Rheology and Morphology of a Trachybasaltic Lava Flow: a Case Study from the Cima Volcanic Field (CA)

Abstract:

Subliquidus rheology of a trachybasaltic lava was measured in the laboratory for the first time. Field observations of the parent flow focused on surface morphology characterization, which was later quantified in terms of surface roughness. The studied lava flow was emitted during the Holocene by a monogenetic cinder cone in the Cima Volcanic Field (CA). Surface morphology transitions from smooth pahoehoe ropes near the vent to jagged `a`a blocks over the majority of the flow. A variety of 2 m² outcrops were photographed using a hand-held DSLR camera, and their surface texture was reconstructed with photogrammetry. The roughness of each outcrop, effectively described by the standard deviation between the real photogrammetric point cloud and the best-fitting surface, was quantified at different spatial scales, ranging from 0.5 cm to 200 m. We found that the roughness of the flow increases linearly as spatial resolution decreases, with a slope break corresponding to the average size of the outcrop lava blocks. The rheology of Cima lavas was determined by concentric cylinder viscometry in the 1220 °C to 1160 °C temperature range. The obtained rheological flow curves indicate a Bingham rheology, with clearly detected yield strength ranging from 25 Pa at the higher temperatures up to 650 Pa at the lower temperatures. Plagioclase crystallization begins at 1170 °C, likely playing a key role in promoting yield strength escalation. Viscosity increases by one order of magnitude (from 94 to 1116 Pa·s) over the 60 °C span of cooling considered, remaining consistently lower than most basaltic melts due to the high alkali content (6 wt%). The rheological and morphological results are being integrated, in order to assess if it is possible to identify the rheological fingerprint of the active flow on the preserved flow morphology. The composition-dependence of the morphological pahoehoe to `a`a transition in a rheological map is being assessed by comparing our results to literature data on Mauna Ulu. Preliminary data suggest that such transition occurs at lower temperatures for trachybasalts than it does for basalts.