General patterns of flow field growth: Aa and blocky lavas

Abstract

Lava flow fields consist of one or more flows. Four ideal emplacement regimes are recognized: (a) that for single flows and (b) that for flow fields dominated by (1) widening, (2) thickening, or (3) lengthening, as a result of generating new flows. Most aa and blocky lavas belong to the flow field widening or single‐flow regimes. These two regimes are analyzed assuming advance is controlled by the distal core of a flow, where motion is treated as steady, uniform, and laminar. Because of low deformation rates, the distal core is also approximated to a Newtonian fluid. Widening and, possibly, lengthening are ultimately limited by crustal resistance. After a critical cooling interval, new flows are generated from the upper reaches of the flow field. A simple relation is derived linking flow field dimensions and underlying slope to eruption duration, independent of terms involving gravity or lava chemistry and rheology. The relation well describes field data from several volcanoes (involving lava compositions from K phonolitic tephrite to dacite). This supports the premise that the overall growth of aa and blocky flow fields is systematic and also suggests that such growth may be predictable at time scales greater than, or similar to, the emplacement times of major flows.