Obsidian: Where geology meets archaeology

Obsidian was one of the favorite stone materials for Native American tool makers (chert and other volcanic rocks were also widely used).

Obsidian’s glass-like characteristics could produce sharp edges for projectile points, swords and useful for processing animal hides and meat handling.

Obsidian is a volcanic rock derived from viscous, silica-rich lava called rhyolite that solidified rapidly, without growing crystals.

It is found at many of the volcanic sources around New Mexico and throughout the Southwest (See draft map from M.S. Shackley).

Each volcanic eruption produces lava with a unique geochemical signature, and therefore detailed analyses of obsidian by geochemists often allow for the identification of the origin of the obsidian.

Besides the obsidian found right at the lave eruption site, many obsidian debris are transported down river. As obsidian is strong it can travel long distances (up to more than 100 miles) in active streams.

Dr. Steve Shackley at the Geoarchaeological XRF Laboratory in Albuquerque has studied many eruption deposits in New Mexico and has compared these with obsidian samples from Pueblo and earlier Native locations.

For example, more than 50 obsidian samples from the former Pottery Mound Pueblo located in western Valencia County were found to have originated from eruption sites near Mt. Taylor, with a few samples from the Valles Caldera area. He also studied many locations in the Rio Grande Basin sediments along former riverbeds where obsidian chunks have been transported from the eruption sites.

Sampling of ancient riverbeds in Tijeras Canyon, for example, were shown to have obsidian from five distinct eruption sites. The research indicated that the obsidian collected for tools by Pueblo and earlier Native Americans were not only collected from the areas where the lavas erupted, but also that these early toolmakers would have found more proximate locations in riverbed sediments where good quality and large obsidian pieces could be collected for tools.

Studies of obsidian at archaeology sites distant from an obsidian eruption location and from any obsidian-containing sediment layers also show that trading of obsidian material must have occurred between tribes.

This type of study illustrates the merging of geological testing and archaeology.