flows are mostly pahoehoe type.
Hornito: A small steep-sided spatter cone or pinnacle formed by effusion of lava from an opening through the top crust of an active lava flow, rather than from a vent. Typically with more fluid pahoehoe lavas.
Hotspot: The surface or shallow subcrustal expression of a magma plume, usually associated with submarine eruption of seamounts and volcanic islands in ocean basins (oceanic lithosphere).
Hotspot track: Linear or arcuate trails or chains of submarine seamounts and volcanic islands across the ocean floor. Many of the better-known examples show a relatively consistent age progression, from an active or recently active volcano to volcanoes as old as Cretaceous; e.g. Hawaii-Emperor chain (Pacific), New England chain (Atlantic).
Hyaloclastite: Fragmental volcaniclastics that form when lava is quenched rapidly under water, beneath ice, or in saturated sediment, such that is shatters into angular fragments. Hyaloclastitic debris is commonly glassy and highly angular, with straight, arcuate, or bubble texture margins.
Hyalotuff: Fragmental deposits formed by explosive, phreatomagmatic eruptions when magma comes into contact with seawater or groundwater. Ash particles are generally angular and commonly have bubble or vesicle textures. Cf. Hyaloclastites.
Hydroplaning A term applied to sediment gravity flows and dilute pyroclastic density currents – where the head of these flows lifts above the substrate. Flume experiments show that a layer of water/fluid beneath the flow can reduce drag, such that the flow head rises and in doing so increases its velocity. If the velocity increases is sufficient, the head can detach (at least temporarily) from the main body of the flow. This mechanism offers one explanation for surging at the head of many flows.
Hydrovolcanic explosion: A general term for explosive eruptions or explosions caused by lava coming into contact with surface water, seawater, or groundwater. This category includes phreatic and phreatomagmatic eruptions. Both types produce fragmental debris and steam.
Ignimbrite: A recent definition states ” …the rock or deposit formed from pumice and ash- through to scoria and ash-rich pyroclastic density currents” regardless of thickness, areal extent, volume, composition, crystal content, relationship with topography, or temperature (usually >500°C) . They are usually regarded as concentrated PDCs where grain-to-grain dispersive pressures and/or fluidization maintain flow support. As such they are fundamentally different to pyroclastic surges and block and ash flows.
Inertia: Inertia is generally defined as a force that resists the change in motion of a body; here motion refers to a vector that describes velocity and direction, and ‘body’ refers to anything composed of matter, including a body of fluid. Inertia was codified by Newton in his Laws of Motion – in the 1st Law as the Law of Inertia, and in the 3rd, as the Action-Reaction law. Inertial forces are central to the quantification of fluid mechanics expressed in Froude and Reynolds numbers.
Intrusion: (a Noun) The forcing of magma into pre-existing rock – it is the name of a process, but also refers to the igneous body thus emplaced. Cf. intrusive – an adjective.
Intrusive: (adjective). An intrusive rock is formed by intrusion of magma. It describes the style of emplacement of the rock body.
Jokulhlaup: An Icelandic word for sudden bursts of water or lahars during a subglacial eruption. Also refers to the sudden breaching of glacial lakes.
Juvenile fragments: In volcaniclastic deposits, the granular material derived directly by fragmentation of new magma. Airfall deposits and pyroclastic density currents consist almost entirely of juvenile debris. C.f. accidental clasts plucked from existing rocks in a vent, or the substrate to ground-hugging flows. Also called Essential clasts. Cf. Accidental pyroclasts, Cognate pyroclasts.
Kelvin Helmholtz instabilities (or waves): At the top of a PDC near the flow head, intense shear at the contact between the rapidly moving flow and overlying air leads to instabilities manifested as billows, vortices and waves.
Laccolith: An intrusive body having a dome-shaped top and relatively flat basal boundary. Feeder dykes are usually present. Cf. Lopolith.
Lahar: A terrestrial gravelly mudflow, or debris flow consisting largely of volcanic debris. Most flows are initiated on the flanks of volcanoes. They develop during and after eruptions, initiated by seismic tremors, or periods of high rainfall that saturate soils and reduce their shear strength. Lahars are capable of carrying vehicle-sized blocks and can be very destructive. Flow run-out is commonly several kilometres. Cf. block and ash flow.
Lapilli: Primary volcaniclastic particles, derived directly from volcanic eruptions, and ranging in size from 2 mm to 64 mm. See Accretionary lapilli.
Lateral blast: An ground-hugging eruption that is triggered by the collapse of a volcanic flank or lava dome. These blasts produce hot, fast-moving block and ash flows, pyroclastic density currents and pyroclastic surges. Mt. St. Helens eruption in 1980 is an iconic recent example.
Lava: Rock melt that is erupted at the Earth surface; beneath the surface the melt is called magma.
Lava delta: A delta-like platform that builds by lava flows entering the sea or a lake. Rapid cooling can generate much coarse angular debris that can become unstable and slide or flow down the delta foreslope. Hydrovolcanic explosions may also cause instability. They commonly form on oceanic islands e.g., the Kīlauea 2018 eruption Soule et al., 2021, OA.
Lava dome: Hemispherical to spine-shaped extrusion of viscous magma in the craters or upper flanks of volcanoes. Lava domes are inherently unstable: debris derived from lava cooling and cracking may spall and accumulate at the dome base; or domes may collapse under the influence of gravity or from internal pressures. Dome collapse commonly generates block and ash flows, or hot pyroclastic flows and surges. Cf. endogenic and exogenic domes.
Lava dome collapse: Domes of viscous lava that grow on volcano summit craters or flanks are inherently unstable. Their collapse from gravitational instability or internal pressures can generate PDCs, commonly as block and ash flows, or pyroclastic surges.
Lava fountain: A pulse or jet of lava erupted to 300 m and more, caused by rapid gas expansion of the shallow magma. Fountains occur at single vents, fissures, and in segments of lava lakes. Lavas tend to be fluid and basaltic. The fallout can accumulate as ash, lapilli, and block tephra, or it can form lava flows. Recent examples include fissure eruption from the Southwest rift zone of Kilauea from 2018-2024, and the fissure eruptions on Reykjanes Peninsula, Iceland.
Lava lake: The accumulation of lava in summit craters. Eruption may be quietly effusive, or as fire fountains from fissures around the crater walls. Crusts that form rapidly on the lake surface are usually broken by lava surges and reworked into the melt. When activity ceases the lava solidifies into a solid plug.
Lava tube: Long, tunnel-like openings through which lava flows; primarily in basaltic pahoehoe lavas. They usually begin life as open lava streams where crust mantles gradually coalesce into a more solid roof. Tubes commonly branch to form networks. When flow ceases, the lava may drain leaving the tube open, or solidify within the tube. The roof of a tube can collapse at any time during or after lava flow.
Lobe and cleft: Lobes, billows, and intervening pockets at the head of a PDC where turbulence is generated by frictional forces across the substrate and the contact with air.
Lopolith: An intrusive body similar to a laccolith but having a flat top and inverted dome-shaped base. Also fed by dykes.
Maar eruptions: A phreatomagmatic, or hydromagmatic eruption caused by magma intruding shallow groundwater. These highly explosive events produce low-relief craters that extend beneath the local watertable and are capable of bringing deep basement rocks to the surface. They subsequently fill with water and form closed lakes or coastal embayments. Volcaniclastics usually include airfall and pyroclastic surge deposits. Accretionary lapilli are common. They may be linked to diatremes.
Mafic: Refers to the composition of igneous rock low in SiO2 (45%-55%), and usually lacking quartz. Includes common rock types such as basalt, andesite, trachyte, diorite, and gabbro. The word is contracted from magnesium and iron (ferrous-ferric) that are important elements in the common ferro-magnesian minerals of mafic rocks. Cf. felsic, ultramafic. It applies to extrusive and intrusive rocks.
Magma: Fluid rock melt or partial melt at depth in the crust and mantle. Most magmas originate in the mantle and ascend through the crust, usually cooling and crystallizing as they rise according to Bowen’s Reaction Series. The melt is referred to as lava when it erupts from the surface. Cf. lava
Magmatic arc: Also called volcanic arcs. A chain of volcanoes and associated intrusions that form in the plate above a subduction zone. Arcs generally parallel the deep oceanic trenches. At mantle depths, dewatering of oceanic crust in the subducting slab lowers the melting point of mantle rock. The partial melts rise because of buoyancy.
Magmatic eruption: Explosive eruptions where magma fragmentation is controlled by decompression of magma volatiles such as CO2 and water. Cf. phreatomagmatic, phreatic, effusive eruptions.
Monogenetic: A volcanic edifice built by a single eruption or eruption episode.
Monzonite: An intrusive rock, intermediate between granite and diorite in composition and mineralogy, having SiO2 averaging 57%, quartz <10%, and feldspar dominated by potassium feldspar that composes 33-66% of total feldspar. Medium to coarse grained, with hornblende, biotite, and aegirine pyroxene. Quartz monzonites have >10% quartz.
Mud volcano: A small volcano or dome-shaped mound, 1-2 m high, built by the eruption of mud. Eruptions may be quietly effusive or violently explosive. They can be accompanied by exhalation of gas including methane that can ignite. They are relatively common in geothermal fields (geysers, boiling pools and so on), near active faults or regions prone to seismic activity, on large river deltas, and above oil fields.
Newtonian fluid: A rheological class wherein a fluid has no yield strength (cf. plastics), and deforms continuously (strain) with increasing stress, independent of viscosity. Water is the best known example. cf. Plastic, hydroplastic rheology
Nuees ardentes: The French name, introduced by Alfred Lacroix in 1903, for particularly mobile pyroclastic density currents. See N. Wallenstein et al., 2018 for a discussion on the origin of the name.
Obsidian: Black volcanic glass, usually associated with rhyolite lava flows and domes. May be spherulitic, or flow banded. In thin section it is amorphous under polarized light.
Pahoehoe flows: Relatively fluid lavas that develop smooth, ropy, billowing or tendril-like textures across the flow top. They occur mostly in basaltic lavas, and are commonly associated with tumuli and spatter cones. Pahoehoe flows also advance by budding and lava breakout. Highly mobile flows can move at speeds up to 40 km/hour.
Partial melting: Most rocks consist of several minerals, each of which has a different melting point. When rocks begin to melt, those minerals with the lowest melting points will be the first to contribute to magmas – the rock will be partially melted, producing a kind of crystal mush. Partial melting is a critical stage of magma formation in the mantle. See Flux melting.
PDC: The acronym for Pyroclastic density current
Peléan eruption: Explosive eruptions in moderately viscous rhyolite-andesite magma, that produces a relatively low eruption column, not unlike Vulcanian types. However, Peléan eruptions are noted for the large, glowing, pyroclastic flows that develop from the collapse or explosive disintegration of a viscous lava dome. Named after the eruption of Mt. Pelée in 1902.
Pele’s hair: Hair-like threads of volcanic glass produced from spattered lava as it cools and stretches. The threads can occur singly or as dense tangles.
Peperite: A mix of brecciated lava and sediment, formed by the explosive injection of magma into water-saturated sediments. Brecciation is partly due to rapid quenching, and to forcible injection of superheated steam. They occur where basaltic lavas flow across lake beds or swamps and partly intrude their sediments.
Pericline twins are similar to albite twins but are oriented at different angles, commonly at 90o to albite twins.
Perthites: One of the key identifiers of potassium feldspars under a polarizing microscope is perthitic texture, which is a mix of two different exsolution feldspar phases – albite and orthoclase. Exsolution occurs during crystallization from the melt.
Phaneritic: A general term to describe volcanic and intrusive rocks where individual crystals can be see without the aid of a microscope. Cf. Aphanitic.
Phonolite: Phonolite is a sodium-potassium rich volcanic rock of intermediate composition, usually associated with partial melting in the lower crust or upper mantle. They are common effusive constituents of seamounts and volcanic islands.
Phreatic eruptions: Explosive eruptions where rock heated by magma comes into contact with groundwater or seawater, but does not involve new magma (i.e. the magma itself is not incorporated into the eruption). Hyaloclastites are a common product of this eruption type.
Phreatomagmatic eruption: Explosive eruptions where rising magma comes into contact with water (sea-lake water, groundwater); Surtsey (Iceland) is an iconic example. Hyalotuffs are a common depositional product of phreatomagmatic eruptions. They tend to be glassy, and finer grained than magmatic eruptions because of the intense reaction between hot magma and water.
Pillow lavas: Bulbous, spheroidal to tubular bodies of lava extruded, toothpaste-like, on to the sea or lake floor. They tend to accumulate in piles where newly formed pillows bud from, and grow around those formed earlier, creating a tight, albeit irregular packing arrangement. Pillows that become detached may roll to the base of the pile. Chilled margins may contain small pipe vesicles. The interstices between pillows usually fill with hyaloclastite fragments formed by shattering of rapidly quenched lava. Pillow lavas most commonly form in subaqueously extruded basaltic magmas but are known from other lava types. Mid-ocean spreading ridges contain humongous volumes of them.
Pipe vesicle: A narrow tube (a few mm across) that protrudes inwards or upwards from the base of a lava flow, resulting from injection of superheated steam derived from underlying soil water and vegetation.
Plinian eruption: A sustained, violent, explosive eruption of viscous siliceous magma that continues for hours or days, producing an eruption column that can reach heights of 20-30 km. Collapse of the column produces pyroclastic flows that reach speeds of 400-700 km/hour. Volumes of ash and blocks produced range from 1-100 cubic km. fine ash and aerosols that enter the upper troposphere and stratosphere can encircle the globe. Magma withdrawal can result in caldera collapse. VEI = 5-7. Named after Pliny the Younger who witnessed Vesuvius’ eruption in 79AD.
Plug: Solidified lava within the conduit or vent of a volcanic edifice. Its resistance to erosion commonly results in an upstanding spire or pinnacle that remains after the enveloping edifice has been removed by erosion. Iconic examples include Sugarloaf Mountain in Rio de Janeiro, Brazil, Pico Cão Grande in São Tomé and Príncipe, and Devils Tower in Wyoming USA.
Progressive laminae: Stoss face laminae that accrete up-flow (i.e. against the current) in bedforms produced during deposition from pyroclastic surges.
Pumice: Light grey-brown, highly vesicular, glassy volcanic rock, usually of dacite or rhyolite composition, formed by the expansion of gas in erupting lava. Commonly seen as floating fragmental debris. Oceanic rafts of pumice indicate submarine eruption.
Pyroclastic density current: (PDC).Pyroclastic density current (PDC): The general name for ground-hugging, gravity-driven mixtures of gas and volcaniclastic debris, derived from explosive eruptions, including pyroclastic flows, welded and non-welded ignimbrites, block and ash flows, pyroclastic surges, and base-flows. They are fast moving (several 10s to 100s of km/hour), and hot (up to 700° C – 1300°F). Fragment debris is predominantly juvenile. Flow is strongly controlled by pre-existing topography. Mechanisms of clast support range from fluidization, turbulence, and dispersive pressures from grain-to-grain collisions; more than one mechanism may operate in a single flow. Clast concentrations are also variable (dilute versus concentrated flows). Flows can be generated by direct lateral blasts, collapse of felsic lava domes, eruption column collapse. Run out distances range from 1- 100 km.
Pyroclastic flow: As a general term it has largely been replaced by pyroclastic density current.
Pyroclastic surge: PDCs that are dilute, ground-hugging, turbulent flows of hot, juvenile volcanic particles and a fluid phase of superheated steam and air. Deposition from turbulent flows commonly leaves deposits (grain size) graded, and because of shear along the base of the flow, deposition associated with traction currents will produce stratification and bedforms. Deposition takes place at supercritical to critical flows.
Regressive laminae: Bedform laminae that accrete down-flow (i.e. in the direction of current flow), most commonly from pyroclastic surges. cf. progressive laminae.
Reynolds number: Derived by Osbourne Reynolds in the mid 19th century, to describe the transition from laminar to turbulent flow. Reynold’s number Re expresses the ratio of inertial (resistance) forces to viscous (resistance) forces:
Re = ρVD/μ
with fluid density = ρ, fluid viscosity μ, mean velocity of flow V, that reflects shear rate and inertia forces, and Tube diameter D that influences the degree of turbulence. Re is dimensionless.
Rhyolite: An extrusive volcanic rock – the highest in SiO2 content at 69%-77%. Tends to be light coloured white to pale grey or pink. Fine grained or replete with quartz and plagioclase and potassium feldspar (e.g., sanidine) phenocrysts, with subordinate biotite, hornblende, and clinopyroxenes like augite. Can also occur as glass, or obsidian. High volatile content produces pumice. As magma, it tends to be highly viscous, forming domes and spines; flow banding is common. Spherulitic structures consisting of fibrous quartz or feldspar form during the late stage of cooling and devitrification.
Run out: The distance traveled by a pyroclastic density current or sediment gravity flow, from start to finish.
Scoria: Mostly lapilli-sized fragments of vesicular, porous pyroclasts, generally of basaltic or andesitic composition – hence dark brown-red colours. Some scoria fragments may be strung out into lacy threads. Cf. Pumice.
Seamount: Submarine volcanoes usually associated with oceanic crust, and formed from deep mantle-derived magma plumes and possibly from shallow rejuvenated plumes. Seamount heights range from a few 100 m, to over 8000 m – they are called volcanic islands where the edifice pokes its vent above the waves. They are similar to terrestrial shield volcanoes in size and gross morphology. They commonly occur in straight or arcuate chains. Iconic examples like Hawaii-Emperor chain and Louisville chain also show consistent age progression. Active seamounts are found in the middle of oceanic plates and close to plate margins; flanking mid-ocean ridges, subduction zones, and along fracture zones. Seamounts that broach the surface may provide habitats for coral reefs. Once volcanic activity ceases, the edifice will gradually sink under its own weight (an isostatic response).
Sector collapse: The collapse of a large portion of a volcanic edifice, usually on steep flanks can occur during an eruption, or long after. Collapse during an eruption may trigger lateral blasts that produce pyroclastic flows (as occurred on Mt St. Helens in 1980). The sector usually breaks up into blocks that produce avalanches and lahars, rather than failing as a coherent unit. Flank collapse into the sea can result in tsunamis.
Shield volcano: Shallow-sided, relatively symmetrical edifices built predominantly of basaltic lava flows and variable volcaniclastic deposits. Slopes range from 1o-10o. Oceanic shields are probably built upon thick piles of pillowed lava – The Hawaiian Island shields are some of the largest in the world, but many other shields are much smaller. Basal diameters range from a few to many 10s of km. The largest shield volcano in the solar system is Olympus Mons on Mars, that is about 600 km in diameter, and 21 km high above the adjacent plains.
Siliciclastic: Sediments composed predominantly of detrital, silica-based minerals; the most common components are quartz, feldspar, and lithic fragments. Heavy minerals such as magnetite, zircon, and tourmaline are important constituents, usually in trace amounts. This broad category includes all grain sizes. It does not include clastic carbonates.
Solfatara: A fumarole where the gas phase is dominated by sulphur compounds like hydrogen sulphide and sulphur dioxide.
Solid solution series: Minerals that share the same basic chemical formula but have different proportions of key elements in their crystal lattice such that crystal form may vary. In sedimentary rocks the most important examples are the alkali (K-Na end-members) and plagioclase (Na-Ca end-members) feldspar groups. Olivine also forms a series with fayalite and forsterite end-members. A mineral’s position in a series reflects the composition and temperature of, for example, the original igneous melts (in the case of feldspar and olivine.
Spatter cone: A small cone-like structure formed by the agglutination of lava spatter along the margins of a fissure or vent. Commonly associated with basaltic fire fountains.
Spherulites: Spherical structures that grow from rapidly quenched fluids. In volcanology, they are commonly found in glassy rhyolites and dacites where they have crystallized directly from the original melt. Usually <10 mm diameter, and tend to occur in clusters or flow bended layers. Each spherulite contains quartz and plagioclase crystallites organized radially.
Stationary waves: Also called standing waves. Surface waves formed during the transition from subcritical to supercritical flow. They are the surface manifestation of, and are in-phase with antidune bedforms on the channel floor; the waves migrate upstream in concert with the deposition of backset laminae on the stoss slopes of antidunes. Stationary waves that break (upstream) have become unstable. Unstable wave eventually decay and surge downstream.
Stratosphere: The stratified atmospheric layer above the troposphere, that extends 30-50 km altitude. It contains most of the ozone. Temperatures in the stratosphere are maintained by ultraviolet radiation absorption in molecules like ozone (O3). Ultraplinian eruption columns may rise to stratospheric levels.
Strombolian eruption: Mild explosive eruptions of relatively fluid magma, that produce incandescent bombs, scoria and lapilli size fragments largely restricted to the cinder cone or volcano flanks. Fire fountains are small. VEI = 1-3.
Subcritical flow: Defined by Froude as the conditions in surface flows where inertial forces dominate and Fr<1. It corresponds to lower flow regime bedforms such as ripples and larger dune structures, that usually are out of phase with surface waves. Also called tranquil flow. cf. antidunes, supercritical flow.
Supercritical flow: Defined by Froude as the conditions in surface flows when gravitational forces dominate (over inertial forces) and the Froude number Fr > 1. The corresponding stream flow surface conditions manifest as an acceleration of flow such that stationary waves (critical flow) break upstream forming chutes. This corresponds to upper flow regime conditions. cf. subcritical flow.
Surtseyan eruption: Violent explosive eruptions caused by the interaction of magma with sea-lake water or groundwater. These are primarily phreatomagmatic eruptions. Eruption columns reach a few 100 metres. Eruptions are a continuous series of jets that can last for weeks, gradually building tephra cones and rings. Named after Surtsey (Iceland), 1963).
Syenite: A plutonic rock derived from partial melting of the lower crust – upper mantle, generally classified as alkalic – >65% feldspar dominated by alkali feldspar (orthoclase) with subordinate muscovite-biotite, pyroxene (augite), and hornblende. SiO2 is 60%-65% but there is no quartz. Its compositional extrusive equivalent is trachyte.
Tephra: As originally defined by Thorarinsson (1941), it includes all air-borne volcaniclastics ejected directly by volcanic eruptions. It does not include subaqueous ejecta. Thus, tephra can include the finest ash particles and the largest blocks. Cf. Tuff.
Trachyte: An extrusive rock with 60%-65% SiO2 but not containing quartz. The primary mineral is alkali feldspar. It is usually coarse-grained – porphyritic with large phenocrysts of K and Na feldspar in a dark groundmass. The plutonic equivalent is syenite.
Tropopause: The boundary between the troposphere and stratosphere – it marks changes in the dynamics of air flow from mixed (troposphere) to stratified, abrupt temperature gradients, and some chemistry (e.g. ozone). It averages 16-18 km high over the tropics, and 6-8 km over the poles, but changes seasonally and with weather systems. It is an important boundary for high altitude volcanic eruption columns.
Troposphere: The lowest layer if air – the layer we live in. It contains most of the water vapour; it determines most of our weather. It is a layer of fluid mixing; Cf. the Stratosphere.
Tuff: This is a volcaniclastic rock name restricted to tephras that are finer-grained than 64 mm. The term can be qualified with prefixes such as fine ash tuff, or medium lapilli tuff. The lithified equivalent for block/bomb tephras is volcanic breccia.
Ultramafic: refers to igneous rock composition where SiO2 is <45%. The two main rock types are peridotite (olivine + pyroxene) and komatiite (extrusive equivalent to peridotite). As in the term mafic, the word is contracted from magnesium and iron (ferrous-ferric) that are important elements in the common ferro-magnesian minerals.
Ultraplinian eruption: With a VEI of 7-8, these are the most powerful eruptions known. They occur in viscous siliceous magmas and produce eruption columns to 50 km altitude (into the stratosphere). The volume of material erupted ranges from 100-1000 cubic kilometres. Eruptions of this magnitude, including some larger Plinian eruptions, can have a significant effect on global climates because of the volume of fine ash and aerosols in the upper atmosphere. Geologically young examples include Yellowstone supervolcano (632 Ka) falls into the latter category, as did Toba (northern Sumatra, 74Ka), and the most recent event at Taupo a mere 1800 years ago.
Vesicularity: Vesicles are subspherical to elongate pores that form during the rise and eruption of magma, as volatile gasses and water vapour, that originally were dissolved in the magma, depressurise. Vesicles are the frozen remnants of these gas bubbles. An extreme example of vesicularity is pumice, that originates as magma ‘froth’. During burial, vesicles are filled with minerals like zeolites and calcite – filled vesicles are called amygdaloids.
Volcanic ash: Sedimentary particles derived directly from volcanic eruptions, ranging from clay-sized material to 2 mm. Subdivisions into fine, medium, coarse, very coarse ash are analogous to Wentworth sand size scale. Cf. lapilli, Wentworth grain size.
Volcanic explosivity index (VEI): A measure of the explosiveness of eruptions, or the amount of kinetic energy involved (Newhall & Self, 1982), based on the erupted volume (as lava or fragmental debris), eruption column height, and the degree of particle fragmentation. The scale is logarithmic. Each category is labelled according to its ‘appearance’, ranging from non-explosive, quietly effusive lava flows (zero explosivity), to colossal super-eruptions (Yellowstone, Toba, Taupo) at 8.
Volcanic island: Refers to seamounts that have breached the sea surface such that active volcanism involves subaerial processes. Good examples include Hawaii islands, Canary Islands, and Cook Islands.
Volcaniclastic: Fragmental debris derived from volcanic eruptions. This includes air-fall ash (tephras), ballistics, and pyroclastic flow and surge deposits (e.g. hot and cold ignimbrites) that are derived directly from eruption events such as collapsing eruption plumes; also called primary volcaniclastics. Material that is redeposited by terrestrial lahars or subaqueous sediment gravity flows (turbidites, debris flows), or redistributed by rivers are secondary volcaniclastics.
Volcanic gas: All active volcanic centers emit gas, pre-, post- and during eruptions. On average, 96% of volcanic gases are water vapour, the remaining components being CO2, SO2 (most common), plus a little helium, nitrogen, carbon monoxide, hydrogen sulphide, and a few halides. Under normal circumstances, volcanic CO2, helps maintain the balance from the perspective of greenhouse forcing.
Volcanic quartz: A variety of monocrystalline quartz that typically shows well developed crystal faces and pointy, pyramid-like terminations. It is common in acid volcanic rocks like rhyolite and dacite. Crystal margins may show small bubble-like indentations. This variety of quartz is a good provenance indicator.
Vulcanian eruption: Magmas tend to be more viscous than Hawaiian and Strombolian eruption, and involve more violent phreatic and phreatomagmatic events that produce ash to bomb sized ejecta. Eruption plumes can reach 10 km altitude and more. Airfall tephra is more widespread; pyroclastic flows develop from collapse of eruption columns. VEI = 2-4.
Yield strength Viscous fluids have finite strength, called the yield strength where the fluid will not deform or flow below a critical stress. Fluids (or solids) that behave in this manner are referred to as hydroplastic or plastic,
Zoning (in crystals): Zoning commonly displays as concentrically arranged crystal growths, where the composition changes outwards from the crystal interior. Zoned crystals may also be twinned.
