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 micr ographs taken serially through different focal depths. These measurements a gree well with the theoretically predicted and experimentally measured dist ribution 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 Row. Measurements of the detai led orientation distribution integrated with theoretical models provide a t ool for inferring flow dynamics and the timing of magmatic processes. (C) 1 998 Elsevier Science B.V. All rights reserved.