Ga - - billions of years ago; thousands of millions of years ago.

GABBRO - - a mafic (dark-colored), coarse-grained, intrusive igneous rock. It
is the approximate intrusive equivalent of basalt and normally contains
easily-seen plag-ioclase feldspar crystals in the shape of elongate, reflective
laths with fine lines running parallel to the long lengths of the plagioclase
laths. Blocky material around the plagioclase laths usually is greenish-black
augite.

GEOCLINE - - a sag in the earth's crust created by subduction in the ocean
near a continent. The subduction usually creates an island arc which, with the
continental shore, forms the edge of the geocline. The sag is created from a
combination of back-arc spreading and the weight of sediment that accumulates in
it from both the continent and the island arc. The sediments so accumulated
form much of a suture zone (orogen) during any continent-continent collision
from plate-tectonic motions.

GEOSYNCLINE - - old term for "geocline".

GLACIER - - a body of moving ice, usually at least 100 feet thick so that the
ice crystals on the bottom deform to effect movement.

GLACIAL LOBE - - a large, tongue-like protrusion from the margin of an ice
sheet.

GNEISS - - a foliated (layered) rock formed by high temperature and high
pressure regional metamorphism in which bands or thin lenses of granular
minerals al-ternate with bands or thin lenses of minerals with flaky and/or
elongate, tabular shapes. Often the alternating bands are of different colors.
Often the rock is com-posed of orthoclase, quartz, and biotite, but composition
is not an essential factor in its definition.

GRABEN - - an elongate structural depression bounded by normal (gravity)
faults.

GRADESD BED - - a type of bedding in which each layer displays a gradual
change in particle size, usually from coarse at the base to fine at the top. It
is characteristic of turbidity currents (underwater landslides).

GRANITE - - a coarse-grained, felsic (light-colored) igneous rock composed of
ortho-clase, quartz, and sodium-rich plagioclase, often with biotite.

GRAYWACKE - - a quartz sandstone which includes noticeable amounts of mud
and/or mica. Sometimes called a "dirty sandstone".

GREENSTONE - - basalt subjected to low temperature- and pressure-metamorphism.
The resulting rock has a greenish-black color due to the presence of chlorite (a
dark green mica) and epidote (a yellowish-green mineral) which developed during
metamorphism.

GROUND WATER - - subsurface water, below the water table.

GROUP - - two or more formations in a stratigraphic column which formed by
similar events or processes.

H

HANGING WALL - - the wall rock above an inclined fault plane.

HARDNESS - - in minerals, resistance to scratching. Usually listed as a
number on a scale of 1 through 10 (Moh's Scale of Hardness).

HEADWARD EROSION - - the lengthening of a young valley or gully by water
erosion at the head of its valley.

HEMATITE - - a red-colored iron oxide mineral. Small amounts of it create
red, pink, and reddish-gray colors in rocks, especially sedimentary rocks.

HORNFELS - - a fine-grained rock composed of a mosaic of equidimensional
grains without preferred orientation (foliation) and typically formed by contact metamor-phism.

I

IGNIMBRITE - - the rock formed by the widespread deposition and consolidation
of volcanic ash flows and nuee ardentes . The term includes welded tuff
(recrystal-lized ash flows) and volcanic breccias.

INTERMEDIATE - - said of an igneous rock composition that is transitional
between felsic and mafic. It has a silica (quartz) content of 50% to 60%.
Andesites and dior-ites are typical intermediate igneous rock.

INTERNAL DRAINAGE - - surface drainage in which the water does not reach the
ocean, such as drainage toward the central part of an interior basin.

INTRUSIVE - - magma (or resulting igneous rock after cooling) which was
squirted into already-existing rocks underneath the land surface.

ISLAND ARC - - a curved chain of volcanic islands rising from the deep-sea
floor and near to a continent caused by subduction processes and occurring on
the continent side of the subduction zone. Its curve generally is convex toward
the open ocean.

ISOSTATIC REBOUND - - the adjustment of the lithosphere of the earth to
maintain equilibrium among units of varying mass and density. Excess mass above
is bal-anced by a deficit of density below, and vice versa . Weight added onto
the earth's surface depresses the lithosphere slowly, and weight removed permits
the litho-sphere to rise slowly, but usually not to the same elevation it had
before it was de-pressed.

K

KNICKPOINT - - a point of abrupt change or inflection in the longitudinal
profile of a stream or of its valley. It is usually a waterfall or a set of
rapids.

L

LACUSTRINE - - pertaining to, produced by, or inhabiting a lake.

LAHAR - - a water-based landslide or mudflow of cooled and solidified chunks
of pyroclastic material on the flank of a volcano.

LAMINATIONS - - layers, often thin.

LAVA - - fluid (molten) rock that issues from a volcano or fissure.

LIMBS - - rock layers on either side of the axis of a fold. Rock layers in
any limb all dip in the same direction, either away from the axis of an
anticline or toward the axis of a syncline.

LIMESTONE - - a non-clastic (non-grained) sedimentary rock consisting chiefly
of the mineral calcite (calcium carbonate). Common impurities are chert, clay
(mud), or dolomite. If the dolomite content exceeds 50%, the rock is called
dolostone.

LIMONITE - - a yellow-colored iron oxide mineral. Small amounts of it create
yellow, orange, and yellowish-brown colors in rocks, especially sedimentary
rocks.

LITHOLOGY - - the physical character of a rock, such as color, mineralogy, and
grain size.

LITHOSPHERE - - the zone of brittle rock between the earth's surface and the
asthenosphere (a zone of ductile deformation about 200 km below the surface.)
The lithosphere consists of the entire crust and a smidgen of the uppermost
mantle. It has an ultramafic igneous composition (mostly magnesium, silicon,
and oxygen). The lithosphere forms the "plates" of plate tectonics.

M

Ma - - millions of years ago.

MAGMA - - naturally-occurring molten rock material generated within the earth
and capable of intrusion and extrusion.

MAGNETITE - - an ore mineral of iron which is an iron oxide. It is black in
color, has a submetallic luster, and is strongly attracted to a magnet.

MAFIC - - a dark-colored igneous rock composed chiefly of ferromagnesian (magnes-ium-iron-rich) minerals, like augite, and gray-colored, calcium-rich
plagioclase.

MANTLE - - the layer of the earth between the crust and the core. It has the
composition of a magnesium-rich silicate rock.

MATRIX - - the smaller-grained stuff filling in around bigger clasts.

MARBLE - - metamorphosed limestone. (Dolomitic marble is metamorphosed
dolostone.)

MECHANICAL WEATHERING - - the process of weathering by which physical
pro-cesses break down a rock into fragments, involving no chemical change.

METAMORPHISM - - mineralogical, chemical, or structural changes in rock due to
changes in temperature, pressure, or chemical environment, generally at depth.

METASEDIMENT - - a sediment or sedimentary rock that has been subjected to
metamorphism.

METASOMATISM - - selected addition and removal of chemical elements by hot,
briny fluids, which soak rocks during metamorphism. Often, the "parent" rock
changes to resemble granite, even though it never was igneous.

MICA - - a group of minerals characterized by low hardness and breakage in one
direction along thin layers, like pages in a book.

MICROCONTINENT - - a small chunk of earth's crust having an overall granitic
composition, like "normal-sized" continents. A typical example is the island of
New Guinea north of Australia.

MONOCLINE - - a local steepening in an otherwise uniform gentle dip (tilt) of
layers of rock; a "flexure" or "draping" of rocks over something, often a buried
fault.

MORAINE - - a mound or ridge of unstratified and unsorted glacial till
deposited by direct action (melting) of glacial ice.

m.y.b.p - - millions of years before (the) present; now designated as "Ma".

N

NONCONFORMITY - - an unconformity (gap in time) where the older rock below the
nonconformity surface is crystalline igneous or metamorphic. A nonconformity
usually represents a large gap in time.

NORMAL FAULT - - a fault along which vertical motion has occurred along a
tilted fault plane and where the hanging wall appears to have moved down,
relative to the footwall. It is due to tension (extension) and results in
crustal lengthening. Synonym : "gravity fault".

NUEE ARDENTE - - a swiftly-flowing, turbulent, gaseous cloud, sometimes
incan-descent, erupted from a volcano and containing ash (glass shards) and
other pyro-clastic fragments in its lower part.

O

OLIVINE - - a mineral group with the formula (Fe,Mg)2SiO4. Olivines exist in
mafic and ultramafic igneous rocks as small, sand-sized grains with a
yellow-green color.

ORDOVICIAN - - the second-oldest time period of the Paleozoic Era. It covers
the span of time from about 500 Ma to about 440 Ma.

OROGEN - - the rocks that comprise a suture zone.

OROGENY - - a mountain-building episode,created by plate-tectonic motions, and
usually lasting several millions to tens of millions of years

ORTHOCLASE FELDSPAR - - mineral group with the formula (K,Na)AlSi3O8. It is
blocky-to-tabular in shape; breaks along perpendicular sets of flat, reflective
sur-faces; is harder than a knife blade; and usually is pink, white or beige in
color.

OUTWASH - - glacial debris re-eroded and re-deposited by meltwater streams.
Outwash forms a "valley train".

P

PAHOEHOE - - a Hawaiian term for basaltic lava flows typified by smooth, ropy
sur-faces.

PALEOSOL - - a buried soil; a "soil of the past".

PALEOZOIC - - the oldest of three eras of the Phanerozoic Eon. The Paleozoic
Era spans the time from 540 Ma to 225 Ma.

PEDIMENT - - gently-inclined erosional surface veneered with fluvial
(stream-depos-ited) gravel and occurring between mountain fronts and valley or
basin bottoms.

PEGMATITE - - very coarse-grained igneous rock, with most grains interlocking
and greater than one centimeter in diameter. The composition usually is
granitic. Peg-matites represent the last and most water-rich portion of a magma
to crystallize and hence contain high concentrations of minerals present only in
trace amounts in typical granites. Often these minerals are rich in such
elements as lithium, boron, fluorine, niobium, tantalum, uranium, and rare-earth
elements like cerium.

PENEPLAIN - - a nearly flat erosional surface, usually near sea level,
presumably produced by the long-continued processes of mass wasting, sheetwash,
and stream erosion.

PHANEROZOIC - - the eon of geologic time from 540 Ma to the present.

PHYLLITE - - a metamorphic rock intermediate between slate and mica schist.
It has a silky sheen on undulating cleavage surfaces.

PILLOW LAVA - - a general term for those lavas displaying pillow structures
(globs of lava with curved tops and "pinched" bottoms) and considered to have
formed under water.

PLAGIOCLASE FELDSPAR - - a group of common rock-forming feldspar minerals
composed of sodium, calcium, aluminum, silicon, and oxygen, and having the
formula (Na,Ca)(Al,Si)2Si2O8. Plagioclases are blocky-to-tabular in shape,
break along perpendicular sets of flat reflective surfaces, are harder than a
knife blade, and usually are white or gray in color. Often they have a pattern
of fine, parallel lines on some reflective surfaces.

PLATE - - see "lithosphere".

PLATE TECTONICS - - the study of the tectonics of lithospheric plates. See
"tecton-ics".

PLAYA - - the shallow central basin of a desert plain, often containing slit,
clay, and salt deposits, and occasionally flooded after rains in adjacent
mountains.

PLUTONIC - - pertaining to igneous rock formed at great depth. A plutonic
rock is coarse-grained due to slow cooling underground.

PORPHYRY - - an igneous rock of any composition that contains conspicuous
pheno-crysts (larger, well-formed mineral grains) in a finer-grained groundmass.
The pre-sence of phenocrysts implies two stages (speeds) of cooling of the
original magma.

PROGRADING SHORELINE - - one that is being built forward or seaward by
depo-sition and accumulation - - for example, in a delta.

PROTEROZOIC - - the eon of geologic time from 2500 Ma to 540 Ma.

PROVENANCE - - source area where sediments are formed or originate, usually
from solid rock by weathering processes.

PROXIMAL - - igneous or sedimentary deposits consisting of coarser particles
depos-ited near to their source areas.

PYROCLASTIC - - pertaining to clastic rock material formed by aerial expulsion
from a volcanic vent.

PYROXENE - - a group of common rock-forming minerals containing silicon,
oxygen, aluminum, and some combination of the metals iron, magnesium, and
calcium. Typical pyroxenes include augite, pigeonite, and jadeite. Pyroxenes
are hard, blocky in appearance, and break along flat, reflective surfaces that
meet at right angles.