Geology Reference
In-Depth Information
Aa -A Hawaiian term for basaltic lava flows with a
rough, blocky surface.
Accretion -A tectonic process by which exotic rock
masses (terranes) are physically annexed to
another landmass after the two collided.
Accretionary wedge -A prism of sediment that builds
up on the leading edge of a moving tectonic
plate.
Agate -A cryptocrystalline quartz (chalcedony) which is
commonly banded and colored.
Agglomerate -A pyroclastic rock made up of coarse,
usually angular fragments, much like volcanic
breccia.
Alluvium -A general term for all detrital deposits
resulting from the operations of modern rivers,
thus including the sediments laid down in river
beds, flood plains, lakes, fans at the foot of
mountain slopes, and estuaries.
Antecedent stream -A stream that was established
before local uplift began and incised its valley
at the same rate the land was rising.
Andesite -A fine-grained volcanic rock containing
plagioclase feldspar (andesine) and one or
more mafic constituents such as pyroxene,
amphibole or biotite.
Anticline -Arched beds with limbs dipping away from
the axis. Simple anticlines have older beds
exposed in the center of the arch.
Aphanitic rock- This term pertains to a texture of rocks
in which the crystals are too small to
distinguish with the unaided eye.
Archipelago -A curved volcanic chain on land or an
island arc in an ocean setting.
Arkose -A sandstone containing 25 percent or more of
feldspars usually derived from coarse-grained
silicic igneous rocks.
Bacarc basin -A sedimentary trough situated between a
volcanic island arc and the mainland.
Basalt -A dark-colored, fine-grained igneous rock
containing mostly calcic fledspar and pyroxene
with or without olivine.
Basin and range structure -Regional structure typified
by parallel fault-block mountain ranges,
separated by sediment-filled basins. These
occur in southeastern Oregon.
Batholith -A large body of coarse-grained intrusive
igneous rock with a surface area greater than
40 square miles. Smaller intrusive bodies are
called stocks.
Bed -A layer or stratum of sedimentary rock.
Brackish -Water with very low salinity that is usually
derived from fluvial runoff into near-shore
marine waters.
Breccia -Fragmental rock whose pieces are angular
unlike water worn material. There are fault
breccias, talus breccias and eruptive volcanic
breccias.
Calcite -A mineral composed of calcium carbonate
(CaC03) and the chief constituent of
limestone.
Caldera -A large basin-shaped, generally circular,
volcanic depression, the diameter of which is
many times greater than that of the included
volcanic vent or vents. It is formed by collapse,
such as at Crater Lake and the Newberry
caldera.
Chert -A compact siliceous rock of varying color
composed of microorganisms or precipitated
silica grains. It occurs as nodules, lenses, or
layers in limestone and shale.
Cinder cone -A cone-shaped peak built up almost wholly
of volcanic fragments erupted from a vent.
Cirque -The rounded saucer-shaped glacially carved
basin at the head of an alpine glacial valley
which commonly contains a cirque lake or tarn.
Clastic rock -A consolidated sedimentary rock com-
posed of the cemented fragments broken or
eroded from pre-existing rocks of any origin by
chemical or mechanical weathering. Examples
are conglomerate, sandstone, and siltstone.
Columnar jointing -Cracks that separate rock into
columns commonly six-sided but generally
ranging from 4-8 sides. Such jointing usually
forms in volcanic rocks due to shrinkage when
cooling.
Concretion -A nodular or irregular mass developed by
localized deposition of material from solution
generally around a central nucleus. It is
harder than the enclosing rock such as limey
concretions in shale.
Conformable -Accumulation of generally parallel strata,
without appreciable break in deposition.
Conglomerate -A sedimentary rock made up of rounded
pebbles and cobbles coarser than sand.
Conodont -Phosphatic microfossils of fish-teeth used
to age date many of the older Oregon marine
rocks.
Contact -The surface between two different types of
rocks or formations which is shown as a line on
geologic maps.
Correlation -A means of determining equivalent ages of
formations or geologic events by studying
physical similarities or differences, by
comparing fossils or by comparing absolute
dates from radioactive minerals.
Crust -The outer layer of the earth above the
Mohorovicic discontinuity usually basaltic
in composition beneath ocean basins and
granitic in composition under continents.
Crystal -The regular polyhedral form, bounded by plane
surfaces which is the outward expression of a
periodic or regularly repeating internal
arrangement of atoms.
Crystalline rock -An inexact term for igneous and
metamorphic rocks as opposed to sedimentary
rocks.
Delta -A fan-shaped alluvial deposit where a stream
enters a standing body of water, usually the
ocean or a large lake.
Density -Mass per unit volume.
Diatom -Microscopic, single-celled plants (algae)
growing in marine or fresh water. Diatoms
secrete siliceous skeletons that may accumulate
in sediments in enormous numbers and which
are referred to as diatomite or diatomaceous
earth.
Dike -A tabular body of igneous rocks that cuts across
the structure of adjacent rocks or cuts massive
rocks.
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