Calculated effect of vesicles on the thermal properties of cooling basaltic lava flows

Abstract

This paper investigates the effects of porosity, in the form of vesicles and bubbles, on the transfer of heat within basaltic lava. This study was undertaken to provide input for realistic cooling models of basaltic lava flows and to help explain some recent field measurements by Jones (1992, 1993). These field observations indicate that the surfaces of vesicular pahoehoe flows cool significantly more rapidly than that of dense flows. It has been suggested that thermal radiation across vesicles is responsible for this enhanced cooling rate. It is shown here that, for vesicles in typical pahoehoe flows, radiation across vesicles may enhance the effective thermal conductivity and thermal diffusivity, but only at high temperatures ( T > 800°C) and high vesicularities ( φ > 40%). It is also found that convection of the gas phase within bubbles will not occur unless the cavities are larger than about 1 cm. Furthermore, previous work has demonstrated that porosity greatly reduces the thermal conductivity and thermal inertia of cold lava. It is shown here that this should also be true at high temperatures. Examining only radiation across vesicles severely understates the effect of porosity on the cooling of the surface of lava flows. Thermal inertia, developed to describe diurnal surface temperature variations, is the most appropriate thermal parameter in this case. Thermal inertia is a measure of how quickly surface temperature responds to heating or cooling, with low thermal inertia materials responding more quickly. It is shown here that porosity greatly reduces thermal inertia at all temperatures and that this provides a more general explanation for the field observation.