Basalt and other fine-grained volcanic rock

Basalt is a dark, fine-grained volcanic rock, but archaeologists use the word loosely for many gray fine-grained volcanic stones, grouped as FGV. In the Pacific Northwest, geochemistry shows that most so-called basalt tools are really andesite, dacite or basaltic andesite, because true basalt has too little silica to flake well. More silica gives sharper edges, and these rocks are tougher than obsidian, so they hold up for rough work. The Columbia River Basalt Group supplied huge amounts, and a basaltic andesite workshop at Pataha Canyon in Washington was a large biface workshop. Dark gray, fine-grained basalt boulders near Eureka, Nevada, also made high-quality artifacts.

By silica content (weight percent SiO2) these rocks range from basalt (under 52 percent) through andesite (52 to 63) and dacite (63 to 68) to rhyolite (over 68). The Columbia River Basalt Group's Grande Ronde Basalt includes basaltic andesites that can be told from the more siliceous andesites and dacites of the Saddle Mountain Basalt by geochemistry. Glassy texture indicates obsidian; fine-grained rock with crystals too small to see is aphanitic. The more siliceous andesites, dacites and rhyolites are easier to flake than basalt.