The rate and extent of degassing (i.e., the loss of volatile species from the silicate melt upon decompression or heating) and crystallization are dominant controls on eruption behavior. We are working with Dr. Hugh Tuffen (Lancaster University), Dr. Pierre Beck (University of Grenoble), Mark Tobin (Australian Synchrotron, and Michael C. Martin (LBNL-ALS) on quantifying the rates of crystallization and vesiculation based on measurement of H2O concentrations in effusive obsidians from various locales. We have discovered that water concentration gradients exist around bubbles and spherulitic crystals in obsidian. We have compared these measurements with numerical models for H2O diffusion in rhyolite melt, and determined both vesiculation and crystallization timescales. This information has allowed us to evaluate the hazardous explosive activity evident in many lava domes and flows.