Appendix G—
Glossary

Terms and explanations in this glossary reflect the latest usage in the volcanological literature cited within the text. Wherever possible, the definitions are adapted from those in the American Geological Institute Glossary (1980).

A

Aa lava
Hawaiian term for lava flows characterized by a rough, jagged, spiny surface.

Accretionary lapilli
More or less spherical masses of cemented ash ranging in diameter from a few millimeters to several centimeters; sometimes referred to as volcanic bailstones, pisolites , or volcanic mudballs .

Adiabatic
Refers to a process that causes no change in the heat content of a system; for example, when a gas or fluid is compressed or expanded.

Advection
Spatial transfer of physical properties, such as heat, through a system.

Agglutinate
Vitric pyroclasts consisting of a cluster of smaller pyroclasts that are annealed or "welded" to one another.

Aphanitic
Texture of a fine-grained igneous rock in which the crystalline components are not visible to the naked eye.

Aphyric
Igneous texture in which phenocrysts are absent in a fine-grained or glassy groundmass.

Aquagene
Synonym for hyaloclastic .

Aquifer
Body of rock or unconsolidated deposits with enough permeability to conduct a significant amount of groundwater.

Aquitard
Rocks that confine, but do not necessarily prevent leakage from an aquifer.

Argillic
Type of rock alteration in which the products are mostly kaolin- and smectite-group minerals.

Ash fall
Rain of airborne volcanic ash from an eruption cloud and the resulting deposit. Synonym: Ash fallout .

Ash flow
Synonym for pyroclastic flow but referring to pyroclastic flows that consist of ash.

Asthenosphere
Region below the Earth's lithosphere in which magmas may be generated and seismic waves are strongly attenuated. Synonym: upper mantle .

Audiomagnetotelluric
Geophysical survey technique that measures global electric and magnetic fields in audiofrequencies.

Authigenic
New minerals formed in place; for example, authigenic clays formed during alteration of volcanic glass by hot water.

B

Batholith
Large, usually composite mass of plutonic rock.

Block
Pyroclast larger than 64 mm that was solid when ejected. It may be juvenile or torn from older rocks in the vent or crater walls.

Blocky
Grain shape characterized by planar or curviplanar surfaces that intersect at nearly right angles.

Bomb
Pyroclast larger than 64 mm that was partly or completely fluid when ejected.

Breccia
Fragmental rock composed of highly angular coarse fragments with or without a fine-grained matrix.

C

Calc-alkalic
Igneous rocks in which the weight percent of SiO₂ is between 56 and 61 and the weight percents of CaO and K₂ O+Na₂ O are nearly equal. Also denotes igneous rock suites composed dominantly of feldspar with lesser amounts of quartz, biotite, amphibole, pyroxene, and other accessory minerals.

Caldera
Volcanic crater formed by collapse during an eruption.

Caprock
Rock layer or zone with low permeability, often related to authigenic cementation; restricts fluid migration from a hydrothermal system below it.

Cinder
Synonym for scoria .

Cinder cone
Synonym for scoria cone .

Clast
Individual fragment or grain within a fragmental rock. Within volcanic rocks, clasts derive from mechanical disintegration of the rising magma, fracturing of

conduit wall rocks, and/or pieces of substrata incorporated by flows.

Coignimbrite
Usually refers to fallout tephra formed during an ignimbrite eruption, including breccias composed of large clasts emplaced near the vent by a pyroclastic flow and fine-grained ash elutriated from the top of a pyroclastic flow by the turbulent rise of hot gases.

Comminution
Decrease in size of a substance to a powder or dust by crushing, grinding, or rubbing. Synonym: pulverization .

Composite cone
Large volcanoes that consist of multiple volcanic landforms and interbedded pyroclastic rocks, lavas, and volcanic sediments. Synonym: Stratovolcano or stratocone .

Connate
Originating at the same time as the adjacent material; for example, water trapped within a rock at the time of deposition.

Crossbed
Bed inclined at an angle to the plane of stratification.

Cryptodome
Uplift of the ground surface by intrusion at very shallow depths. The final land-form consists of a lava dome with a thin carapace of older rock and soil.

D

Dense-rock equivalent (DRE)
Volume of igneous rock erupted after all pore space caused by vesiculation, fracturing, and intergranular porosity has been subtracted. This calculation is used to determine the volume of magma that reached the surface during an eruption.

Devitrification
Conversion of glass to crystalline materials.

Diagenesis
Chemical and physical changes in a rock after its deposition and during and after lithification.

Diamicton
Poorly sorted clastic deposit that contains a wide range of particle sizes; for example, a lapilli clast in a matrix of fine ash.

Diapir
Rock mass that has risen buoyantly to cut or deform overlying rocks. Diapirs can refer to magma bodies, salt, or mud.

Diatreme
General term for a volcanic vent or conduit drilled upward through enclosing rocks by the explosive energy of gas-charged magmas. The surface expression of a diatreme is a maar crater or, after extensive erosion, a tuff or tuff-breccia neck.

Dike swarm
Group of closely spaced dikes intruded during the same intrusive/eruptive episode.

Dry eruption cycle
Eruptions producing pyroclastic products that show textural features characteristic of decreasing water:magma ratios during the course of the eruption.

Dry surge
Pyroclastic surge containing steam that is mostly superheated. Surge deposits show little textural evidence of the presence of moisture during emplacement.

E

Endogenous
Geologic process (or its resultant features or rocks) that originates within the Earth; refers to plutonic and volcanic rocks.

Energy line
Loci of points where a potential energy surface intersects a topographic surface. The term's application to pyroclastic flows has evolved from studies of rock-fall debris streams.

Equant
Refers to a rock fragment or crystal that has the same or nearly the same diameter in all directions.

Eruption cycle
Sequence of events or changes in behavior during a volcanic eruption. Synonym: eruption (Fisher and Schmincke, 1984).

Eruption plume
Convoluted, rolling mass of partly condensed water vapor, magmatic gases, dust, and ash that often rises to great heights. Synonym: eruption cloud .

Eruption unit
Deposits of volcanic material from an eruptive pulse, eruptive phase, or eruption (Fisher and Schmincke, 1984).

Euhedral
Refers to a mineral grain completely bounded by its own crystallographic faces, with no interference by surrounding minerals.

Exogenous
Refers to a geologic process at or above a planetary surface; for example, an extrusive volcanic dome. Contrast with endogenous .

Expansion wave
Perturbation in fluid flow that propagates at the local sound speed while causing the flow to accelerate, expand in volume, and decrease in density and pressure.

Explosion pit
Bowl-shaped crater surrounded by a low ejecta rim; generally formed by phreatic or phreatomagmatic explosions in which little or no juvenile material is ejected.

Exsolution
Process whereby an initially homogeneous solid separates into two or more distinct crystalline phases without addition or removal of material.

Extrusive
Refers to igneous rock that has been erupted onto a planetary surface.

F

Facies
Physical, chemical, and biological variations of rock bodies deposited within a specific geologic time interval.

Felsic
Somewhat archaic general term for silicic igneous rocks.

Fissure vents
Long, narrow cracks or fissures along which eruptions occur.

Flank vents
Vents located on the flanks of a larger volcano, which have their own plumbing systems that are independent of those from the larger volcano.

Fluidal
Refers to pyroclasts with smooth, round surfaces formed by surface tension within a droplet.

Fluvial
Pertaining to a river and its deposits.

Fumarole
Vent from which volcanic gases are emitted.

G

Geobarometry
Method that uses pressure-sensitive mineral reactions to indirectly determine the pressure conditions under which a rock formed.

Geothermometer
Mineral or mineral assemblage whose compositions are fixed within known temperature limits.

Glomeroporphyritic
Igneous texture denoting clustering of phenocrysts in a finely crystalline or glassy groundmass.

Granulometry
Measurement of grain sizes.

Groundmass
Fine-grained material between phenocrysts in a porphyritic volcanic rock.

H

Hawaiian eruption
Eruption of low-viscosity gas-charged magmas as lava fountains and thin, rapidly moving lava flows.

Heat flow
Amount of heat leaving the Earth; determined by measuring the thermal gradient (degrees per kilometer of depth), thermal conductivity of rocks (W/m-K), and other factors such as permeability. One heat flow unit (HFU) = 41.84 mW/m² = 10^-6 cal/cm² -s.

Hot-dry-rock (HDR)
Geothermal resource derived from the thermal energy within rocks that lack permeability or have very low permeability. Heat transfer within these resources is by conduction only; however, the resource can be exploited by creating fractures and circulating fluids through those fractures.

Hot spring
Thermal spring whose temperature is above that of the human body (Meinzer, 1923).

Hyaloclastite
Volcaniclastic rocks generated by nonexplosive to mildly explosive granulation of volcanic glass when magmas are quenched on contact with water. These are common products of deep-sea volcanism.

Hyalotuff
Pyroclastic rocks generated by phreatomagmatic explosions in shallow surface water.

Hydration
Transfer of H₂ O from a fluid phase into the structure of a mineral or glass.

Hydraulic fracturing
Fracturing of rock by fluid overpressure.

Hydroclast
Clasts formed during hydroclastic eruptions.

Hydrofracture
Shortened synonym for hydraulic fracturing .

Hydrogeochemistry
Chemistry of ground and surface waters and the fluids in hydrothermal systems.

Hydromagmatic
General term for all processes—subsurface or surface—involving interaction of magma or magmatic heat with meteoric or connate water in the Earth.

Hydrothermal systems
Natural hot water or steam systems caused by circulation of groundwater through permeable rock units over a natural source of heat; for instance, elevated heat flow in regions where there is a thin crust or a magma body.

Hydrothermal eruption
Synonym for phreatic eruption .

Hydrothermal reservoir
Combination of a heat source and permeable rock that allows the convective circulation of hot fluids; an accumulation of hot water and steam.

Hydrovolcanic
Term encompassing all volcanic activity that results from the interaction between lava, magmatic heat, or gases and meteoric or connate water at or near the surface of the Earth. Synonym: Phreatomagmatic .

I

Ignimbrite
Rock unit of consolidated tuff deposited by pyroclastic flows.

Intraplate volcanism
Volcanism occurring within tectonic plates, away from plate margins.

Isentropic
Refers to an idealized process that causes no change in entropy.

Isobaric
Refers to an idealized process that causes no change in pressure or a system in which all locations have equal pressure.

Isopach
Line on a map drawn through points of equal thickness of a designated rock unit.

Isopleth
Line on a map drawn through points of equal or constant size, composition, or abundance. In volcanology, these lines show areal distributions for values such as the diameter of pumice or lithic clasts.

Isotherm
Line connecting points of equal temperature.

Isothermal
Refers to an idealized process that causes no change in temperature or a system in which all locations have equal temperature

J

Jet
High-velocity stream of pressurized fluid forced out of a narrow or restricted opening.

L

Laccolith
Shallow igneous intrusion with a flat floor and deformed roof that is found in sequences of bedded rocks.

Lag breccia
Concentrations of large, comparatively dense lithic clasts at the base of a pyroclastic flow unit. These clasts are deposited close to the vent during sedimentation from the pyroclastic flow.

Lahar
Debris flow or mudflow of pyroclastic material that is mainly derived from pyroclastic deposits. Indonesian term for volcanic mudflow .

Lapilli (singular: lapillus)
Pyroclasts between 2 and 64 mm.

Lava fountain
Jet of molten lava driven by hydrostatic pressure and/or expansion of magmatic gases; erupted nearly vertically from vent.

Lava lake
Lake of molten lava in a crater; also applies to a solidified lake.

Liquidus
Loci of points in a temperature-composition diagram that represent conditions of maximum solubility for a solid component or phase in the liquid phase. In a binary system, it is a line and in a ternary system, a surface. It can also refer to the temperatures above which the system is completely liquid.

Lithic pyroclasts
Pyroclasts consisting of previously formed rocks; fragments of igneous, metamorphic, or sedimentary rock; or pieces of earlier lavas from the vent that are included in a pyroclastic deposit.

Lithophysae (singular: lithophysa)
Spherical structures composed of radially oriented, finely crystalline minerals—usually alkali feldspar and silica minerals; found in glassy silicic lavas and densely welded tuffs. Some lithophysae are hollow.

Lithosphere
Solid portion of the Earth; in most definitions, this includes the crust and part of the upper mantle.

Littoral cone
Tuff cone formed on or adjacent to a lava flow where it entered a body of water. Such cones are formed by steam explosions. Synonym: rootless cone .

M

Maar
Small volcano characterized by a crater that is wide (several hundred meters to several kilometers in diameter) relative to its height and whose floor commonly lies below the general level of surrounding topography. Maar volcanoes can form during hydrovolcanic eruptions or very energetic gas eruptions.

Mafic
Refers to an igneous rock that is composed chiefly of ferromagnesian minerals and is low in SiO₂ content.

Magma
Molten rock within a terrestrial planet.

Maturity
Progression of a volcano through a lifetime of eruptive periods, during which it changes in composition, magnitude, volcanic landforms, and associated hydrothermal activity. Progression from first activity to complete dormancy.

Megabreccia
Clasts larger than a single outcrop that are slump deposits formed by wall collapse during caldera collapse.

Merapian
Volcanic activity caused by disintegration and collapse of a silicic dome or lava flow.

Mesobreccia
Concentrations of small lithic clasts interlayered with caldera-filling tuffs.

Microlites
Small crystals, usually of tabular or prismatic habit, that are often contained in a glassy matrix.

Microseismicity
Natural background noise that affects seismographs. Within geothermal areas, systematic variations in microseismic activity can be used to study fluid motions and natural fracturing within the system.

Mineral spring
Spring whose water contains enough mineral matter to give it a definite taste (Gary et al ., 1973). By convention, this term is applied only to nonthermal springs (Mariner et al. , 1990).

Moberg
Table mountain formed by a fissure eruption of basalt under a glacier.

Mohorovicic discontinuity
Seismic-velocity discontinuity that separates the Earth's crust from the underlying mantle; its depth ranges from 5 to 10 km beneath the ocean floor to 35 to 60 km under the continents.

Monogenetic volcano
Volcano constructed during a single phase of eruptive activity.

N

Noncondensible
Characteristic of a fluid when it passes directly from a gas to a solid with decreasing temperature.

Nuée ardente
"Glowing cloud" that consists of a mixture of pyroclasts and hot gases and moves rapidly down the flanks of a volcano as a density current. Synonym: pyroclastic flow .

O

Overpressure
Pressure in excess of lithostatic or atmospheric pressure.

P

Pahoehoe lava
Hawaiian term for a basaltic lava flow typified by a smooth, billowy, or ropy surface.

Palagonite
General term used to describe the alteration products of basaltic glass. Mineral phases in palagonite include smectite clays, iron oxides, calcite, and zeolites. Products may be formed during lithification of the deposit, weathering, or hydrothermal alteration of the glass phases.

Paleosol
Buried soil horizon of the geologic past.

Paragenesis
Characteristic order or sequence in which a group of minerals is formed; a reflection of the physical and chemical conditions under which the rock was formed.

Parasitic vents
Volcanic vent occurring on the flank of a larger volcano that is linked to the central conduit of the larger volcano. In contrast, flank vents occur on the flanks of a larger volcano but are not linked to its central plumbing system.

Peléean eruption
Characterized by explosions of moderate to extreme violence in which solid or viscous hot fragments of new lava are ejected—commonly as pyroclastic fallout and pyroclastic flows. These eruptions are usually associated with silicic magmas.

Peperite
Breccia-like rock formed as lava intrudes water-saturated sediment.

Peralkaline
Igneous rock in which molecular proportions of Na₂ O+K₂ O are greater than those of Al₂ O₃

Perlite
Glassy silicic lava or welded tuff with a relatively high water content. Perlitic

texture is characterized by multiple spheroidally curved cracks.

Petrogenesis
Study of rock origins.

Phenocryst
Large, conspicuous crystal in a porphyritic igneous rock.

Phi (f)
Logarithmic scale of particle diameters where f = -log₂ (diameter in millimeters).

Phreatic eruption
Explosion that follows transformation of groundwater into steam. No incandescent or juvenile material is erupted.

Phreatomagmatic eruption
Explosive volcanic eruption caused, at least in part, by interaction of magma with meteoric water (groundwater or shallow surface water). Synonym: hydrovolcanic .

Phreatoplinian eruption
Plinian eruption during which interaction of vesiculating magma and near-surface water produces very fine ash and abundant steam in an eruption column that reaches great heights.

Pillow lava
Lavas displaying pillow- or tube-like forms that have formed in a subaqueous environment.

Pit crater
Small crater formed by collapse during magma withdrawal; not always a vent.

Platy
1. Particle texture in which length is more than three times its thickness.
2. Planar flow structure within a lava flow.

Plinian eruption
Paroxysmal ejection of large volumes of ash and pumice as a well-defined eruption column or "jet;" often precedes caldera collapse. The resulting tephra fallout covers an area of more than 500 km² .

Pluton
Igneous intrusion.

Polygenetic
Resulting from more than one formation process, derived from more than one source, or originating at various places and times.

Porphyritic
Refers to the texture of an igneous rock in which the larger crystals (phenocrysts ) are set in a fine-grained groundmass.

Proppant
Particles within a fracture that hold the fracture open.

Pumice
Highly vesicular pyroclasts with very low bulk density and thin vesicle walls.

Pyroclast
Any fragment ejected during an explosive volcanic eruption.

Pyroclastic flow
Eruption cloud consisting of hot pyroclasts and gases that are driven by gravity and move across the ground as a density current. Many flows are generated by collapse of a particle-laden eruption column. Most flows move at high velocity downslope and along drainage systems, but some have enough energy to move across hills and valleys as well.

R

Rarefaction wave
Expansion wave that initially moves in the opposite direction from an associated

shock wave; for instance, down a volcanic conduit during an explosive eruption.

Resurgent caldera
Caldera in which the downdropped block is uplifted by magmatic intrusion following crater formation.

Reworked
Refers to any geologic material that has been removed or displaced naturally and incorporated into a younger geologic unit.

Rheomorphic tuff
Densely welded tuff that has flowed after deposition; often so similar to a silicic lava flow that it is difficult to distinguish between them in the field.

Ring faults
Steep, cylindrical, or semicylindrical faults bounding a collapse crater or caldera. Ring dikes , commonly associated with ring faults, have moved along these faults after caldera collapse.

S

Sandwave
Any sand dune or wave-like bedform in clastic rocks. A large and asymmetrical bedform.

Scoria
Vesicular, coarse-grained (lapilli-size or coarser) pyroclasts of basaltic or basaltic andesite composition. Vesicle walls are usually thick (much thicker than those in pumice pyroclasts). Synonym: Cinder .

SEM
Scanning electron microscope.

Shard
Glass fragment (vitric pyroclast) that has been broken from a vesicle wall (in magmatic eruptions) or formed during chilling and fragmentation (in phreato-magmatic eruptions). It may be flat, curved, blocky, or Y-shaped, depending upon the bubble-wall segment from which it was broken.

Shock wave
Hydrodynamic discontinuity in a fluid that is set up when the flow suddenly changes from subsonic to supersonic. Shock waves are characterized by instantaneous increases in temperature, pressure, and density.

Sideromelane
Basaltic glass; clear, brown glass found in basaltic ash and lava flow margins.

Silicic
Refers to a silica-rich igneous rock or magma. In most classifications, the amount of SiO₂ is at least 65%. Granite and rhyolite are typical silicic rocks.

Silica sinter
Chemical sedimentary rock deposited as a hard encrustation by precipitation from hot springs.

Soda spring
Mineral spring whose waters effervesce carbon dioxide (Mariner et al ., 1990).

Solidus
On a temperature-composition diagram, the loci of points in a system above which solid and liquid are in equilibrium and below which the system is completely solid.

Spatter
Accumulation of fluid, coarse pyroclasts around a vent. In most spatter accumulations, the pyroclasts are welded to one another.

Stable isotope
Nuclide that does not undergo radioactive decay.

Stratocone or stratovolcano
Volcano made up of many different volcanic landforms and deposits: interbedded lavas, pyroclastic deposits, and sedimentary deposits. Synonym: composite cone .

Strombolian eruption
Weak-to-violent, sporadic, ballistic eruptions of tephra, generally of moderately fluid basaltic or andesitic magma. Scoria cones are constructed by this type of activity.

Subplinian eruption
Small-scale Plinian eruption—intermediate between Strombolian and Plinian activity—that is characterized by pumice and ash deposits covering less than 500 km² .

Supercritical
Thermodynamic state of a system at which its temperature is greater than its critical temperature and in which it displays both liquid and vapor behavior.

Superheat
Heat in excess of that (a) required for a fluid (water) to exist as a saturated vapor or (b) necessary to cause complete melting of a solid (magma).

Surge
Density current pulse that moves laterally outward from an explosion column either by directed blast or column collapse. Surge deposits are thinly bedded and may consist of dune-like beds, inversely graded flat-lying beds, and massive beds.

Surtseyan eruption
Eruptions characterized by steam explosions that eject new lava fragments as pyroclastic surges and fallout. Activity typified by the eruptions of the volcano Surtsey in Iceland, where rising basaltic magma explosively vaporized near-surface water. Synonyms: phreatomagmatic and hydrovolcanic .

T

Tachylite
Hyalocrystalline pyroclasts; that is, pyroclasts consisting of groundmass minerals separated by glass. These are basaltic—or sometimes andesitic—and, in hand-specimen and under transmitted-light microscopes, they appear black or brown.

Tephra
Collective term (generally plural) used for all material—regardless of size—ejected during an explosive volcanic eruption. From the Greek term for volcanic ash, it was originally used by Aristotle and was revived by Thorarinsson in 1944.

Tephrochronology
Use of tephra layers for correlation and dating.

Thermal regime
Systematic patterns of heat flow within the Earth's asthenosphere and crust that are linked to dynamic conditions imposed by tectonic patterns, ground-water movement, rock chemistry, or plutonic/volcanic history.

Thermal spring
Spring whose water temperature is appreciably higher than the local mean annual atmospheric temperature; either a warm spring or a hot spring (Meinzer, 1923).

Thermal resource
Total heat contained in a body of rock; for example, a magma body.

Thermite
Incendiary mixture of fine aluminum powder and a metallic oxide, which when ignited yields intense heat through an oxidation-reduction reaction.

Tuff
Consolidated pyroclastic rock.

Tuff ring
Volcano constructed by pyroclastic deposits with low depositional slopes (2 to 10°) that encircle a relatively wide crater.

Tuff cone
Volcano composed of indurated ash with 20 to 30° slopes.

U

Ultraplinian
Highly energetic eruption in which the area of pumice and ash fall exceeds 50,000 km² .

V

Vapor-phase alteration
Alteration of tuffs that results from crystallization of silica minerals (tridymite and cristobalite), alkali feldspar, and minor amounts of other minerals within porous zones of pyroclastic flows and lava flows. The alteration occurs when gases released during compaction of hot ash deposits interact with glass shards and pumice.

Vesicle
Bubble formed when molten rock approaches the ground surface and volatiles come out of solution at the lower pressures. Vesicularity refers to the vesicle volume in a rock.

Vitric
Glassy; for instance, a vitric tuff , which consists mostly of glass pyroclasts.

Vitrophyre
Igneous rock with a glassy groundmass.

Volcaniclastic
Clastic rock containing volcanic material in any proportion, regardless of its origin or environment.

Vulcanian eruption
Moderate to violent ejection of solid or very viscous hot fragments of new lava in short-lived, cannon-like bursts. Ash and fine ash are emitted with gases and ascend to form a cauliflower-like eruption cloud.

W

Warm spring
Thermal spring whose temperature is appreciably above the local mean annual temperature but below that of the human body (Meinzer, 1923).

Wet eruption cycle
Eruption sequence producing pyroclastic deposits that have textural features indicative of increasing water:magma ratios.

Wet surge
Pyroclastic surge containing saturated steam. This designation reflects the physical state of the tephra after deposition; wet surge deposits show textural and diagenetic evidence of being wet when emplaced.

Working fluid
Any fluid that produces pressure-volume work during changes in pressure and/or temperature. In geothermal systems, the working fluid is water.

X

Xenocryst
Crystal that resembles a phenocryst but is foreign to the body of rock in which it is found.

Xenolith
Foreign rock fragment in an igneous rock; lithic clast or lithic fragment.