MEASURING BASALTIC FLOW VISCOSITY FROM CRUSTAL THICKNESS

Cooling of basaltic lava flows quickly produces a glassy crust that dramatically affects the thermal infrared (TIR) emitted energy from its surface. As the crust thickens it also impacts the development and frequency of flow folding, which has been used as a proxy for composition and crustal thickness. To better understand this TIR effect and to develop an approach to remotely monitor crustal growth and viscosity, actively folding and cooling basalt flows at Kilauea, Hawaii have been analyzed using TIR camera data. The TIR data collected in May 2010 were acquired at 30 frames per second in order to capture the details of folding, temperature, and spatial patterns at different stages of cooling. These measurements revealed a distinct trend in wavelength and, by extrapolation, the crust thicknesses and viscosity. When folds first appear, the surface brightness temperature was ~806°C, the fold wavelength was 15.9 mm, and the crust was several microns thick. As the flow continued to cool, the wavelengths and amplitudes became larger and the insulating crust grew thicker. After the surface temperature cooled to ~730°C, the initial folds were incorporated into a larger second generation class of folding and the dominant wavelength increased to 31.8 mm. The ratio of the second generation wavelength to the first generation wavelength indicates that the folding at this initial stage is due to heat loss rather than compression or flow thickening. After the surface temperature reached ~688°C, the crust thickened to the millimeter scale and the multiple fold generations were difficult to distinguish. With this thicker crust, the ratio of the surface viscosity to the interior viscosity became larger. Because this viscosity ratio can be related to crustal thickness and fold wavelength, it therefore becomes possible to determine either the variation in viscosity or the crust thickness as the flow cools. We have performed these measurements for the first time on basaltic pahoehoe toes during the first few minutes following emplacement. These measurements will be used to further constrain the emitted energy from these flows and could be applied to other lava flow compositions, types, locations, and eruptive environments.