Topographic Constraints on Lava Flow Patterns: the December 2010 Eruption of Piton de La Fournaise (La Réunion)

The December 2010 lava flow of Piton de La Fournaise consists of four separate branches, all fed by the same fissure over about 15 hours, but then flowing in slightly different directions. This provided us a unique chance to address the long-standing question of how pre-existing topography controls lava flow patterns. Each flow branch was mapped and sampled from vent to toe. The morphology of all branches can be described using a combination of three basic cross-section types (perpendicular to flow direction): (1) single, stable channel; (2) unstable, braided channels; and (3) dispersed flow. These flow morphologies depend on pre-existing underlying topography, so that a steep slope results in a stable channel, while a gentle slope results in a braided channel. Downflow slope variations can drive the flow morphology to switch back and forth between a single stable channel and multiple braided channels several times along its length. However, in our volume-limited study case, if slope falls below a critical threshold the flow becomes dispersed and stops shortly thereafter. The exact transition values from stable to braided channel and vice-versa depend on parameters other than topography, such as magma effusion rate, as illustrated by the comparative analyses of the different flow branches. The findings of this study allow us to interpret and explain the observed flow patterns on the basis of pre-existing volcano topography and effusion rates estimates.