A LABORATORY MODEL OF SURFACE CRUST FORMATION AND DISRUPTION ON LAVA FLOWS THROUGH NON-UNIFORM CHANNELS

Abstract

Crust formation on basaltic lava flows dictates conditions of both flow cooling and emplacement. For this reason, flow histories are dramatically different depending on whether lava is transported through enclosed lava tubes or through open channels. Recent analog experiments in straight uniform channels (Griffiths et al. J Fluid Mech 496:33–62, 2003) have demonstrated that tube flow, dictated by a stationary surface crust, can be distinguished from a mobile crust regime, where a central solid crust is separated from channel walls by crust-free shear zones, by a simple dimensionless parameter ϑ, such that ϑ<25 produces tube flow and ϑ>25 describes the mobile crust regime. ϑ combines a previously determined parameter ψ, which describes the balance between the formation rate of surface solid and the shear strain that disrupts the solid crust, with the effects of thermal convection (described by the Rayleigh number Ra). Here we explore ways in which ϑ can be used to describe the behavior of basaltic lava channels. To do this we have extended the experimental approach to examine the effects of channel irregularities (expansions, contractions, sinuosity, and bottom roughness) on crust formation and disruption. We find that such changes affect local flow behavior and can thus change channel values of ϑ. For example, gradual widening of a channel results in a decrease in flow velocity that causes a decrease in ϑ and may allow a down-flow transition from the mobile crust to the tube regime. In contrast, narrowing of the channel causes an increase in flow velocity (increasing ϑ), thus inhibiting tube formation. We also quantify the fraction of surface covered by crust in the mobile crust regime. In shallow channels, variations in crust width (d c) with channel width (W) are predicted to follow d c~W 5/3. Analysis of channelized lava flows in Hawaii shows crustal coverage consistent with this theoretical result along gradually widening or narrowing channel reaches. An additional control on crustal coverage in both laboratory and basaltic flows is disruption of surface crust because of flow acceleration through constrictions, around bends, and over breaks in slope. Crustal breakage increases local rates of cooling and may cause local blockage of the channel, if crusts rotate and jam in narrow channel reaches. Together these observations illustrate the importance of both flow conditions and channel geometry on surface crust development and thus, by extension, on rates and mechanisms of flow cooling. Moreover, we note that this type of analysis could be easily extended through combined use of FLIR and LiDAR imaging to measure crustal coverage and channel geometry directly.