Textural characterization of the pāhoeho-'a'a transition in Hawaiian basalt

Abstract

Pāhoehoe and ‘a‘ā are the most common surface morphologies in basaltic lava flows, yet no predictive models exist to link physical parameters of flow emplacement to changes in their surface textures and rheological properties. We have characterized changes of vesicularity, vesicle deformation, and crystallinity across the pāhoehoe–‘a‘ā transition as preserved in two different eruptions, a brief, high-velocity effusion of lava from Kīlauea Volcano on 1 February 1996, and a small breakout from an ephemeral vent within a larger channel produced during the 1868 eruption of Mauna Loa. This allowed us to compare conditions leading to the pāhoehoe–‘a‘ā transition for both open channel flow (Kīlauea 1996) and reactivation of lava from an ephemeral vent (Mauna Loa 1868). Textural changes across the transition include (a) decrease in vesicularity and vesicle number density, (b) increase in microlite crystallinity and crystal number density, and (c) increase in vesicle deformation. The results support past qualitative descriptions of the transition and highlight the importance of plagioclase crystallinity in controlling lava rheology and surface morphology.