Gravity instability in the Holocene Big and Little Glass Mountain rhyolitic obsidian flows, northern California

Abstract

Field relations on the surfaces of the Holocene Big and Little Glass Mountain rhyolitic obsidian flows in northern California suggest that regularly spaced areas of coarsely vesicular pumice are diapirs which rose from the base of the flow in response to a density inversion inherent in the flow stratigraphy. The flows contain three lithologic units, defined on the bases of color, texture, and specific gravity, which are arranged in the following vertical sequence: a basal layer of coarsely vesicular, brownish grey pumice ( ρ∼- 0.8 to 2.0), a central core of black, glassy obsidian ( ρ ∼- 2.2), and a surface crust of white to pinkish grey, finely vesicular pumice ( ρ ∼- 2.0). The flows were modeled as multilayered fluid systems with unstable density stratifications, and the spacings of coarse pumice diapirs were equated with the dominant wavelengths of viscous folding theory. Wavelengths of 45 m, 56 m, and 104 m were calculated using measurements of unit thicknesses and estimates of viscosity ratios for the three lobes. Dome spacings of 43 m, 70 m, and 60 m were measured on the same three flow lobes. Discrepancies between predicted and measured spacings are attributed to local variations in thicknesses and viscosities of the flow units. Elongation and surface folding of diapirs are cited as evidence that they emerged before the flows stopped moving.