A

AA - - a Hawaiian term for lava flows having a rough, jagged, and clinker
surface.

ALLUVIAL FAN - - an outspread, gently-sloping mass of detritus deposited by
moving water where there is an abrupt change in gradient from steep to gentle,
as from mountain gorge to valley floor.

ALLUVIAL - - material deposited by a stream or by running water.

AMPHIBOLE - - a mineral group composed mostly of some mixture of magnesium,
iron, and calcium, with aluminum, silicon, and oxygen. It includes common
rock-forming minerals characterized by flat, reflective breakage surfaces in two
directions, which meet at about 60o angles. The most common amphibole minerals
are hornblende, tremolite, actinolite, cummingtonite, and hypersthene.

AMPHIBOLITE - - a crystalline metamorphic rock consisting mainly of amphibole
and plagioclase, with little or no quartz. The amphibole usually is hornblende.
Officially, it is non-foliated (not layered), but the name often is applied to hornblende-plagioclase schist as well.

ANDESITE - - a medium-gray-colored, fine-grained, usually extrusive igneous
rock. It is intermediate in composition (half-way between felsic and mafic) and
is the extrusive equivalent of diorite.

ANGLE OF REPOSE - - the maximum slope or angle at which loose material remains
stable.

ANTECEDENT STREAM - - a stream that was established before local uplift began
and incised (eroded) its channel at the same rate the land was rising.

ANTICLINE - - a fold in rocks in which the strata dip outward on both sides
(limbs) away from the axis. A fold, which is concave downward, as in the shape
of a barn roof.

ARCHEAN - - the eon of geologic time from 4000 Ma to 2500 Ma.

ARGILLITE - - a compact rock derived from siltstone, mudstone, or shale which
has been hardened or consolidated by pressure, cementation, or heat. It does
not break into thin layers, as do shale and slate. It is regarded as a product
of weak metamorphism.

ARKOSE - - a sandstone containing more than 10% feldspar (usually orthoclase)
grains.

AUGITE - - a dark greenish-black mineral of the pyroxene group. An essential
miner-al of many mafic igneous rocks. It usually exists as irregular, blocky
masses. It does break along flat, reflective surfaces, which meet at right
angles.

AULACOGEN - - a tectonic trough on a craton (shield), bounded by convergent
Normal (gravity) faults. They are formed when triple junctions are created at
the beginning of continental rifting, but fail to continue the process of
separation like the other two parts of the triple junction.

AUREOLE - - a zone surrounding an igneous intrusion in which contact
metamorphism of the "country rock" has taken place. The rock in the aureole
often becomes more brittle than and changes color slightly from the original
rock.

AXIAL PLANE - - a planar (two-dimensional) surface that connects the hinge
lines of the strata in a fold. It is found by connecting all the places where
strata in the fold change dip direction.

AXIS - - the linear map trace of the axial plane of a fold.

B

BACK-ARC SPREADING - - the pulling apart of the earth's crust by tensional
forces between a continental craton (shield) and an offshore subduction - island
arc zone. It forms parallel to the map trace of the subduction zone and often
creates a linear trough between the island arc and the adjacent continent. One
present-day ex-ample is the Sea of Japan.

BAJADA - - an alluvial apron which, together with the pediment, makes up the
piedmont slops in a basin.

BASALT - - a dark-colored, extrusive igneous rock. It is the fine-grained
equivalent of gabbro. It is the major rock of ocean basins.

"BASEMENT" - - older, usually crystalline igneous and/or metamorphic rock
occurring underneath younger stuff.

BASIN-AND-RANGE - - a type of regional block faulting where the valleys
(basins) and mountains (ranges) are elongate and parallel to one another. The
basins often are partially or completely filled with detritus carried into them
from erosion of the adjacent ranges.

BATHOLITH - - old magma chamber, now exposed by extensive erosion of the
earth's surface as a large area of igneous rock, usually granite.

BIOTITE - - black to brownish-black mica.

BLOCK FAULTING - - a type of normal faulting in which the crust is divided
into structural blocks of different elevations and orientations; often results in basin-and-range topography.

BOLIDE - - an asteroid, meteor, comet, or any other chunk of stuff moving
inside our solar system. Usually used to describe one of these particles which
has collided with earth.

b.p. - - "before present time" or "before the present".

BRECCIA - - a coarse-grained, clastic rock composed of angular and broken rock
fragments in a finer-grained matrix. It is usually sedimentary in origin, but
may also be igneous (volcanic breccia).

b.y.b.p. - - billions of years before (the) present; now designated as "Ga"

C

CALCITE - - the major mineral form of the compound calcium carbonate, CaCO3.
It is a common rock-forming mineral and is the chief constituent of limestone
and marble.

CALCAREOUS - - any rock which has enough carbonate material so that it reacts
with hydrochloric (or any other strong) acid, producing bubbles of carbon
dioxide. Usually, the carbonate material is calcite, but it may be dolomite,
siderite, smithsonite, or witherite.

CARBONACEOUS - - adjective for a rock containing very small grains of carbon
distributed evenly throughout the rock, giving it a black color.

CARBONATE - - any mineral or rock which effervesces with hydrochloric (or any
other strong) acid to release carbon dioxide gas. The chemical formula must
include the (CO3)-2 ion.

CHEMICAL WEATHERING - - the process of weathering by which chemical reactions
transform rocks and minerals into new chemical combinations (compounds). One
example is the chemical weathering of feldspars into clays (muds).

CHERT - - an extremely fine-grained ("cryptocrystalline") variety of quartz.
It foms massive chunks (not crystals), is opaque, and has a white, tan, or beige
color

CHLORITE - - a dark-green mica mineral, usually formed during metamorphism at
low temperatures and pressures.

CLAST - - an individual piece of pre-existing rock.

CLASTIC - - pertaining to a sedimentary rock composed principally of fragments
derived from pre-existing rocks, rather than chemical precipitates.

CLEAVAGE - - the tendency to break along planes of weakness. It usually
refers to flat, reflective surfaces in some minerals, but may also be applied to
foliated meta-morphic rocks, like slate.

COMPRESSION - - a system of forces that decreases volume or shortens distance.

CONCRETION - - a hard, compact aggregate of mineral matter in sedimentary or
pyroclastic igneous rocks (ash or cinders), subspherical to irregular in shape,
formed by precipitation from water solution, usually around some nucleus, such
as a small chunk of shell or bone. Concretions generally are different in
composition from the rock in which they occur and represent a concentration of
some minor con-stituent in that rock.

CONGLOMERATE - - a coarse-grained clastic sedimentary rock composed of
round-ed or sub-rounded fragments larger than two millimeters in diameter and
cemented together.

CONTINENTAL SHELF - - that part of the continental margin that is between the
shoreline and the continental slope. Usually is extends vertically to a depth
of about 600 feet. It is the zone where sunlight penetrates and is the most
productive area of marine life in the ocean. It is characterized by its very
gentle slope, usually about 0.1o.

CONTINENTAL SLOPE - - that part of the continental margin that lies between
the continental shelf and the bottom of the ocean. Sunlight does not penetrate
this area, and mostly it is home to scavengers. It is characterized by a
relatively steep slope of 3o to 6o.

CONVECTION - - bodily movement of material from one place (usually hotter) to
another (usually colder). Often in sub-circular patterns called "convection
cells".

COULEE - - a dry or intermittent stream valley or a long, trench-like gorge
that once carried meltwater from an ice sheet.

"COUNTRY ROCK" - - the rock present in any area before some other event
affects it, such as faulting or an igneous intrusion.

CRATON - - a part of the earth's crust that has attained stability and has
been little deformed in recent geologic time. It usually refers to the oldest
core of a continent. Synonym : "shield".

CROSS-BEDS - - cross-stratification in which the beds are more than one
centimeter thick. Usually, the tilted layers within the bed are truncated at
the top and meet the bottom at shallow angles.

CRUST - - the outermost layer of the earth. It includes the oceanic crust
(about 5-10 miles thick) and the continental crust (50-75 miles thick). The
bottom of the crust is the Mohorovicic Discontinuity ("Moho").

D

DECOMPRESSIONAL MELTING - - the melting of already-hot rock by the release of
pressure without an increase in temperature.

DELTA - - the nearly flat alluvial tract of land at the mouth of a river that
commonly forms a triangular or fan-shaped plain resembling the Greek letter
"delta".

DIABASE - - an intrusive, mafic (dark-colored) igneous rock consisting
essentially of calcium-rich plagioclase and some pyroxene mineral, usually
augite. Its grain size (texture) lies between coarse-grained gabbro and
fine-grained basalt.

DIFFERENTIATION - - separation of mineral and/or chemical parts during
formation of an igneous rock as it forms from a cooling magma.

DIKE - - a tabular (table-top-shaped) body of igneous rock that cuts across
the struc-ture of the country rocks. It is a discordant igneous intrusion.

DIORITE - - a medium-gray-colored, coarse-grained, intrusive igneous rock,
often with a "salt-and-pepper" appearance. It is intermediate in composition
(half-way between felsic and mafic) and is the intrusive equivalent of andesite.

DIP - - the amount and direction of tilt of a geologic surface from
horizontal, measured in degrees. The geologic surface usually is a layered rock
unit or a fault plane.

DISTAL - - igneous or sedimentary deposits made up of fine particles deposited
far from their sources.

DOLERITE - - see "diabase".

DOLOMITE - - calcium magnesium carbonate, CaMg(CO3)2 , a common rock-forming
mineral. It typically forms dolostone (similar to limestone) and dolomitic
marble.

DOLOSTONE - - rock equivalent of the mineral dolomite, usually including some
impurities such as clay.

DOME - - an uplift or anticlinal structure, circular or elliptical in outline,
in which the rocks dip away in all directions.

DOWNTHROWN - - said of that side of a fault that appears to have moved
downward compared with the other side; opposite of "upthrown".

DRAG FOLD - - a minor fold formed in an incompetent (ductile) bed lying
between more competent (rigid) beds, produced by movement of the competent beds
in op-posite directions relative to one another. Often, it appears in layered
rocks, which "curl" towards a fault plane, as the rocks initially, began to
fold, then faulted as pres-sures increased.

DRIFT - - all material of glacial origin, whether deposited directly from

melting ice (till) or by meltwater streams (outwash).

E

EOLIAN - - pertaining to the wind. Erosion and deposition accomplished by the
wind.

EXTRUSIVE - - said of lava (or the resulting igneous rock after cooling) that
has been erupted onto the land surface.

F

FACIES - - the aspect, appearance, and characteristics of a rock unit, usually
reflect-ing the conditions of its origin, and usually used to differentiate it
from adjacent or associated units formed at the same time (sedimentary facies)
or at different temp-eratures (metamorphic facies).

FAULT - - a fracture along which there has been differential movement of the
oppo-site sides.

FAULT SCARP - - the cliff or escarpment formed by a fault that reaches the
earth's surface.

FELDSPAR - - the most common group of rock-forming minerals. They contain
sili-con, oxygen, aluminum, and some combination of the metals sodium,
potassium, and calcium. They are hard, opaque, and break along flat, reflective
surfaces, which meet at right angles. Orthoclase feldspar usually is white,
beige, or pink in color, whereas plagioclase feldspar usually is white or gray
in color.

FELDSPATHIC - - containing noticeable feldspar grains.

FELSIC - - an igneous rock composed chiefly of light-colored feldspar minerals
(orthoclase, chiefly, plus some sodium-rich plagioclase), usually accompanied by
quartz and some biotite (black) mica.

FISSURE - - an extensive crack, break, or fracture in rocks.

FLUVIAL - - pertaining to streams and river deposits.

FOLIATION - - a planar arrangement of textural or structural features in any
type of rock.

FOOTWALL - - the mass of rock beneath an inclined fault plane.