About Obsidian and Obsidian Sourcing

Obsidian is a natural silica-rich volcanic glass, typically of rhyolitic composition, produced when magma from a deep source erupts onto or near the earth’s surface and rapidly chills against air, water, or some colder rock. Wherever it occurs in the world, obsidian was extensively used in prehistoric and historic times by native peoples to manufacture a wide variety of tools, like projectile points, spears, knives, scrapers and sharp flakes.

Obsidian cobble from Little Glass Butte, Oregon. Specimen is ca. 14 cm. long and 11 cm. high.
Obsidian boulder from Whitewater Ridge, Oregon, ca. 25 cm. long X 25 cm. high.

Obsidian sample from Mt. Edziza,
British Columbia, Canada. Scale in centimeters.

Because most obsidian eruptions are highly uniform in chemical composition, it is possible to analyze geological samples to determine the “fingerprint” or trace element profile of each source using a variety of instrumental techniques. Non-destructive energy dispersive x-ray fluorescence (edxrf) analysis is used at GRL to determine the trace and selected minor element composition of geological obsidian samples and archaeological artifacts made from obsidian (see Laboratory Equipment and Methods).

Obsidian sourcing refers to research concerned with establishing chemical correlations between geologic parent materials (obsidian sources) and archaeological artifacts made from obsidian. Chemical correlations between geologic obsidian samples and archaeological artifacts allow archaeologists to determine the parent geologic source of origin for artifacts, and to then use the information to study source-use patterning in an obsidian assemblage at a site, or region, by artifact type and time period with the ultimate goal of investigating continuity and change in prehistoric trade/exchange relationships and group mobility patterns through time.

Ross Barbed spear point from Mound 7 at Mound City, Ohio, manufactured from volcanic glass from Obsidian Cliff, Wyoming. (see Hughes 2006 in Resume) Scale in centimeters.

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