Ignimbrite: The rock formed by the widespread deposition and consolidation of
ash flows and Nuees Ardentes. The term was originally applied only to densely
welded deposits but now includes non-welded deposits.
Intensity: A measure of the effects of an earthquake at a
particular place. Intensity depends not only on the magnitude of the
earthquake, but also on the distance from the epicenter and the local
A descriptive term applied to igneous rocks that are transitional between basic and acidic
with silica (SiO2) between 54% and 65%.
Intrusion: The process of emplacement of magma in pre-existing
rock. Also, the term refers to igneous rock mass so formed within the surrounding rock.
A surface of fracture in a rock.
Pyroclastic material derived directly from magma reaching the surface.
An area surrounded by a lava flow.
A body of igneous rocks with a flat bottom and domed top. It
is parallel to the layers above and below it.
A torrential flow of water-saturated volcanic debris down the slope of a
volcano in response to gravity. A type of mudflow.
Landsat: A series of unmanned satellites orbiting at about
706 km (438 miles) above the surface of the earth. The satellites carry
cameras similar to video cameras and take images or pictures showing features
as small as 30 m or 80 m wide, depending on which camera is used.
Lapilli: Literally, "little stones." Round
to angular rock fragments, measuring 1/10 inch to 2 1/2 inches in diameter,
which may be ejected in either a solid or molten state.
Lava: Magma which has reached the surface through a volcanic eruption. The term
is most commonly applied to streams of liquid rock that flow from a crater
or fissure. It also refers to cooled and solidified rock.
Lava Dome: Mass of lava, created by many individual flows, that has built a dome-shaped pile of lava.
Lava Flow: An outpouring of lava onto the land surface from a
vent or fissure. Also, a solidified tongue like or sheet-like body formed by
Lava Fountain: A rhythmic vertical fountainlike eruption of lava.
Lava Lake (Pond):
A lake of molten lava, usually basaltic, contained in a vent, crater, or broad depression of a shield volcano.
Lava Shields: A shield volcano made of basaltic lava.
Lava Tube: A tunnel formed when the surface of a lava flow
cools and solidifies while the still-molten interior flows through and
Limu O Pele (Pele Seaweed): Delicate, translucent sheets of spatter
filled with tiny glass bubbles.
Lithic: Of or pertaining to stone.
Lithosphere: The rigid crust and uppermost mantle of the earth.
Thickness is on the order of 60 miles (100 km). Stronger than the
Luster: The reflection of light from the surface of a mineral.
Maar: A volcanic crater that is produced by an
explosion in an area of low relief, is generally more or less circular, and
often contains a lake, pond, or marsh.
Mafic: An igneous composed chiefly of one or more dark-colored
Magma: Molten rock beneath the surface
of the earth.
Magma Chamber: The subterranean cavity
containing the gas-rich liquid magma which feeds a volcano.
Magmatic: Pertaining to magma.
Magnitude: A numerical expression of the amount of energy
released by an earthquake, determined by measuring earthquake waves on
standardized recording instruments (seismographs.) The number scale for
magnitudes is logarithmic rather than arithmetic. Therefore, deflections
on a seismograph for a magnitude 5 earthquake, for example, are 10 times
greater than those for a magnitude 4 earthquake, 100 times greater than
for a magnitude 3 earthquake, and so on.
Mantle: The zone of the earth below the crust and above
Matrix: The solid matter in which a fossil or crystal is
embedded. Also, a binding substance (e.g., cement in concrete).
Miocene: An epoch in Earth's history from about 24 to 5 million years ago.
Also refers to the rocks that formed in that epoch.
Moho: Also called the Mohorovicic discontinuity. The surface or discontinuity
that separates the crust from the mantle. The Moho is at a depth of 5-10 km
beneath the ocean floor and about 35 km below the continents (but down to 60 km
below mountains). Named for Andrija Mohorovicic, a Croatian seismologist.
Monogenetic: A volcano built by a single eruption.
Mudflow: A flowage of
water-saturated earth material possessing a high degree of fluidity during
movement. A less-saturated flowing mass is often called a debris flow. A
mudflow originating on the flank of a volcano is properly called a
Myth: A fictional story to explain the origin of some person,
place, or thing.
A French term applied to a highly heated
mass of gas-charged ash which is expelled with explosive force and
moves hurricane speed down the mountainside.
Obsidian: A black or
dark-colored volcanic glass, usually composed of rhyolite.
The earth's crust where it underlies oceans.
A Hawaiian term for lava with a smooth, billowy, or ropy surface.
Click here to view
a photo of pahoehoe.
Pali: Hawaiian word for steep hills or cliffs.
A natural spun glass formed by blowing-out during quiet fountaining of fluid
lava, cascading lava falls, or turbulent flows, sometimes in association with pele tears. A single strand, with a diameter of
less than half a millimeter, may be as long as two meters.
Small, solidified drops of volcanic glass behind which trail pendants of Pele
hair. They may be tear-shaped, spherical, or nearly cylindrical.
Peralkaline: Igneous rocks in which the molecular proportion of aluminum oxide
is less than that of sodium and potassium oxides combined.
Phenocryst: A conspicuous, usually large, crystal embedded
in porphyritic igneous rock.
Phreatic Eruption (Explosion): An explosive volcanic eruption
caused when water
and heated volcanic rocks interact to produce a violent expulsion of steam
and pulverized rocks. Magma is not involved.
Phreatomagmatic: An explosive volcanic eruption that results from the
interaction of surface or subsurface water and magma.
Interconnected, sack-like bodies of lava formed underwater.
A vertical conduit through the Earth's crust below a volcano,
through which magmatic materials have passed. Commonly filled with
volcanic breccia and fragments of older rock.
Pit Crater: A crater formed by sinking in of the surface, not
primarily a vent for lava.
Plastic: Capable of being molded into any form, which is retained.
Plate Tectonics: The theory that the earth's crust is broken into
about 10 fragments (plates,) which move in relation
to one another, shifting continents, forming new ocean crust, and
stimulating volcanic eruptions.
Pleistocene: A epoch in Earth history from about 2-5 million years to
10,000 years ago. Also refers to the rocks and sediment deposited in
Plinian Eruption: An explosive eruption in which a steady,
turbulent stream of fragmented magma and magmatic gases is released at a high
velocity from a vent. Large volumes of tephra and tall eruption columns
Plug: Solidified lava that
fills the conduit of a volcano. It is usually more resistant to erosion
than the material making up the surrounding cone, and may remain standing
as a solitary pinnacle when the rest of the original structure has eroded
Plug Dome: The steep-sided, rounded mound formed when
viscous lava wells up into a crater and is too stiff to flow away. It
piles up as a dome-shaped mass, often completely filling the vent from which it
Pluton: A large igneous intrusion formed at great
depth in the crust.
Originating in various ways or from various sources.
Precambrian:All geologic time from the beginning of Earth history to 570
million years ago. Also refers to the rocks that formed in that epoch.
Light-colored, frothy volcanic
rock, usually of dacite or rhyolite composition, formed by the expansion
of gas in erupting lava. Commonly seen as lumps or fragments of pea-size
and larger, but can also occur abundantly as ash-sized
Pyroclastic: Pertaining to fragmented (clastic) rock
material formed by a volcanic explosion or ejection from a volcanic vent.
Flow: Lateral flowage of a turbulent mixture of hot gases and
pyroclastic material (volcanic fragments, crystals, ash, pumice, and glass
shards) that can move at high
speed (50 to 100 miles an hour.) The term also can refer to the deposit
Quaternary: The period of Earth's history from about 2 million
years ago to the present; also, the rocks and deposits of that age.
Relief: The vertical difference between the summit of a mountain and the adjacent valley or plain.
Renewed Volcanism State:
Refers to a state in the evolution of a typical Hawaiian volcano during which
--after a long period of quiescence--lava and tephra erupt intermittently. Erosion
and reef building continue.
Repose: The interval of time between volcanic eruptions.
Rhyodacite: An extrusive rock intermediate in composition between
dacite and rhyolite.
Rhyolite: Volcanic rock (or lava) that
characteristically is light in color, contains 69% silica or more,
and is rich in potassium and sodium.
Ridge, Oceanic: A major submarine mountain range.
Rift System: The oceanic ridges
formed where tectonic plates are separating and a new crust is being
created; also, their on-land counterparts such as the East African
Rift Zone: A zone of volcanic features associated with underlying dikes.
The location of the rift is marked by cracks, faults, and vents.
Ring of Fire: The regions of mountain-building
earthquakes and volcanoes which surround the Pacific
Scoria: A bomb-size (> 64 mm) pyroclast that is irregular in
generally very vesicular. It is usually heavier, darker, and more
crystalline than pumice.
Seafloor Spreading: The mechanism by which new seafloor crust
is created at oceanic ridges and slowly spreads away as plates are
Seamount: A submarine volcano.
Seismograph: An instrument that records seismic
waves; that is, vibrations of the earth.
Seismologist: Scientists who study earthquake waves and what they tell
us about the inside of the Earth.
Seismometer: An instrument that measures motion of the ground
caused by earthquake waves.
Shearing: The motion of surfaces sliding past one another.
Earthquake waves that move up and down as the wave itself
moves. For example, to the left.
Shield Volcano: A
gently sloping volcano in the shape of a flattened dome and built almost
exclusively of lava flows.
Shoshonite: A trachyandesite composed of olivine and augite phenocrysts in a
groundmass of labradorite with alkali feldspar rims, olivine, augite, a
small amount of leucite, and some dark-colored glass. Its name is derived
from the Shoshone River, Wyoming and given by Iddings in 1895.
Silica: A chemical combination of silicon and oxygen.
Sill: A tabular body of intrusive igneous rock, parallel to
the layering of the rocks into which it intrudes.
Skylight: An opening formed by a collapse in the roof of a lava tube.
Solfatara: A type of fumarole, the gases of which are characteristically sulfurous.
A low, steep-sided cone of spatter built up on a fissure or vent. It is
usually of basaltic material.
Spatter Rampart: A ridge of congealed pyroclastic material (usually basaltic) built up on a fissure or vent.
Specific Gravity: The density of a mineral divided by the density of
Spines: Horn-like projections formed upon a lava dome.
Stalactite: A cone shaped deposit of minerals hanging from the roof of a cavern.
Stratigraphic: The study of rock strata, especially of their
distribution, deposition, and age.
Stratovolcano: A volcano composed of both
lava flows and pyroclastic material.
Streak: The color of a mineral in the powdered form.
Strike-Slip Fault: A
nearly vertical fault with side-slipping displacement.
Strombolian Eruption: A type of volcanic eruption
jetting of clots or fountains of fluid basaltic lava from a central crater.
Subduction Zone: The zone of convergence of two tectonic plates,
one of which usually overrides the other.
Surge: A ring-shaped cloud of gas and suspended solid debris that
radially outward at high velocity as a density flow from the base of a
vertical eruption column accompanying a volcanic eruption or crater
Talus: A slope formed a the base of a steeper slope, made of fallen
and disintegrated materials.
Tephra: Materials of all types and
sizes that are erupted from a crater or volcanic vent and deposited from
Tephrochronology: The collection, preparation, petrographic
approximate dating of tephra.
Tilt: The angle between the slope of a part of a volcano and some reference. The reference may be the slope of
the volcano at some previous time.
Trachyandesite: An extrusive rock intermediate in composition
between trachyte and andesite.
Trachybasalt: An extrusive rock intermediate in composition
between trachyte and basalt.
Trachyte: A group of fine-grained, generally porphyritic, extrusive
igneous rocks having alkali feldspar and minor mafic minerals as the main
components, and possibly a small amount of sodic plagioclase.
Tremor: Low amplitude, continuous earthquake activity often
associated with magma movement.
Tsunami: A great sea wave produced by a submarine
earthquake, volcanic eruption, or large landslide.
formed of pyroclastic material.
Tuff Cone: A type of volcanic cone formed by the interaction of
basaltic magma and water. Smaller and steeper than a tuff ring.
Tuff Ring: A wide, low-rimmed, well-bedded accumulation of
hyalo-clastic debris built around a volcanic vent located in a lake, coastal
zone, marsh, or area of abundant ground water.
Tumulus: A doming or small mound on the crest of a lava flow caused by
pressure due to the difference in the rate of flow between the cooler crust
and the more fluid lava below.
Ultramafic: Igneous rocks made mostly of the mafic minerals hypersthene,
augite, and/or olivine.
Unconformity: A substantial break or gap in the geologic
record where a rock unit is overlain by another that is not next in
stratigraphic sucession, such as an interruption in continuity of a
depositional sequence of sedimentary rocks or a break between eroded
igneous rocks and younger sedimentary strata. It results from a
change that caused deposition to cease for a considerable time, and
it normally implies uplift and erosion with loss of the previous
The opening at the earth's surface through which volcanic materials issue
Vesicle: A small air pocket or cavity formed in volcanic rock
Viscosity: A measure of resistance to flow in a liquid
(water has low viscosity while honey has a higher viscosity.)
Volcano: A vent in the surface of the Earth through which
magma and associated gases and ash erupt; also, the form or structure
(usually conical) that is produced by the ejected material.
A generally curved linear belt of volcanoes above a subduction zone,
and the volcanic and plutonic rocks formed there.
Volcanic Complex: A persistent volcanic vent area
that has built a complex combination of volcanic
Volcanic Cone: A mound of loose material that was ejected
Volcanic Neck: A massive pillar of rock more resistant to erosion than
the lavas and pyroclastic rocks of a volcanic cone.
Vulcan: Roman god of fire and the forge after whom
volcanoes are named.
Vulcanian: A type of eruption consisting of the explosive ejection of
incandescent fragments of new viscous lava, usually on the form of blocks.
Water Table: The surface between where the pore space in rock is filled with water and where the
the pore space in rock is filled with air.
A crystal that resembles a phenocryst in igneous rock, but is a foreign to
the body of rock in which it occurs.
Xenoliths: A foreign inclusion in an igneous rock.
Audubon Society, The Once and Future Mountain (July,
Bullard, Fred M., Volcanoes of the Earth (London: University of
Texas Press, 1976)
Decker and Decker, Volcanoes (W.H. Freeman and
Foxworthy and Hill, Volcanic Eruptions of Mount St.
Helens: The First 100 Days (U.S. Geological Survey)
Korosec, The 1980
Eruption of Mount St. Helens (Washington State
Department of Natural Resources)
Tilling, Eruptions of Mount St. Helens: Past, Present
and Future (U.S. Geological Survey)
Bates, R.L., and Jackson, J.A., Glossary of Geology (American Geological
Takahashi, T.J., and Griggs, J.D., Hawaiian Volcanic Features: A
Photoglossary (U.S. Geological Survey Professional Paper 1350, v. 2,