Orientation distribution of microlites in obsidian

The shape and three-dimensional orientation distribution of microlites are measured in obsidian from Little Glass Mountain, CA. Measurements are made from thin sections using an image series of high magnification digital micrographs taken serially through different focal depths. These measurements agree well with the theoretically predicted and experimentally measured distribution of long slender rods in a Newtonian fluid undergoing simple shear flow. In this type of flow, rods in a dilute suspension rotate periodically, spending most of the time aligned with the flow. Measurements of the detailed orientation distribution integrated with theoretical models provide a tool for inferring flow dynamics and the timing of magmatic processes.