AA
lava: 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 of fluidal-shaped volcanic bombs that 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.
Andesite
magma: 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 100°C.
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 earth’s 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 a simple
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 as compound cooling units (Cas
and Wright, 1987, p. 253-255).
Coulée: 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 as lahars.
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 dome’s
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 earth’s
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 earth’s 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 earth’s 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 earth’s 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
(98°F/37°C)
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 earth’s 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 (Pele’s tears, Pele’s 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 planet’s 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 earth’s lithosphere (crust and upper
mantle. This term is synonomous with tectonic
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 planet’s 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 planet’s surface.
Magnetic
polarity: The
direction of the magnetic poles (either normal or reversed) that is preserved
in igneous rocks after 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 earth’s 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 earth’s 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”(Rodolfo, 2000, p. 973-974). Mudflows are common in both volcanic and
non-volcanic environments.
Nested
caldera: A type
of caldera which is found within a larger, older caldera structure.
Nueés
ardente: The term
used for a “glowing avalanche” resulting from a small-volume block and ash flow
produced by the collapse of an actively growing lava dome (LaCroix, 1904). In recent years, the term has unfortunately
been more widely used as a synonym for “ignimbrite”. Its use should be restricted to the original
definition of LaCroix (Cas and Wright, 1987, p. 225).
Orogeny: A term which describes the process of forming
mountains, particularly by folding and thrusting (AGI, 1976, p. 308).
Outwash: Sediments deposited by glacial meltwater
beyond the active glacial ice. Outwash
sediments are commonly characterized by poorly bedded gravels interlayered with
well-bedded (and commonly cross-bedded) sands.
Pahoehoe
lava: A
Hawaiian term to describe lava flows with smooth, continuous surfaces (Kilburn,
2000, p. 291). Pahoehoe flows may have a variety of surfaces described as
smooth, ropy (characterized by rope-like, commonly braided flow folds on the
lava flow’s surface) , or shelly (vesicular and cavernous; Cas and Wright,
1987, p. 66-67). Pahoehoe toes and lobes form when largely degassed mafic magma
issues from tubes relatively far from the erupting vent.
Peicemeal
caldera: A type
of caldera characterized by an internal structure composed of several
individual fault-bounded blocks (Lipman, 1997).
Piecemeal calderas may result from non-uniform subsidence of a caldera
formed from a single eruption, or may be the result of subsidence following a
series of large eruptions (multicyclic; Lipman, 1997; Lipman, 2000, p.
655-656).
Pele’s
hair: A type
of achnelith composed of thin, hair-like strands of volcanic glass. These thin, cylindrical strands of volcanic
glass are commonly golden in color, have diameters between 1-500mm in diameter,
and may be up to 1 meter in length. They
are formed from stretched magma droplets emitted into the atmosphere during
fire fountaining and strombolian eruptions (Vergniolle and Mangan, 2000, p.
447).
Pele’s
tears: A type
of achnelith composed of small droplets of shiny black volcanic glass that have
been ballistically molded and quenched during flight into spherical, dumbbell,
or tadpole shapes. These droplets
generally range from a few millimeters to a few centimeters in size, are
generally dense, but locally may be quite vesicular (Vergniolle and Mangan,
2000, p. 447).
Pele: The mythological Polenesian goddess of
volcanoes. In Hawaii, this temperamental
goddess makes her home in Kilauea’s fiery vent, Halemaumau (Sigurdsson and
Lopes-Gautier, 2000, p. 1297).
Pelean
eruption: A type
of volcanic eruption characterized by a ground hugging glowing avalanche
(pyroclastic flow) resulting from a mixture of hot volcanic gases, ash, and
incandescent lava fragments. Pelean eruptions may occur when pyroclasts are
blown out of a central volcanic vent and then collapse onto the earth’s surface
to form a pyroclastic flow (Tilling, 1985).
Pelean eruptions may also occur as a result of the explosive
disintegration of a lava dome (as was the case for the lava dome on Mt. Pelee,
Martinique in 1902).
Peperite: A genetic term for a rock formed by in-situ
disintigration and mixing of molten magma or lava with wet, poorly consolidated
sediment (Batiza and White, 2000, p. 361).
A breccia-like deposit formed from the extrusive or intrusive mixture of
lava or magma with wet sediment (Schmidt and Schminke, 2000, p. 383).
Perlite: Hydrated obsidian, generally light grey in
color, that is commonly characterized by rounded, onion-skin-like fractures
(perlitic cracks). Apache’s tears are
unhydrated clumps of fresh obsidian that are commonly found within regions
containing perlite.
Phreatic
eruption: A steam
eruption, commonly associated with water, mud, and other earth materials, that
is caused when groundwater, heated by a magma, flashes (and explosively
expands) into steam (Harris, 2000, p. 1301).
Phreatic eruptions expel no juvenile (magmatic) material, and are
commonly the precursor to magmatic eruptive activity.
Phreatomagmatic
eruption: A type
of explosive volcanic eruption that occurs when water (groundwater or surface
water) comes in contact with hot magma.
The quenching of the magma by the water causes the magma to violently
fragment into juvenile (cognate) particles that are bounded by fracture
surfaces and by rounded walls of broken vesicles. Due to the moisture present, accretionary
lapilli are also common in volcanic deposits resulting from phreatomagmatic
eruptions (Williams and McBirney, 1979, p. 247-248).
Pillow
breccia: A
mixture of coarse, typically glassy fragments and broken to whole pieces of pillow lava formed
from the shattering of pillow lava crusts (Batiza and White, 2000, p. 361). Pillow breccias commonly form in areas where
pillow lavas are not strong enough to maintain their competence along steep
submarine slopes or scarps.
Pillow
hyaloclastite:
Hyaloclastite deposits immediately surrounding, and intimately
associated with, pillow lavas.
Pillow
lava: A type
of submarine lava flow consisting of interconnected, elongated lava tubes. Cross-sections of individual lava tubes
resemble pillows with convex upper surfaces and flat or concave lower surfaces
(Schmidt and Schminke, 2000, p. 383).
Both radial and concentric cooling fractures may be present along the
margins of individual pillows, and these fractures are brought on by thermal
contraction during cooling. Growth of
the pillow tubes takes place as the outer, commonly striated outer glassy
surface of the pillow tube fractures, and a new tube “buds” from the fracture
in a manner similar to the way that toothpaste is squeezed out of a tube.
Pipe-like
alteration zone: A
type of narrow, cylindrical or inverted-cone shaped, discordant hydrothermal
alteration zone that is typically confined to a narrow region in close
proximity to a synvolcanic structure (e. g.
synvolcanic fault). Pipe–like
alteration zones are commonly formed by the highest temperature hydrothermal
fluids within a hydrothermal cell (Morton and Franklin, 1987).
Plate
(piston)-type caldera: A
type of caldera in which the caldera floor subsides more or less evenly as one
coherent block. Plate-(piston)-type
calderas are believed to result from single, large volume pyroclastic eruptions
from relatively shallow depth (hypabyssal) magma chambers.
Plinean
eruption: Named
for Pliny the Younger (who witnessed the destruction of Pompeii by eruptions
from Mt. Vesuvius), a type of violently explosive volcanic eruption that ejects
large volumes of tephra high into the atmosphere (Harris, 2000, p. 1301).
Pluton: A body of rock which has formed beneath the
earth from crystallization and consolidation from a magma (AGI, 1976, p.
334). Plutons may be considered extinct
magma chambers (Marsh, 2000, p. 191).
Large plutons (>40 square miles in area) are called “batholiths”.
Ponded
flow: A term
used to describe a lava flow that has ponded within a depression or a volcanic
vent. A lava lake is a specific type of ponded flow that occurs a volcanic
conduit.
Pumice: Solidified fragments of quenched, highly
vesicular (>60%) silicic magma or lava (Cashman et al., 2000, p. 421). The highly vesicular nature of pumice results
from large volumes of gas rapidly expanding within a rapidly cooling
magma. The low density of pumice
commonly permits it to float on water for extended periods of time. Hot pumice,
however, has been shown experimentally to sink rapidly upon interacting with
water (Whitham and Sparks, 1986 ).
Pyroclastic: Refers to processes resulting from the
explosive fragmentation of a magma or lava.
May also be used to describe the deposits formed by explosive volcanic
activity and directly deposited by transport processes resulting directly from
this activity (Cas and Wright, 1987, p. 8).
Pyroclastic is a Greek term which means “fire-broken” (Harris, 2000, p.
1301).
Pyroclastic
fall: The
“rain-out” of pyroclasts through the atmosphere from an eruption jet or
eruption plume during an explosive volcanic eruption (Wilson and Houghton,
2000, p. 545; Houghton et al, 2000, p. 555).
Pyroclastic
fall deposit:
Volcaniclastic (pyroclastic) deposits formed from the rain-out of clasts
through the atmosphere from an eruption jet and/or plume during an explosive
eruption (Houghton et al., 2000, p. 555).
Fall deposits typically exhibit mantle bedding, are well sorted, and
commonly show well-developed planar stratification (Cas and Wright, 1987, p.
95-96).
Pyroclastic
flow: A dense,
hot, dry, high particle concentration mixture of gas and hot rock fragments
(ash, pumice, blocks, etc.) that travels along the ground surface, typically at
high velocity (generally on the order of hundreds of feet or meters per second;
Harris, 2000, p. 1301) away from a volcano.
The high speeds of pyroclastic flows are possible because they flow over
a thin layer of hot, commonly expanding and escaping gases. Most of the material within a pyroclastic
flow is contained within concentrated particle dispersion located at the flow’s
base (Wilson and Houghton, 2000, p. 545).
Pyroclastic
flow deposit:
Pyroclastic (volcaniclastic) deposits that are left by pyroclastic flows
(Cas and Wright, 1987, p. 96). The
deposits are usually topographically controlled (infilling valleys and
topographic depressions), massive, and poorly sorted. Depending upon their thickness and heat
retention, pyroclastic flow deposits may coalesce into welded tuffs. Pumice-rich pyroclastic flow deposits are
often called “ignimbrites”.
Pyroclastic
surge: A type
of turbulent, low density (low particle concentration) pyroclastic cloud or
pyroclastic density current. Being more
dilute than pyroclastic flows, surges can sweep over ridges, hills, and other
topographic boundaries. Two kinds of
surges are known: wet surges have temperatures <100°C and contain
steam that condenses into water droplets that surge along the ground surface
with gas and pyroclasts; and dry surges, which have temperatures >100°C, and
form by either hydrovolcanic eruptions with low water/magma ratios, or by
magmatic eruptions driven solely by expanding magmatic gases (Valentine and
Fisher, 2000, p. 571).
Pyroclastic
surge deposit:
Pyroclastic deposits that are left by pyroclastic surges. These deposits mantle topographic features
but also generally thicken within topographic depressions. These deposits are generally well-sorted, and
are enriched in crystals and lithic fragments relatively to pyroclastic flow
deposits. Surge deposits commonly
exhibit unidirectional sedimentary bedforms, including low angle cross-bedding,
dune forms, climbing dune forms, pinch and swell structures, and chute and pool
structures (Cas and Wright, 1987, p. 98).
Quenching: The rapid cooling of magma to form glass
(Batiza and White, 2000, p. 361).
Fuel-coolant interactions commonly lead to quenching. Abrupt quenching may cause a rapid volume
decrease which leads to fragmentation of the glass (cooling-contraction
granulation).
Reticulite: An exceptionally porous type of scoria
containing porosities ranging from 95-99%98% (Vergniolle and Mangan, 2000, p.
447; McPhie et al., 1993, p. 27).
Commonly referred to as “thread-lace” scoria, reticulite is made up of a
honeycomb-like network of thin glass fibers.
Rhyolite: A volcanic rock containing greater than 68%
silica (by weight). Rhyolites are
composed primarily of alkali feldspars (sanadine and orthoclase) and quartz
(>10% by volume), with lesser amounts of sodic plagioclase (albite,
oligoclase), hornblende, or biotite.
Accessory minerals include zircon, apatite, and tourmaline. Due to their high silica content (and thus
high degree of polymerization), rhyolite lava are very viscous and commonly
form lava domes, mesa lavas, or coulees. Rhyolitic magmas with high gas
contents typically explode violently to form pyroclastic flows, pyroclastic
surges, and pyroclastic falls.
Rhyolite
magma: A magma
which contains greater than 68% silica by weight.
Rift: A linear topographic feature formed by
crustal extension. Rift structures
associated with volcanism are commonly composed of a graben with a central high
region, which is usually the site of active volcanism (for example, along the
mid-ocean ridges).
Ring
fracture/Ring fault:
The arcuate bounding faults upon which caldera (cauldron) subsidence
takes place. Ring fractures (faults) define the structural limits of calderas.
Most observed ring faults are nearly vertical or dip steeply inward (toward the
center of the caldera), and this is thought to be a result of doming of the
caldera structure following its initial formation (Lipman, 2000, p. 649-650).
Ropy
Pahoehoe: A type
of pahoehoe lava characterized by flexible crusts that are bent into tight
folds as lava flows. These tight folds
form lava surfaces that appear to be made up of a series of braided ropes
(Kilburn, 2000, p. 295).
Satellite
vent: A secondary vent on a volcano, commonly located
on the volcano’s flank.
Scoria: Solidified fragments of quenched, highly
vesicular (>60%) mafic magma or lava (Cashman et al., 2000, p. 421). The highly vesicular nature of scoria results
from rapid cooling of gas-rich lava.
Scoria
cone: Small volcanic
landforms formed from focused (single-vent) subaerial strombolian eruptions of
basalt or basaltic-andesite magma. These
features have an inverted cone-shaped profile and are generally circular in
plan, although elongate scoria cones can be formed from multiple-vent volcanic
eruptions (Cas and Wright, 1987, p. 371-372).
Seismic
wave: A term
for elastic earth waves formed by either earthquakes or explosions. Seismic waves include both surface waves as
well as body waves.
Seismicity: The phenomenon of earth movement or seismic
activity.
Seismograph: A scientific instrument used to detect and
record seismic waves.
Semiconformable
alteration zone: A
regional zone of hydrothermal alteration typically characterized by a
sheet-like or cloud-like geometry.
Semiconformable alteration zones are generally quite extensive in
permeable rock units (e.g. tuffs, medium- to coarse-grained clastic sediments
and sedimentary rocks), and are generally patchy in less permeable rock units
(e.g. lavas, intrusions). These zones
commonly are found along the periphery of “pipe-like” alteration zones, which
are generally confined to regions in close proximity to synvolcanic structures
(e.g. synvolcanic faults zones) (Morton and Franklin, 1987).
Shelly
pahoehoe: A type of
pahoehoe lava characterized by highly vesicular, extremely fragile crusts that
form over hollow lava blisters. The
surfaces of these blisters break easily when stepped upon, giving the
impression of walking on eggshells (Kilburn, 2000, p. 295).
Shield
volcano: A broad,
low-relief volcano constructed by flows of relatively fluid lava (e.g. basalt:
Spudis, 2000, p. 698). Flank slopes on
shield volcanoes are typically < 5° (Zimbelman, 2000, p. 771).
Silica: The chemical compound silicon dioxide, SiO2.
Silicic: A term used to describe silica-rich volcanic
rock or magma (Miller, 1989). A chemical
classification for a type of rock or magma containing >62% SiO2
(Peterson and Tilling, 2000, p. 958) or 63% SiO2 (Cas and Wright,
1987, p. 16) by weight.
Silicic
lava: A lava
with a silica content greater than 62% (by weight). Synonymous with the term “felsic lava”
(Peterson and Tilling, 2000, p. 957).
Sinter: A type of fragile, commonly white or grey
rock formed by precipitation of silica from cooling hydrothermal solutions at
or near a hydrothermal vent. Precipitation of siliceous sinter (often with
associated sulfide minerals and precious metals) commonly occurs in neutral and
acid hydrothermal systems under the influence of biogenic agents such as algae
and bacteria (Cas and Wright, 1987, p. 316).
Slabby
pahoehoe: A type
of pahoehoe lava with a surface composed of slabs of broken lava crust that are
up to meters across and up to several centimeters thick (Kilburn, 2000, p.
295).
Solfatara: A type of
steam vent or dry fumarole that is characterized by quiet discharge
(<20 m/s), and that precipitates a significant amount of sulfur (Hochstein
and Browne, 2000, p. 850-851).
Spatter
bomb: A glassy
pyroclast greater than 64mm in diameter that takes on a fluidal shape by the
force of ejection (Vergniolle and Mangan, 2000, p. 447).
Spherulite: Typically rounded, radiating arrays of
crystal fibers produced by the high temperature devitrification of volcanic
glass. In felsic rocks, the crystal
fibers are generally composed of alkali feldspar and a silica polymorph (either
quartz or cristobalite), whereas in mafic rocks the fibers commonly consist of
plagioclase and/or pyroxene. Spherulites
typically have diameters of 0.1-2.0 cm, but can be much larger (commonly up to
20 cm). Isolated spherulites are
generally spherical, but adjacent spherulites may impinge upon one another to
produce long chains that are often aligned with flow foliation (McPhie et al.,
1993, p. 24-25).
Spreading
center/Spreading ridge:
Places on the ocean floor characterized by active volcanism and where
separation of lithospheric plates takes place.
Stratovolcano: A generally steep sided volcano composed of
alternating layers of lava flows, pyroclastic deposits, and commonly,
volcaniclastic sedimentary deposits (Walker, 2000, p. 283). Stratovolcanoes 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. Also called a “composite
volcano”.
Stony
rhyolite: Very
finely crystalline rhyolite lava (Cas and Wright, 1987, p. 84).
Strombolian
eruption: Volcanic
eruptions of basaltic magma, slightly more violent than Hawaiian eruptions,
that produce large amounts of scoria and ash around a central vent to form a
cone. Strombolian eruptions are
typically pulsating and have periods of several seconds (Wolf and Sumner, 2000,
p. 321). The deposits consist of lava
spatter, vesicular bombs, scoria lapilli, and mafic ash (Vespermann and
Schminke, 2000, p. 683). Named after
Stromboli, an Italian volcano.
Subduction
zone: A
sloping region at collisional plate boundaries where one tectonic plate
overrides another tectonic plate. In
most regions, continental crust overrides oceanic crust which is then consumed
in the subduction zone (continental – oceanic plate boundary), but in many
areas, oceanic crust may be overridden by another plate of oceanic crust
(oceanic – oceanic plate boundaries).
Deep oceanic trenches commonly occur as the surface landform associated
with subduction zones. Melting of the
subducting slab commonly produces magma which rises to the earth’s surface to
produce volcanic arcs.
Surtseyan
eruption:
Hydrovolcanic eruptions dominated by jets of wet tephra (scoria and ash)
that result in the formation of tuff cones.
The term “surtseyan” is generally used for volcanoes erupting through
seawater. Named after Surtsey, a volcano
which emerged from the sea off the coast of Iceland in 1963 (Vespermann and
Schminke, 2000, p. 683).
Synvolcanic: A term used to describe a process or feature
that was active or produced during volcanic activity.
Synvolcanic
fault: A fault
or geological structure present or produced at the time of volcanic activity.
Tectonic: A general term used to describe the forces
involved in the deformation of the earth’s crust. Commonly used to also describe the geological
structures or features produced by such deformation.
Tectonic
plate: One of
the large segments of the earth’s lithosphere (crust and upper mantle, up to
250km thick) that comprise the earth’s outer shell. At the present time, there
are 16 major tectonic plates that “float” on top of the asthenosphere, the
plastic layer in the earth’s mantle.
Tephra: A general term used by volcanologists to
describe all fragmental volcanic ejecta produced during explosive volcanic
eruptions (Dehn and McNutt, 2000, p. 1271).
This includes ash (<2mm diameter fragments), lapilli (2-64 mm
diameter fragments and fragements greater than 64 mm in diameter known as bombs
(semi-solid or plastic ejecta) or bombs (solid ejecta) (Tilling et al., 1987).
Thread-lace
scoria: See
“reticulite”.
Trap-door
caldera: A type
of caldera formed when one part of the caldera floor subsides to a greater
depth than the other side of the caldera floor.
In general, trap-door calderas have a partial ring fracture (associated
with the side of greatest caldera collapse) and a hinge area (associated with
the side of least collapse). Trap-door
calderas may represent either calderas that have undergone incomplete collapse,
or calderas formed from eruptions from shallow asymmetrical magma chambers
(Lipman, 1997: Lipman, 2000, p. 654.
Tremor: A continuous vibration of the ground around
active volcanoes (Vergniolle and Mangan, 2000, p. 447). Tremors defined on seismographs may have
either a regular sine-wave appearance (harmonic tremor) or an irregular,
pulsating appearance (spasmodic tremor) (McNutt, 2000, p. 1015).
Tuff: A lithified volcaniclastic rock composed
primarily of ash, with up to minor volumes of lapilli and/or blocks and bombs
(Fisher, 1966). Originally used as a
non-genetic rock name, common use today typically implies (incorrectly) that
the tephra comprising the rock was deposited while hot. Similar deposits that have no indication of
being hot while deposited are commonly referred to as “tuffaceous” (McPhie et
al., 1993, p. 8).
Tuff
cone: A type
of hydroclastic volcano that is generally higher than (generally >50 m
high), and has steeper external flanks (commonly >25°) than tuff rings or
maars (Vespermann and Schminke, 2000, p. 684).
Craters within tuff cones are generally higher in elevation than the
adjacent land surface. Tuff cones are
made up primarily of juvenile clasts deposited from lateral surges, airfall,
and associated volcaniclastic remobilization processes.
Tuff
ring: A type
of hydroclastic volcano, generally <50m high, defined by craters with low
depth/width ratios that sit at or above the elevation of the adjacent land
surface. The rims around tuff rings are
composed of juvenile and accidental clasts and are deposited in beds with dips
<25° (Vespermann and Schminke, 2000, p. 684).
Tumescence: The doming or uprising of a volcano commonly
due to inflation of a shallow magma chamber.
Regional tumescence commonly occurs prior to a major pyroclastic
eruption, but may also occur following an eruption as less volatile magma is
emplaced into the shallow crust (Smith and Bailey, 1968).
Tuya: A flat-topped, steep-sided volcano that
erupted into a lake thawed into a glacier by volcanic heat (Smellie, 2000, p.
403). Commonly referred to as a “table
mountain”.
Unconformity: A surface of erosion that separates younger
strata from older rocks (AGU, 1976, p. 448).
Variolite: A spherulite-like radiating aggregate
composed of feathery, needle-like crystals of plagioclase and pyroxene that
occur in mafic volcanic rocks (typically basalt). Variolites may result from devitrification,
but are commonly believed to be formed in subaqueous rocks by quench-induced crystallization
(Cas and Wright, 1987, p. 420).
VEI
index: The
Volcanic Explosivity Index, which is a measure of the size of an eruption based
on its magnitude, intensity and destructive power. The VEI is measured on an eight-point scale,
where “8” is the most destructive and powerful eruption (Cioni et al., 2000. p.
477).
Vent: A surface opening through which volcanogenic
materials are erupted (Davidson and DeSilva, 2000, p. 663). Typically thought of as a hole in a planet
from which volcanic products (magma, ash, etc.) are erupted (Spudis, 2000, p.
697).
Vesicle: A frozen bubble in a volcanic rock. Vesicles are formed when magma crystallizes
around a gas bubble (Spudis, 2000, p. 697).
Vesicular: A textural term describing volcanic rocks
filled with frozen gas bubbles (vesicles).
Vesicular
tuff: Tuffs
containing millimeter to centimeter-sized, irregular to round vesicles which
are interpreted to form during trapping of air or vapor in wet ash deposits
(Vespermann and Schminke, 2000, p. 683).
Vesuvian
eruption: Commonly
used as a synonym for a “Plinean” eruption (e.g. Tilling, 1985), , but also
used to describe basaltic eruptions which involve long-sustained gas streaming
with little ash being released (as in the 1906 eruption of Vesuvius; Cas and
Wright, 1987, p. 130).
Viscosity: A measurement of the ratio of shear stress to
the rate of shear strain in a fluid (Williams and McBirney, 1979, p. 20). In common language, how easily a fluid flows. Considered the most important physical
property of a magma because it largely determines eruptive style as well as
volcano morphology. Magma viscosity generally increases as the silica content
of the magma increases (due to silica polymerization) and as the temperature of
the magma decreases. Magma viscosity may
also be affected by the presence of trace elements (e.g. Ti) or volatiles (e.g.
H2O, CO2, SO2, etc.). In general, common magmas increase in
viscosity in the following order: komatiite, basalt, andesite, dacite,
rhyodacite, rhyolite.
Volcaniclastic: A non-genetic term used to describe any
fragmental aggregate of volcanic parentage (Cas and Wright, 1987, p. 8). Rocks formed by the fragmentation of volcanic
materials (either magma or volcanic rocks) irrespective of the method of
fragmentation. Pyroclastic rocks and
epiclastic rocks are both considered to be “volcaniclastic”.
Volcanic
bomb: 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
Volcanic
cycle: A
general term used to describe a period of increased volcanic activity.
Volcanic
field: A region
comprising a large number of volcanic edifices.
Volcanic fields are usually associated with basaltic volcanism, and
commonly comprise a number of small, monogenetic volcanoes (e.g. cinder cones,
maars, tuff cones, tuff rings, small shield volcanoes, lava domes). Fields may form in linear trends associated
with tectonic structures (such as faults), on the flanks of larger composite or
shield volcanoes, or within calderas (Connor and Conway, 2000, p. 331).
Volcanic
landslide: A landslide that occurs along the flank of a
volcano.
Volcano: A mound, hill or mountain constructed by the
extrusion of lava and/or pyroclastic material from beneath the ground (Fisher
et al., 1997, p. 43). A vent in the earth’s crust from which molten lava,
pyroclastic materials, volcanic gases, etc. issue (AGU, 1976, p. 457).
Vulcanian
eruption: An
explosive volcanic eruption generally expelling less than 1km3 of
material, but with an eruption column that may reach heights of up to 10-20km
(Nakada, 2000, p. 945). These eruptions
last on the order of seconds to minutes (Morrissey and Mastin, 2000, p. 463).
Welding: The sintering together of hot, glassy
fragments, irrespective of shape and size, by compactional lithostatic load
(Cas and Wright, 1987, p. 165).
Welded
tuff: A hard
pyroclastic rock compacted by internal heat and pressure from overlying
pyroclastic deposits.
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