Emplacement dynamics and timescale of a Holocene flow from the Cima Volcanic Field (CA): Insights from rheology and morphology

Abstract

We present a rheological and morphological study of a Holocene lava flow emitted by a monogenetic cinder cone in the Cima Volcanic Field, eastern California. Our field observations focused on surface morphology, which transitions from smooth core extrusions near the vent to jagged ‘a’ā blocks over the majority of the flow, and on channel and levée dimensions. We collected airborne photogrammetry data and used it to generate a digital elevation model. From this, the total flow volume was estimated and surface roughness was quantified in terms of standard deviation of the real surface (5cm resolution) from the software-generated 1m-average plane. Sample textural analyses revealed that the near-vent portion of the flow is significantly more crystalline (ϕxtal=0.95±0.04) than the main flow body (ϕxtal=0.66±0.11). The rheology of Cima lavas was determined experimentally by concentric cylinder viscometry between 1550°C and 1160°C, including the first subliquidus rheology measurements for a continental intraplate trachybasaltic lava. The experimentally determined effective viscosity increases from 54Pa·s to 1361Pa·s during cooling from the liquidus (~1230°C) to 1160°C, where crystal fraction is 0.11. The lava viscosity over this range is still lower than most basaltic melts, due to the high alkali content of Cima lavas (~6wt% Na2O+K2O). Monte Carlo simulations were used to account for and propagate experimental uncertainties, and to determine which rheological model (Bingham, power law, or Herschel-Bulkley) provides the best-fit of the obtained rheological data. Results suggest that Bingham and Herschel-Bulkley models are statistically indistinguishable from each other, and that both fit the data better than a power law model. By combining field observations and experimental results, we reconstructed the eruption temperature and few days-long emplacement history of the Cima flow.