GeoMan's Glossary of Earth Science
Terms
Oregon university
A
Aa: Hawaiian term used to describe a lava flow whose surface is broken into
rough angular fragments. Aa flows commonly develop from pahoehoe flows
as they cool and lose gas.
Absolute Date: An estimate of the true age of a mineral or rock based on the
rate of decay of radioactive minerals.
Acre-foot: The volume of water required to cover one acre of land to a depth
of one (1) foot. One acre-foot is equal to 325,851 gallons of water.
Active volcano: A volcano that is erupting; or one that, while not erupting at
the present, has erupted within (geologically) recent time and is considered
likely to do so in the (geologically) near future.
Adiabatic rate: The rate of temperature change in the atmosphere due to the
raising or lowering of an air mass. The "dry adiabatic rate" is 5.5 deg. F. per
1000 feet, while the "wet" rate is 3.5 deg. F. per 1000 feet.
Alluvium: A general term for clay, silt, sand, gravel or similar unconsolidated
material deposited by a stream or other body of running water.
Andesite: Intermediate volcanic rocks containing 54 to 62 percent silica and
moderate amounts of iron and magnesium. Andesite minerals commonly
include plagioclase and hornblende, with lesser amounts of mica, pyroxene,
and various accessory minerals. Andesites are aphanitic in texture and are
usually medium dark in color. They occur with composite volcanic cones
associated with convergent plate margins.
Aquifer: A water-bearing layer of rock or sediment capable of holding and
transmitting fluid (such as water, gas, or oil).
Aquifer, Confined (or Artesian): An aquifer overlain by a non-permeable
layer or layers, in which pressure will force water to rise above the aquifer.
Aquifer, Perched: An aquifer containing unconfined groundwater separated
from an underlying body of groundwater by an unsaturated zone.
Aquifer, Principal: The aquifer or combination of related aquifers in a given
area that is the important economic source of water to wells.
Aquifer, Secondary: Any aquifer that is not the main source of water to wells
in a given area.
Aquifer, Unconfined (or Water Table): An aquifer in which the upper
surface is the water table.
Aquiclude: An impermeable geologic formation or stratum which will not
hold or transmit fluid.
Aquitard: A geologic formation or stratum that significantly retards fluid
movement.
Artesian Well: A well in an aquifer where the groundwater is confined under
pressure and the water level will rise above the top of the confined aquifer.
Artificial Recharge: The unnatural addition of surface waters to groundwater.
Recharge could result from reservoirs, storage basins, leaky canals, direct
injection of water into an aquifer, or by spreading water over a large land
surface.
Ash: Fine particles of rock material ejected during an explosive volcanic
eruption (commonly intermediate to felsic events). Ash may be either solid or
molten when first erupted, and generally measures less than 0.10 inch in size
(larger particles have other names).
Ashfall (subaerial): Volcanic ash that has fallen through the air. The resulting
deposit is usually well sorted and exhibits a finely layered structure.
Ash flow: A turbulent mixture of gas and rock fragments, most of which are
ash-sized particles, ejected violently from a crater or fissure. The mass of
pyroclastics is normally of very high temperature and moves rapidly down the
slopes, or even along a level surface.
Avalanche: A large mass of material falling or sliding rapidly due to the force
of gravity. In many cases, water acts as a catalyst and/or lubricant. Avalanches
often are classified by what is moving, such as a snow, ice, soil, or rock
avalanche. A mixture of these materials is commonly called a debris flow.
B
Basalt: Volcanic rock (or magma) that is generally dark in color, contains 45
to 54 percent silica, and is rich in iron and magnesium. An eruption of basaltic
magma is generally quiet, and results in flows (both vesicular and nonvesicular) and breccias. Undersea eruptions commonly result in the formation
of "pillow lavas." Basalt represents the initial differentiated material erupted
by the earth at spreading centers, and is considered by GeoMan to be the
"blood of the earth."
Bedrock A general term for any consolidated rock.
Bentonite A clay material composed principally of the mineral
montmorillonite. It has a great affinity for fresh water and when hydrated will
increase its volume more than seven times. Water/bentoninte suspensions are
essentially impermeable. Commonly used as a sealant for ponds.
Biostratigraphy: The study and classification of rocks and their history based
on their fossil content.
Block: Angular chunk of solid rock ejected during a volcanic eruption.
Bomb: Fragments of molten or semi-molten rock, several inches to several
feet in diameter, which are blown out during an explosive volcanic eruption.
Because of their semi-plastic condition, bombs are often modified in shape
during their flight or upon impact.
Breccia: Angular fragments of material, commonly formed by physical
weathering processes or explosive volcanic activity.
Brittle-Ductile Transition Zone: The location at depth within the earth's
crust where the temperature and pressure have risen to such a high level that
directed stress results in plastic deformation as opposed to fracturing and
faulting.
C
Caldera: The Spanish word for cauldron, a basin-shaped volcanic depression;
by definition, at least a mile in diameter. Such large depressions are typically
formed by the subsidence of volcanoes. Crater Lake occupies the best-known
caldera in the Cascades.
Calorie: A unit of heat energy. The amount of heat required to raise the
temperature of 1 gram (cubic Centimeter) of water 1 degree Celsius. Also, the
substance which gives food its flavor.
Chattermarks: Erosional features associated with alpine glaciers.
Cinder cone: A volcanic cone built entirely of loose fragmented material
(pyroclastics.)
Collection: The accumulation of precipitation into surface and underground
areas, including lakes, rivers, and aquifers.
Comet: An object which circles the sun in a non-circular orbit. Commonly
made up of a large mass of rock debris and ice. Basically, a giant snockball
from space.
Composite volcano: A steep volcanic cone built by both lava flows and
pyroclastic eruptions.
Condensation: The change of state of water from the vapor to the liquid
phase. Results in liberation of 80 calories per cubic centimeter.
Connate water: Water included in the groundwater which is derrived from
the rock itself, as opposed to water which has percolated down from the
surface.
Continental crust: Solid, outer layers of the earth, including the rocks of the
continents.
Continental drift: The theory that horizontal movement of the earth's surface
causes slow, relative movements of the continents toward or away from one
another.
Continental shelf: Portions of the continental land masses covered by sea
water. Extend varying distances outward from the exposed continental
margins. Usually, the continental shelf will be wider along a passive
continental margin, and narrower along an active margin.
Crater: A steep-sided, usually circular depression formed by either explosion
or collapse at a volcanic vent.
D
Dacite: Volcanic rock (or lava) that characteristically is light in color and
contains 62 to 69 percent silica and moderate amounts of sodium and
potassium.
Debris avalanche: A flow of unsorted masses of rock and other material
downslope under the influence of gravity. Water is commonly involved as a
catalyst and/or lubricant. For example: a rapid mass movement that included
fragmented cold and hot volcanic rock, water, snow, glacial ice, trees and
other debris, and hot pyroclastic material was associated with the May 18,
1980 eruption of Mt. St. Helens. Most of the deposits in the upper valley of the
North Fork Toutle River and in the vicinity of Spirit Lake are from the debris
avalanche resulting from the eruption.
Density: A measure of how tightly packed the atoms of a substance are.
Measured in grams per cubic centimeter. Varies by the mineral or substance.
For example, gold has a high density, while quartz has a low density. See also
"specific gravity."
Detachment plane: The surface along which a landslide disconnects from its
original position.
Dew point: The temperature (elevation) where adiabatic cooling results in the
initiation of condensation of water vapor into cloud droplets.
Di-polar: The arrangement of the hydrogen atoms of a water molecule at 105
deg. across the oxygen results in a slight electrical charge to the molecule. It
also results in water molecules looking like Mickey Mouse instead of Alfred
E. Newman.
Dome: A steep-sided mass of viscous (doughy) lava extruded from a volcanic
vent, often circular in plane view and spiny, rounded, or flat on top. Its surface
is often rough and blocky as a result of fragmentation of the cooler, outer crust
during growth of the dome.
Dormant volcano: This term is used to describe a volcano which is presently
inactive but which may erupt again. The major volcanic cones of the Cascade
Mountains (in Washington, Oregon, and California) are believed to be
dormant rather than extinct.
Drift (glacial): General term for material deposited by a glacier.
E
Ejecta: Material that is thrown out by a volcano, including pyroclastic
material (tephra) and, from some volcanoes, lava bombs.
Erosion: The movement of weathered material downslope under the influence
of gravity. Water acts as a catalyst and as a lubricant. Some common types of
erosion includes landslides, rockfalls, creep, etc. Erosion takes weathered
material and puts it in a river so it can be transported to the beach (see
Strickler's 3rd Law of GeoFantasy).
Erratic (glacial): Large rocks or other debris deposited by a glacier, usually in
an area far removed from its source. Commonly used to indicate a big chunk
of debris which is clearly out of place and shouldn't even be where it is.
Eruption: The process by which solid, liquid, and gaseous materials are
ejected into the earth's atmosphere and onto the earth's surface by volcanic
activity. Eruptions range from the quiet overflow of liquid rock to the
tremendously violent expulsion of pyroclastics.
Eruption cloud: The column of gases, ash, and larger rock fragments rising
from a crater or other vent. If it is of sufficient volume and velocity, this
gaseous column may reach many miles into the stratosphere, where high
winds will carry it long distances.
Eruptive vent: The opening through which volcanic material is emitted.
Evaporation: The change of state of water from the liquid to vapor phase.
Requires the addition of 80 calories per cubic centimeter.
Evapotranspiration: Water used by plants and animals and subsequently
returned directly to the atmosphere.
Evolution: The theory that living organisms mutate and change, generally
from simple to increasingly complex forms.
Extinct volcano: A volcano that is not presently erupting and is not likely to
do so for a very long time in the future.
F
Fault: A crack or fracture in the earth's surface in which there has been
movement of one or both sides relative to the other. Movement along the fault
can cause earthquakes or, in the process of mountain-building, can release
underlying magma and permit it to rise to the surface as a volcanic eruption.
Firn: The intermediate "granular" stage which occurs during the conversion of
snow to glacial ice.
Fissures: Elongated fractures or cracks on the slopes of a volcano. Fissure
eruptions typically produce liquid flows, but pyroclastics may also be ejected.
Flank eruption: An eruption from the side of a volcano (in contrast to a
summit eruption.)
Fossil: Evidence of past life on earth. Can include the preserved hard and soft
parts of plants and animals, tracks and burrows, whole organisms preserved
intact in amber or tar, and fossilized dung. ANY evidence of life constitutes a
fossil.
Floodplain: The low relief lands bordering a stream or river, common to the
mature and old age stages of stream development. Floodplains store excess
water in times of high water, and excess sediments in times of low water.
Beware of building your dream house on a floodplain - they tend to get rather
wet at irregular intervals.
Fumarole: A vent or opening through which issue steam, hydrogen sulfide, or
other gases. The craters of many dormant volcanoes contain active fumaroles.
G
GeoMan: Your's truly, and a real GeoGeek.
Geothermal energy: Energy derived from the internal heat of the earth.
Geothermal power: Power generated by using the heat energy of the earth.
Glacial ice: Naturally occurring ice which exhibits internal plastic flow and
deformation.
Glacial abrasion: A copmmon mechanical weathering process where rock
and debris frozen into the sides and bottom of a glacier act like sandpaper and
wear down the bedrock the glacier is mocing across.
Glacial quarrying (plucking): A common mechanical weathering process in
alpine glaciated terrain where glacial ice frozen into cracks in the bedrock
literally "pluck" rock material from the valley floor.
Glacial polish: Polished bedrock surfaces left behind after melting of glacial
ice. The polishing is probably due to very fine grained rock flour carried at the
base of the ice.
Graben: An elongate crustal block that is relatively depressed (downdropped)
between two fault systems.
Groundwater: Water stored beneath the surface in open pore spaces and
fractures in rock.
H
Harmonic tremor: A continuous release of seismic energy typically
associated with the underground movement of magma. It contrasts distinctly
with the sudden release and rapid decrease of seismic energy associated with
the more common type of earthquake caused by slippage along a fault.
Heat transfer: Movement of heat from one place to another.
Horizontal blast: An explosive eruption in which the resultant cloud of hot
ash and other material moves laterally rather than upward.
Hot-spot volcanoes: volcanoes related to a persistent heat source in the
mantle.
Hydrologic cycle: The transfer of water between numerous temporary storage
reservoirs. These include the ocean, rivers and streams, glacial ice, beer cans,
dogs and cats, groundwater, and the atmosphere.
Hydrothermal reservoir: An underground zone of porous rock containing
hot water.
I
Isostasy: The vertical readjustment of the surface of the earth due to the
addition or removal of weight. Commonly associated with the advance and
retreat of glacial ice.
K
Kinetic energy: The energy of motion.
L
Lahar: A torrential flow of water-saturated volcanic debris down the slope of
a volcano in response to gravity. A type of mudflow. Also known as a
"glowing avalanche."
Langley: The unit of solar energy relating to the amount which reaches a
specific area of the earth's surface. In general, more "langleys" reach the
surface of the earth at the equator than at the poles.
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 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 outpouring
lava.
Lava tube: A tunnel formed when the surface of a lava flow cools and
solidifies, while the still-molten interior flows through and drains away.
Leeward: The side facing away from the wind. Usually hot and dry.
Loess: Very fine-grained sediments deposited by wind action. Usually
associated with the margins of continental ice sheets. Large expanses of loess
are currently responsible for much of the corn and wheat in Nebraska and
Kansas.
M
Magma: Molten rock beneath the surface of the earth.
Magma chamber: The subterranean cavity containing the gas-rich liquid
magma which feeds a volcano.
Mantle: The zone of the earth below the crust and above the core.
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.
Marker horizon (or bed): A distinctive horizon which is used for regional
correlation of lithology. A good marker horizon is distinctive, widespread, and
represents a relatively short period of geologic time. For example, ash from a
volcanic eruption, debris from a meteorite impact, etc. It is GeoMan's opinion
that humans will represent one of the earth's finest marker horizons in several
hundred million years. Our effect on the surface is certainly distinctive and
widespread, and, at the rate we are going, it is likely that our species will have
a relatively short lifespan (speaking in terms of geologic time, of course).
Metamorphic: From the Greek "meta" (change) and "morph" (form).
Commonly occurs to rocks which are subjected to increased heat and/or
pressure. Also applies to the conversion of snow into glacial ice.
Mineral: A naturally occurring, inorganic, crystalline solid with a definite
internal structure and chemical composition.
Moraine: General term for material deposited beneath, along the sides, and/or
at the terminus of a glacier. Also, what we get here in Oregon during the fall,
winter, and spring. See also till.
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 lahar.
N
Norite: An igneous rock with mafic composition.
Nuée ardente: A French term applied to a highly heated mass of gas-charge
ash which is expelled with explosive force down the mountainside. Common
to intermediate volcanoes. Can be quite deadly.
O
Obsidian: A black or dark-colored volcanic glass, usually of rhyolitic (felsic)
composition.
Oceanic crust: The earth's crust where it underlies oceans.
P
Pahoehoe: Hawaiian term for a fluid volcanic eruption resulting in broad
basaltic shield volcanoes. The highly fluid magma flows readily, and hardens
into ropey forms as it cools.
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.
Pillow lava: Interconnected, sack-like bodies of lava formed underwater.
Plastic deformation (or flow): Permanent bending or folding of rock (or ice)
as a result of directed pressure. In rock, usually occurs below the BrittleDuctile Transition Zone, and is commonly associated with metamorphism.
Plate tectonics: The theory that the earth's crust is broken into about
fragments (plates,) which move in relation to one another, shifting continents,
forming new ocean crust, and causing volcanic eruptions.
Plug: Solidified lava that fills the conduit of a volcano. Plugs (also called
volcanic necks) are 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 away.
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 emerged.
Pluton: A large igneous intrusion formed at great depth in the crust.
Potential energy (gravitational): The stored energy of a substance. Water has
a lot of this if there is an elevation difference. Potential energy can be
converted to kinetic energy if the water (or other substance) is allowed to
move.
Precipitation: Any condensed water falling from the atmosphere to the
surface of the earth. Common types include rain, snow, sleet, and hail.
Problem: A situation which is generally uncomfortable, or otherwise
undesirable. I have several - how about you?
Pumice: 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 particles.
Pyroclastic: Pertaining to fragmented (clastic) rock material formed by a
volcanic explosion or ejection from a volcanic vent.
Pyroclastic flow: Lateral flowage of a turbulent mixture of hot gases and
unsorted 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 so formed.
R
Rhyolite: Volcanic rock (or lava) that characteristically is light in color,
contains 69 percent silica or more, and is rich in potassium and sodium. It is
fine grained, which although different in texture, has the same composition as
granite
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 like the East
African Rift.
Ring of Fire: The regions of mountain-building earthquakes and volcanoes
which surround the Pacific Ocean.
Rock flour: Finely ground rock material, usually associated with glaciers (or
faults). Can be mixed with water and formed into loaves which, when baked
for 45 minutes at 350 deg., are totally unedible.
S
Seafloor spreading: The mechanism by which new seafloor crust is created at
oceanic ridges and slowly spreads away as tectonic plates separate.
Sea level: The top of the ocean, where the water meets the atmosphere.
Sediment: Rock debris commonly produced by mechanical or chemical
weathering processes.
Seismograph: An instrument that records seismic waves; that is, vibrations of
the earth. Used to record and measure earthquakes.
Shearing: The motion of surfaces sliding past one another.
Shield volcano: A gently sloping volcano in the shape of a flattened dome,
built almost exclusively of mafic lava flows. The Hawaiian Islands are a good
example.
Silica: A chemical combination of silicon and oxygen.
Sniceball: A snowball which has been stored in the freezer for several months
(or more). Useful for surprising unwelcome visitors during the spring and
summer months.
Snirtball: A combination of snow and dirt. Snirtballs are produced by
accident when the total snowfall on bare ground is less than 0.537 inches.
Snockball: A combination of snow and rock: generally an innocent-looking
snowball with a dense, rocky core. Snockballs are always premeditated, and
are not known to occur naturally on earth. Giant snockballs from space (also
called comets) may be responsible to the initial introduction of water onto our
planet.
Snowball: A spherical accumulation of water in the crystalline form.
Snowline: The lower limit of any year's permanent snowfall. Separates the
Zone of Accumulation from the Zone of Ablation.
Solid state: In metamorphism, indicates the change of mineral identity
without melting. All ion migration occurs while the rock (or pre-glacial ice) is
still solid.
Specific gravity: A measure of how tightly packed the atoms of a substance
are. Varies by the mineral or substance. Example, gold has a high specific
gravity, while quartz has a low specific gravity. See also "density."
Spines: Horn-like projections formed upon a lava dome.
Spring: The time between winter and summer.
Spring: A surface flow of groundwater which occurs any time the water table
intersects the surface.
Stratovolcano: A volcano composed of both lava flows and pyroclastic
material. Also called "Composite" volcanoes. Common at convergent
boundaries. Excellent examples in the U.S. include Mt. St. Helens, Mt. Shasta,
and the other peaks of the Cascade Range of California, Oregon, and
Washington.
Striations (glacial): Grooves eroded into bedrock by rock debris frozen into
the base of a glacier.
Strike-slip fault: A nearly vertical fault with side-slipping displacement.
Subduction zone: The zone of convergence of two tectonic plates, one of
which usually overrides the other.
Sublimation: The direct change from the solid to the vapor phase (without
passing through the liquid phase). Commonly occurs in ice and snow fields on
sunny days above the snowline.
T
Tephra: Materials of all types and sizes that are erupted from a crater or
volcanic vent and deposited from the air.
Till (glacial): General term for material deposited by a glacier. See also
moraine.
Tsunami: A great sea wave produced by a submarine earthquake, volcanic
eruption, or large landslide. Commonly (but erroneously) called a "tidal
wave," tsunamis can cause great damage due to flooding of low coastal areas.
Tuff: Rock formed of pyroclastic material.
V
Vapor (water): Water in the gaseous state.
Vent: The opening at the earth's surface through which volcanic materials
reach the surface.
Vesicular basalt: Holes and other openings in basaltic flow which are the
result of trapped gas bubbles. Vesicles are often filled at a later date with a
wide variety of materials, including, quartz, agate, zeolites, and many other
minerals.
Viscosity: A measure of resistance to flow in a liquid (molasses in January has
high viscosity while molasses in August has lower viscosity).
Volcanic Neck: Solidified lava that fills the conduit of a volcano. Volcanic
necks (also called plugs) are 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 away.
Vulcan: Roman God of fire and the forge, after whom volcanoes are named.
W
Water cycle: The transfer of water between numerous temporary storage
reservoirs. These include the ocean, rivers and streams, glacial ice, beer cans,
dogs and cats, groundwater, and the atmosphere.
Weathering (surface): "Making little ones out of big ones." Waethering
includes the processes which mechanically and chemically break down the
mountains into little pieces, so they can be eroded and transported to the beach
(see Strickler's 3rd Law of GeoFantasy).
Well: A hole dug into the ground in the attempt to intersect water or other
subsurface fluids.
Windward: The side facing into the wind.
Z
Zone of ablation (or wastage): The area below the snowline where snow
melt exceeds snowfall, and material is lost from a glacier.
Zone of accumulation: The area above the snowline where snowfall exceeds
snow melt, and material is added to a glacier.