The Interpretation of Lava Flow Morphology

Abstract

Summary It is postulated that lavas are non-Newtonian liquids with a yield stress and that it is the yield stress which determines flow dimensions. An appropriate theory was developed for the unconfined flow of ideal Bingham liquids on inclined planes. The occurrence of structures similar to IevCes on lava flows was predicted. The theory was verified by laboratory measurements on flows of suspensions of kaolin. These flows showed similarities to lava flows. Data from lava flows was also found to be in general agreement with the theory which was then used to interpret the shapes of two lunar lava flows. It was possible to estimate yield stresses and flow rates for these lavas. 1. Effects limiting the flow of lava Lava flows show great variations in size, shape and surface features. The final form of a flow must be determined by the physical properties of the lava, its temperature and rate of extrusion and local conditions such as gravitational field strength and topography. The aim of the work presented in this paper is to isolate the parameters which have the greatest effect on flow morphology and to elucidate the relationship between the conditions at the start of a flow and the final form of the flow. At present there is no detailed knowledge of this kind but, were it available, the value of air- and satellite-photographs of lava flows would be greatly enhanced. The hypothesis on which this work is based is that flowing lava is a non-Newtonian liquid and it is its non-Newtonian properties which are mainly responsible for the shapes of flows. If lava were an ideal Newtonian liquid it would flow downhill and would continue to flow even after the supply at the vent had ceased until it ponded in a depression. Furthermore the flow would spread laterally until it was restricted by topography or until surface tension prevented spreading by which time it would be extremely thin. Observations show that lava does not behave like this. Commonly it comes to rest on a slope as soon as the supply of fresh lava ceases and many flow fronts are high and steep although unconfined by topographic features. It is clear that there is some process which limits the flow of lava, brings it to rest on slopes and prevents its lateral spreading. The most obvious and apparently generally accepted process is the solidification of lava due to cooling. For this to be a feasible process the time taken for the development of a strong enough skin to prevent lateral motion must be realistic. The solid skin of a flow experiences an outward force due to the hydrostatic pressure of the lava and this must be balanced by an inward force which is the result of tension in the curved skin. If the skin can withstand the tension the lava will not flow laterally.