THE INITIAL COOLING OF PAHOEHOE FLOW LOBES

In this paper we describe a new thermal model for the initial cooling of pahoehoe lava flows. The accurate modeling of this initial cooling is important for understanding the formation of the distinctive surfac e textures on pahoehoe lava flows as well as being the first step in m odeling such key pahoehoe emplacement processes as lava flow inflation and lava tube formation. This model is constructed from the physical phenomena observed to control the initial cooling of pahoehoe flows an d is not an empirical fit to field data. We find that the only signifi cant processes are (a) heat loss by thermal radiation, (b) heat loss b y atmospheric convection, (c) heat transport within the flow by conduc tion with temperature and porosity-dependent thermal properties, and ( d) the release of latent heat during crystallization. The numerical mo del is better able to reproduce field measurements made in Hawai'i bet ween 1989 and 1993 than other published thermal models. By adjusting o ne parameter at a time, the ef feet of each of the input parameters on the cooling rate was determined. We show that: (a) the surfaces of po rous flows cool more quickly than the surfaces of dense flows, (b) the surface cooling is very sensitive to the efficiency of atmospheric co nvective cooling, and (c) changes in the glass forming tendency of the lava may have observable petrographic and thermal signatures, These m odel results provide a quantitative explanation for the recently obser ved relationship between the surface cooling rate of pahoehoe lobes an d the porosity of those lobes (Jones 1992, 1993). The predicted sensit ivity of cooling to atmospheric convection suggests a simple field exp eriment for verification, and the model provides a tool to begin studi es of the dynamic crystallization of real lavas, Future versions of th e model can also be made applicable to extraterrestrial, submarine, si licic, and pyroclastic flows.