Comparing laboratory and remotely sensed measurements of rheological properties at Obsidian Dome, Inyo Craters, California.

Abstract:

Topographic ridges on the surface of obsidian lavas are believed to form as a result of layer-parallel compressional stresses amplifying subtle surface fold perturbations due to a viscosity contrast between the lava crust and interior. The geometry of surface ridges including wavelength and amplitude, have been used to infer the rheology of obsidian lavas and other planetary landforms using a modification of Biot’s fold theory. Viscosity estimates using fold geometry assume that the viscosity decreases exponentially from the cool crust to the hot interior. Here we test that assumption by comparing crustal and interior viscosity estimates made using ridge geometries identified from LiDAR and unoccupied aerial system (UAS) derived digital elevation models (DEMs) to laboratory measurements of natural obsidian lava samples. Obsidian Dome is a small 600-year-old obsidian lava located in the Inyo Craters, CA. Obsidian Dome has abundant topographic ridges and two scientific drill cores that were collected in the 1980s. Drill cores provide a high resolution suite of samples through the thickness of the upper surface of the lava in two locations, near the vent and near the flow margin. Estimates of crustal and interior viscosities from the folding model were calculated using ridge geometries determined using power-spectrum densities. We then measured the apparent viscosity (η_app), volatile content (X), and thermal history (T-t) of natural samples using a uniaxial parallel-plate viscometer, Fourier-transform infrared spectroscopy (FTIR), and relaxational geospeedometry. The folding model predicts a crustal viscosity of 10^12.7 Pa s and an interior viscosity of 10^10.2 Pa s at a depth of five meters. Assuming an eruption temperature of 792 °C, our experimental measurements suggest that the viscosity does not decrease exponentially as a function of depth, due to differences in expected thermal profiles of the upper surface. Differences as much as 10^2.5 Pa s between viscosity values from the fold theory and experimental measurements are observed. Timing of ridge formation is also constrained and only possible for up to 12 hours after eruption. Our results suggest using fold theory to estimate rheological properties of obsidian lavas and other planetary landforms must be applied with caution.