Fracturing as a quantitative indicator of lava flow dynamics

Abstract

The traditional classification of lava flows into pahoehoe, aa and blocky varieties reflects differences in how a flow can fracture its surface during advance. Pahoehoe and aa lavas have a low strength upon eruption and require surface cooling to produce a crust that can fracture. Among pahoehoe lavas, applied stresses are small enough to allow the growth of a continuous crust, which is broken intermittently as the flow advances by propagating a collection of lava tongues. Among aa lavas, in contrast, applied stresses are large enough to maintain persistent crustal failure. Blocky lavas are distinguished by having a significant strength upon eruption, so that motion can induce surface fracturing even without cooling. The transition from pahoehoe to aa thus coincides with a change from intermittent to persistent failure of chilling crust, whereas the transition from aa to blocky lava occurs when lava beneath the crust can also autobrecciate. These fracturing characteristics have been used to quantify the transitions between flow regimes and suggest that shear fracture may dominate tensile failure. They also (1) explain the restricted variations with slope of pahoehoe and aa flow thickness, as well as the maximum advance rate for which pahoehoe can form, equivalent to the minimum rate required for aa lava, and (2) provide a revised method for forecasting the maximum potential lengths of aa flows. Applications include improved hazard assessments during effusive eruptions and new evaluations of the emplacement conditions for very large-volume pahoehoe lava flows.