Mount Etna and the 1971 eruption - Lengths of lava flows

Abstract

The principal factor influencing the length of a lava flow is the rate of effusion. With a high rate the lava flows rapidly from the source and tends to form an extensive and far-reaching flow which is simple in character (i.e. made of a single flow unit). With a low rate the lava tends to pile up layer upon layer to form a local accumulation of limited lateral extent near the source, and this accumulation is strongly compound in character (i.e. divisible into flow units). The initial viscosity affects the length indirectly by controlling the thickness of the extrusion, and this thickness control is capable of accounting for the fact that the median length of low-viscosity basaltic extrusions is 3.2 times that of high-viscosity andesite, trachyte and rhyolite ones. Other factors, such as the local topography, are thought to be relatively unimportant, an exception being when lava is ponded in a topographic depression. Measurement of the rate of effusion may be critical in any attempt to predict the distance that a lava flow will travel, such as the one which threatened Fornazzo and other towns and villages on Etna in 1971.