Textural, thermal, and topographic constraints on lava flow system structure: the December 2010 eruption of Piton de la Fournaise

Abstract

In this study, we examine the channel-fed ‘a‘ā lava flow system that was emplaced during a very short (less than 15 h long) eruption at Piton de la Fournaise (La Réunion) in December 2010. The system had four branches, the longest of which was 1100 m long. Three branches were emplaced over a smooth-surfaced pāhoehoe flow field with a vertical relief of 1–2 m and did not undergo burial by subsequent events. The fourth branch erupted from the same vent as the 1957 eruption and re-used the pre-existing channels of that eruption. In the proximal–medial sections of the three systems that were unconfined, we identified channelized flow sections that were characterized by the presence of either a single channel or multiple braided channels. These fed short (30–260 m long) zones of dispersed flow in the distal sections. We subsequently investigated the role of lava rheology (as controlled by downflow variations in crystal and bubble content) and pre-existing topography in triggering the transitions between single-channel and braided channel flow sections. Crystal content was 10 to 70 vol% and vesicle content was 18 to 55 vol%; cooling rates over distance (derived from glass chemistry) were 11 °C/km to 27 °C/km. However, downflow textural and thermal evolution appeared to neither affect, nor be affected by, whether the channel was single or braided. Instead, the channel network architecture could be related to even modest underlying slope variations. Here, a slope increase resulted in channel confluence, and a slope decrease resulted in channel bifurcation. This process was reversible, in that downflow slope variation could drive the channel network architecture to switch back and forth between a single channel and multiple braided channels several times along its length. Dispersed flow is always present immediately behind the flow front, irrespective of underlying topography. Three previous studies of basaltic lava flows found that steeper slopes favored braided channels, the opposite of what was observed here. We suggest that the underlying substrate and lava type may exert a control on this behavior, but further studies remain necessary.