The initial development of long lava flows is investigated using simple theory and field evidence. Order-of-magnitude estimates of the evolving thickness and the extending length of lava are obtained by scaling arguments based on the simplification that the bulk structure can be modelled initially as a Newtonian fluid. A scaling analysis suggests that the rate of advance of the leading front evolves primarily due to temporal variations in the effusion rate and minimally due to topography. The apparent viscosity of the bulk flow increases with time at subsequent stages when effects due to cooling become important. Theoretical results are applied to the study of long lava flows that descended on Etna, Kilauea and Lonquimay volcanoes. We determine that lava flows at Kilauea extended initially like a Newtonian fluid with constant viscosity, implying that thermal effects did not significantly influence the dynamic properties of the bulk flow. In contrast, effects due to cooling played a major role throughout the advance of lava flows at Etna and Lonquimay. We show that the increasing length and volume of an active emplacement field can be monitored to estimate its evolving viscosity, which in turn allows the further advance of the lava to be predicted.
- Effusion rate