Glossary

A Hawaiian term for lava that has a rough, jagged, spiny, and often clinkery surface.In thick aa flows, the surface comprises rubble composed of loose, rough lapilli and blocks that generally hides a thick, more massive flow interior (Tilling et al., 1987).The thickness of the surface crust of aa lavas is controlled by cooling (Kilburn, 2000, p. 291).

Active volcano:A volcano that is currently erupting, one that has erupted during recorded history, or one that has erupted during recorded history and is likely to erupt again (Foxworthy and Hill, 1982).

Accessory fragment:A lithic fragment composed of country rock that has been explosively ejected during an eruption (Cas and Wright, 1987, p. 54).Accessory fragments within pyroclastic deposits may be difficult to distinguish from accidental fragments.In general terms, referred to as a xenolith.

Accidental fragment:A clast picked up locally by pyroclastic flows and surges (Cas and Wright, 1987, p. 54).Accidental fragments may be difficult to distinguish from accessory fragments.In general terms, referred to as a xenolith.

Accretionary lapilli:Spherical aggregates (commonly with a concentric structure) formed by the accretion of moist ash in eruption clouds (White and Houghton, 2000, p. 495).Also used for all ash aggregates, including mud lumps (Houghton et al., 2000, p. 513).

Achnelith:A type of juvenile fragment characterized by smooth, glassy molded surfaces formed from lava spray from extremely fluid mafic eruptions (Walker and Croasdale, 1972).

Agglomerate:A course, pyroclastic deposit composed of a large proportion offluidal-shaped volcanic bombsthat are formed, in the strictest sense, by a fall deposit in the immediate vicinity of a volcanic vent. It is best applied to describe bomb and scoria deposits that build strombolian cones, and should never be used as a non-generic term for a volcanic breccia (Cas and Wright, 1987, p. 359).

Aerosol:Fine liquid or solid particles suspended in the atmosphere.Aerosols composed of tiny droplets of sulfuric acid are commonly formed during explosive volcanic eruptions.

Airfall:Volcanic ash that has fallen through the air from an eruption cloud.Airfall deposits are characteristically well-sorted and well-layered, and typically exhibit mantle bedding (Foxworthy and Hill, 1982; Cas and Wright, 1987).

Alteration mineral assemblages:Mineral assemblages found in rocks that result from chemical reactions between the original rock and an agent of alteration (for example, hot volcanic vapors or hydrothermal fluids).

Amygdaloidal:A volcanic texture comprising vesicles (rounded holes resulting when magma cools around gas bubbles) which have been subsequently filled by secondary minerals.

Amygdule:An individual vesicle which has been subsequently filled-in by secondary minerals.

Andesite:A grey to grey-green colored volcanic rock containing 53% to 63% silica (compositionally between basalt and dacite).Minerals commonly found in andesite include intermediate composition plagioclase and hornblende.

Andesitemagma:A magma with a chemical composition ranging from 53% to 63% which, upon crystallization, forms an andesite.

Armoured lapilli:A type of accretionary lapilli composed of a crystal, pumice, or lithic fragment core which is surrounded by a rim of fine to coarse ash (McPhie et al., 1993, p. 29).

Ash:A textural term for volcanic fragments less than 2mm in diameter (Fisher, 1966; Schmid, 1981).Ash is the typical product of explosive volcanic eruptions.

Ash cloud:A cloud of ash produced during pyroclastic eruptions (Miller, 1989).These clouds can result from rapid rising of the hot, buoyant ash-rich eruptive plume, or can be derived by elutriation at the top of a pyroclastic flow (Cas and Wright, 1987).

Ash flow:A type of pyroclastic flow comprising dominantly ash-sized particles. Hot ash flows may be called glowing avalanches or nuee ardentes, and if their volume is large enough, may eventually form deposits known as welded tuffs.These types of flows are extremely dangerous and historically have killed hundreds of thousands of people.

Atmospheric shock wave:A strong compressional shock wave caused by a combination of volcanic ejecta and sonic waves.

Avalanche:A large mass of material or mixtures of materials (e.g snow, ice, rock, soil, etc.) that is falling or sliding rapidly due to the force of gravity.Debris avalanches are avalanches composed of a mixture of earth materials (Foxworthy and Hill, 1982).

Ballistic fragment:An explosively ejected rock fragment that follows a ballistic (arced) trajectory.

Basalt:A dark colored (usually dark grey, dark green, or black), low silica content (45% to 53% SiO2) volcanic rock.Minerals commonly found in basalt include intermediate to calcium-rich plagioclase, pyroxene, and commonly olivine.Accessory minerals commonly include ilmenite and magnetite.

Basaltic magma:A low viscosity, low silica (45% to 53% silica) magma that, upon crystallization, forms the volcanic rock basalt.

Base surge:A turbulent, low-density cloud of rock debris, water, and/or steam that moves over the ground surface at extremely high speeds.Base surges are commonly the result of directed volcanic explosions.Base surge deposits are commonly composed of cross-bedded deposits comprising ash and lapilli.

Bimodal:A term used to describe a material composed of two distinctly compositionally and/or texturally different components.Commonly used to describe volcanic terrains that have nearly equal proportions of felsic and mafic volcanic rocks.

Blocks:Fragments of solid rock greater than 64 millimeters in diameter that are ejected during volcanic eruptions.Blocks are commonly composed of accessory fragments made up of crystallized magma associated with the eruption (e.g. pieces of a lava dome).

Blocky lava:Lava flows that are characterized by highly fractured surfaces which contain fragments of debris (usually flow fragments) up to several meters in diameter.The size of the surface fragments in blocky lavas is controlled by the rheology of the lava in the interior of the flow (Kilburn, 2000, p. 291).

Boiling lake:A lake which has a temperature of nearly 100C.Examples include the Boiling Lake on Dominica and a lake of mud on Saint Lucia (Bardintzeff and McBirney, 2000, p. 159).

Bombs:Juvenile fragments of semi-solid or plastic magma ejected during a volcanic eruption.Based on their shapes after they hit the ground and cool, bombs are given various textural names including breadcrust bombs, cow-dung (cow pie) bombs, spindle bombs (fusiform bombs) and ribbon bombs.

Caldera:Large, circular to elongate, volcanic collapse depressions that form from the rapid extrusion of magma form a shallow subterranean magma chamber.In general, the diameter of a caldera is much greater than any of its individual volcanic vents (Williams and McBirney, 1979, p. 207).

Caldera cycle:A commonly observed evolutionary sequence recognized in many caldera complexes.From oldest to youngest, the seven stages of the caldera cycle are: 1) regional tumescence and generation of ring fractures; 2) ignimbrite (pyroclastic) eruption(s); 3) caldera collapse; 4) pre-resurgent volcanism and intra-caldera sedimentation; 5) resurgent doming; 6) major ring fracture volcanism; and 7) terminal fumarolic and/or hot spring activity.

Cinders:A term to describe generally highly vesicular, mafic lava lapilli.

Cinder cone:A small, generally conical-shaped volcano formed by accumulation of ejected cinders and other volcanic debris that falls back to the earth close (proximal) to the location of the volcanic vent (Gardner et al., 1995).

Clay (minerals):A group of aluminum-bearing hydrous phyllosilicate minerals (for example, kaolinite).

Clay (textural):A sedimentary grain size classification for particles less than 1/256 mm in diameter, regardless of mineralogy.

Cognate lithic fragment:Non-vesiculated juvenile magmatic fragments that have silicified from the erupting magma (Cas and Wright, 1987, p. 54).

Columnar jointing:A type of fracture pattern resulting from the thermal contraction of hot volcanic rocks after their crystallization which commonly is expressed in elongate, pentagonal or hexagonal columns oriented perpendicular to the cooling surface.Columnar jointing is common in all compositions of lava flows, although it is generally best developed in mafic (basalt) lava flows and in felsic welded tuffs.

Composite volcano:A generally steep sided volcano composed of a mixture of lava flows, pyroclastic deposits, and volcaniclastic sedimentary deposits.Composite volcanoes commonly have increasing slopes toward their summits since they generally have mainly lava flows and sedimentary deposits near their base and pyroclastic (tephra) deposits near their summits.

Conduit:The underground passage or passages through which magma makes it way to the earths surface.

Cooling unit:A group of hot pyroclastic deposits (ignimbrites) that cools at more or less the same time.A deposit from a single eruption that shows simple variations in the degree of welding is known as asimple cooling unit.When many ignimbrites occur over an extremely short period of time, each individual ignimbrite may be deposited, and start to weld over a previous deposit or group of deposits that are cooling and undergoing welding.The resulting deposits have several zones of partial and dense welding, and since they more or less cool together, are known ascompound cooling units(Cas and Wright, 1987, p. 253-255).

Coule:A type of rhyolite lava flow that forms when lava issues from one side of a volcanic vent and produces a lava flow which is elongate in plan view (Cas and Wright, 1987, p. 81).

Crater:A steep sided, usually bowl or funnel shaped depression that commonly occurs at the top of a volcanic cone, and is often a vent for eruptions (Lipman, 2000, p. 643). Volcanic craters may be formed by either explosion or collapse in the vicinityof the volcanic vent.

Curie point:The temperature at which a body loses (by heating) or preserves (by cooling) its permanent magnetization.As rocks cool, the electromagnetic field aligns magnetic minerals in the magma, and their orientation is preserved as the rocks cool below the Curie point.

Dacite:A generally light-colored, relatively silica rich (65% to 68 % SiO2) volcanic rock (extrusive equivalent of a quartz diorite or a tonalite).Dacitic magmas have a relatively high viscosity, and their associated volcanic eruptions may produce thick, muffin-shaped lava flows (lava domes) or, commonly, may be explosive and produce abundant tephra resulting in ash falls, ash flows, and surges.Dacites typically contain intermediate plagioclase (andesine or oligoclase) and quartz (10%) with pyroxene and/or hornblende with minor biotite and/or sanadine (volcanic K-feldspar).

Debris flow:A type of mass flow comprising a dense, cohesive, flowing mixture of sediment (mud through boulder sized materials, generally 50% by volume), water, and commonly, organic debris.Debris flows generally move downslope in laminar fashion due to the force of gravity (Vallance, 2000, p. 601; Carey, 2000, p. 627). Debris flows generated at volcanoes are commonly referred to aslahars.

Decompressive melting:Melting that occurs when rocks undergo a decrease in pressure.This commonly occurs in the vicinity of hot spots as mantle rocks rise to shallower levels in the earth due to convective rise and upwelling (Sigurdsson, 2000, p. 15). Melting occurs as a result of decreasing pressure, not increasing temperature.

Deposit:Earth materials that have accumulated by some natural process (Gardner et al., 1995). Deposits may be the result of volcanic (e.g. lavas or pyroclastic), sedimentary (either clastic or chemical), or hydrothermal (precipitation) processes.

Devitrification:The solid-state transformation of volcanic glass into crystalline materials (AGI, 1976, p. 117).Devitrification tends to be more prevalent in densely-welded tuffs, but may also occur in less densely-welded or unwelded pyroclastic and/or volcaniclastic deposits.The main products of devitrification are cristobalite (SiO2) and alkali feldspar (KAlSi3O8) (Cas and Wright, 1987, p. 258).

Diatreme:A funnel-shaped, pipe-like volcanic conduit, usually filled with volcaniclastic debris, emplaced by the explosive energy of gas-charged magmas.Diatremes are believed to result from hydrovolcanic fragmentation and subsequent wall rock collapse (Vespermann and Schminke, 2000, p. 683), and may reach depths up to 2500 meters.Diamond-bearing diatremes are economically important and are referred to as kimberlite pipes.

Dike:A discordant, sheetlike body igneous body formed from the injection of magma into a fracture within the brittle crust of the earth (Carrigan, 2000, p. 219: Marsh, 2000, p. 191).Generally a tabular igneous body which cross-cuts the planar structures in the adjacent rocks.

Directed blast:A hot, low density mixture of gas, rock debris, and ash that is propelled by a volcanic eruption and generally moves along the ground at high speeds (Miller, 1989).

Dome:A steep-sided mass of lava that is generally formed immediately above the volcanic vent from which it was extruded.Domes are generally circular in plan and have a relatively small surface area relative to other types of lava flows.Domes may be spiny, rounded, or flat on top, and often have rough, blocky surfaces formed by the fragmentation of the domes crust during intrusion.Domes may grow by extrusion of lava onto the outer surface of a previously formed dome (exogenous dome) or may be formed by inflation of a pre-existing dome (endogenous dome).Domes are most commonly the result of extrusion of viscous lava (primarily of the composition of rhyolite and dacite, but andesite may occur as well).

Dormant volcano:A volcano that is not currently erupting, but is thought to be likely to erupt in the future.

Downsag caldera:A type of caldera characterized by inward sloping topography, inward tilted wall rocks, and an apparent absence of large displacement caldera bounding faults (Lipman, 1997).Downsag calderas are believed to result from small volume eruption from a deep-seated subvolcanic intrusion.

Epithermal mineralization:A mineral deposit formed from relatively low temperature (generally 350 C) hydrothermal solutions at shallow levels (2km) in the earths crust.Epithermal mineralization is a common feature on many volcanoes.

Eruption:The expulsion of volcanic materials (magma, volcanic gases) from a vent or fissure at the earths surface.In a general sense, eruptions are considered to be relatively large explosions which result in the expulsion of volcanic materials at or onto the earths surface.

Extinct volcano:A volcano that is not presently erupting and is unlikely to do so in the future (Foxworthy and Hill, 1982).

Facies:A part of a rock body that can be differentiated from another part of a related rock body by textural or compositional variations.The general appearance or composition of one part of a rock body as contrasted with other parts (AGI, 1976, p. 155).

Facies changes:The textural and compositional changes that occur laterally and/or vertically within related rock bodies.

Fire fountain:A spray of lava emitted from a vent or a fissure composed of a highly fluid mixture of basaltic magma and gas (Vespermann and Schminke, 2000, p. 683: Spudis, 2000, p. 697).Deposits from fire fountains produce mantling deposits composed of dense, plastic juvenile fragments and ash known as agglomerates.

Flow banding:A foliation commonly observed in intermediate and felsic lavas, that results from shearing of the lava during laminar flow (Cas and Wright, 1987, p. 78).In rhyolite flows, flow banding is commonly exhibited by alternating bands comprising volcanic glass and spherulites (small, radiating bodies of devitrified glass).

Fuel-coolant interaction:The interaction of magma (fuel) with external water (coolant) that may result in thermal explosions (Vespermann and Schminke, 2000, p. 683).

Fumarole:A vent which releases volcanic gases.These include steam (H2O), carbon dioxide (CO2), sulfur dioxide (SO2), hydrogen sulfide (H2S), as well as other volatile gases emitted from subterranean magmas.

Fumarolic activity:Volcanic gas emissions, with or without an accompanying change in the temperature or compositions of the gasses/fluids emitted (USGS Glossary of Volcano and Related Terminology).

Geyser:A special type of hot spring characterized by intermittent discharged of water and volcanic gases brought about by expansion of a vapor phase (generally steam) in the subsurface.

Graben:An elongate crustal block that has moved downward relative to bounding fault systems (Foxworthy and Hill, 1982).

Heterolithic:A clastic (volcaniclstic) deposit containing of a variety of different types of rock fragments.

Hot spot:An area, generally located in the middle of a lithospheric plate, characterized by anomalous heat flow.Mantle material rises toward the earths surface and undergoes decompressive melting at hot spots which may form volcanoes (as in Hawaii) or cause partial melting of the overlying crust which leads to the formation of volcanoes (e.g. Yellowstone region).

Hot spring:A thermal spring containing water at a higher temperature than the human body (98F/37C)

Hydrothermal:Pertains to hot water or the action of hot water which has been heated by or in association with magma (Gardner et al., 1995).

Hydrothermal alteration:Changes in rocks or minerals brought about by metasomatism with hydrothermal fluids (generally hot water).

Hydrothermally altered:Minerals or rocks that have undergone hydrothermal alteration.

Hydrothermal system:The system comprising the rocks, fluids, vapors, and conduits associated with hydrothermal activity.In general, hydrothermal systems have the following components: 1) a shallow magma chamber or cooling intrusion (provides the heat for the system); 2) fluids which can be of magmatic, meteoric, or connate origin, that are heated by the intrusion and flow through the rocks adjacent to (or sometimes within) the heat source; 3) fractures or high permeability zones which allow transfer of fluids from one part of the system to another part of the system. In most cases, this transfer is believed to be the result of buoyancy contrasts between the colder and warmer fluids within the system.

Hydrovolcanic eruptions:A general term for eruptions caused by the mixing of magma with water (Vespermann and Schminke, 2000, p. 683).Encompasses hydroclastic, hydromagmatic, and phreatomagmatic eruptions.

Hyaloclastite:A deposit comprising small, angular glass fragments formed by nonexplosive shattering of lava or magma flowing into water, ice, or water-saturated sediment (Batiza and White, 2000, p. 361: Schmidt and Schmincke, 2000, p. 383).

Igneous:Refers to the processes associated with magma, or the rocks formed via the solidification of magma.

Igneous rock:A variety of rock formed via crystallization from a magma.The two major classes of igneous rocks are volcanic (crystallized at or near the earths surface, for example, basalt) and plutonic (crystallized at depth within the earth, for example, gabbro).

Ignimbrite:A term used for pyroclastic flow deposits, that is synonymous with ash tuff (Lipman, 2000, p. 643).According to Cas and Wright (1987, p. 98), the term should only be used to describe pumiceous pyroclastic flow deposits.

Island arc:A curved chain of islands, generally convex towards the open ocean, which is bounded on its convex side by a deep oceanic trench (typically a subduction zone) and generally a deep sea basin (AGI, 1976, p. 234).

Jokulhlaup:The Icelandic term for glacial outburst floods which are commonly caused by subglacial volcanic eruptions.

Juvenile fragment:Glassy or partially crystallized fragments which represent samples of an erupting magma.These include fragments such as pumice, scoria, reticulate, achneliths (Peles tears, Peles hair), and various types of volcanic bombs (Cas and Wright, 1987, p. 47-53).

Lahar:The Indonesian term for a debris flow or a mudflow originating on a volcano (Harris, 2000, p. 1301).Lahars are generally composed of volcanic materials, but can contain significant amounts of non-volcanic materials derived from erosion during flow.Most volcanologists prefer this term to be used for the process and not the sedimentary deposits that it forms, but unfortunately, this distinction has been largely ignored in the geological literature.Many lahars are composed of sand and coarser materials, and thus, can be distinguished from mudflows which predominantly contain silt- or clay-sized grains (Rodolfo, 2000, p. 973).

Landslide:A general term for relatively dry, gravity-induced movements of rock, sediment and/or soils (commonly with associated organic debris and/or human-made construction materials (e.g. houses, buildings, roads, etc.)) that are perceptible to the human eye.

Lapilli:A textural term for fragments in volcanic rocks and volcanic deposits that range from 2mm to 64mm in diameter (Fisher, 1966; Schmid, 1981).

Lateral blast:A volcanic eruption which is directed horizontally instead of vertically. Lateral blasts may be caused by sudden decompression of a shallow magma chamber residing within the flanks of a volcano (for example, the 1980 eruption of Mt. St. Helens), or along the base or side of a lava dome (for example, the 1902 eruption of Mt. Pelee in Martinique)(Nakada, 2000, p. 945).

Lava:The term used for magma that has been erupted on to a planets surface.

Lava flow:An outpouring of lava from a vent or fissure that spreads along the ground surface, as well as the crystallized rock resulting from solidification of the outpouring (Peterson and Tilling, 2000, p. 957).

Lava lake:A region typically within the summit of a shield volcano which contains partially crystallized or molten lava which lies immediately above a volcanic conduit which joins the lava lake to the magma chamber.Strong magma convection within volcanic conduits sustains lava lakes within their respective volcanic vents (Walker, 2000, p. 285).

Lava tube:A hollow region, commonly found within crystallized pahoehoe lava flows, which was filled with hot, flowing lava during a volcanic eruption.Lava tubes are formed when the top surface of a channelized lava flow crystallizes, and the magma flowing in the interior of the lava flow drains during and/or immediately following a volcanic eruption.

Lithophysae:Radial aggregates of fibrous crystals which have formed around an expanding vesicle in a melt which is capable of flowing (Cas and Wright, 1987, p. 84).Lithophysae are commonly the result of vapor-phase crystallization within a rhyolitic magma.They should not be confused with spherulites, which are similar-shaped structures formed from devitrification of volcanic glass.

Lithic:Fragments of previously-formed rocks or dense fragments that occur within volcaniclastic deposits.Lithic fragments may be accessory fragments, accidental fragments, or juvenile fragments.

Lithospheric plates:The series of rigid slabs that comprise the earths lithosphere (crust and upper mantle.This term is synonomous withtectonic plates.

Littoral:An adjective describing physical features or processes associated with shorelines of oceans, seas, or lakes (Peterson and Tilling, 2000, p. 957).

Lobate lava:A submarine lava comprising elongate, flattish lobes with smooth, outer glassy skins (Batiza and White, 2000, p. 361).

Maar:A type of monogenetic volcano, generally formed by subterranean phreatic or phreatomagmatic eruptions that occur as magma explosively interacts with ground water or subsurface moisture. Maar craters are cut into the surrounding country rock, vary from 10-500 meters deep, and range from a few hundred meters to 3 km in diameter.Maar volcanoes are generally surrounded by low, shallowly outward-dipping beds of well-bedded volcanic ejecta that rapidly decrease in thickness away from the vent.The volcanic deposits are mainly emplaced by base surges and fallout, and commonly contain very little (or in the case of phreatic eruptions, no) juvenile volcanic materials (Vespermann and Schminke, 2000, p. 685: Cas and Wright, 1987, p. 376-377).

Mafic:A compositional term for igneous rocks which contain 45%-55% SiO2(by weight).Mafic rocks are generally dark colored, and are characterized by mineralogy including pyroxene and calcium-rich plagioclase, variable amounts of olivine, and accessory minerals such as ilmenite and magnetite.Examples of mafic rocks include basalt and gabbro.

Mafic lava:A lava with a silica content (by weight) ranging from 45-55% (AGI, 1976, p. 447; Peterson and Tilling, 2000, p. 957).

Magma:A term used to describe subsurface molten rock (Jeanloz, 2000, p. 41).Magmas are generally considered to be silicate melts (Grove, 2000, p. 133; Wallace and Anderson, 2000, p. 149), but may also be composed of carbonatitic liquids (Spera, 2000, p. 171).Magmas are composed of up to three components (liquid, crystalline solids, and gas (or supercritical fluid) bubbles; Grove, 2000, p. 133), and may be fully liquid or partially crystalline.Lavas are magmas that have erupted on to a planets surface.

Magma chamber:A subterranean region composed of magma that may have a conduit or set of conduits leading to a volcanic vent or vents on a planets surface.

Magnetic polarity:The direction of the magnetic poles (either normal or reversed) that is preserved in igneous rocksafter they cool below their Curie temperature (USGS Glossary of Volcano and Related Terminology)

Magnitude:A numerical measure of the size of an earthquake based on the amount of seismic energy released.The magnitude of an earthquake is determined by measuring the highest-amplitude waves and correcting for distance and the type of seismometer used (McNutt, 2000, p. 1015).The seismic magnitude scale is logarithmic, with each increase in one unit on the scale equivalent to a tenfold increase in the wave amplitude.

Mantle:The part of the earths interior lying above the outer core and below the Mohorovicic discontinuity.The mantle is commonly divided into three parts: the upper mantle (depths down to ~400 km), the transition zone (~400-670 km depth), and the lower mantle (~670-2900 km depth).

Mantle bedding:Pyroclastic deposits generated by ash fall which maintain a uniform thickness and drape over all but the steepest topography (Cas and Wright, 1987, p. 96).

Mantle plume:An elliptical, drop-shaped mass of mantle that ascends toward the earths crust due to its relatively lower density relative to the adjacent mantle.The density contrast is commonly the result of higher heat content of the plume, but may also be the result of chemical anomalies within the mantle (Perfit and Davidson, 2000, p. 89: Sigurdsson, 2000, p. 271).Mantle plumes are associated with intraplate rifting and volcanism.Mantle plumes are the hypothetical cause of hot spots (Hooper, 2000, p. 345).

Megabreccia:Coarse, heterolithic breccia deposits formed during caldera collapse, which contain fragments which are generally greater than one meter in diameter (Lipman, 1976).Megabreccia fragments may be so large that individual fragments may not be readily recognizable on the scale of an outcrop.

Mesa lava:Generally rhyolitic in composition, a lava flow with an approximately circular plan which forms a biscuit-shaped body (Cas and Wright, 1987, p. 81).

Mesobreccia:Heterolithic breccia deposits formed during caldera collapse which contain fragments that are generally less than 1 meter in diameter (Lipman, 1976).

Metamorphic rock:In the strictest sense, rocks that have formed in the solid state in response to pronounced changes in temperature and/or pressure without any change in the bulk chemical composition of the rock.Metamorphic processes are generally confined to regions within the earth below the zones of weathering, cementation, and diagenesis.

Metamorphism:In the stricktest sense (isochemical metamorphism), the process by which consolidated rocks undergo textural and mineralogical changes brought about by changes in temperature and/or pressure.The textural and/or mineralogical changes associated with metamorphism are thermodynamic responses to the physical conditions present in the metamorphic environment.In general, increasing metamorphism results in dehydration of the rocks, as well as an increase in the grain size of the rocks.

Metasomatism:A type of metamorphism characterized by the exchange of chemical species between rocks and their associated altering fluids and/or vapors.

Moat sediments:A general term for sedimentary deposits that occur between the tographic walls and the resurgent central cores of the calderas.In felsic caldera systems, moat sediments are commonly intruded by, and associated with, lava domes.

Monogenetic volcano:A volcano that erupts only once (Walker, 2000, p. 283).

Monolithic:A type of volcaniclastic deposit in which all the clasts present are of the same composition.

Moraine:A topographic feature or landform composed of an accumulation of sediment that has been carried and subsequently deposited by a glacier.

Mudflow:A flowing mixture composed of water and mud (clay- and silt-sized sediments).The term should be used exclusively for mud-dominated mass flows, and should not be used as a substitute for the term lahar(Rodo