UNITED STATES AND CANADA OBSIDIAN SOURCE CATALOG

Obsidian: Glossary of Related Terms

VERY UNFINISHED

A

AA: Atomic absorption (an analytical method).

ACCIDENTAL INCLUSION: Enclosed rocks or crystals having no genetic connection with the igneous rock in which they are found.

ACICULAR: Needle-like in form.

ACICULAR TRICHITE: A hairlike trichite.

ACIDIC: Term applied to Igneous rocks (such as rhyolite and dacite) that contain more than 60% SiO2.

ALKALINE: Generally refers to rocks high in sodium and/or potassium.

ANDESITIC/ANDESITE: Extrusive volcanic rock with an Si02 composition ranging from about 58% to 63%.

APACHE TEARS: See Marakenite.

ARCHAEOMETRY:

ARTIFACT:

ASH-FLOW: Avalanche of very hot gas-rich pyroclastic material erupted explosively from a volcanic vent.

ASH-FLOW TUFF: Ash-flow deposit indurated by compaction and heat.

ASTEROIDAL TRICHITE: Spiderlike arrangements of trichites radiatiating from a central point.

AUTOLITH: An inclusion or fragment of older igneous rock that is genetically related to the rock in which it is found.

B

BANDED:

BASALTIC/BASALT: Extrusive volcanic rock with an Si02 content ranging from about 45% to 53%.

BASALTIC ANDESITE: Extrusive volcanic rock with an sio2 content ranging from about 53% to 58%.

BELONITE: An elongated or acicular crystallite having rounded or pointed ends.

BIFACE: Artifact with patterned flake scars on both faces.

BIREFRINGENCE:

BLANK:

BRECCIA: Fragmented rock whose components are angular.

C

CALC-ALKALINE:

CALDERA: Large basin-shaped volcanic depression roughly circular in form.

CHAINE OPERATOIRE:

CHARACTERIZATION: A distinguishing attribute or characteristic.

CHEMICAL ANALYSIS:

CHEMICAL SOURCE: See Geochemical Source.

CONCHOIDAL FRACTURE: Smoothly-curved fracture surface; typical of obsidian and quartz.

CRYSTALLITE: A broad term applied to a minute body of unknown mineralologic composition or crystal form which does not polarize light.

D

DACITE/DACITIC: Volcanic rock with an Si02 composition ranging from about 63% to 70%.

DEBITAGE:

DENSITY:

DEVITRIFICATION: Process by which glassy rocks break up into definite minerals; obsidian devitrifies to perlite.

DIAGNOSTIC ARTIFACT: An artifact, often a projectile point, with a stylistic or other characteristic distinctive of a certain time period or cultural affiliation.

DIFFUSION FRONT:

E

EDGE-MODIFIED PIECE (EMP):

EDXRF: Energy-dispersive X-ray fluoresence.

EN ECHELON: Approximately parallel and offset structural features such as faults.

ENDOGENOUS DOME:

ERRATICS: See Glacial Erratic

EUTECTIC POINT: The lowest melting temperature obtainable with mixtures of given components where the components r~~ain in a liquid state.

EXOGENOUS DOME: Volcanic dome built by the discharge of viscous flows from a summit vent.

F

FELSIC/FELSITE: Light-colored. General term for a light-colored igneous rock with a grain size so small that constituents cannot be megascopically determined.

FIAME: Dar, vitric lenses in welded tuff formed by the collapse of fragments of pumice. These sometimes resemble obsidian.

FISSION-TRACK AGE: The age of a rock or mineral as determined by the fissiontrack dating method.

FLOAT:Rocks found separated from parent veins or strata by weathering processes.

FLUVIAL: Pertaining to river or stream action.

G

GEOCHEMICAL:

GEOCHEMICAL SOURCE:

GLACIAL ERRATIC: Ice-transported rocks.

GRABEN: Block that has been downthrown along bordering faults relative to rocks on either side.

GRAVER: An artifact with a point or points; assumed to have been used to incise or form organic materials or soft stone.

H

HOLOHYALINE: Said of an igneous rock that is composed entirely of glass.

HYDRATION:

HYDRATION RATE: Rate at which water hydrates into the surface of obsidian.

HYDRATION RIM: Visible rim (under a microscope) that forms on the surface of obsidian as the glass slowly hydrates with water.

I

IGNEOUS:

IGNIMBRITE: See Ash-Flow Tuff.

IN SITU: In its natural position or place.

INDEX OF REFRACTION: See Refractive Index.

INNA: Instrumental neutron activation analysis.

ISOTROPIC:

:

J

JET:

K

K-AR AGE: See Potassium-Argon Age.

K-FELDSPAR:

KNAPPING STATION: Term applied to an archaeological site where lithic materials were worked; characterized by small fragments of waste material.

L

LAHAR: Landslide or mudflow of pyroclastic material on the side of a volcano.

LAPILLI: Pyroclastic material ranging in size from 2 to 64 rom.

LATH: Long, thin crystal.

LITHOIDAL: Term applied to devitrified glass.

LITHOPHYSAE: Hollow, bubblelike structures composed of concentric shells of finely crystalline materials; often found in rhyolites and obsidians.

LONGULITE: A cylindrical or conical belonite thought to have been formed by the coalescence of globulites.

M

MAFIC: Dark-colored.

MAGMA:

MAGNETIC POLARITY: Refers to the direction of the north and south magnetic poles. The magnetic polarity of the poles periodically reverses and the direction of the polarity can be recorded in volcanic rocks erupted in the past (paleomagnetic polarity). Reversed polarity refers to reversed magnetic poles, i.e., the polarity of the north and south poles reverse: normal polarity refers to polarity as it exists today. The last reversal took place about 690,000 years ago.

MAJOR ELEMENT: Elements occurring in abundances greater than about 0.1 percent or 1000 parts per million.

MARAKENITE: Rounded glassy obsidian masses found associated with deposits of perlite; Apache Tear.

MARGARITE: Bead-like string of globulites commonly found in glassy igneous rocks.

MAZAMA TEPHRA/MAZAMA ASHFALL: Tephra or volcanic ash originating about 7,000 radiocarbon years ago from the present site of Crater Lake in the southern Cascades.

MICROLITE: A microscopic crystal with determinable optical properties.

MICROLITH: A microlite.

MICRON: One-millionth of a meter.

MICROLITIC STRUCTURE: A general term referring to the variety of microscopic structures classified as microlites and crystallites.

MICROPHENOCRYST: Microscopic phenocryst set in a very fine-grained glassy groundmass.

N

NEUTRON ACTIVATION ANALYSIS:

NORMAL PALEOMAGNETIC POLARITY:

O

OBSIDIAN:

OBSIDIAN HYDRATION ANALYSIS:

OBSIDIAN HYDRATION YEARS:

OBSIDIAN-LIKE VITROPHYRE: Vitrophyre that is macroscopically similar to obsidian; sometime associated with the densely-welded zones in welded ash-flows. Also see Vitrophyre.

P-Q

PALEOMAGNETIC POLARITY:

PERALKALINE:

PERLITIC/PERLITE: Volcanic glass similar in composition to obsidian but with a higher water content; devitrified obsidian.

PETROGRAPHIC/PETROGRAPHY: The branch of geology dealing with the description and classification of rocks.

PHENOCRYST: A relatively large crystal set in a finer-grained groundmass.

PLINIAN:

PORPHYRITIC: Textural term for igneous rocks in which larger crystals are set in a finer groundmass that may be crystalline or glassy.

POST-MAZAMA: Event occurring after the Mazama ashfall of about 7,000 radiocarbon years ago.

POTASSIUM-ARGON AGE: The age of a rock or mineral as determined by the potassilimargon dating method.

PPM: Parts per million.

PREFORM:

PRE-MAZAMA:

PRIMARY OBSIDIAN SOURCE: Source of obsidian found in the vicinity of its vent.

PRISMATIC MICROLITE: Rod-shaped microlite.

PYROCLASTIC:General term applied to volcanic material that has been explosively or aerially ejected from a volcanic vent.

QUARRY SITE: An archaeological site centered around around a source of a natural raw material.

R

RADIOCARBON YEARS: The age of organic material (charcoal, bone, shell, etc.) as determined by the radiocarbon dating method. Calibration methods have been developed so that calendrical dates may be calculated from radiocarbon dates.

RADIOMETRIC DATING: Determination of the age of geological or archaeological materials by any of several methods based on the decay of naturally-occurring radioactive isotopes.

REFRECTIVE INDEX: A characterizing number which expresses the ratio of the velocity of light in a vacuum to the velocity of light in a substance.

REVERSED PALEOMAGNETIC POLARITY:

RHYODACITE/RHYODACITIC: A rather vague term often used synonomously with rhyolite.

RHYOLITE/RHYOLITIC: Volcanic rocks with an Si02 content greater than 70%.

RHYOLITE DOME: steep-sided volcanic dome created by the extrusion of viscous, rhyolitic lavas.

S

SCOPULITE: Rodlike or stemlike crystallite that terminates in branches or plumes.

SECONDARY OBSIDIAN SOURCE: Source of obsidian transported from the original primary source by a variety of natural and cultural processes.

SELVAGE: The edge of a volcanic body; sometimes refers to a volcanic dome or flow in which a glassy border is formed by the rapid chilling of the lavas.

SILICA:

SOURCE:

SPHERULITE: small, radiating, and usually concentrically arranged aggregation of minerals of a spherical or spheroidal shape formed by the rapid growth of crystals in a rigid glass.

T

TACHYLYTE/TACHYLITE: Volcanic glass of basaltic composition.

TEKTITE:

TEMPORALLY-DIAGNOSTIC ARTIFACT:

TEPHRA: Ejecta blown through the air by explosive volcanic eruptions.

THIN SECTION: A very thin slice (approxmately 30 microns) of rock mounted on a slide for microscopic petrographic examination.

TIME-SENSTIVE ARTIFACT:

TRACE-ELEMENT: A chemical element that occurs in very low concentrations, generally less than 1000 ppm.

TRACHYTIC: Textural term describing microlites arranged in a parallel or subparallel manner bending around phenocrysts and corresponding to flow lines in the lavas in which they are found.

TRICHITE: A straight or curved hairlike crystallite, usually black. Trichites occur singly or radially arranged in clusters and are found in glassy igneous rocks.

TUFF:

TYPE: Classification of an artifact so that the class reflects cultural or behavioral characteristics.

U

UNCONFORMITY:

UNIFACE: Artifact with patterned flake scars on one face.

V

VENT:

VESICULAR:

VITREOUS: Glassy; amorphous.

VITROPHYRE: Any porphyritic igneous rock having a glassy groundmass.

W

WELDED ASH-FLOW:

WELDED TUFF:

X-Y-Z

XENOLITH:

XRF: X-ray fluorescence.

XRF ANALYSIS: X-ray fluorescence analysis; an analytical method used to determine the trace and major-element composition of rocks.

REFERENCES

American Geological Institute (1972) Glossary of Geology.

Irvine, T.N. and W.R.A. Baragar (1971) A Guide to the Chemical Classification of Common Volcanic Rocks, Canadian Journal of Earth Sciences 8:523-548.

Middlemost, E.A.K. (1972) A Simple Classification of Volcanic Rocks, Bulletin Volcanologique 36:382-397.

Shackley, M. Steven (2005) Obsidian: Geology and Archaeology in the North American Southwest.. The University of Arizona Press, Tucson, Arizona.

Williams, Howel and Alexander R. McBirney (1979) Volcanology. Freeman, Cooper, and Co., San Francisco, California.

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Last Updated: 10/19/2018