Lava Flows on Slopes: Numerical Modeling of Flow Transitions and Collapse

Lava flow models based on morphology and surface structures are easily applied to planetary images because the structures involved can have dimensions on the order of tens of meters to kilometers. Previous laboratory work has demonstrated that most of the common structures found on lava flows form as part of a continuum of morphological regimes, depending only on the growth rate of solid surface crust. This in turn depends on the relative magnitudes of effusion rate and cooling rate, expressed as a single dimensionless number, Ψ , both for natural lavas and for laboratory simulations. Recent laboratory studies show that increasing the underlying slope has a similar morphologic effect as raising the eruption rate. As underlying slope increases, lava flows are also more susceptible to flow front collapse caused by oversteepening of flow margins, leading to pyroclastic flows and block-and-ash flow deposits. Certain deposits around venusian pancake domes have been interpreted as having this sort of explosive origin, and recent analyses suggest that explosive volcanism and pyroclastic flows may be more common on Mars than previously thought. The goal of this research is to understand the effects of slope on lava flow stress fields, how these stresses influence transitions between morphologic regimes, and what factors control flow phenomena such as flow front collapse. We have created preliminary two-dimensional elastic finite element models of lava flows on slopes. For given material properties and flow shapes, we can calculate the distribution of stress as a function of slope in order to determine where the lava flow is most likely to fracture, to identify potential failure surface orientations, and to determine where new material may extrude. We use laboratory observations of changing flow geometry at specific times or volume increments to consider "snapshots" of stress development within a growing flow on a fixed slope. This approach may also be applied to natural examples, where measurements of volcanic dome growth and collapse provide an opportunity for comparison with and verification of laboratory and numerical work. We are currently developing more complex models that incorporate thermal effects and the formation of a brittle shell around a viscous fluid interior to more realistically simulate lava flow dynamics.