LONG VALLEY AND YELLOWSTONE CALDERAS ERUPT STRESSED QUARTZ CRYSTALS

The crystal cargo carried by a volcanic eruption may preserve a record of the forces and stresses that drive eruptions. To test this idea we used synchrotron X-ray microdiffraction at beamline 12.3.2 of the Advanced Light Source, Lawrence Berkeley National Laboratory, to measure crystal deformation in quartz from explosive and effusive eruptions from Yellowstone and Long Valley calderas. All the crystals from each eruption preserve elastic strains that were generated by stresses ranging from 90 to 220 MPa. Crystals from individual eruptions preserve similar residual stresses, marked with consistent means and standard deviations. To isolate the contributing processes and environments that produced the deformation, we analyzed the residual stresses in quartz crystals from eruptions with a range of eruption styles, magnitudes, ages, and volumes. We find that the residual stresses from effusive and explosive eruptions, large and small eruptions, and proximal and distal samples, are similar. The preserved stresses must record subsurface processes, which may be generated in the pre-eruptive reservoir or during eruptive ascent. In magma mushes, crystals impinge upon each other to create force chains between particles. The mean stress generated by this impingement should be equal to the confining pressure. Indeed, we find that the magnitudes of preserved residual stresses in quartz from the Bishop Tuff and Yellowstone eruptions align with petrologic estimates of storage pressures. Alternatively, the residual stresses preserved in the quartz crystals may be an in situ measure of the fragmentation threshold in the conduit. Our measurements of 90 to 220 MPa correspond to experimental limits for the shear stresses associated with failure of rhyolite, which have been shown to be ~100 MPa. Prior to fragmentation, viscous deformation in the conduit produces similar stresses in both the melt and crystals. When those stresses become large enough the melt fragments and the strained crystals preserve a record of the stress. Both explosive and effusive fragmentation events could thus produce the same strain because it is related to the shear stresses required for the brittle failure of viscous, crystal-bearing glass.