Influence of volatile loss on thickness and density profiles of active basaltic flow lobes

A bulk density increase due to degassing during emplacement may have a sign ificant influence on the thickness of a lava flow and the rate at which it advances. We present a theoretical model of a lava flow that loses enough v olatiles to cause density changes along the path of the flow. We assume tha t the flow is emplaced as a single, isolated unit and the bulk rheology (e. g., viscosity) is a function of distance from the vent. This type of model is applicable to solitary lobes of basaltic aa and isolated sheets of pahoe hoe that advance as a fluid continuum with bulk lava density changes as a f unction of distance along the flow path. Equations for the flow thickness a nd the bulk density profiles are derived from mass and volume conservation. Formulas are tabulated for thickness and density profiles for various comb inations of flow rates, theologic changes, and degassing rate functions. We also tabulate formulas for estimating parameters associated with the form and rate of degassing from field data. The Mauna Loa 1984 "1 flow" is a typ ical example of a flow showing evidence of a bulk density increase and is u sed to estimate the model parameters. Thickness and density profiles are th en computed for a range of plausible lava densities, two different rate fun ctions for the loss of volatiles, and two different models of viscosity cha nge. Results indicate that the thickness profile of a lava flow can be sign ificantly affected when there is a large difference between the density at the vent and at the flow front. For relatively high rates of degassing, the flow profile has a maximum thickness located progressively closer to the v ent as the rate of degassing increases. For depth-dependent degassing, an i ncrease in viscosity acts to thicken the flow, which increases the rate of degassing, thus mitigating the thickening influence of the increasing visco sity. Degassing while a flow is active can increase the duration of emplace ment by as much as 60%. We find that the flow thickness profiles are sensit ive to the choice of flow rate and the initial density, regardless of the f orm of the degassing function. The nature of the flow rate can significantl y affect the shape of the profile as well as the flow front thickness. When the rate of volatile loss depends on the flow thickness, the density profi le depends explicitly on the way the rheology changes along the flow path. In all cases, density increases during emplacement counter the tendency of a flow to thicken due to increases in viscosity or resistance to flow with distance. Thus the parameters that define the rate of degassing, and the co nsequent density change along the path of a flow, emerge as important varia bles for a quantitative understanding of flow emplacement.