Factors affecting obsidian composition:

Erupting volcano

Cultural factors:


Natural factors:

Mineralogy of magma (rock minerals melted under extreme heat and pressure to form molten lava).

Temperature & pressure affect mineral formation during molten and cooling phases; fractional crystallization.

Weathering of cooled rock (surface hydration).

Post-depositional factors:

Weathering (hydration) of exposed surfaces.

Obsidian artifacts



Obsidian flows are spatially limited, discrete sources on the landscape.


Individual flows are highly homogenous, with order-of-magnitude differences between flows.


The provenance principle is readily applied and obsidian artifacts can be easily linked to a known source.

Oregon State University Archaeometry Lab