Rheology of Pahoehoe Lava Surfaces Across Their Glass Transition: Preliminary Results on Thermal Histories and Melt Viscosities, Pu'u 'O'o-Kupaianaha Eruption, Hawaii

Glassy pahoehoe lava flow surfaces from the ongoing Pu'u 'O'o-Kupaianaha eruption on Hawaii are being analyzed using relaxation geospeedometry to determine their thermal histories upon vitrification. This technique is based on a calorimetric quantification of the relaxation process of enthalpy that allows modeling of the thermal path involved in cooling a liquid across its glass transition to form a glass. We are investigating various pahoehoe flow morphologies including spongy (S-type), pipe- vesicle bearing (P-type) and "blue-glassy" sub-types, collected over the last ten years of the eruption. Initial results on S-type pahoehoe surface glass transition temperatures (Tg) show minor variations around 670 deg C. Tg is taken as the temperature of the peak of the heat capacity trace determined during reheating the vitrophyres at constant pressure. Cooling rates modeled across the glass transition interval for S-type samples vary between 10 and 40 K/min. These rates are linear approximations of the thermal history of the flow surfaces during their cooling across their glass transition intervals. The intervals cover a temperature range of approximately 100 K to temperatures as low as 630 deg C, where the melt structure finally "freezes in" to form the glass. Ultimately, these limiting temperatures mark the transition from ductile to brittle behavior of the flow surface. A recently developed model based on the equivalence of the activation energies for enthalpy relaxation and viscous flow allows us to predict melt viscosities for these samples. The model gives melt viscosities for S-type pahoehoe flow surfaces of log₁₀ 10.2 +/- 0.3 Pa s at their glass transition. The cooling rates determined are in excellent agreement with thermal data obtained directly at active flows during the eruption via remote sensing techniques. This agreement provides validation of our technique and a case for a comprehensive study on the cooling processes involved in generating pahoehoe flow fields on Hawaii. We hope to be able to correlate individual thermal histories with specific flow morphologies in order to constrain rheological processes such as in-situ surface vesiculation and lava lobe inflation. Ultimately, our results will provide valuable input parameters for computational flow propagation models employed for hazard assessment and risk mitigation on Hawaii and elsewhere.