Silicic volcanic domes, caldera margins, and rhyolitic lava flows
Core Principles & Analysis
Chemically equivalent to granite and rhyolite (≥70 wt% SiO₂) with less than 1 wt% water, high-viscosity felsic lava extruded at flow margins cools too rapidly for atomic diffusion to nucleate crystals, solidifying as a metastable volcanic glass.
Lacking crystal cleavage planes, obsidian breaks with a smooth conchoidal fracture whose freshly knapped edge (~3 nanometers thick) is sharper than honed surgical steel, making it a prized trade commodity for Paleolithic, Neolithic, and Mesoamerican blades.
Over geologic timescales (millions of years), groundwater diffuses into obsidian at a predictable rate used in archaeological obsidian hydration dating, eventually devitrifying the glass into perlite and radially clustered white cristobalite spherulites ('snowflake obsidian').