Q

QUARTZ - - mineral with the formula SiO2. Usually it is clear, or translucent
white, but it can be other colors due to impurities or radiation damage. It is
not scratched by a knife and breaks along irregular surfaces.

QUARTZITE - - metamorphosed sandstone.

R

RADIOMETRIC DATING - - age determination based on nuclear decay of
naturally-occurring radioactive isotopes, which are impurities in mineral
grains.

REGRESSION - - the slow lowering of sea level and/or raising of the edge of a
contin-ent such that the shoreline slowly moves away from the center of the
continent, exposing more land above sea level. Marine sediments deposited
during regres-sion get coarser in size as you move vertically upward through the
pile.

RELIEF - - the difference between highest elevation and lowest elevation in an
area.

REVERSE FAULT - - a fault along which vertical motion has occurred along a
tilted fault plane and where the hanging wall appears to have moved up, relative
to the footwall. It is due to compression and results in crustal shortening.

RHYOLITE - - a group of extrusive, felsic light-colored) igneous rocks which
are typically porphyritic, with phenocrysts of quartz and/or orthoclase feldspar
in a cryptocrystalline groundmass. The presence of phenocrysts indicates two
stages (speeds) of cooling of the original magma.

RIFT - - a break in the earth's crust due to separation of the two sides by
tensional forces. Often a graben forms along the rift.

S

SALTATION - - movement of individual particles as variable leaps or jumps
powered by wind or water; a "roll-and-bounce" motion.

SANDSTONE - - a sedimentary rock consisting predominantly of quartz grains
between 2 mm and 1/16 mm in size which have been more or less cemented together
by silica, iron oxides (limonite, hematite), calcite, and/or clay.

SCHIST - - a medium- to coarse-grained, foliated (layered) metamorphic rock
created by regional metamorphism to medium or high temperatures and shearing
pres-sures. Commonly, schists include quartz, feldspars, and micas, but mineral
compo-sition is not an essential factor in its definition. Schists are strongly
foliated, with well-developed parallelism of more than 50% of the minerals present.

SEICHE - - an oscillation of a body of water in an enclosed or semi-enclosed
basin, caused by local changes in atmospheric pressure, wind, tidal currents,
and earthquakes.

SHALE - - a very fine-grained, thinly layered sedimentary rock composed of
clay and/or mud grains less than 1/256 mm in size. Shales break easily along
their layering, especially along weathered surfaces. They feel smooth to the
touch, not gritty.

SHATTER CONE - - a funnel-shaped fracture in a rock (in three dimensions)
caused by a sudden release of energy nearby. The tip of the funnel shape points
in the direction of the location of energy release. Usually several cones form,
nested within one another like stacked soup bowls with sharply pointed bottoms.

SHIELD - - see "craton".

SILICA - - any material with the composition SiO2. Often refers to a
Jello-like sub-stance with that composition which can act as a rock cement or
solidify by itself as chert, flint, or jasper.

SILICATE - - a mineral or rock with a predominant composition that includes
silicon and oxygen.

SILICATE MELT - - molten rock (magma or lava) that is mostly silicon and
oxygen in composition, usually also including some combination of aluminum,
magnesium, iron, calcium, sodium, and potassium.

SILL - - a tabular (table-top-shaped), igneous intrusion concordant (parallel
to) the layering in the country rock which it intrudes.

SILTSTONE - - a fine-grained, layered sedimentary rock composed primarily of
grains between 1/256 mm and 1/16 mm in size. Siltstones tend to be "flaggy" and
contain hard thin layers. They feel grittier than shales or mudstones.

SLATE - - shale or other fine-grained rock metamorphosed at low temperatures
and shearing pressures. Mineral grains re-grow in the solid in parallel fashion
so that the resulting slate breaks along flat, smooth, usually reflective
surfaces.

SLUMP - - the downward slipping of a mass of rock or unconsolidated material
moving as a unit along a spoon-shaped surface, usually with backward rotation.

SORTED - - separated by grain size.

STRATIGRAPHIC COLUMN - - a diagram or list of a sequence of rock units
arranged so that the oldest occurs on the bottom of the stack.

STRATIGRAPHY - - the study of vertical sequences of rock layers.

STRIKE - - the trend of a linear feature on a map or on a generally flat
ground surface. The linear feature usually is the eroded end of a layered rock
unit or the inter-section of a fault plane with the land surface.

STRUCTURAL GEOLOGY - - the branch of geology that deals with the description,
representation, and analysis of structures, chiefly on a moderate to small
scale.

STRUCTURES - - the three-dimensional orientations and relative positions of
the rock masses of an area, including the sum total of features resulting from
such processes as faulting, folding, and igneous intrusion. The features
generally are megascopic, usually best seen on an outcrop rather than in hand
sample.

SUBDUCTION - - the process of one lithospheric plate descending beneath
another.

SUPERCONTINENT - - an amalgamation of two or more "typically-sized" continents
into one, very large continent. The amalgamation is accomplished by plate
conver-gence and the development of suture zones (orogens) where they meet.

SUPERPOSED STREAM - - a stream that incises (erodes) its channel down into
older rocks and rock structures (such as folds or faults) that were buried under
some younger rock or sediment. Usually, the superposed stream is established on
the younger material first, then regional uplift forces the downcutting of the
stream channel.

SUTURE - - the collision zone created by the plate-tectonic convergence of two
(or more) continents. A subduction zone usually includes some combination of
geo-cline, island arc, and ocean-floor remnant (the remaining part that was not
sub-ducted as the continents approached one another). The rocks and sediments
caught up in the suturing event are highly folded, thrust-faulted, and (in the
core of the resulting orogen) metamorphosed and selectively melted.

SYNCLINE - - a fold in rocks in which the strata dip inward from both sides
(limbs) toward the axis. A fold, which is concave upward.

T

TECTONICS - - a branch of geology dealing with the broad architecture of the
outer part of the earth. It deals with major structural or deformational
features and their relations, origins, and historical evolution. It is closely
related to structural geology, but tectonics deals with larger features.

TENSION - - stress that tends to pull a body apart; elongation; opposite of
com-pression.

TERRACE - - a relatively level bench or step-like surface breaking the
continuity of a slope.

TERRAIN - - a tract or region of the earth's surface considered as a physical
feature.

TERRANE - - a term applied to a group of related rocks and to the area in
which they crop out. Adjacent terranes do not have rocks which formed in the
same geogra-phical area or were formed by similar geological processes.

THRUST FAULT - - a fault on which the hanging wall appears to have moved
up-ward, relative to the footwall, at an angle of 45o or less. It is caused by
horizontal compression. It is a reverse fault with a shallowly dipping fault
plane.

TILL - - debris deposited directly by melting ice in a glacier.

TILLITE - - glacial till cemented into a solid rock.

TRANSGRESSION - - the slow raising of sea level and/or lowering of the edge of
a continent such that the shoreline slowly moves toward the center of the
continent, exposing less land above sea level and flooding the edge of the
continent. Marine sediments deposited during transgression get finer in size as
you move vertically upward through the pile.

TRENCH - - a narrow, elongate depression of the deep-sea floor, having steep
sides and oriented parallel to the trend of an adjacent continent. It lies
between the continental margin and the abyssal plain. Usually it forms the
surficial trace of a subduction zone.

TRIPLE JUNCTION - - three fractures in the lithosphere which meet at roughly
120o angles on a map. They form during the initial stages of continental
rifting. Grabens usually form along the fracture patterns. As rifting
continues, two of the three "arms" of the triple junction continue to widen,
perhaps creating a new ocean basin be-tween the separating continental chunks,
while the third one "fails" (an aulocogen).

TUFF - - a general term for all consolidated pyroclastic rocks, but usually
referring to the finer-grained variety; the coarser variety usually is called
volcanic breccia.

TURBIDITE - - debris deposited from an underwater landslide.

U

ULTRAMAFIC - - said of an igneous rock composed chiefly of the minerals
hyper-sthene, augite, or olivine; a magnesium-rich rock with less than 45%
silica (silicon dioxide). It is the typical composition of the earth's mantle.

UNCONFORMITY - - a break or gap in the geologic record, as recorded in
adjacent rocks.

UNDERFIT STREAM - - a stream that appears too small to have eroded the valley
in which it flows. Usually the valley was carved by a much larger, ancient
river or by a glacier.

V

VALLEY TRAIN - - the sediment deposited by glacial meltwater streams.

VENT - - the opening at the earth's surface through which volcanic materials
are ex-truded. Its conduit usually is pipe-like in shape.

VESICLE - - a small cavity in an aphanitic (fine-grained) or glassy igneous
rock, formed by the expansion of a bubble of gas or steam during the
solidification of the rock.

W

WATER TABLE - - the paper-thin boundary underground between the zone of
saturation (all openings permanently filled with water) and the zone of aeration
(openings having water only temporarily or not at all).

WEATHERING - - the physical and/or chemical break-up of solid rock on the
earth's surface, producing loose debris.