What causes some eruptions to be more explosive than others? Is it the
total driving gas fuel, or how fast the gas escapes? This talk examines
both the volatile content and the speed of magma ascent immediately
prior to eruption. Chemical zonation preserved inside glass pockets and
crystals provides some of the fastest clocks in geology. These
timescales of chemical diffusion operate over minutes to hours in the
run-up to eruption.