Kamus Geologi
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A
aa
lava - Lava whose surface is covered with random
masses of angular jagged blocks.
ablation - As
applied to glacier ice, process by which ice below snow line is wasted by evaporation and melting.
abrasion - Erosion of rock material by friction of solid particles moved by gravity, water,
ice, or wind.
absolute
time - Geologic time measured in years. Compare with relative time.
acid
rain - Rain higher in acid than normal. Forms from sulfur dioxide
(SO2) from the burning of fossil fuels; combines with water to form
sulfuric acid.
acidic
lava - Lava containing 70 percent or more of
silica.
actynolite - A metamorphic ferromagnesian mineral; an asbestos.
adiabatic
rate - In a body of air moving upward or downward, change in
temperature without gain or loss of temperature to air through with it moves.
aftershock -
Earthquake that follows a larger earthquake and originates at or near focus of larger earthquake. Major shallow
earthquakes are generally followed by many
aftershocks, which decrease
in number as time goes on but may continue for days or even months.
agate -
Variety of chalcedony with alternating layers of
chalcedony and opal.
A
horizon - Soil zone immediately below surface, from
which soluble material and fine-grained particles have been moved downward by
water seeping into soil. Varying amounts of organic matter give A horizon color ranging from gray to black.
Airy
hypothesis - Explains isostasy by assuming earth’s crust has same density everywhere and differences in
elevation result from differences in thickness of outer layer.
albite - Feldspar in which diagnostic positive ion is Na+; sodic feldspar, Na(AlSi3O8).
One of plagioclase feldspars.
alkali
rocks - Igneous rocks in which abundance of
alkalies is unusually high, generally indicated by soda pyroxenes, soda amphiboles, and/or feldspathoids.
alluvial
fan - Land counterpart of a delta: an assemblage of sediments marking
place where a stream moves from a steep gradient to a flatter gradient and suddenly
loses transporting power. Typical of arid and semiarid climates but not
confined to them.
almandite - A
deep-red garnet of iron and aluminum formed during regional metamorphism.
alpha
decay - Radioactive decay taking place by loss of
an alpha particle from nucleus. Mass of element decreases by 4, and atomic number decreases by 2.
alpha
particle - A helium atom lacking electrons and therefore having a double positive charge.
Alpine,
or mountain, glacier - See valley glacier.
amphibole
group - Ferromagnesian silicates with a double
chain of silicon-oxygen tetrahedra. Common example: hornblende. Contrast with pyroxene group.
amphibolite - A
faintly foliated metamorphic rock developed during regional metamorphism of simatic rocks. Composed mainly of hornblende and plagioclase feldspars.
amphibolite
facies - An assemblage of minerals formed during regional metamorphism at moderate to high pressures between 450 and 700º C.
andalusite - A
silicate of aluminum built around independent tetrahedra, Al2SiO5.
Characteristic of middle-grade metamorphism. Compare with kyanite, which has same composition and
forms under similar conditions but which has different crystal habit. Contrast with sillimanite, which has same composition but
different crystal habit and forms at highest metamorphic temperatures.
andesite - A
fine-grained igneous rock with no quartz or orthoclase, composted of about 75 percent
plagioclase feldspars, balance ferromagnesian silicates. Important as lavas; possibly derived by fractional crystallization from basaltic magma. Widely characteristic of mountain-making processes around borders of
Pacific Ocean. Confined to continental sectors.
andesite
line - A map line designating the petrographic boundary of Pacific Ocean. Extrusive rocks on Pacific side of line are
basaltic; on other side, andesitic.
angstrom - A
unit of length equal to one hundred-millionth of a centimeter (10-8
cm). Abbreviation, Å.
angular
momentum - A vector quantity, product of mass times radius of orbit times velocity.
The energy of motion of solar system.
angular
unconformity - An unconformity in which older strata dip at different angle from that of younger
strata.
anorthite - Feldspar in which diagnostic positive ion is Ca2+; calcic feldspar, Ca(Al2Si2O8).
One of plagioclase feldspars.
anorthosite - A plutonic igneous rock composed of 90
percent or more of feldspar mineral anorthite. Pyroxene and some olivine usually make up balance of the rock.
Antarctic
bottom water - Seawater that sinks to ocean floor off Antarctica
and flows equatorward beneath North Atlantic deep water.
Antarctic
intermediate water - Seawater that sinks at about 50ºS and
flows northward above North Atlantic deep water.
anthracite -
Metamorphosed bituminous coal of about 95 to 98 percent
carbon.
anticline - A
configuration of folded, stratified rocks in which rocks dip in two directions away from a crest, as
principal rafters of a common gable roof dip away from ridgepole. Reverse of syncline. The "ridgepole," or
crest, is called axis.
aphanitic
texture - Individual minerals present but in particles so small
that they cannot be identified without a microscope.
aquifer - A
permeable material through which groundwater moves.
arête - A
narrow, saw-toothed ridge developed between cirques.
arkose - A detrital sedimentary rock formed by cementation of individual grains of sand size and predominantly composed of quartz and feldspar. Derived from disintegration of granite.
arroyo -
Flat-floored, vertically walled channel of an intermittent stream typical of
semiarid climates; often applied to such features of southwestern United States.
Synonymous with wadi and wash.
artesian
water - Water under pressure when tapped by a well and able to
rise above level at which first encountered. It may or may not flow out at
ground level.
asbestos - A
general term applied to certain fibrous minerals displaying similar physical
characteristics although differing in composition. Some asbestos has fibers
long enough to be spun into fabrics with great heat resistance, such as those
for automobile brake linings. Types with shorter fibers are compressed into
insulating boards, shingles, etc. Most common asbestos mineral (95 percent of United States
production) is chrysotile, a variety of serpentine, a metamorphic mineral.
aseismic
ridge - An oceanic ridge without extensive seismic activity as,
for example, the Hawaiian Island-Emperor Seamount ridge in the Pacific Ocean.
ash -
Volcanic fragments of sharply angular glass particles, smaller than cinders. See also volcanic ash.
asphalt - A
brown to black, solid or semisolid bituminous substance.
Occurs in nature but is also obtained as residue from refining of certain hydrocarbons ("artificial
asphalt").
asteroids -
Orbiting small bodies believed to be either fragments of a disintegrated planet or of matter that never completed
planet forming process.
asthenosphere - A
zone within earth's mantle where plastic movements occur to
permit isostatic adjustments. Begins 50 to 100 km below surface and extends
perhaps to 500 km.
astrogeology - Geology of celestial bodies other than
earth. Generally restricted in meaning to geology of extraterrestrial bodies of
our planetary system.
Atlantic
Ridge - Belt of mountains under Atlantic
Ocean from Iceland
to Antarctica and nearly midway between continents; also called mid-Atlantic Ridge.
atoll - A
ring of low coral islands arranged around a central lagoon.
atom - A
building block of matter; combination of protons, neutrons, and electrons, of which 103 kinds are now
known.
atomic
mass - The nucleus of an atom contains 99.95 percent of its mass. Total number of protons and neutrons in nucleus is called mass number.
atomic
number - Number of positive charges on nucleus of an atom; number of protons in nucleus.
atomic
reactor - A huge apparatus in which a radioactive core heats water under pressure
and passes it to a heat exchanger.
atomic
size - Radius of an atom (average distance from center to
outermost electron of neutral atom). Commonly
expressed in angstroms.
augite - A rock-forming ferromagnesian silicate mineral built around single chains of silicon-oxygen tetrahedral.
aureole - A
zone in which contact metamorphism has taken place.
axial
plane - A plane through a rock fold that includes the axis and divides the fold as symmetrically as possible.
B
backset
beds - Inclined layers of sand developed on gentler dune slope to windward. These beds may
constitute a large part of total volume of a dune, especially if there is
sufficient vegetation to trap most sand before it can cross over to slip face.
bank-full
stage - Stage of flow at which a stream fills its channel up to
level of its bank. Recurrence interval averages 1.5 to 2 years.
barchan - A
crescent-shaped dune with wings, or horns, pointing downwing. Has gentle
windward slope and steep lee slope inside horns; about 30 m high and 300 m wide
from horn to horn. Moves with wind about 15 m/year across flat, hard surface
where limited supply of sand is available.
barrier
island - A low, sandy island near shore and parallel to it,
on a gently sloping offshore bottom.
barrier
reef - A reef separated from a landmass by a lagoon of varying
width and depth opening to sea through passes in reef.
basal
plane - Lower boundary of zone of concentric folding.
basal
slip - Movement of an entire glacier over underlying ground surface.
basalt - A
fine-grained igneous rock dominated by dark-colored minerals, consisting of plagioclase feldspars (over 50 percent) and ferromagnesian silicates. Basalts and andesites represent about 98 percent of all
extrusive rocks.
base
flow - That portion of stream flow attributable to groundwater flow.
base
level - Level below which a stream cannot erode. There may be temporary base levels along a stream's
course, such as those established by lakes or resistant layers of rock. Ultimate base level is sea level.
basement
complex - Undifferentiated rocks underlying oldest identifiable rocks
in any region. Usually sialic, crystalline, metamorphosed. Often, but not
necessarily, Precambrian.
basic
lava - Lava containing less than 50 percent
silica.
basin -
See dome.
batholith - A discordant pluton that increases in size
downward, has no determinable floor, and shows an area of surface exposure
exceeding 100 km2.
bauxite -
Chief ore of commercial aluminum. A mineraloid mixture of hydrous aluminum
oxides.
bay
barrier - A sandy beach, built up across mouth of a bay
so that it is no longer connected to main body of water.
bed -
(1) A rock mass of large horizontal extent bounded, especially above, by
physically different material. (2) A deposit, as of ore, parallel to local
stratification.
bedding -
(1) A collective term used to signify existence of beds or layers, in sedimentary rocks. (2) Sometimes synonymous
with bedding plane.
bedding
plane - Surface separating layers of sedimentary rocks. Each bedding plane marks
termination of one deposit and beginning of another of different character,
such as surface separating a sand bed from a shale layer. Rock tends to separate, or break, readily
along bedding planes.
bed
load - Material in movement along stream bottom or, if wind is
moving agency, along surface. Contrast with material carried in suspension or solution.
belt
of soil moisture - Subdivision of zone of aeration. Belt from which water may
be used by plants or withdrawn by soil evaporation. some water passes down into intermediate belt, where it may be held by
molecular attraction against influence of gravity.
bench
mark - A point of known elevation and position - usually
indicated on a map by the letters B. M., with the altitude given (on American
maps) to the nearest foot. (Some maps now use meters.)
Benioff
zone - A seismic zone dipping beneath a continental margin and having a deep-sea
trench as surface expression.
bergschrund - Gap,
or crevasse, between glacier ice and headwall of a cirque.
berms - In
coastline terminology berms are storm-built beach features that resemble small terraces; on seaward edges are low ridges
built up by storm waves.
beta
decay - Radioactive decay taking place by loss of a
beta particle (electron) from a neutron in nucleus. Mass of element remains same, but atomic number increases by 1.
B
horizon - Soil zone of accumulation below A horizon. Here is deposited some material
moved down from A horizon.
big-bang
theory - Theory that presently expanding universe originated as
primeval cosmic fireball in very short period of time 10 to 12 billion years ago. Compare steady-state theory.
binding
energy - Amount of energy that must be supplied to break an
atomic nucleus into its component fundamental particles. It is equivalent to mass that disappers when fundamental
particles combine to form a nucleus.
biochemical
rock - A sedimentary rock made up of deposits
resulting directly or indirectly from life processes of organisms.
biotite -
"Black mica," ranging in color from dark brown to green. Rock-forming ferromagnesian silicate mineral with tetrahedra in sheets.
bituminous
coal - Soft coal, containing about 80 percent carbon
and 10 percent oxygen.
blowout - A
basin scooped out of soft, unconsolidated deposits by process of deflation. Ranges from a few meters to
several kilometers in diameter.
body
wave - Push-pull or shake earthquake wave travelling through a body of a medium;
distinguished from waves travelling along free surface.
bornite - A mineral, Cu5FeS4; an
important ore of copper.
bottomset
bed - Layer of fine sediment deposited in a body of standing
water beyond advancing edge of a growing delta, which eventually builds up on
bottomset beds.
boulder
size - Volume greater than that of a sphere with diameter of 256
mm.
boulder
train - Series of glacier erratics from the same bedrock source,
usually with some property that permits easy identification. Arranged across
country in a fan, with apex at source and widening in direction of glacier
movement.
Bowen's
reaction series - Series of minerals for which any early-formed phase tends to react with melt that remains
to yield a new mineral further along in the series. Thus early-formed crystals of olivine react with remaining liquids to form augite crystals; these in turn may further
react with liquid then remaining to form hornblende. See also continuous reaction series and discontinuous reaction series.
braided
stream - Complex tangle of converging and diverging stream channels
separated by sand bars or islands. Characteristic of flood plains where amount of debris is
large in relation to discharge.
breccia -
Clastic rock made up of angular fragments of such
size that an appreciable percentage of rock volume consists of particles of
granule size or larger.
brittle - A
property of material whereby strength in tension is greatly different from strength
in compression. Substance ruptures easily with little or no flow.
brown
clay - An extremely fine-grained deposit found on the deep ocean
floors, most abundantly in the Pacific Ocean.
Derived from the continents and drifted to the open ocean,
where the clay-sized particles settle.
burial metamorphism - Changes resulting
from pressures and temperatures in rocks buried to depth of several
kilometers.
C
calcic
feldspar - Anorthite, Ca(Al2Si2O8).
calcite - A mineral composed of calcium carbonate, CaCO3.
caldera -
Roughly circular, steep-sided volcanic basin with diameter at least three or
four times depth. Commonly at summit of a volcano. Contrast with crater.
caliche -
Whitish accumulation of calcium carbonate in soil profile.
capillary
fringe - Belt above zone of saturation, in which underground water is lifted against gravity
by surface tension in passages of capillary size.
capillary
size - "Hairlike," or very small, such as tubes from
0.0025 to 0.25 cm in diameter.
carbohydrate -
compound of carbon, hydrogen, and oxygen. Carbohydrates are chief products of
life process in plants.
carbonate
mineral - Mineral formed by combination of complex ion (CO3)2- with a
positive ion. Common example: calcite, CaCO3.
carbon
cycle - Process in sun's deep interior by which radiant energy is generated in formation of
helium from hydrogen.
carbon
14 - Radioactive isotope of carbon, which has halflife of
5,730 years. Used to date events up to about 50,000 years ago.
carbon
ratio - A number obtained by dividing amount of fixed carbon in coal by sum of fixed carbon and volatile
matter and multiplying by 100. This is same as percentage of fixed carbon,
assuming no moisture or ash.
cassiterite - A mineral, tin dioxide, SnO2. Ore of tin with specific gravity 7; nearly 75 percent of
world's tin production is from placer deposits, mostly from cassiterite.
cataclastic
metamorphism - Textural changes in rocks in which brittle minerals and rocks are broken and flattened
as a result of intense folding or faulting; produces fragmentation of rocks
as coarse-grained breccias and fine-grained mylonites.
cementation -
Process by which a binding agent is precipitated in spaces among individual
particles of an unconsolidated deposit. Most common cementing agents are calcite, dolomite, and quartz; others include iron oxide, opal, chalcedony, anhydrite, and pyrite.
cement
rock - Clayey limestone used in manufacturing hydraulic
cement. Contains lime, silica, and alumina in varying proportions.
chalcedony -
General name applied to fibrous cryptocrystalline silica with waxy luster.
Deposited from aqueous solutions and frequenly found lining or filling cavities
in rocks. Agate is a variety with alternating layers
of chalcedony and opal.
chalcocite - A mineral, copper sulfide, Cu2S;
sometimes called copper glance; one of most important ore minerals of copper.
chalcopyrite - A mineral, a sulfide of copper and iron, CuFeS2;
sometimes called copper pyrite or yellow copper ore.
chalk -
Variety of limestone made up in part of biochemically
derived calcite, in form of skeletons or skeletal
fragments of microscopic oceanic plants and animals mixed with very
fine-grained calcite deposits of biochemical or inorganic-chemical origin.
chemical
rock - In terminology of sedimentary rocks chemical rock is composed
chiefly of material deposited by chemical precipitation, whether organic or inorganic
(compare with detrital sedimentary rock). Chemical
sedimentary rocks may have either clastic or nonclastic (usually crystalline) texture.
chemical
weathering - Weathering of rock material by chemical processes that
transform original material into new chemical combinations. Thus chemical
weathering of orthoclase produces clay, some silica, and
a soluble salt of potassium.
chert -
Granular cryptocrystalline silica, similar to flint but usually light in color. Occurs as compact massive rock or as nodules.
chlorite -
Family of tetrahedral sheet silicates of iron, magnesium, and aluminum,
characteristic of low-grade metamorphism. Green color, with cleavage like mica except that chlorite small scales are
not elastic.
C
horizon - Soil zone that contains partially
disintegrated and decomposed parent material. Lies directly under B horizon and grades downward into
unweathered material.
chromite - Mineral oxide of iron and chromium, FeCr2O4,
only ore of commercial chromium. One of first minerals to crystallize from magma; is concentrated within the magma.
chrysotile -
Metamorphic mineral; an asbestos, fibrous variety of serpentine. Silicate of magnesium, with
tetrahedra arranged in sheets.
chute or chute cutoff - Applied
to stream flow, chute refers to new route taken by a stream when main flow is
diverted to inside of a bend, along trough between low ridges formed by
deposition on inside of bend, where water velocity is reduced. Compare with neck cutoff.
cinder
cone - Structure built exclusively or
predominantly of pyroclastic ejecta dominated by cinders. Parasitic to a major volcano, seldom exceeds 500 m in height. Slopes up 30 to 40º. Eample:
Parícutin.
cinders -
Volcanic fragments; small, slaglike, solidified pieces of magma 0.5 to 2.5 cm across.
cirque - A
steep-walled hollow in a mountainside at high elevation, formed by
ice-plucking and frost action and shaped like a half bowl or
half amphitheater. Serves as principal gathering ground for ice of a valley glacier.
clastic
texture - Texture shown by sedimentary rocks from deposits of minerals and rock fragments.
clay
minerals - Finely crystalline, hydrous silicates formed from weathering of such silicate minerals as feldspar, pyroxene, and amphibole. Most
common clay minerals belong to kaolinite, montmorillonite, and illite groups.
clay size -
Volume less than that of a sphere with diameter of 0.004 mm.
cleavage -
(1) Mineral cleavage: property possessed by
many minerals of breaking in certain preferred direction along smooth plane
surfaces. Planes of cleavage are governed by atomic pattern and represent
direction in which atomic bonds are relatively weak. (2) Rock cleavage: property possessed by
certain rocks of breaking with relative ease along parallel planes or nearly
parallel surfaces. Rock cleavage is designated as slaty, phyllitic, schistose, and gneissic.
coal - Sedimentary rock composed of combustible
matter derived from partial decomposition and alteration of plant cellulose and
lignin.
cobble
size - Volume greater than that of a sphere with diameter of 64
mm and less than that of a sphere with diameter of 256 mm.
coesite -
High-pressure form of quartz, with density of 2.92. Associated with impact craters and cryptovolcanic structures.
col -
Pass through a mountain ridge. Created by enlargement of
two cirques on opposite sides of ridge until
headwalls meet an are broken down.
cold
glacier - Glacier in which no surface melting occurs
during summer months and whose temperature is always below freezing.
colloidal
size - between 0.0002 and 0.001 mm.
color -
Sensation resulting from stimulation of retina by light waves of certain lengths.
column -
Column, or post, of dripstone joining floor and roof of a cave;
result of joining of stalactite and stalagmite.
columnar
jointing - Pattern of jointing that blocks out columns of rock. Characteristic of tabular basalt flows or sills.
compaction -
Reduction in pore space between individual grains from pressure of overyling sediments or
pressures of earth movement.
composite
volcanic cone - Composed of interbedded lava flows and pyroclastic material and
characterized by slopes of close to 30º at summit, progressively reducing to 5º
near base. Example: Mayon.
compression -
Squeezing stress that tends to decrease volume of a
material.
concentric
folding - Elastic bending of an originally horizontal sheet with all
internal movements parallel to a basal plane (lower boundary of the fold).
conchoidal
fracture - A mineral's habit of fracturing to produce
curved surfaces like interior of a shell (conch). Typical of glass and quartz.
concordant
pluton - An intrusive igneous body with contacts parallel to
layering or foliation surfaces of rocks into which it has intruded.
concretion - An
accumulation of mineral matter formed around a center, or axis, of deposition after a sedimentary
deposit has been laid down. Cementation consolidates the deposit as a
whole, but the concretion is a body within hos rock that represents local concentration of
cementing material: enclosing rock is less firmly cemented than the concretion.
Commonly spheroidal or disk-shaped and composed of such cementing agents as calcite, dolomite, iron oxide, or silica.
cone
of depression - A dimple in the water table, which forms as water is pumped
from a well.
cone
sheet - A dike, part of a concentric set dipping
inward, like an inverted cone.
conglomerate - Detrital sedimentary rock made up of more
or less rounded fragments of such size that an appreciable percentage of volume
of rock consists of particles of granule size
or larger.
connate
water - Water trapped in a sedimentary deposit when the deposit
was laid down.
contact
metamorphism - Metamorphism at or very near contact
between magma and rock during intrusion.
continent -
One of the principal land masses of the earth, usually regarded as including Africa, Antarctica, Asia, Australia,
Europe, North America,
and South America.
continental
drift - Slow, lateral movement of continents; involves rigid plates that may carry both continental and
oceanic areas as they move.
continental
glacier - An ice sheet that obscures mountains and plains of a large section of
a continent. Continental glaciers exist on Greenland
and Antarctica.
continental
margin - Continental rise, continental slope, and continental shelf together make up the
continental margin, particularly on the trailing edge of a continent.
continental
rise - In some places base of continental slope is marked by somewhat
gentler continental rise, which leads downward to deep ocean floor.
continental
shelf - Shallow, gradually sloping zone extending from sea margin
to a depth at which there is marked or rather steep descent into ocean depths
down continental slope. Seaward boundary of
shelf averages about 130 m in depth.
continental
slope - Portion of ocean floor extending downward from seaward
edge of continental shelves. In some places, such as south of Aleutian
Islands, slopes descend directly to ocean deeps. In other places,
such as off eastern North America, they grad
into somewhat gentler continental rises, which in turn lead to
deep ocean floors.
continuous
reaction series - Branch of Bowen's reaction series comprising
plagioclase feldspars, in which reaction of
early-formed crystals with later liquids takes place continuously - that is,
without abrupt phase change.
contour
interval - The difference in elevation represented by adjacent
countour lines.
contour
line - A map line connecting points representing places on the
earth's surface that have the same elevation. It thus locates the intersection
with the earth's surface of a plane at any arbitrary elevation parallel to the datum plane. Contours represent the
vertical, or third, dimension on a map, which has only two dimensions. They
show the shape and size of physical features such as hills and valleys. A
depression is indicated by an ordinary contour line except that hachures, or
short dashes, are used on one side and point toward the center of the
depression.
convection -
Mechanism by which material moves because its density differs from that of surrounding
material. Density differences are often brought about by heating.
convection
cell - Pair of convection currents adjacent to each other.
convection
current - Closed circulation of material sometimes developed during convection. Convection currents normally
develop in pairs, each pair called convection cell.
convergent
plate boundary - Boundary between two plates moving toward each other. Compare divergent plate boundary.
coquina - A
coarse-grained, porous, friable variety of clastic limestone made up chiefly of fragments of
shells.
core - Innermost
zone of earth, surrounded by mantle.
Coriolis
effect - Tendency of any moving body, on or starting from surface
of earth, to continue in direction in which earth's rotation propels it.
Direction in which the body moves because of this tendency combined with
direction in which it is aimed determines ultimate course of the body relative
to earth's surface. In the northern hemisphere Coriolis effect causes a moving
body to veer or try to veer to right of its direction of forward motion; in the
southern hemisphere, to left. Magnitude of effect is proportional to velocity
of a body's motion. This effect causes cyclonic storm-wind circulation to be
counterclockwise in the northern hemispher and clockwise in the southern
hemisphere and determines final course of ocean currents relative to trade
winds.
correlation -
Process of establishing contemporaneity of rocks or events in one area with rocks or
events in another area.
crater -
Roughly circular, steep-sided volcanic basin with diameter less than three
times depth. Commonly at summit of a volcano (contrast with caldera). Applied also to depressions
caused by meteorites by either direct or explosive
impact.
craton -
Relatively immobile part of the earth, generally large central portion of a continent.
creep -
Applied to soils and surficial material, slow downward
plastic movement. As applied to elastic solids, slow permanent yielding to stresses less than yield point if applied for a short time
only.
crevasse -
(1) Deep crevice, or fissure, in glacier ice. (2) Breach in a natural levee.
cross
bedding - See inclined bedding.
crosscutting
relationships, law of - A rock is younger than any rock across which
it cuts.
crust -
Outermost shell of the earth. Continental crust averages 35 km thick, density 2.6 t/m3; oceanic crust,
about 5 km thick, density 3 t/m3.
cryptocrystalline -
State of matter in which there is actually orderly
arrangement of atoms characteristic of crystals but in units so small (material is
so fine grained) that crystalline nature cannot be determined with an ordinary
microscope.
cryptovolcanic
structure - Geologic structure in which rocks appear disrupted by volcanic activity
but in which there are no associated volcanic rocks.
crystal - Solid with orderly atomic arrangement,
which may or may not develop external faces that give it crystal form.
crystal
form - Geometrical form taken by a mineral, giving external expression to
orderly internal atomic arrangement.
Curie
temperature - Temperature above which ordinarily magnetic material loses
magnetism. On cooling below this temperature, it regains magnetism. Example:
Iron loses magnetism above 760º C and regains it while cooling below this
temperature. Thus 760º C is the Curie temperature of iron.
current
bedding - See inclined bedding.
cutoff -
See chute cutoff, neck cutoff.
D
datum
plane - The reference surface from which all altitudes on a map
are measured. This is usually mean sea level.
debris
slide - Small, rapid movement of largely unconsolidated material
that slides or rolls downward to produce irregular topography.
decomposition -
Synonymous with chemical weathering.
deep
focus - Earthquake focus deeper than 300 km. Greatest depth of
focus known is 700 km.
deep-sea
trenches - See island-arc deeps.
deflation -
Erosive process in which wind carries off unconsolidated material.
deformation
of rocks - Any change in original shape or volume of rock masses; produced by mountain-building forces. Folding, faulting, and plastic flow are common modes
of rock deformation.
delta -
Plain underlain by an assemblage of sediments accumulated where a stream flows
into a body of standing water, its velocity and transporting power suddenly
reduced. Originally named after Greek letter delta () because many are roughly
triangular in plan, with apex pointing upstream.
dendritic
pattern - An arrangement of stream courses that, on a map or viewed
from the air, resemble branching habit of certain trees, such as oaks or maples.
density
current - Current due to differences in density of water from place to place caused
by changes in temperature and variations in salinity or amount of material held
in suspension.
depositional
remanent magnetism - Magnetism resulting from tendency of
magnetic particles such as hematite to orient themselves in earth's
magnetic field as they are deposited. Orientation is maintained as soft
sediments are lithified and thus records earth's field when particles are laid
down. Abbreviation, DRM.
desert
varnish - Thin, shiny layer of iron and manganese oxides that coats
some desert-rock surfaces.
detrital
sedimentary rock - Rock formed from accumulation of minerals and rocks derived from erosion of previously existing rocks or
from weathered products of these rocks.
diabase - Rock of basaltic composition, essentially
labradorite and pyroxene, characterized by ophitic texture.
diamond - A mineral composed of elemental carbon; hardest substance known.
Used as a gem and, in industry, for cutting tools.
differential
erosion - Process by which different rock masses or different parts of same rock
erode at different rates.
dike - Tabular discordant pluton.
dike
swarm - Group of approximately parallel dikes.
dilatancy -
Tendency of rocks to expand along minute fractures immediately prior to failure; stress may be from earth movements or from
controlled laboratory experiments.
diorite -
Coarse-grained igneous rock with composition of andesite (no quartz or orthoclase), composed of 75 percent
plagioclase feldspars and balance ferromagnesian silicates.
dip -
(1) Acute angle that a rock surface makes with a horizontal plane.
Direction of dip is always perpendicular to strike. (2) See magnetic declination.
dipole -
Any object oppositely charged at two points. Most commonly refers to a molecule that has concentrations of positive charge or negative charge at two different points.
dipole
magnetic field - Portion of earth's magnetic field best described by a dipole passing through earth's center and
inclined to earth's rotation axis. See also nondipole magnetic field and external magnetic field.
dip
pole - See magnetic pole.
dip-slip
fault - Fault in which displacement is in direction
of fault's dip.
discharge -
With reference to stream flow, quantity of water that passes a given point in
unit time. Measured in cubic meters per second or, often, cubic feet per second
(abbreviation, cfs).
disconformity - Unconformity in which beds on opposite
sides are parallel.
discontinuity -
Within earth's interior, sudden or rapid changes with depth in one or more
physical properties of materials constituting the earth, as evidenced by
seismic data.
discontinuous
reaction series - Branch of Bowen's reaction series including minerals olivine, augite, hornblende, and biotite, for which each series change
represents abrupt phase change.
discordant
pluton - An intrusive igneous body with boundaries that cut across
surfaces of layering or foliation in rocks into which it has intruded.
disintegration -
Synonymous with mechanical weathering.
divergent
plate boundary - Boundary between two plates moving apart. New oceanic-type lithosphere is created at the opening.
Compare convergent plate boundary.
divide -
Line separating two drainage basins.
dolomite - Mineral composed of carbonate of calcium
and magnesium, CaMg(CO3)2. Also used as rock name for formations composed largely
of mineral dolomite.
dome -
Anticlinal fold without clearly developed linearity of
crest so that beds involved dip in all directions from a central area,
like an inverted but usually distorted cup. Reverse of basin.
drainage
basin - Area from which a given stream and its tributaries receive
water.
drift -
Any material laid down directly by ice or deposited in lakes, oceans, or
streams as result of glacial activity. Unstratified glacial drift is called till and forms moraines; stratified forms outwash plains, eskers, kames, and varves.
dripstone -
Calcium carbonate deposited from solution by underground water entering a cave in zone of aeration. Sometimes called travertine.
DRM -
See depositional remanent magnetism.
drumlin -
Smooth, streamlined hill composed of till. Long axis oriented in direction of
ice movement: Blunt nose points upstream, and gentler slope tails off
downstream. In height drumlins range from 8 to 60 m, with
average somewhat less than 30 m. Most drumlins are between 0.5 and 1 km in
length, the length commonly several times width. Diagnostic characteristics are
shape and composition of unstratified glacial drift, in contrast to kames, or stratified glacial drift and
random shapes.
dune -
Mound or ridge of sand piled by wind.
dust-cloud
hypothesis - Hypothesis that solar system was formed from condensation
of interstellar dust clouds.
dust size -
Volume less than that of sphere with diameter 0.06 mm; used in reference to
particles carried in suspension by wind.
E
ecliptic -
Apparent path of the sun in the heavens; plane of planets' orbits.
eclogite
facies - Metamorphic rocks of gabbroic composition, consisting primarily
of pyroxene and garnet.
elastic
deformation - Nonpermanent deformation, after which body returns to
original shape or volume when deforming force removed.
elastic
energy - Energy stored within a solid during elastic deformation and released during elastic rebound.
elastic
limit - Maximum stress that produces only elastic deformation.
elastic
rebound - Recovery of elastic strain, when material breaks or deforming
force is removed.
elastic
solid - A solid that yields to applied force by
changing shape, volume, or both but returns to original conditions when force
is removed. Amount of yield is proportional to force.
elasticity - A
property of materials that defines extent to which they resist small
deformations, from which they recover completely when deforming force is
removed. Elasticity equals stress divided by strain.
electric
charge - Property of matter resulting from imbalance between
number of protons and number of electrons in given piece of matter. The
electron has negative charge; the proton, positive charge. Like charges repel each
other; unlike charges attract each other.
electric
current - Flow of electrons.
electric
turbine generator - Apparatus that uses steam from heat exchanger to drive turbine and
generate electricity.
electron -
Fundamental particle of matter, the most elementary unit of
negative electrical charge. Mass, 0.00055 u (atomic mass unit or just mass unit).
electron
shell - Imaginary spherical surface representing all possible
paths of electrons with same average distance from nucleus and with approximately same energy.
electron-capture
decay - Radioactive decay that takes place as an
orbital electron is captured by a proton in nucleus. Mass of element remains constant, but atomic number decreases by 1.
element -
Unique combinations of protons, neutrons, and electrons that cannot be broken down by
ordinary chemical methods. Fundamental properties of an element are determined
by number of protons, each element assigned a number corresponding to its
number of protons. Combinations containing from 1 through 103 protons are now
known.
end
moraine - Ridge or belt of till marking farthest advances of a glacier. Sometimes called terminal moraine.
energy -
Capacity for producing motion. Energy holds matter together and can become mass or be derived from mass. Takes such
terms as kinetic, potential, heat, chemical, electrical, and atomic energy; one form of energy can be
changed to another.
energy
level - Distance from atomic nucleus at which electrons orbit. May be thought of as shell
surrounding nucleus.
epicenter -
Area on surface directly above focus of earthquake.
epidote -
Silicate of aluminum, calcium, and iron characteristic of low-grade metamorphism and associated with chlorite and albite in greenschist facies. Built around
independent tetrahedra.
epidote-amphibolite
facies - Assemblage of minerals formed between 250 and 450º C
during regional metamorphism.
erg -
Unit of energy expressing capacity for doing work.
Equal to energy expended when force of 1 dyne acts through distance of 1 cm.
erosion -
Movement of material from one place to another on earth's surface. Agents of
movement include gravity, water, ice, and wind.
erosional
flood plain - Flood plain created by lateral erosion and gradual retreat of valley walls.
erratic - In
terminology of glaciation a stone or boulder carried by ice to a place where it
rests on or near bedrock of different composition.
escape
velocity - Minimum velocity an object must have to escape from
gravitational field. For moon this is about 2.83 km/s and for earth
about 11.2 km/s.
esker -
Winding ridge of stratified glacial drift, steep-sided, 3 to 15 m high, and
from a fraction of a kilometer to over 160 km long.
eugosyncline -
Part of geosyncline in which volcanism is
associated with clastic sedimentation, generally located away from craton.
eustatic
change - Change in sea level produced entirely by increase or
decrease in amount of water in oceans; hence of worldwide proportions.
eutrophic - In
lake aging pertains to an old-age lake and indicates high supply of nutrients
supporting high biologic productivity.
evaporation -
Process by which liquid becomes vapor at temperature below
boiling point.
evaporite - rock composed of minerals precipitated from solutions
concentrated by evaporation of solvents. Examples: rock salt, gypsum, anhydrite.
exfoliation -
Process by which rock plates are stripped from larger rock
mass by physical forces.
exfoliation
dome - Large, rounded domal feature produced in homogenous
coarse-grained igneous rocks and sometimes in conglomerates by process of exfoliation.
external
magnetic field - Component of earth's field originating from activity above
earth's surface. Small when compared with dipole and nondipole components of field,
which originate beneath surface.
extravasate - To
cause (molten lava) to pour forth from a volcanic vent.
extrusive rock - Rock solidified from mass of magma poured or blown out upon earth's
surface.
F
facies -
Assemblage of mineral, rock, or fossil features reflecting environment in
which rock was formed. See sedimentary facies, metamorphic facies.
fault -
Surface of rock rupture along which has been differential
movement. A fracture in the continuity of a rock formation caused by a shifting
or dislodging of the earth's crust, in which adjacent surfaces are
differentially displaced parallel to the plane of fracture. Also called shift.
feldspars - Silicate minerals composed of silicon-oxygen
and aluminum-oxygen tetrahedra linked together in three-dimensional networks
with positive ions fitted into interstices of negatively charged framework of
tetrahedra. Classed as aluminosilicates. When positive ion is K+, mineral is orthoclase; when Na+, mineral is albite; when Ca+, mineral is anorthite.
felsite -
General term for light-colored, fine-grained igneous rocks.
ferromagnesian
silicate - Silicate in which positive ions are dominated by iron,
magnesium, or both.
fibrous
fracture - Mineral habit of breaking into splinters or
fibers.
fiery
cloud - Avalanche of incandescent pyroclastic debris mixed with steam and
other gases, heavier than air, and projected down
a volcano's side. Also called nuée ardente.
fiord -
Glacially deepened valley now flooded by the sea to form long, narrow,
steep-walled inlet.
firn -
Granular ice formed by recrystallization of snow. Intermediate between snow and
glacier ice. Sometimes called névé.
fissility -
Property of splitting along closely spaced planes more or less parallel to bedding. Presence distinguishes shale from mudstone.
flashy
stream - Stream with high flood peak of short duration, which may
be caused by urbanization.
flint -
Granular cryptocrystalline silica, usually dull and
dark. Often occurs as lumps or nodules in calcareous rocks, such as Cretaceous chalk beds of southern England.
flood
basalt - Basalt poured out from fissures in floods
that tend to form great plateaus. Sometimes called plateau basalt.
flood
frequency - Time within which a flood of a given size can be expected
to occur.
flood
plain - Area bordering a stream, over which water spreads in time
of flood.
flood
plain of aggradation - Flood plain formed by building up of valley
floor by sedimentation.
fluid -
Material that offers little or no resistance to forces tending to change its
shape.
focus -
Source of given set of earthquake waves.
fold - Bend, flexure, or wrinkle
in rock produced when rock was in a plastic
state.
fold
mountains - Mountains consisting primarily of elevated,
folded sedimentary rocks.
foliation -
Layering in some rocks caused by parallel alignment of minerals; textural feature of some metamorphic rocks. Produces rock cleavage.
footwall -
One of blocks of rock involved in fault movement. One that would be under
feet of person standing in tunnel along or across fault; opposite hanging wall.
fore
dune - Dune immediately behind shoreline of ocean
or large lake.
foreset
beds - Inclined layers of sediment deposited on advancing edge of
growing delta or along lee slope of advancing sand dune.
foreshock -
Relatively small earthquake that precedes larger earthquake by a few days or
weeks and originates at or near focus of larger earthquake.
fossil -
Evidence of past life, such as dinosaur bones, ancient clam shell, footprint of
long-extinct animal, or impression of leaf in rock.
fossil
fuels - Organic remains used to produce heat or power by
combustion. Include coal, petroleum, and natural gas.
fractional
distillation - Recovery - one or more at a time - of fractions of complex
liquid, each of which has different density.
fractionation -
Process whereby crystals that formed early from magma have time to settle appreciably before
temperature drops much further. They are thus effectively removed from
environment in which they formed.
fracture - As
mineral characteristic, way in which
mineral breaks when it does not have cleavage. May be conchoidal (shell shaped),
fibrous, hackly, or uneven.
fracture
cleavage - System of joints spaced fraction of centimeter apart.
freeboard
of continents - Relative elevation of continents with respect to sea level.
fringing
reef - Reef attached directly to landmass.
frost
action - Process of mechanical weathering caused by repeated
cycles of freezing and thawing. Expansion of water during freezing cycle
provides energy for process.
fumarole -
Vent for volcanic steam and gases.
fundamental
particles - Protons, neutrons, and electrons, which combine to form atoms. Each particle is defined in terms of
its mass and its electric charge.
fusion -
Process by which nuclei of lighter atoms join to form nuclei of heavier atoms.
G
gabbro -
Coarse-grained igneous rock with composition of basalt.
galaxy -
Family of stars grouped in space. The earth belongs
to Milky Way galaxy, which contains about 100 billion stars.
galena - A mineral; lead sulfid, PbS. Principal ore of
lead.
garnet -
family of silicates of iron, magnesium, aluminum, calcium, manganese, and
chromium, which are built around independent tetrahedra and appear commonly as
distinctive 12-sided, fully developed crystals. Characteristic of metamorphic rocks; generally cannot be
distinguished from one another without chemical analysis.
gas -
(1) State of matter that has neither independent shape
nor volume, can be compressed readily, and tends to expand indefinitely. (2) In
geology "gas" is sometimes used
to refer to natural gas, gaseous hydrocarbons that occur in rocks, dominated by methane. Compare "oil," referring to petroleum.
geode -
Roughly spherical, hollow or partially hollow accumulation of mineral matter from a few centimeters to
nearly 0.5 m in diameter. Outer layer of chalcedony lined with crystals that project toward hollow center.
Crystals, often
perfectly formed, usually quartz although calcite and dolomite also found and - more rarely -
other minerals. Geodes most commonly found in limestone and more rarely in shale.
geographic
poles - Points on earth's surface marked by ends of axis in
rotation.
geologic
column - Chronologic arrangement of rock units in columnar form, with oldest
units at bottom and youngest at top.
geologic-time
scale - Chronologic sequence of units of earth time.
geologize - To
study geology or make geologic investigations.
geology -
Organized body of knowledge about the earth, including physical geology and historical geology, among others.
geomagnetic
poles - Dipole best approximating earth's observed
field is one inclined 11.5º from axis of rotation. Points at which ends of this
imaginary magnetic axis intersect earth's surface are known as geomagnetic
poles. They should not be confused with magnetic, or dip, poles or virtual geomagnetic poles.
geophysical
prospecting - Mapping rock structures by methods of experimental
physics. Includes measuring magnetic fields, force of gravity, electrical
properties, seismic-wave paths and velocities, radioactivity, and heat flow.
geophysics -
Physics of the earth.
geosyncline -
Literally, "earth syncline." Term now refers, however,
to a basin in which thousands of meters of sediments have accumulated, with
accompanying progressive sinking of basin floor explained only in part by load
of sediments. Common usage includes both accumulated sediments themselves and
geometrical form of basin in which they are deposited. All folded mountain ranges were built from geosynclines, but
not all geosynclines have become mountain ranges.
geothermal
field - Area where wells drilled to obtain elements contained in solution in hot
brines and to tap heat energy.
geothermal
gradient - See thermal gradient.
geyser -
Special type of thermal spring that intermittently ejects
its water with considerable force.
glacier - A
mass of ice, formed by recrystallization of snow, that flows forward or has
flowed at some past time under influence of gravity. By convention we exclude
icebergs even though they are large fragments broken from seaward end of
glaciers.
glacier
ice - Unique form of ice developed by compression and recrystallization of snow
and consisting of interlocking crystals.
glass -
Form of matter that exhibits properties of a solid but has atomic arrangements, or lack
of order, of a liquid.
Glossopteris
flora - A late-Paleozoic assemblage of fossil plants named for seed fern
Glossopteris, a plant in the flora. Widespread in South America, South Africa,
Australia,
India,
and Antarctica.
gneiss - Metamorphic rock with gneissic cleavage. Commonly formed by metamorphism of granite.
gneissic
cleavage - Rock cleavage where surface may be a few
hundredths of a millimeter to a centimeter or more apart.
goethite -
Hydrous iron oxide, FeO(OH).
Gondwanaland -
Hypothetical continent thought to have broken up in
Mesozoic. Resulting fragments are postulated to form presentday South America, Africa, Australia, India, and Antarctica.
graben -
Elongated, trenchlike, structural form bounded by parallel normal faults created when block that forms
trench floor moves downward relative to blocks that form sides.
grade -
Term used to designat extent to which metamorphism has advanced. Found in such
combinations as "high-grade" or "low-grade metamorphism."
Compare with rank.
graded
bedding - Type of bedding shown by sedimentary deposit when
particles become progressively finer from bottom to top.
gradient -
Slope of stream bed or land surface; expressed in percent, as feet per mile, or
meters per kilometer, or in degrees.
granite -
Coarse-grained igneous rock dominated by light-colored minerals, consisting of about 50 percent orthoclase, 25 percent quartz, and balance of plagioclase feldspars and ferromagnesian silicates. Granites and granodiorites comprise 95 percent of all intrusive rocks.
granitization -
Special type of metamorphism by which solutions of magmatic origin move through solid rocks, change ions with them, and convert
them into rocks that achieve granitic character without having passed through
magmatic stage.
granodiorite -
Coarse-grained igneous rock intermediate in composition
between granite and diorite.
granular
texture - Composed of mineral grains large enough to be seen by
unaided eye; also called phaneritic texture.
granulite
facies - Gneissic rocks produced by deep-seated high-grade regional metamorphism.
graphic
structure - Intimate intergrowth of potassic feldspar and quartz with long axes of quartz crystals lining up parallel to feldspar
axis. Quartz part is dark and feldspar is light in color; so pattern suggests Egyptian
heiroglyphs. Commonly found in pegmatites.
graphite -
"Black lead." A mineral composed entirely of carbon. Very
soft because of crystalline structure; diamond, in contrast, has same composition
but its hardest substance known.
gravity
anomaly - Difference between observed and computed values of
gravity.
gravity
fault - Fault in which hanging wall appears to have moved downward
relative to footwall. Also called normal fault.
gravity
meter - An instrument for measuring force of gravity. Also called
gravimeter.
gravity
prospecting - Mapping force of gravity at different places to determine
differences in specific gravity of rock masses and, through this, distribution
of masses of different specific gravity. Done with gravity meter (gravimeter).
graywacke - A
variety of sandstone generally characterized by hardness, dark color, and angular grains of quartz, feldspar, and small rock fragments set in matrix of clay-sized
particles. Also called lithic sandstone.
greenschist - Schist characterized by green color. Product of regional metamorphism of simatic rocks. (Green color is imparted by mineral chlorite.)
greenschist
facies - Assemblage of minerals formed between 150 and 250º C
during regional metamorphism.
groundmass -
Finely crystalline or glassy portion of porphyry.
ground
moraine - Till deposited from a glacier as veneer over landscape and
forming gently rolling surface.
groundwater - Underground water within zone of saturation.
groundwater
table - Upper surface of zone of saturation for underground water. An irregular surface
with slope or shape determined by quantity of groundwater and permeability of earth materials. In
general, highest beneath hills and lowest beneath valleys. Also referred to as water table.
guyot -
Flat-topped seamount rising from ocean floor like a volcano but planed off on top and covered
by appreciable water depth. Synonymous with tablemount.
gypsum -
Hydrous calcium sulfate, CaSO4 · 2H2O. A soft, common mineral in sedimentary rocks, where it sometimes
occurs in thick beds interstratified with limestones and shales. Sometimes occurs as layer under bed
of rock salt since it is one of first minerals
to crystallize on evaporation of seawater. Alabaster is a
fine-grained massive variety of gypsum.
H
hackly
fracture - Mineral's habit of breaking to produce
jagged, irregular surfaces with sharp edges.
half-life -
Time needed for one-half of nuclei in sample of radioactive element to decay.
halide -
Compound made from a halogen, such as chlorine, iodine, bromine, or fluorine.
halite - A mineral; rock salt or common salt, NaCl. Occurs widely disseminated or
in extensive beds and irregular masses precipitated from seawater and
interstratified with rocks of other types as true sedimentary rock.
hanging
valley - A valley that has greater elevation than the valley to
which it is tributary, at point of junction. Often (but not always) created by
deepening of main valley by a glacier. Hanging valley itself may or may
not be glaciated.
hanging
wall - One of blocks involved in fault movement. One that would be hanging
overhead for person standing in tunnel along or across fault; opposite footwall.
hardness - Mineral's resistance to scratching on a
smooth surface. Mohs scale of relative hardness consists of 10 minerals, each
scratching all those below it in scale and being scratched by all those above
it:
1.
talc
2.
gypsum
3.
calcite
4.
fluorite
5.
apatite
6.
orthoclase
7.
quartz
8.
topaz
9.
corundum
10.
diamond
head -
Difference in elevation between intake and discharge points for a liquid. In geology most commonly of interest in
connection with movement of underground water.
heat
energy - Special manifestation of kinetic energy in atoms. Temperature of a substance depends
on average kinetic energy of component particles. When
heat is added to a substance, average kinetic energy increases.
heat
exchanger - Unit in atomic-power generation that uses water (heated
under pressure by atomic reactor) to form steam from water in
another system and to drive a turbine for electricity generation.
heat
flow - Product of thermal gradient and thermal conductivity
of earth materials.
Average over whole earth, 1.2 ± 0.15 µcal/cm2-s.
Average over whole earth, 1.2 ± 0.15 µcal/cm2-s.
heave - To
displace or move (a vein, lode, or stratum, for example).
height -
The vertical difference in elevation between an object and its immediate
surroundings.
hematite -
Iron oxide, Fe2O3. Principal ore mineral for about 90 percent of commercial
iron produced in United States. Characteristic red color when powdered.
hinge
fault - Fault in which displacement preceptibly
dies out along strike and ends at definite point.
historical
geology - Branch of geology that deals with history of the
earth, including record of life on earth as well as physical changes in earth
itself.
horn -
Spire of bedrock left where cirques have eaten into a mountain from more than two sides around a
central area. Example: Matterhorn of the Swiss Alps.
hornblende - A rock-forming ferromagnesian silicate mineral with double chains of silicon-oxygen tetrahedral. An amphibole.
hornfels -
Dense, granular metamorphic rock. Since this term is commonly
applied to a metamorphic equivalent of any fine-grained rock, composition is variable.
hornfels
facies - Assemblage of minerals formed as temperatures greater
than 700º C during contact metamorphism.
horst - Elongated
block bounded by parallel normal faults in such a way that it stands
above blocks on both sides.
hot
spot - Localized melting region in mantle near base of lithosphere, a few hundred kilometers in
diameter and persistent over tens of millions of years. Existence of heat is
assumed from volcanic activity at surface.
hot
spring - Thermal spring that brings hot water to
surface. Water temperature usually 6.5º C or more above mean air temperature.
hydration -
Process by which water combines chemically with other molecules.
hydraulic
gradient - Head of underground water divided by distance of
travel between two points: If head is 10 m for two points 100 m apart,
hydraulic gradient is 0.1 or 10 percent. When head and distance of flow are
same, hydraulic gradient is 100 percent.
hydraulic
mining - Use of strong water jet to move deposits of sand and gravel from original site to
separating equipment, where sought-for mineral is extracted.
hydrocarbon -
Compound of hydrogen and carbon that burns in air to form water and oxides of
carbon. There are many hydrocarbons. The simples, methane, is chief component of natural gas. Petroleum is a complex mixture of
hydrocarbons.
hydroelectric
power - Conversion of energy to electricity by free fall of
water. This method supplies about 4 percent of world's electrical energy.
hydrograph -
Graph of variation of stream flow over time.
hydrologic
cycle - General pattern of water movement by evaporation from sea to atmosphere, by precipitation onto land, and by return to
sea under influence of gravity.
hydrothermal
solution - Hot, watery solution that usually emanates from magma in late stages of cooling. Frequently
contains, and deposits in economically workable concentrations, minor elements that, because of incommensurate
ionic radii or electronic charges, have not been able to fit into atomic
structures of common minerals of igneous rocks.
hysteresis -
Retardation of recovery from elastic deformation after stress is removed.
I
icecap -
Localized ice sheet.
ice
sheet - Broad, moundlike mass of glacier ice of considerable extent that has
tendency to spread radially under own weight. Localized ice sheets are
sometimes called icecaps.
igneous -
(1) Formed by solidification from a molten state. Used of rocks.
(2) Of or relating to rock so formed; pyrogenic.
igneous
rock - Aggregate of interlocking silicate minerals formed by cooling and solidification of magma.
illite -
Clay mineral family of hydrous aluminous
silicates. Structure is similar to that of montmorillonite, but aluminum replaces 10
to 15 percent of silicon, which destroys montmorillonite's property of
expanding with addition of water because weak bonds replaced by strong
potassium-ion links. Structurally illite intermediate
between montmorillonite and muscovite. Montmorillonite converts to
illite in sediments; illite, to muscovite under conditions of low-grade metamorphism. Illite is commonest clay
mineral in clayey rocks and recent marine sediments and is
present in many soils.
ilmenite -
Iron titanium oxide. Accounts for much of unique abundance of titanium on moon.
inclined
bedding - Bedding laid down at angle to horizontal.
Also referred to as cross bedding or current bedding.
index
minerals - Chlorite, low-grade metamorphism; almandite, middle-grade metamorphism; sillimanite, high-grade metamorphism.
induced
magnetism - In terminology of rock magnetism one of components of rock's natural remanent magnetism. It is parallel
to earth's present field and results from it.
inertia
member - Central element of a seismograph, consisting of weight suspended
by wire or spring so that it acts like pendulum free to move in only one plane.
infiltration -
Soaking into ground of water on surface.
inosilicate - Mineral with crystal structure containing silicon-oxygen tetrahedra in single or
double chains.
intensity -
Measure of effects of earthquake waves on human beings, structures, and
earth's surface at particular place. Contrast with magnitude, which is measure of total energy released by an earthquake.
intermediate
belt - Subdivision of zone of aeration. Belt that lies between
belt of soil moisture and capillary fringe.
intermediate
lava - Lava composed of 60 to 65 percent silica.
intrusive
rock - Rock solidified from mass of magma that invaded earth's crust but did not reach surface.
ion -
Electrically unbalanced form of an atom or group of atoms, produced by gain or
loss of electrons.
ionic
bond - Bond in which ions are held together by electrical attraction
of opposite charges.
ionic
radius - Average distance from center to outermost electron of an ion. Commonly expressed in angstroms.
island-arc
deeps - Arcuate trenches bordering some continents; some reach depths of 9,000 m or
more below sea surface. Also called deep-sea trenches or trenches.
isoclinal
folding - Beds on both limbs nearly parallel, whether fold upright, overturned, or recumbent.
isograd -
Line connecting those rocks comprising the same metamorphic facies; line connecting similar
temperature-pressure values; line marking the boundary
between two metamorphic facies.
isoseismic
line - Line connecting all points on surface of earth where intensity of shaking from earthquake waves is same.
isostasy -
Ideal condition of balance that would be attained by earth materials of
differing densities if gravity were the only force governing heights relative to each other.
J
jasper -
Granular, cryptocrystalline silica usually colored red by hematite inclusions.
joint -
Break in rock mass with no relative movement of rock
on opposite sides of break
joint
system - Combination of intersecting joint sets, often at approximately right
angles.
juvenile water -
Water brought to surface or added to underground supplies from magma.
K
kame -
Steep-sided hill of stratified glacial drive. Distinguished from drumlin by lack of unique shape and by stratification.
kame
terrace - Stratified glacial drift deposited between wasting glacier and adjacent valley wall. When ice
melts, this material stands as a terrace along valley wall.
Kansan -
Of, or relating to, the second glacial stage of the Pleistocene in North
America.
kaolinite -
Clay mineral, hydrous aluminous silicate, Al4Si4O10(OH)8.
Structure consists of one sheet of silicon-oxygen tetrahedra each sharing
three oxygens to give ratio of (Si4O10)4-
linked with one sheet of aluminum and hydroxl. Composition of pure kaolinite
does not vary as for other clay minerals, montmorillonite and illite, in which ready addition or
substitution of ions takes place.
karst
topography - Irregular topography characterized by sink holes,
streamless valleys, and streams that disappear underground - all developed by
action of surface and underground water in soluble rock such as limestone.
Kerguelen-Gaussberg
Ridge - Belt of mountains under Indian Ocean between India
and Antarctica.
kettle -
Depression in ground surface formed by melting of a block of ice buried or
partially buried by glacial drift, either outwash or till.
kinetic
energy - Energy of movement. Amount possessed by an
object or particle depends on mass and speed.
kyanite - A silicate mineral characteristic of
temperatures of middle-grade metamorphism. Al2SiO5
in bladed blue crystals is softer than a knife along the
crystal. Its crystalline structure is based on
independent tetrahedra. Compare with andalusite, which has same composition and
forms under similar conditions but has different crystal habit. Contrast with sillimanite, which has same composition but
different crystal habit and forms at highest metamorphic temperatures.
L
L -
Symbol for earthquake surface waves.
laccolith - Concordant pluton that has domed up strata
into which it intruded.
lag
time - On stream hydrograph time interval between center of
mass of precipitation and center of mass of
resulting flood.
laminar
flow - Mechanism by which fluid (such as water) moves slowly along a
smooth channel or through a tube with smooth walls with fluid particles
following straight-line paths parallel to channel or walls. Contrast with turbulent flow.
landslide -
General term for relatively rapid mass movement, such as slump, rock slide, debris slide, mudflow, and earthflow.
lapilli - Pyroclastic debris in pieces about walnut
size.
large
waves - Earthquake surface waves.
latent
heat of fusion - Number of calories per unit volume that must be added to a
material at melting point to complete process of melting. These calories do not
raise temperature.
lateral
moraine - Ridge of till along edge of valley glacier. Composed largely of
material fallen to glacier from valley walls.
laterite -
Tropical soil rich in hydroxides of aluminum and
iron and formed under conditions of good drainage.
lava - Magma poured out on surface of earth or rock solidified from such magma.
leading
edge - That margin that occurs on the side of a continent which is riding on a plate in motion toward another plate;
commonly this margin is very active tectonically.
left-lateral
fault - Strike-slip fault where ground opposite you
appears to have moved left when you face it.
levee
(natural) - Bank of sand and silt built by river during floods,
where suspended load deposited in greatest quantity close to river. Process of
developing natural levees tends to raise river banks above level of surrounding
flood plains. Break in natural levee
sometimes called crevasse.
lignite -
Low-grade coal, with about 70 percent carbon and 20
percent oxygen. Intermediate between peat and bituminous coal.
limestone - Sedimentary rock composed largely of mineral calcite, CaCO3, formed by either
organic or inorganic processes. Most limestones have clastic texture, but nonclastic,
particularly crystalline, textures are common. Carbonate rocks,
limestone and dolomite, constitute estimated 12 to 22
percent of sedimentary rocks exposed above sea level.
limonite -
Iron oxide with no fixed composition or atomic structure; a mineraloid. Always of secondary origin, not
a true mineral. Is encountered as ordinary rust or
coloring material of yellow clays and soils.
liquefaction -
Process of changing soil and unconsolidated sediments into
water mixture immediately following earthquake; often results in foundation
failure, with sliding of ground under building structures.
liquid -
State of matter that flows readily so that the mass assumes form of container but retains
independent volume.
lithic
sandstone - See graywacke.
lithification -
Process by which unconsolidated rock-forming materials are converted into
consolidated or coherent state.
lithosphere -
Rigid outer layer of earth; includes crust and upper part of mantle. Relatively strong layer in contrast
to underlying asthenosphere.
loess -
Unconsolidated, unstratified aggregation of small, angular mineral fragments, usually buff in color. Generally believed to be
wind-deposited; characteristically able to stand on very steep to vertical
slopes.
longitudinal
dune - Long ridge of sand (dune) oriented in general direction of wind
movement. A small one is less than 3 m high and 60 m long. Very large ones are
called seif dunes.
longitudinal
wave - Push-pull wave.
lopolith - Tabular concordant pluton shaped like spoon bowl,
with both roof and floor sagging downward.
M
magma -
Naturally occurring silicate melt, which may contain suspended silicate crystals, dissolved gases, or both. These conditions may be met
in general by a mixture containing as much as 65 percent crystals but no more
than 11 percent dissolved gases.
magmatic
segregation - The process by which heavy, early crstallized minerals settle out from a magma and become concentrated.
magnetic
declination - Angle of divergence between geographic meridian and
magnetic meridian. Measured in degrees east and west of geographic north.
magnetic
inclination - Angle that magnetic needle makes with surface of earth.
Also called dip of magnetic needle.
magnetic
pole - North magnetic pole is point on earth's surface where
north-seeking end of a magnetic needle free to move in space points directly
down. At south magnetic pole the same needle points directly up. These poles
are also known as dip poles.
magnetic
reversal - Shift of 180º in earth's magnetic field such that
north-seeking needle of magnetic compass would point south rather than to north
magnetic pole.
magnetite - A mineral; iron oxide, Fe3O4.
Black; strongly magnetic. Important ore of iron.
magnetosphere -
Region 1,000 to 64,000 km above earth, where magnetic field traps electrically
charged particles from sun and space. First believed to consist of two bands,
Van Allen belts.
magnetostratigraphy -
Use of magnetized rocks to determine history of events in
record of changes in earth's magnetic field in past geologic ages.
magnitude -
Measure of total energy released by an earthquake. Contrast
with intensity, which is measure of effects of
earthquake waves at particular place.
manganes
nodule - A nodule found on the deep ocean floors and
composed largely of iron and manganese, with smaller amounts of such metals as cobalt, titanium, copper, and
nickel.
mantle -
Intermediate zone of earth. Surrounded by crust, it rests on core at depth of about 2,900 km.
marble - Metamorphic rock of granular texture, with no rock cleavage, and composed of calcite, dolomite, or both.
maria -
Dark-toned "seas" of moon. Mark moons topographically low areas.
marl -
Calcareous clay or intimate mixture of clay and particles of calcite or dolomite, usually shell fragments.
marsh
gas - Methane, CH4; simplest paraffin hydrocarbon. Predominant component of natural gas.
mass - A
number that measures quantity of matter. It is obtained on earth's surface
by dividing weight of a body by acceleration due to gravity.
mass
movement - Surface movement of earth materials induced by gravity.
matter -
Anything that occupies space. Usually defined by describing its states and
properties: solid, liquid, or gas; possesses mass, inertia, color, density, melting point, hardness, crystal form, mechanical strength, or chemical properties. Composed
of atoms.
meander -
(1) Turn or sharp bend in stream's course. (2) To turn, or bend, sharply.
Applied to stream courses in geological usage.
mechanical
weathering - Process by which rock is broken down into smaller and
smaller fragments as result of energy developed by physical forces. Also
known as disintegration.
medial
moraine - Ridge of till formed by junction of two lateral moraines when two valley glaciers join to form single ice
stream.
mélange -
Heterogenous mixture of rock materials. Mappable body of deformed
rocks that may be several kilometers in length and consists of highly sheared
clayey matrix, thoroughly mixed with angular native and exotic blocks of
diverse origin and geologic age.
Mercalli
intensity scale - Scale to evaluate intensity of earthquake shaking on basis of
effects at given place.
mesotrophic - In
lake aging, stage between oligotrophic and eutrophic.
metal -
Substance fusible and opaque, good conductor of electricity, and with
characteristic luster. Examples: gold, silver, aluminum. Of the elements 77 are metals.
metallic
bonding - Special kind of bonding in atoms of metallic elements whereby outermost electrons are not shared or exchanged but
are free to move around and connect to any atoms in solid. Relative freedom of movement of
electrons accounts for high level of electrical conductivity in metals.
metalloid - Element of some metallic and some nonmetallic characteristics. There are nine
metalloids.
metamorphic
facies - Assemblage of minerals that reached equilibrium during metamorphism under specific range of
temperature and pressure.
metamorphic
rock - "Changed-form rock." any rock changed in texture or composition by heat, pressure, or chemically active fluids after original formation.
metamorphic
zone - Area subjected to metamorphism and characterized by certain metamorphic facies formed during process.
metamorphism - A
process whereby rocks undergo physical or chemical changes
or both to achieve equilibrium with conditions other than those under which
they were originally formed (weathering arbitrarily excluded from
meaning). Agents of metamorphism are heat, pressure, and chemically active fluids.
metasomatism -
Process whereby rocks are altered when volatiles exchange
ions with them.
meteor -
Transient celestial body that enters earth's atmosphere with great speed,
becoming incandescent from heat generated by air resistance.
meteoric
water - Groundwater derived primarily from precipitation.
meteorite -
Stone or metallic body fallen to earth from outer
space.
methane -
Simplest paraffin hydrocarbon, CH4. Principal
consituent of natural gas. Sometimes called marsh gas.
micas -
Group of silicate minerals characterized by perfect
sheet or scale cleavage resulting from atomic pattern, in
which silicon-oxygen tetrahedra linked in sheets.
Biotite is ferromagnesian black mica. Muscovite is potassic white mica.
microseism -
"Small shaking." Specifically limited in technical usage to earth waves generated by sources other than
earthquakes and, most frequently, to waves with periods of from 1 to about 9 s, from
sources associated with atmospheric storms.
mid-Atlantic
Ridge - See Atlantic Ridge.
midocean
ridge - Continuous, seismically active, median mountain range extending through North and South
Atlantic, Indian, and South Pacific Oceans.
mineral -
Naturally occurring solid element or compound, exclusive of
biologically formed carbon components. Has definite composition or range of
composition and orderly internal atomic arrangement (crystalline structure), which gives unique
physical and chemical properties, including tendency to assume certain
geometrical forms known as crystals.
mineral
deposit - Occurrence of one or more minerals in such concentration and form as
to make possible removal and processing for use at profit.
mineraloid -
Substance that does not yield definite chemical formula and shows no sign of crystallinity. Examples: bauxite, limonite, and opal.
miogesyncline -
That part of a geosyncline in which volcanism is absent,
generally located near craton.
modulus
of elasticity - Slope of graph line relating stress to strain in elastic deformation.
Mohorovicic
discontinuity (Moho) - Base of crust marked by abrupt increases in
velocities of earth waves.
molecule -
Smallest unit of compound that displays properties of that compound.
Monel
metal - Steel containing 68 percent nickel.
monocline -
Double flexure connecting strata at one level with same strata at another
level.
montmorillonite -
Clay mineral family, hydrous aluminous silicate
with structural sandwich of one ionic sheet of aluminum and hydroxl between two
(Si4O10)4- sheets. Sandwiches piled on each
other with water between and with nothing but weak bonds to hold them together.
As result, additional water can enter lattice readily, causing mineral to swell
appreciably and further weakening attraction between structural sandwiches.
Consequntly a lump of montmorillonite in a bucket of water slumps rapidly into a loose, incoherent
mass. Compare with other clay minerals, kaolinite and illite.
moon - A
natural satellite.
moraine -
General term applied to certain landforms composed of till.
mountain -
Any part of landmass projecting conspicuously above its surroundings.
mountain
chain - Series of more or less parallel ridges, all of which
formed within a single geosyncline or on its borders.
mountain
structure - Structure produced by deformation of rocks.
mudcracks -
Cracks caused by shrinkage of drying deposit of silt or clay under surface
conditions.
mudflow -
Flow of well-mixed mass of rock, earth, and water that behaves like a fluid and moves down slopes with
consistency similar to that of newly mixed concrete.
mudstone -
Fine-grained, detrital sedimentary rock made up of silt- and clay-sized particles.
Distinguished from shale by lack of fissility.
muscovite -
"White mica." Nonferromagnesian rock-forming silicate mineral with tetrahedra arranged
in sheets. Sometimes called potassic mica.
mylonite -
Fine-grained rock formed by grinding during intense folding or faulting associated with cataclastic metamorphism.
N
native state - State in which an element occurs uncombined in nature.
Usually applied to metals, as in "native copper,"
"native gold," etc.
natural
gas - Gaseous hydrocarbons that occur in rocks. Dominated by methane.
natural remanent magnetism - Magnetism of rock. May or may not coincide with present
magnetic field of earth. Abbreviation, NRM.
natural
resources - Energy and materials made available by
geological processes.
neck
cutoff -
Breakthrough of a river across narrow neck separating two meanders, where downstream migration of one
has been slowed and next meander upstream has overtaken it. Compare with chute cutoff.
negative
charge -
Condition resulting from surplus of electrons.
nesosilicate - Mineral with crystal structure containing silicon-oxygen tetrahedra arranged as
isolated units.
neutron - Proton and electron combined and behaving like
fundamental particle of matter. Electrically neutral with mass of 1.00896 u. If isolated, decays to
form proton and electron.
névé - Granular ice formed by recrystallization of snow.
Intermediate between snow and glacier ice. Sometimes called firn.
Nichrome - Steel alloy with 35 to 38 percent
nickel.
nickel
steel -
Steel containing 2.5 to 3.5 percent nickel.
nodule - Irregular, knobby-surfaced mineral body that differs in composition
from rock in which formed. Silica in form of chert or flint is common component of nodules. They
are commonly found in limestone and dolomite.
nonclastic
texture -
Applied to sedimentary rocks in which rock-forming
grains are interlocked. Most sedimentary rocks with nonclastic texture are crystalline.
nonconformity - Unconformity separating younger rocks from distinctly different older high
metamorphosed or igneous rocks.
nondipole magnetic field - Portion of earth's
magnetic field remaining after dipole field and external field are
removed.
nonferromagnesians - Silicate minerals that do not contain iron
or magnesium.
nonmetal - Element that does not exhibit metallic
luster, conductivity, or other features of metal. Of the elements 17 are non-metals.
normal
fault - Fault in which hanging wall appears to have moved downward
relative to footwall; opposite of thrust fault. Also called gravity fault.
North Atlantic deep water - Seawater in Arctic
that sinks in North Atlantic and drifts southward as far as 60º S.
northeast
trades -
Winds blowing from the northeast toward the equator in the northern hemisphere.
NRM - See natural remanent magnetism.
nuée
ardente -
"Hot cloud." French term applied to highly heated mass of gas-charged lava ejected from vent or pocket at volcano summit more or less horizontally
onto an outer slope, down which it moves swiftly, however slight the incline,
because of its extreme mobility.
O
obduction - Process whereby part of the subducted plate and/or associated igneous rocks and deep-sea sediments are
broken off and pushed up onto the overriding plate.
oblique
slip fault - Fault with components of relative
displacement along both strike and dip.
oil
shale - Shale containing such proportion of hydrocarbons as to be capable of yielding petroleum on slow distillation.
oligotrophic - In lake aging pertains to
a youthful lake and indicates water low in accumulated nutrients and high in
dissolved oxygen.
olivine - Rock-forming ferromagnesian silicate mineral that crystallizes early from magma and weathers readily at earth's
surface. Crystal structure based on isolated (SiO4)4-
ions and positive ions of iron, magnesium, or both. General formula: (Mg, Fe)2SiO4.
oölites - Spheroidal grains of sand size, usually composed of calcium
carbonate, CaCO3, and thought to have originated by inorganic
percipitation. Some limestones largely made up of oölites.
ooze - Deep-sea deposit consisting of 30 percent or more by
volume of hard parts of very small, sometimes microscopic, organisms. If
particular organism dominant, its name is used as modifier, as in globigerina
ooze, or radiolarian ooze.
open-pit
mining -
Surface mining represented by sand and gravel pits, stone quarries, and
copper mines of some western states.
ophitic - Rock texture in which lath-shaped plagioclase crystals are enclosed wholly or in part in
later-formed augite, as commonly occurs in diabase.
order of crystallization - Chronological
sequences in which crystallization of various minerals of an assemblage takes place.
ore
deposit - Metallic minerals in concentrations that can be
worked at profit.
orogeny - Process by which mountain structures develop.
orthoclase - Feldspar in which K+ is diagnostic positive
ion; K(AlSi3O8).
orthoquartzite - Sandstone composed completely - or almost
completely - of quartz grains. Quartzose sandstone is
synonym.
outwash
plain -
Flat or gently sloping surface underlain by outwash.
overbank
deposits -
Sediments (usually clay, silt, and fine sand) deposited on flood plain by river overflowing banks.
overturned
fold - Fold with at least one limb rotated through more than 90º.
oxide
mineral - Mineral formed by direct union of an element with oxygen. Examples: ice,
corundum, hematite, magnetite, cassiterite.
P
P -
Symbol for earthquake primary
waves.
pahoehoe
lava - Lava whose surface is smooth and billowy,
frequently molded into forms resembling huge rope coils. Characteristic of basic lavas.
paired
terraces - Terraces that face each other across stream
at same elevation.
paleomagnetism - Study of earth's magnetic
field as has existed during geologic time.
Pangaea - Hypothetical continent from which all others are
postulated to have originated through process of fragmentation and drifting.
parabolic
dune - Dune with long, scoop-shaped form that,
when perfectly developed, exhibits prabolic shape in plan, with horns pointing upwind. Contrast barchan, in which horns point downwind.
Characteristically covered with sparse vegetation; often found in coastal
belts.
paternoster
lakes -
Chain of lakes resembling string of beads along glaciated valley where ice
plucking and gouging have scooped out series of basins.
peat - Partially reduced plant or wood material, containing
approximately 60 percent carbon and 30 percent oxygen. An intermediate material
in process of coal formation.
pebble
size -
Volume greater than that of sphere with diameter of 4 mm and less than that of
a spere of 64 mm.
pedalfer - Soil characterized by accumulation of iron salts or iron and aluminum salts in B horizon. Varieties of pedalfers include
red and yellow soils of southeastern United States and podsols of northeastern quarter of United
States.
pedocal - Soil characterized by accumulation of
calcium carbonate in its profile. Characteristic of low rainfall. Varieties
include black and chestnut soils of northern Plains states and red and gray desert
soils of drier western states.
pegmatite - Small pluton of exceptionally coarse texture, with crystals up to 12 m in length,
commonly formed at margin of batholith. Nearly 90 percent of all
petmatites are simple pegmatites of quartz, orthoclase, and unimportant percentages of micas; others are extremely rare
ferromagnesian pegmatites and complex pegmatites. Complex pegmatites have as
major components sialic minerals of simple pegmatites but also
contain a variety of rare minerals.
pelagic
deposit -
Material formed in deep ocean and deposited there. Example: ooze.
pendulum - Inertia member so suspended that, after
displacement, restoring force will return it to starting position. If displaced
and then released, oscillates, completing one to-and-fro swing in time called period.
Pennsylvanian - Of, belonging to, or
being the geologic time, system of rocks, and sedimentary deposits of the sixth
period of the Paleozoic Era, characterized by the development of coal-bearing
rock formations.
perched
water table -
Top of zone of saturation that bottoms on
impermeable horizon above level of general water table in area. Is generally near
surface and frequently supplies a hillside spring.
peridotite - Coarse-grained igneous rock dominated by dark-colored minerals, consisting of about 75 percent ferromagnesian silicates and balance
plagioclase feldspars.
period - For oscillating systems, length of
time required to complete on oscillation.
permeability - For rock or earth material, ability to transmit
fluids. Permeability equal to velocity of
flow divided by hydraulic gradient.
petroleum - An oily mixture of hydrocarbons extracted from subsurface
earth structures. Thought to result from physical
and chemical conversion of remains of animals and plants. A fuel in natural or
refined state, yielding on distillation such products as gasoline, kerosene,
naphtha.
phaneritic
texture -
Individual grains large enough to be identified without the aid of a
microscope; also called granular texture.
phase - (1) Homogenous, physically distinct
portion of matter in physical-chemical system not
homogenous, as in three phases of ice, water, and aqueous vapor. (2) Group of
seismic waves of one type.
phenocryst - A crystal significantly larger than crystals
of surrounding minerals.
phosphate
rock - Sedimentary rock containing calcium
phosphate.
photosynthesis - Process by which carbohydrates are compounded from carbon
dioxide and water in presence of sunlight and chlorophyll.
phyllite - Clayey metamorphic rock with rock cleavage intermediate between slate and schist. Commonly formed by the regional metamorphism of shale or tuff. Micas characteristically impart a
pronounced sheen to rock cleavage surfaces. Has phyllitic cleavage.
phyllitic
cleavage - Rock cleavage in which flakes are produced
barely visible to unaided eye. Coarser than slaty and finer than schistose cleavage.
phyllosilicate - Mineral with crystal structure containing silicon-oxygen tetrahedra arranged as
sheets.
physical
geology -
Branch of geology that deals with nature and
properties of material composing the earth, distribution of materials
throughout globe, processes by which they are formed, altered, transported, and
distorted, and nature and development of landscape.
piedmont
glacier - Glacier formed by coalescence of valley glaciers and spreading over plains
at foot of mountains from which valley glaciers come.
pipe - (1) A vertical, cylindrical vein of ore. (2) An eruptive
passageway opening into the crater of a volcano.
placer - A concentration of relatively heavy
and resistant minerals in stream or beach deposits; two
examples are some deposits of gold and of diamonds.
planet - Heavenly body that changes position
from night to night with respect to background of stars.
planetology - Organized body of
knowledge about planetary system.
plastic
deformation -
Permanent change in shape or volume not involving failure by rupture and, once started, continuing
without increase in deforming force.
plastic
solid - Solid that undergoes deformation
continuously and indefinitely after stress applied to it passes a critical
point.
plate - Earth's lithosphere, varying in thickness from
several tens of kilometers to as much as 100 km and including crust and part of upper mantle above asthenosphere.
plate
tectonics -
Theory of worldwide dynamics involving movement and interactions of the many
rigid plates of earth's lithosphere.
plateau
basalt - Basalt poured out from fissures in floods that
tend to form great plateaus. Sometimes called flood basalt.
playa - Flat-floored center of undrained
desert basin.
playa
lake -
Temporary lake formed in a playa.
pleochroic
halo -
Minute, concentric-spherical zones of darkening or coloring that form around inclusions of radioactive minerals in biotite, chlorite, and a few other minerals. About
0.075 mm in diameter.
plume - Pipelike convection cells thought to carry heat and mantle material from lower mantle up to crust, producing hot spots at surface.
pluton - A body of igneous rock formed beneath earth surface
by consolidation from magma. Sometimes extended to include bodies
formed beneath surface by metasomatic replacement of older rock.
plutonic igneous rock - Rock formed by slow crystallization, which yields coarse texture. Once believed to be typical of
crystallization at great depth, but not a necessary condition.
pluvial
lake -
Lake formed during a pluvial
period.
pluvial
period -
Period of increased rainfall and decreased evaporation; prevailed in nonglaciated
areas during time of ice and advance elsewhere.
podsol - Ashy-gray or gray-brown soil of pedalfer group. Highly bleached soil, low
in iron and lime, formed under moist and cool conditions.
point
bars -
Accumulations of sand and gravel deposited in slack waters
on inside of bends of winding, or meandering, river.
Poisson's
ratio -
Ratio of change of diameter per unit diameter to change of length per unit
length in elastic stretching or compression of cylindrical specimen.
polar
compound -
Compund,such as water, with a molecule that behaves like small bar magnet
with positive charge on one end negative charge on other.
polarity
epoch -
Interval of time during which earth's magnetic field has been oriented
dominantly in either normal or reverse direction. May be marked by shorter
intervals of opposite sign, called polarity events.
polarity
event -
See polarity epoch.
polar wandering, or
migration -
Movement of position of magnetic pole during past time in relation
to present position.
polymorphism - Existence of several
different morphologic kinds occurring species or mineral.
porosity - Percentage of open space or
interstices in rock or other earth material. Compare with permeability.
porphyritic - Textural term for igneous rocks in which large crystals, called phencrysts, are set in finer
groundmass, which may be crystalline or glass or both.
porphyry - Igneous rock containing conspicuous pheocrysts
in fine-grained or glassy groundmass.
Portland
cement -
Hydraulic cement consisting of compounds of silica, lime, and alumina.
positive
charge -
Condition resulting from deficiency of electrons.
potassic
feldspar - Orthoclase, K(AlSi3O8).
potential
energy -
Stored energy waiting to be used. Energy that a
piece of matter possesses because of position or
because of arrangement of parts.
prairie
soils -
Transitional soils between pedalfers and pedocals.
Pratt
hypothesis -
Explains isostasy by assuming all portions of the crust have same total mass above certain elevation, called level
of compensation. Higher sections would have proportionately lower density.
precipitation - Discharge of water, in
rain, snow, hail, sleet, fog, or dew, on land or water surface. Also, process
of separating mineral constituents from solution by evaporation (halite, anhydrite) or from magma to form igneous rocks.
precursor - Relating to earthquakes, refers to
events immediately preceding actual shaking of ground. Includes changes in
seismic velocities, groundwater levels, and tilt of ground
surface.
pressure - Force per unit area.
prevailing westerlies - Winds blowing from the
west and characteristic of the middle latitudes both north and south of the
equator.
primary
wave -
Earthquake body wave that travels fastest and advances
by push-pull mechanism. Also known as longitudinal, compressional, or P wave.
proton - Fundamental particle of matter with positive electrical charge of 1
unit (equal in amount, but opposite in effect, to the charge of electron) and mass of 1.00758 u.
proton-proton
fusion -
Rapidly moving protons in hot interior of stars collide and fuse to form atoms of helium from atoms of hydrogen in
continuous buildup of higher elements.
pumice - Pieces of magma up to several centimeters across that
have trapped bubbles of steam or other gases as they were thrown out in eruption.
Sometimes they have sufficient buoyancy to float on water.
push-pull
wave - Wave that advances by alternate compression and rarefaction of medium,
causing particles in path to move forward and backward along direction of
wave's advance. In connection with earth waves, also known as primary wave,
compressional wave, longitudinal wave, or P wave.
pyrite - A sulfide mineral, iron sulfide, FeS2.
pyroclastic
debris -
Fragments blown out by explosive volcanic eruptions and subsequently
deposited on ground. Include ash, cinders, lapilli, blocks, bombs, and pumice.
pyroxene
group - Ferromagnesian silicates with a single
chain of silicon-oxygen tetrahedra. Common example: augite. Compare with amphibole group (example: hornblende), which has a double chain of
tetrahedra.
pyrrhotite - A mineral, iron sulfide. So commonly
associated with nickel minerals that has been called "world's greatest
nickel ore."
Q
quartz - A silicate mineral, SiO2, composed
exclusively of silicon-oxygen tetrahedra, with all oxygens
joined in a three-dimensional network. Crystal form is six-sided prism tapering at
end, with prism faces striated transversely. An important rock-forming mineral.
quartzite - Metamorphic rock commonly formed by metamorphism of sandstone and composed of quartz. No rock cleavage. Breaks through sand grains in contrast to sandstone, which
breaks around grains.
Quaternary -
Of, belonging to, or designating the geologic time, system of rocks, and
sedimentary deposits of the second period of the Cenozoic Era, from the end of
the Tertiary Period through the present, characterized by the appearance and
development of human beings and including the Pleistocene Epoch and the
Holocene Epoch.
R
radar -
Ultrahigh-frequency electromagnetic radiation.
radial
drainage -
Arrangement of stream courses in which streams radiate outward in all
directions from central zone.
radiant
energy -
Electromagnetic waves travelling as wave motion.
radioactivity - Spontaneous breakdown of
atomic nucleus, with emission of radiant energy.
rain
wash -
Water from rain after it has fallen on ground and before concentrated in
definite stream channels.
range - Elongated series of mountain peaks considered to be a part of
one connected unit, such as Appalachian Range or Sierra Nevada Range.
rank - Term used to designate extent to which metamorphism has advanced. Compare with grade. Rank is more commonly employed in
designating stage of metamorphism of coal.
ray
craters -
Lunar craters marked by rays. Young on lunar time
scale.
rays - Light-toned streaks that spread outward from such lunar craters as Tycho, Kepler, and Copernicus.
recessional
moraine -
Ridge or belt of till marking period of moraine formation, probably in period of
temporary stability or slight readvance, during general wastage of a glacier and recession of its front.
recorder - Part of a seismograph that makes record of ground
motion.
rectangular
pattern -
Arrangement of stream courses in which tributaries flow into larger streams at
angles approaching 90º.
recumbent
fold - Fold with axial plane more or less horizontal.
reflection seismic
prospecting -
Uses reflected waves and places seismographs at distances only a fraction
of depths investigated.
refraction seismic
prospecting -
Uses travel times of refracted waves and spreads seismographs over lines roughly four times
depth being investigated.
refractory - Mineral or compound that resists action of
heat and chemical agents.
regional metamorphism - Metamorphism occurring over tens or scores
of kilometers.
rejuvenation - Change in conditions of erosion that causes a stream to begin more
active erosion and a new cycle.
relative
time -
Dating of events by place in chronologic order of occurrence rather than in
years. Compare with absolute time.
relief - The difference in elevation of an
area between tops of hills and bottoms of valleys.
reverse
fault - Fault in which hanging wall appears to have moved upward
relative to footwall; contrast with normal, or gravity fault. Also called thrust fault.
rhyolite - Fine-grained igneous rock with composition of granite.
right-lateral
fault - Strike-slip fault in which ground opposite
you appears to have moved right when you face it.
rigidity - Resistance to elastic shear.
rill - Miniature stream channel which forms along axis of broad,
shallow trough carrying sheet wash, or sheet flow.
ripple
marks -
Small waves produced in unconsolidated material by wind or water. See ripple marks
of oscillation.
ripple marks of oscillation - Ripple marks formed
by oscillating movement of water such as may be found along sea coast outside
surf zone. Symmetrical, with sharp or slightly rounded ridges separated by more
gently rounded troughs.
rock
cycle -
Concept of sequences through which earth materials may pass when subjected to
geological processes.
rock
flour -
Finely divided rock
material pulverized by glacier and carried by streams fed by
melting ice.
rock-forming silicate
minerals - Minerals built around framework of silicon-oxygen tetrahedra: olivine, augite, hornblende, biotite, muscovite, orthoclase, albite, anorthite, quartz.
rock
glacier -
Tongue of rock waste
found in valleys of certain mountainous regions. Characteristically
lobate and marked by series of arcuate, rounded ridges that give it aspect of
having flowed as viscous mass.
rock
slide -
Sudden and rapid slide of bedrock along planes of weakness.
Rossi-Forel
scale -
Scale for rating earthquake intensities, devised in 1878.
runoff - Water that flows off land.
rupture - Breaking apart or state of being
broken apart.
S
S -
Symbol for secondary
wave.
salt - In geology this term usually refers to halite, or rock salt, NaCl, particularly in such
combinations as salt water and salt
dome.
saltation - Mechanism by which a particle moves
by jumping from one point to another.
salt
dome -
Mass of NaCl generally of roughly cylindrical shape and with diameter of about
2 km near top. Such mass has been pushed through surrounding sediments into
present position. Reservoir rocks above and alongside salt domes trap oil and gas.
saltwater
wedge -
Body of water, found in some estuaries, which thins toward head of estuary and is overridden by fresh
water from land.
sand
size -
Volume greater than that of a sphere with diameter of 0.0625 mm and less than
that of a sphere with diameter of 2 mm.
sandstone - Detrital sedimentary rock formed by cementation of individual grains of sand size and
commonly composed of mineral quartz. Sandstones constitute estimated 12
to 28 percent of sedimentary
rocks.
sapropel - Aquatic ooze or sludge rich in organic matter.
Believed to be source material for petroleum and natural gas.
satellite
crater - Crater formed by impact of a fragment
ejected during creation of a primary crater. Also called secondary crater.
scale - The ratio on a map of the distance
between two points on the ground and the same two points on the map. It may be
expressed in three ways:
Fractional scale - If
two points are 1 km apart in the field, they may be represented on the map as
separated by some fraction of that distance, say, 1 cm. In this instance, the
scale is 1 cm to the kilometer. There are 100,000 cm in 1 km; so this scale can
be expressed as the fraction, or ratio, 1:100,000. Many topographic maps of the
United States Geological Survey have a scale of 1:62,500; and many recent maps
have a scale of 1:31,250, and others of 1:24,000.
Graphic scale -
This scale is a line printed on the map and divided into units that are
equivalent to some distance, such as 1 km or 1 mi.
Verbal scale -
This is an expression in common speech, such as "four centimeters to the
kilometer," "an inch to a mile," or "two miles to the
inch."
schist - Metamorphic rock dominated by fibrous or
platy minerals. Has schistose cleavage and is product of regional metamorphism.
schistose
cleavage - Rock cleavage with grains and flakes clearly
visible and cleavage surfaces rougher than in slaty or phyllitic cleavage.
sea-floor
spreading -
Process by which ocean floors spread laterally from crests of main ocean
ridges. As material moves laterally from ridge, new material is thought to
replace it along ridge crest by welling upward from mantle.
seamount - Isolated, steep-sloped peak rising
from deep ocean floor but submerged beneath surface. Most have sharp peaks, but
some have flat tops and are called guyots, or tablemounts. Seamounts are
probably volcanic in origin.
secondary
crater -
See satellite crater.
secondary
wave -
Earthquake body wave slower than primary wave. Shear, shake, or S wave.
secular variation of
magnetic field -
Change in inclination, declination, or intensity of earth's magnetic field.
Detectable only from long historical records.
sedimentary - Of
or relating to rocks formed by the deposition of sediment.
sedimentary
facies -
Accumulation of deposits that exhibits specific characteristics and grades laterally
into other sedimentary accumulations that were formed at same time but exhibit
different characteristics.
sedimentary
rock - Rock formed from accumulations of sediment,
which may consist of rock fragments of various sizes, remains or products of
animals or plants, products of chemical action or of evaporation, or mixtures of these. Stratification
is single most characteristic feature of sedimentary rocks, which cover about
75 percent of land area.
sedimentation - Process by which mineral and organic matters are laid down.
seif
dune -
Very large longitudinal dune. As high as 100 m and as
long as 100 km.
seismic
prospecting -
Method of determining nature and structure
of buried rock formations by generating waves in ground (commonly by small
explosive charges) and measuring length of time these waves require to travel
different paths.
seismic
sea wave -
Large wave in ocean generated at time of earthquake. Popularly but incorrectly
known as tidal wave. Sometimes called tsunami.
seismogram - Record obtained on a seismograph.
seismograph - Instrument for recording
vibrations, most commonly employed for recording earth vibrations during
earthquakes.
seismology - Scientific study of earthquakes and
other earth vibrations.
senile -
Worn away nearly to the base level, as at the end of an erosion cycle.
series - A
group of rock formations closely related in time of origin and distinct as a
group from other formations.
serpentine - Silicate of magnesium common among
metamorphic minerals. Occurs in two crystal habits: platy, known as antigorite;
fibrous, known as chrysotile, an asbestos. "Serpentine" comes from
mottled shades of green on massive varieties, suggestive of snake markings.
S.G. - Symbol for specific gravity.
shake
wave - Wave that advances by causing particles in
path to move from side to side or up and down at right angles to direction of
wave's advance, a shake motion. Also called shear wave, or secondary wave.
shale - Fine-grained, detrital sedimentary rock made up of silt- and
clay-sized particles. Contains clay minerals as well as particles of quartz, feldspar, calcite, dolomite, and other minerals. Distinguished from mudstone by presence of fissility.
shear - Change of shape without change of
volume.
shear
wave - Wave that advances by shearing
displacements (which change shape without changing volume) of medium. This
causes particles in path to move from side to side or up and down at right
angles to direction of wave's advance. Also called shake wave, or secondary wave.
sheet - A
broad, relatively thin deposit or layer of igneous or sedimentary rock.
sheet
flow -
See sheet wash.
sheeting - Joints essentially parallel to ground
surface. More closely spaced near surface and become pregressively farther
apart with depth. Particularly well developed in granitic rocks, but sometimes in other massive rocks
as well.
sheet
wash -
Water accumulating on a slope in thin sheet of water. May begin to concentrate
in rills. Also called sheet flow.
shift -
Fault.
shield
volcano - Volcano built up almost entirely of lava, with slopes seldom as great as 10º at
summit and 2º at base. Examples: five volcanoes on island of Hawaii.
sial - A term coined from chemical symbols for silicon and
aluminum. Designates composite of rocks dominated by granites, granodiorites, and their allies and
derivatives, which underlie continental areas of globe. Specific gravity
considered to be about 2.7.
sialic
rock - Igneous rock composed predominantly of
silicon and aluminum, from whose chemical symbols term is constructed. Average specific gravity,
about 2.7.
silicate
minerals - Minerals with crystal structure containing silicon-oxygen
tetrahedra arranged as isolated units (nesosilicates), single or double chains (inosilicates), sheets (phyllosilicates), or three-dimensional
frameworks (tectosilicates).
silicon-oxygen tetrahedron - Complex ion composed of silicon ion surrounded by
four oxygen ions. Negative charge of 4 units, and represented
by symbol (SiO4)4-. Diagnostic unit of silicate minerals,
and makes up central building unit of nearly 90 percent of materials of earth's
crust.
sill - An
approximately horizontal sheet of igneous rock intruded between older rock
beds.
sillimanite - A silicate mineral,
Al2SiO5, characteristic of highest metamorphic
temperatures and pressures. Occurs in long slender crystals, brown, green, white. Crystalline structure based on independent
tetrahedra. Contrast with kyanite and andalusite, which have same composition but
different crystal habits and form at lower temperatures.
silt
size -
Volume greater than that of a sphere with diameter of 0.0039 mm and less than
that of a sphere with diameter of 0.0625 mm.
Silurian -
Of, belonging to, or being the geologic time, system of rocks, or sedimentary
deposits of the third period of the Paleozoic Era, characterized by the
development of early invertebrate land animals and land plants.
sima - Term coined from silicon and magnesium. Designates
worldwide shell of dark, heavy rocks. Sima believed to be outermost rock
layer under deep, permanent ocean basins, such as Pacific. Originally sima
considered basaltic in composition, with specific gravity
of about 3.0. It has been suggested also, however, that it may be peridotitic
in composition, with specific gravity of about 3.3.
simatic
rock - Igneous rock composed perdominantly of
ferromagnesian minerals. Average specific gravity,
3.0 to 3.3.
sinkhole - Depression in surface of ground
caused by collapse of roof over solution cavern.
sinter - A
chemical sediment or crust, as of porous silica, deposited by a mineral spring.
slate - Fine-grained metamorphic rock with well-developed slaty cleavage.
formed by low-grade regional metamorphism of shale.
slaty
cleavage - Rock cleavage in which ease of breaking occurs
along planes separated by microscopic distances.
slip -
(1) A smooth crack at which rock strata have moved on each other.
(2) A small fault. (3) The relative displacement of formerly
adjacent points on opposite sides of a fault.
slope
failure -
See slump.
slump - Downward and outward movement of rock or unconsolidated material as unit or
as series of units. Also called slope failure.
snowfield - Stretch of perennial snow existing in
area where winter snowfall exceeds amount of snow that melts away during
summer.
snow
line -
Lower limit of perennial snow.
sodic
feldspar - Albite, Na(AlSi3O8).
soil - Superficial material that forms at earth's surface as
result of organic and inorganic processes. Soil varies with climate, plant and
animal life, time, slope of land, and parent material.
soil
horizon -
Layer of soil
approximately parallel to land surface with observable characteristics produced
through operation of soil-building processes.
solar
constant -
Average rate at which radiant energy received by earth from sun.
Equal to little less than 2 cal/cm2-min on plane perpendicular to
sun's rays at outer edge of atmosphere, when earth is at mean distance from
sun.
solar
system -
Sun with group of celestial bodies held by its gravitational attraction and
revolving around it.
sole
mark -
Cast of sedimentary structures
such as cracks, tracks, or grooves formed on lower surface or underside of sandstone bed,
commonly revealed after original underlying sedimentary layer has weathered
away.
solid - Matter with definite shape and volume and
some fundamental strength.
May be crystalline, glassy, or amorphous.
solid
solution -
Single crystalline phase that may vary in composition within
specific limits.
solifluction - Mass movement of soil affected by alternate freezing and thawing.
Characteristic of saturated soils in high latitudes.
sorosilicates - Mineral with crystal structure containing silicon-oxygen
tetrahedra arranged as double units.
southeast
trades -
Winds blowing from the southeast toward the equator in the southern hemisphere.
space
lattice - In
crystalline structure of mineral, three-dimensional array of points
representing pattern of locations of identical atoms or groups of atoms constituting a
mineral's unit cell. There are 230 pattern types.
specific
gravity -
Ratio between weight of given volume of material and weight of equal volume of
water at 4º C.
specific
heat -
Amount of heat necessary to raise temperature of 1 g of any material 1º C.
sphalerite - A mineral; zinc sulfide, ZnS. Nearly always
contains iron, (Zn, Fe)S. Principal ore of zinc. (Also known as zinc blende or
blackjack.)
spheroidal weathering - Spalling off of
concentric shells from rock masses of various sizes as result of pressures built up during chemical weathering.
spring - Place where water table crops out at ground surface and
water flows out more or less continuously.
stack - Small island that stands as isolated,
steep-sided rock mass just off end of promontory. Has
been isolated from land by erosion and weathering concentrated behind end of a
headland.
stage - A
subdivision in the classification of stratified rocks, ranking just below a
series and representing rock formed during a chronological age.
stalactite - Icicle-shaped accumulation of dripstone hanging from cave roof.
stalagmite - Post of dripstone growing upward from cave floor.
star - A heavenly body that seems to stay in same position
relative to other heavenly bodies.
staurolite - Silicate mineral characteristic of middle-grade metamorphism. Crystalline structure based on independent
tetrahedra with iron and aluminum. Has unique crystal habit that makes it striking and
easy to recognize; six-sided prisms intersecting at 90º to form cross or at 60º
to form an X shape.
steady-state
theory -
Theory that universe is developing by continuous creation of matter as newly formed galaxies replace
those expanding out of sight, thus keeping mass density of universe constant. Compare with
big-ban theory.
stock - Discordant pluton that increases in size
downward, has no determinable floor, and shows area of surface exposure less
than 100 km2. Compare with batholith.
stoping - Mechanism by which batholiths have moved into crust by breaking off and foundering of
blocks of rock surrounding magma chamber.
strain - Change of dimensions of matter in response
to stress:
Commonly, unit strain, such as change in length per unit length (total
lengthening divided by original length), change in width per unit width, change
in volume per unit volume.
straticulate -
Having thin layers.
stratification - Structure produced by deposition of
sediments in layers or beds.
stratigraphic
trap - Structure that
traps petroleum or natural gas because of variation in permeability of reservoir rock or termination of inclined reservoir
formation on up-dip side.
stratum - A
bed or layer of sedimentary rock having approximately the same composition
throughout.
streak - Color of fine powder of mineral; may be different from color of
hand specimen. Usually determined by rubbing mineral on piece of unglazed
porcelain (hardness about 7) known as a "streak
plate," which is, of course, useless for minerals of greater hardness.
stream
order -
Hierarchy in which segments of a stream system are arranged.
stream
terrace -
Surface representing remnants of stream's channel or flood plain when stream was flowing at
higher level. Subsequent downward cutting by stream leaves remnants of old
channel or flood plain standing as terrace above present stream level.
strength - Stress at which rupture occurs or plastic deformation begins.
stress - Force applied to material that tends
to change dimensions: commonly, unit stress or total force divided by the area
over which applied. Contrast with strain.
striations - (1) Scratches, or small channels,
gouged by glacial action. Bedrock, pebbles, and boulders may show striations
produced when rocks trapped by ice were ground against
bedrock or other rocks. Striations along bedrock surface are oriented in
direction of ice flow across that surface. (2) In minerals, parallel, threadlike lines, or
narrow bands, on face of mineral. Reflect internal atomic arrangement.
strike - Direction of line formed by
intersection of a rock surface with a horizontal plane.
Strike is always perpendicular to direction of dip.
strike-slip
fault - Fault in which movement is almost in
direction of fault's strike.
strip
mining -
Surface mining in which soil
and rock covering sought-for commodity are
moved to one side. Some coal mining is pursued in this manner.
structural - Of
or relating to the structure of rocks and other aspects of the earth's crust.
structural
relief -
Difference in elevation of parts of deformed stratigraphic horizon.
subduction - Acto of one tectonic unit's
descending under another (commonly slab of lithosphere).
subduction
zone -
Elongate region along which lithospheric block descends relative to another
lithospheric block.
sublimation - Process by which solid material passes
into gaseous state without first becoming liquid.
subsequent
stream -
Tributary stream flowing along beds of less erosional resistance and parallel to beds
of greater resistance. Course determined subsequent to uplift that brought more
resistant beds within sphere of erosion.
subsurface
water -
Water below ground surface. Also referred to as underground water and subterranean water.
subterranean
water -
Water below ground surface. Also referred to as underground water and subsurface water.
subtropical high pressure
zones -
Zones of atmospheric high pressure located about 30º north and 30º
south of the equator.
sulfate
mineral - Mineral formed by combination of complex ion (SO4)2- with
positive ion. Common example: gypsum, CaSO4 · 2H2O.
sulfide
mineral - Mineral formed by direct union of element with sulfur. Examples: argentite, chalcocite, galena, sphalerite, pyrite, and cinnabar.
superposition - Law by which, if series
of sedimentary rocks
has not been overturned, topmost layer is always youngest and lowermost always
oldest.
surface
wave - Wave that travels along free surface of
medium. Earthquake surface waves sometimes represented by symbol L.
suspended
water - Underground water held in zone of aeration by molecular attraction
exerted on water by rock and earth materials and by attraction
exerted by water particles on one another.
suspension - Process by which material is buoyed
up in air or water and moved about without making contact with surface while in
transit. Contrasts with traction.
symmetrical
fold - Fold in which axial plane is essentially vertical. Limbs dip at similar angles.
syncline - A configuration of folded, stratified rocks in which rocks dip downward from opposite directions to
come together in a trough. Reverse of anticline.
synclinal -
Relating to, formed by, or forming a syncline
T
tablemount - See guyot.
tabular - (1) Shape with large area relative to
thickness. (2) Tending to split into thin flat pieces.
taconite - Unleached iron formation of Lake
Superior District. Consists of chert with hematite, magnetite, siderite, and hydrous iron silicates. Ore of iron, averaging
25 percent iron, but natural leaching turns it into ore with 50 to 60 percent
iron.
talc - Silicate of magnesium common among metamorphic minerals. Crystalline structure based on tetrahedra
arranged in sheets; greasy and extremely soft. Sometimes known as soapstone.
talus - Slope established by accumulation of rock fragments at foot of cliff or ridge.
Rock fragments that form talus may be rock waste, slide rock, or pieces broken
by frost action. Actually, term
"talus" widely used to mean rock debris itself.
tarn - Lake formed in bottom of cirque after glacier ice has disappeared.
tectonic -
Relating to, causing, or resulting from structural deformation of the earth's
crust.
tectonic change of sea level -
Change in sea level produced by land movement.
tectonism -
(1) The structural behavior of an element of the earth's crust. (2)
Crustal instability.
tectosilicate - Mineral with crystal structure containing silicon-oxygen tetrahedra arranged in
three-dimensional frameworks.
temporary
base level -
Nonpermanent base level, such as that formed by lake.
terminal
moraine -
Ridge or belt of till
marking farthest advance of a glacier. Sometimes called end moraine.
terminal
velocity -
Constant rate of fall eventually attained by grain or body when acceleration
caused by influence of gravity is balanced by resistance of fluid through which grain falls or air
through which body falls.
terrace - Nearly level surface, relatively
narrow, bordering a stream or body of water and termination in a steep bank.
Commonly term is modified to indicate origin, as in stream terrace and wave-cut terrace.
terrigenous -
Derived from the land, especially by erosive action. Used primarily of
sediments.
terrigenous
deposit -
Material derived from above sea level and deposited in deep ocean. Example: volcanic ash.
Tertiary -
Of, belonging to, or being the geologic time, system of rocks, and sedimentary
deposits of the first period of the Cenozoic Era, extending from the Cretaceous
Period of the Mesozoic Era to the Quaternary Period of the Cenozoic Era,
characterized by the appearance of modern flora and of apes and other large
mammals.
tetrahedron - A four-sided solid. Used commonly in describing silicate minerals as shortened reference to
silicon-oxygen tetrahedron.
texture - General physical appearance of rock, as shown by size, shape, and
arrangement of particles that make it up.
thermal
gradient - In
earch, rate at which temperature increases with depth below surface.
thermal
pollution -
Increase in normal temperatures of natural waters caused by intervention of
human activities.
thermal
spring - Spring that brings warm or hot water to
surface. Temperature usually 6.5º C or more above mean air temperature.
Sometimes called warm spring, or hot spring.
thermoremanent magnetism - Magnetism acquired
by igneous rock as it cools below Curie temperature of magnetic minerals in it. Abbreviation, TRM.
thin
section -
Slice of rock ground so thin as to be translucent.
tholeiite - Group of basalts primarily composed of plagioclase
(approximately An50), pyroxene, and iron oxides as phenocrysts in glassy groundmass of quartz and alkali feldspar; little or no olivine present.
throw -
The amount of vertical displacement of a fault.
thrust
fault - Fault in which hangin wall appears to have
moved upward relative to footwall; opposite of gravity, or normal,
fault. Also called reverse fault.
tidal
current -
Water current generated by tide-producing
forces of sun and moon.
tidal
inlet -
Waterway from open water into a lagoon.
tidal
wave -
Popular but incorrect designation for tsunami.
tide - Alternate rising and falling of surface of coean, other
bodies of water, or earth itself in response to forces resulting from motion of
earth, moon, and sun relative to each other.
till - Unstratified, unsorted glacial drift deposited directly by glacier ice.
tillite - Rock formed by lithification of till.
time-distance
graph -
Graph of travel time
against distance.
topographic
deserts -
Deserts deficient in rainfall because they are either located far from oceans
toward center of continents or cut off from rain-bearing
winds by high mountains.
topography - Shape and physical features of land.
topset
bed -
Layer of sediment constituting surface of delta. Usually nearly horizontal and covers
edges of inclined foreset beds.
tourmaline - Silicate mineral of boron and aluminum with
sodium, calcium, fluorine, iron, lithium, or magnesium. Formed at high
temperatures and pressures through agency of fluids carrying boron and fluorine.
Particularly associated with pegmatites.
township
and range -
Most of the area of the United States has been subdivided by a system of land
survey in which a square 6 mi on a side is the basic unit, called a township
when measured north and south of a given base line, and called a range when
measured east and west of a given principal meridian.
traction - Process of carrying material along
bottom of a stream. Traction includes movement by saltation, rolling, or sliding.
trailing
edge -
The margin of a continent that occurs on the side opposite
the margin moving toward another continental or oceanic plate (leading edge); commonly is less active
tectonically, behaving in a passive manner. The margin of a continent moving
away from the locus of spreading along a midoceanic ridge as, for example, the
eastern coasts of North and South America.
transducer - Device that picks up relative motion
between mass of seismograph and ground and converts this
into form that can be recorded.
transform
fault -
Point at which strike-slip displacements stop and another
structural feature, such as a ridge, develops.
transition
element - Element in series in which inner shell is
being filled with electrons after outer shell has been
started. All transition elements metallic in free state.
transpiration - Process by which water
vapor escapes from a living plant and enters atmosphere.
transverse
dune - Dune formed in areas of scanty vegetation
and in which sand has moved in ridge at right angles to
wind. Exhibits gentle windward slope and steep leeward slope characteristic of other
dunes.
transverse
wave -
See shear, or shake, wave.
travel
time -
Total elapsed time for wave to travel from source to designated
point.
travertine - Form of calcium carbonate, CaCO3,
which forms stalactites, stalagmites, and other deposits in limestone caves or incrustations around
mouths of hot and cold calcareous springs. Sometimes known as tufa, or dripstone.
trellis
pattern -
Roughly rectilinear arrangement of stream courses in pattern reminiscent of
garden trellis, developed in region where rocks of differing resistance to erosion have been folded, beveled, and uplifted.
trenches - See island-arc deeps.
TRM - See thermoremanent magnetism.
tropical
deserts -
Deserts lying between 5º to 30º north and south of equator.
truncated
spur -
Beveled end of divide between two tributary valleys where
they join a main valley that has been glaciated. Glacier of main valley has worn off end of
divide.
tsunami - Large wave in ocean generated at time
of earthquake. Popularly but incorrectly known as tidal wave. Sometimes called seismic sea wave.
tufa - Calcium carbonate, CaCO3, formed in stalactites, stalagmites, and other deposits in limestone caves, as incrustations around
mouths of hot and cold calcareous springs, or along streams carrying large
amounts of calcium carbonate in solution. Sometimes known as travertine or dripstone.
tuff - Rock consolidated from volcanic ash.
tundra - Stretch of Arctic swampland developed
on top of permanently frozen ground. Extensive tundra regions have developed in
parts of North America, Europe,
and Asia.
turbidites - Sedimentary deposits settled out of
turbid water carrying particles of widely varying grade size.
Characteristically display graded bedding.
turbidity
current -
Current in which limited volume of turbid or muddy water moves relative to
surrounding water because of current's greater density.
turbulent
flow -
Mechanism by which fluid (such as water) moves over or past a
rough surface. Fluid not in contact with irregular boundary outruns that slowed
by friction or deflected by uneven surface. Fluid particles move in series of
eddies or whirls. Most stream flow is turbulent; turbulent flow is important in
both erosion and transportation. Contrast with laminar flow.
type locality -
The place or region in which a rock, series of rock, or formation is typically
exposed.
U
ultimate base level - Sea level, lowest
possible base level for a stream.
ultrabasic - Containing magnesium and iron and
only a very small amount of silica. Used of igneous rock.
ultramatic - Ultrabasic.
unconformity - Buried erosion surface separating two rock masses, older exposed to erosion for
long interval of time before deposition of younger. If older rocks were
deformed and not horizontal at time of subsequent deposition, surface of
separation is angular unconformity. If older rocks
remained essentially horizontal during erosion, surface separating them from
younger rocks is called disconformity. Unconformity that develops
between massive igneous or metamorphic rocks exposed to erosion and
then covered by sedimentary rocks is called nonconformity.
unconformable -
Showing unconformity.
underground
water -
Water below ground surface. Also referred to as subsurface water and subterranean water.
uneven
fracture - Mineral habit of breaking along rough,
irregular surfaces.
uniformitarianism - Concept that present is
key to past. This means that processes now operating to modify earth's surface
have also operated in geologic past, that there is uniformity of processes past
and present.
unit
cell - In
crystalline structure of mineral, three-dimensional grouping of atoms arbitrarily selected so that
mineral's strucutre represented by periodic repetition of this unit in space lattice.
unpaired
terrace - A terrace formed when an eroding stream,
swinging back and forth across a valley, encounters resistant rock beneath unconsolidated alluvium and is
deflected, leaving behind single terrace with no corresponding terrace on other
side of stream.
upheaval - A raising of a part of the earth's
crust.
uplift - An
upheaval.
upthrow - An
upward displacement of rock on one side of a fault.
upper -
Of, relating to, or being a later division of the period named.
Upper Carboniferous - Pennsylvanian.
V
valley glacier - Glacier confined to stream valley. Usually
fed from a cirque. Sometimes called Alpine glacier or mountain glacier.
valley
train -
Gently sloping plain underlain by glacial outwash and confined by valley walls.
varve - Pair of thin sedimentary beds, one
coarse, one fine. This couplet has been interpreted as representing a cycle of
1 year or interval of thaw followed by interval of freezing in lakes fringing a
glacier.
vein - A
regularly shaped and lengthy occurrence of an ore; a lode.
ventifact - Pebble, cobble, or boulder that has had its
shape or surface modified by wind-driven sand.
vesicle - Small cavity in aphanitic or glassy igneous rock, formed by expansion of bubble
of gas or steam during solidification of rock.
virtual geomagnetic pole - Pole consistent
with magnetic field as measured at any one locality. Refers to magnetic-field
direction of single point, in contrast to geomagnetic pole, which refers to best fit
of geocentric dipole for entire earth's field. Most
paleomagnetic readings expressed as virtual geomagnetic poles.
viscocity - An internal property of rock that offers resistance to flow. Ratio
of deforming force to rate at which changes in shape are produced.
volatile
components -
Materials in magma, such as water, carbon dioxide, and
certain acids, whose vapor pressures are high enough to cause them to
become concentrated in any gaseous phase that forms.
volcanic
ash -
Dust-sized pyroclastic particle: volume equal to, or less than, that of sphere
with diameter of 0.06 mm.
volcanic
block -
Angular mass of newly congealed magma blown out in eruption. Contrast with volcanic bomb.
volcanic
bomb -
Rounded mass of newly congealed magma blown out in eruption. Contrast with volcanic block.
volcanic
breccia - Rock formed from relatively large blocks of
congealed lava embedded in mass of ash.
volcanic
dust -
Pyroclastic detritus consisting of particles of dust size.
volcanic
earthquakes -
Earthquakes caused by movements of magma or explosions of gases during volcanic activity.
volcanic
eruption -
Explosive or quiet emission of lava, pyroclastics, or volcanic gases at earth's surface, usually from volcano but rarely
from fissures.
volcanic
mountains - Mountains built up from extrusion of lava and pyroclastic debris.
volcanic
neck - Solidified material filling vent, or pipe, of dead volcano.
volcanic
tremor -
Continuous shaking of ground associated with certain phase of volcanic eruption.
vug - Small unfilled cavity in rock, usually lined with crystalline layer of different composition
from surrounding rock.
W
wadi -
See arroyo.
warm
glacier - Glacier which reaches melting temperature
throughout thickness during summer season.
warm
spring - Thermal spring that brings warm water to
surface. Temperature usually 6.5º C or more above mean air temperature.
water
gap -
Gap cut through resistant ridge by superimposed or antecedent stream.
water
table -
Upper surface of zone of saturation for underground water. An irregular surface
with slope or shape determined by quantity of groundwater and permeability of earth materials. In
general, highest beneath hills and lowest beneath valleys.
weathering - Response of materials once in
equilibrium within earth's crust to new conditions at or near contact
with water, air, or living matter. See also chemical weathering and mechanical weathering.
wrinkle
ridges -
Ridges found on surfaces of lunar maria and flooded craters. May be caused by uplift due to
volcanism or to compression.
X
xenolith - Rock fragment foreign to igneous rock in which it occurs. Commonly
inclusion of country rock intruded by igneous rock
Y
yazoo-type river - Tributary unable to enter
main stream because of natural levees along main stream. Flows
along backswamp zone parallel to main stream.
yield
point -
Maximum stress that solid can withstand without
undergoing permanent deformation, either by plastic flow or by rupture.
Z
zone of aeration - Zone immediately below
ground surface, in which openings partially filled with air and partially with
water trapped by molecular attraction. Subdivided into (1) belt of soil moisture. (2) intermediate belt, and (3) capillary fringe.
zone
of saturation -
Underground region within which all openings filled with water. Top of zone of
saturation is called water table. Water contained within zone of
saturation is called groundwater.
zones of regional metamorphism – High-grade, above 700º C; middle-grade, 400 to
700º C; low-grade, 150 to 400º C.
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