VESICULARITY, CRYSTALLINITY, AND IMPLICATIONS FOR RHEOLOGY OF THE KĪLAUEA 2018 LAVA FLOWS

The 2018 Kīlauea lava flows were among the best documented in history, with extensive ground-based observation and Uncrewed Aerial Systems (UAS) overflights. This wealth of data allows for the exploration of temporal and spatial evolution of lava flows of varying vesicularities and crystallinities in both syn- and post-eruptive contexts. To investigate the impact of these on rheology and flow emplacement, we sampled locations with syn-eruptive observations of emplacement along the largest and longest-lived fissure 8 flow field where overflows, squeeze-outs, and breakouts had quenched and preserved flow textures.

Preservation of flow textures is especially important in highly vesicular flows, as was the case with fissure 8’s, which erupted lavas with vesicularities up to ~86%. We collected samples from 22 localities along the fissure 8 flow field, comprising a range of emplacement types, transport times, and distances from the vent. These show variations in volume fraction, shape, and size distribution of vesicles and crystals. Main vesicle textures observed include sub-equant polygonal and collapsed polygonal meshes, spherical networks, and gashes. Some samples show a marked difference in vesicularity from crust to lava flow interior, while others show alternating vesicle-rich and vesicle-poor bands. This range in vesicle textures is not only related to transport distance and pāhoehoe vs. ‘a’ā, but also to emplacement context. For example, pāhoehoe overflows contained multiple different textures and vesicularities, regardless of distance from the vent. Pāhoehoe squeeze-outs ~0.5 km from the vent contain 80-86% vesicles in a sub-equant polygonal texture while thicker, sheet flows the same distance away display collapsed polygonal meshes of ~60% vesicles.

Nondeformable (crystal) and pseudo-deformable (vesicle) cargo affects the rheology of the flow in a manner that depends on the capillary number, Ca, the ratio of shear stresses and surface tension acting upon bubbles. The change in vesicle textures represented by the Ca can increase or decrease the bulk viscosity. Combining syn-eruptive video with textural measurements, rheological experiments, and numerical models will allow us to better constrain the rheology and hazards of these, and other large, fast moving basaltic lava flows.