Volcanic glass dating by water diffusion rind thickness
When obsidian (volcanic glass) is freshly fractured, it exposes a surface that immediately begins absorbing water from the environment. Water molecules diffuse into the silicate glass network at a rate governed by temperature (Arrhenius equation) and glass chemistry. This creates a hydration "rind" — a thin band of structurally altered glass visible under polarized light microscopy, typically 1–20 micrometers thick. The rind thickness x grows as x² = D·t (Fickian diffusion), so age t ∝ x²/D. Calibrating D requires independent dating of samples with known ages, and temperature histories must be estimated from climate proxies. Used to date Paleolithic and Mesolithic flint-knapping sites across the Pacific and Americas.