Surface cooling, advection and the development of different surface textures on active lavas on Kilauea, Hawai'i.

Ball, M. and Pinkerton, Harry and Harris, A. J. L. (2008) Surface cooling, advection and the development of different surface textures on active lavas on Kilauea, Hawai'i. Journal of Volcanology and Geothermal Research, 173 (1-2). pp. 148-156. ISSN 0377-0273

Abstract

The validity of lava flow cooling models were investigated using data from a high resolution, high frame rate thermal imaging camera on Kilauea, Hawai'i during August 2004. Once the surface temperature data for small lava flow and lobes were corrected for emissivity, viewing distance, humidity and viewing angle, it was clear that their upper surfaces were thermally stratified on eruption. The interior of a stationary lava lobe was exposed by removing the crust, and its cooling rate was measured at 30 Hz. For this flow, the measured cooling trend was successfully modelled using a finite difference cooling model. However, modelled and measured cooling rates of a pahoehoe lobe and a small channelised lava flow were only in agreement until the crust began to deform. The departure from the modelled trend at this point is attributed to two factors. One is the complex cooling regime of a highly deformed ropy crust in which exposed lava at the crests of the ropes radiates in all directions compared with lava in the troughs where radiative cooling is less effective. The second, and more important, reason for the difference is the reduced flow rate of the crust once it begins to be compressed. The underlying visco-elastic zone beneath the crust flows more rapidly than the crust and consequently advective heat flow significantly reduces cooling rates. Variations in heat loss from different flow textures and the incorporation of different advective heat components is clearly important in future flow models. One of the critical parameters in such models is the temperature at which different surface textures develop. Temperature profiles along lobes and flows also revealed that rope textures developed on pahoehoe lavas when temperatures were in the range of 700–800 °C, brittle deformation of pahoehoe crust occurred when temperature dropped below 700 °C, and small pahoehoe lobes stopped advancing at temperatures of ~ 600–650 °C. It is important to recognise that the flow front temperatures at which these flows stop advancing will be significantly greater than for larger flows on similar gradients. The higher internal and basal shear stresses of large lava flows will allow the flow to advance until the visco-elastic layer at the front of a front is significantly cooler than the temperatures recorded on these small flows on Kilauea.