Obsidian forms by slow cooling

Obsidian is a natural glass that is a common product of silicic volcanic eruptions. It has been prized throughout human history for its glassy nature, finding use in tools, weapons and ornaments, and in ritual and spiritual practice. The absence of large crystals in obsidian has led to the widespread view that it is formed by rapid cooling of lava, preventing crystal nucleation and growth. Here we show that, on the contrary, the absence of vesicles in obsidian requires relatively slow cooling - on the order of 10^-4 to 10^-8°C/s - to enable the resorption of remnant bubbles. Our bubble-resorption model for obsidian formation is supported by in-situ X-ray computed tomography at magmatic temperatures that reveals bubble shrinkage during resorption on cooling. We validate a numerical model for growth and resorption of bubbles against these results, then apply the model to explore the conditions under which obsidian can form in nature. Our findings revise the accepted thermal histories of obsidian-forming systems, overturning conventional wisdom for the formation of this culturally, archaeologically, and volcanologically important material.