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THIS SECTION IS UNDER DEVELOPMENT
A
- Accidental Inclusion
Enclosed rocks or crystals having no genetic connectionwith the igneous rock in which they are found. Also see Xenolith.
- Acicular
Needle-like in form.
- Acicular Microlite
A hairlike or very thin microlite. Often occurs in dense swarms.
- Acicular Trichite
A hairlike or very thin trichite.
- Andesite
Extrusive volcanic rock with an SiO content ranging from about 58% to 63 percent.
- Apache Tear
Rounded obsidian masses found associated with deposits of perlite. Also see marakenite.
- Ashflow Tuff
See Welded Tuff.
- Asteroidal Trichites
Spiderlike arrangements of trichites radiating from a central point.
B
- Basalt
Extrusive volcanic rock with an SiO2 content ranging from about 45 percent to 53 percent.
- Belonite
An elongated or acicular crystallite having rounded or pointed ends.
- Biface
Artifact with contiguous flakes scares on both surfaces.
- Breccia
Fragmented rock whose components are angular.
C
- Caldera
Large basin-shaped volcanic depression roughly circular in form.
- Characterization
The use of any distinguishing attribute or characteristic to identify a specific source. The trace element pattern or signature of obsidian artifacts is ofteh used to characterize the sample.
- Chemical Source
See Geochemical Source.
- Conchoidal Fracture
Smoothly-curved fracture surface that is typical of obsidian or quartz.
- Crystallite
A broad term applied to a minute body of unknown mineralologic composition or crystal form which does not polarize light.
D
- Dacite
Volcanic rock with an SiO2 content ranging from about 63 percent to 70 percent.
- Dendrochronologic Calibration
- Devitrification
Process by which glassy rocks break down into definite minerals; obsidian devitrified to perlite.
- Diffusion Front
See Hydration Rim Diffusion Front.
E
- Erratics
See Glacial Erratics.
F
- Felsic
A dark-colored igenous rock.
- Float
Surface rocks or nodules found separated from parent veins of strata.
- Fluvial
Pertaining to river or stream activity.
G
- Geochemical Source
- Glacial Erratics
Ice-transported rocks.
- Globulite
- Groundmass
The material between the phenocrysts of a porphyritic igneous rock. It is relatively finer-grained than the phenocrysts and may be crystalline, glassy, or both.
H
- Holohyaline
An igneous rock composed entirely of glass.
- Hydration Rate
- Hydration Rim
- Hydration Rim Diffusion Front
I
- Ignimbrite
- Index of Refraction
- INAA
Instrumental neutron activation analysis.
- Isotropic
Exhibiting the same optical properties (when viewed under polarized light) in all orientations.
K
- K-Ar
See Potassium-Argon Age.
L
- Lahar
- Lath
Long, thin crystal.
- Lithoidal
Term applied to devitrified glass.
- Lithophysae
- Longulite
M
- Mafic
A dark-colored igenous rock.
- Major Elements
Element occurring in abundance greater than about 0.1 percent (1000 ppm).
- Margarite
Beadlike string of globulites commonly found in glassy volcanic rocks.
- Marakenite
Rounded obsidian masses found associated with deposits of perlite. Also see Apache Tear.
- Microlite
A microscopic crystal with determinable optical properties.
- Microlith
A microlite.
- Micron
One-millionth of a meter. Standard unit used to measure obsidian hydration rims.
- Microphenocryst
- Minor Element
N
- NAA
Neutron activation analysis.
O
- Obsidian Hydration Years
- Obsidian-Like Vitrophyre
P
- Peralkaline
- Perlite
- Perlitic Texture
- Petrography
Branch of geology dealing with the description and classification of rocks.
- Phenocryst
A relatively large crystal set in a finer-grained or glassy groundmass.
- Porphyritic
Textural term for igneous rocks in which larger crystals are set in a finer groundmass that may be crystalline or glassy.
- Potassium-Argon Age
The age of a rock or mineral as determined by the potassium-argon dating method.
- ppm
Parts per million.
- Primary Source
- Prismatic Microlite
Rod-shaped microlite.
- Provenance
- Provenancing
- Provenience
- Pyroclastic
Pertaining to clastic rock material formed by volcanic explosion or aereal expulsion from a volcanic vent.
R
- Radiocarbon Years
The age of organic material (charcoal, bone, shell, etc.) as determined by the radiocarbon dating method. Calibration methods are widely used to correct for the differences between radiocarbon and calendar years.
- Rhyodacite
A rather vague term often usued synonymouisly with rhyolite.
- Rhyolite
Extrusive volcanic rock with an SiO content greater than 70 percent.
S
- Scopulite
Rodlike or stemlike crystallite that terminates in branches or plumes.
- Secondary Source
- Selvage
- Source
- Spherulite
T
- Tachylyte
Volcanic glass of basaltic comp[osition.
- Thin Section
A very thin slice (about 30 microns thick) mounted on a slide for microscopic petrographic examination. In obsidian hydration analysis, a small slice from the edge of an artifact is typically prepared as a thin section so that the hydration rim can be measured.
- Trace Element
Element occurring in abundance less than about 0.1 percent (1000 ppm).
- Trichite
A straight or curved hairlike crystallite, usually black in color. Trichites occur singly or radially arranged in clusters and are found in glassy volcanic rocks.
- Tuff
A general term for all consolidated pyroclastic rocks.
V
- Vent
The opening at the Earth's surface through which volcanic materials are extruded.
- Vitreous
Glassy; amorphous. Obsidian has a vitreous texture.
- Vitrophyre
Any porphyritic igneous rock with a glassy groundmass. Also see Obsidian-like Vitrophyre.
V
- Welded Ashflow
See Welded Tuff.
W
- Welded Tuff
A glass-rich pyroclastic rock that has been indurated by the welding together of its glass shards under the combined action of heat, hot gases, and the weight of overlying material.
X
- XRF
X-ray fluorescence (an analytical method used to determine the chemical composition of rocks).
- Xenolith
A rock fragment foriegn to the body of igneous rock in which it is found. Also see Accidental Inclusion.
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