Lava flow shapes and dimensions as reflections of magma system conditions

Abstract

The physical conditions producing variation in the shapes of lava extrusions, from thin flows to domes to spines, are investigated with focus on the influence of the lava thickness. The accumulation of lava above the vent acts to increase the vent pressure and reduce the eruption rate. In turn, the eruption rate determines how the volume and thickness of erupted lava change with time. The spreading of lava and the flow of magma from a deep reservoir through a conduit system are therefore coupled. A fluid dynamical model is developed to evaluate the influence of magma chamber and conduit dimensions, chamber overpressure, magma viscosity and dome bulk viscosity on the eruption behaviour and dimensions of the lava flow. The model assumes laminar flow of Newtonian, constant-density magma from an overpressured reservoir with elastic walls, up a vertical cylindrical conduit, and onto a planar horizontal surface, on which grows a radially symmetrical lava flow of constant bulk viscosity greater than the magma viscosity. Two dimensionless numbers characterize the resulting behaviour: the ratio of a scale erupted lava volume to the total volume available for eruption, and the ratio of the pressure due to a scale thickness of lava to the magma chamber overpressure. The dimensionless numbers are used to map the flow behaviours and reveal two extreme extrusion regimes separated by a narrow transition zone. One extreme corresponds to thin and voluminous flows, and occurs for large, deep, strongly overpressured magma reservoirs, narrow conduits and low viscosity contrasts between the magma and lava. The other extreme corresponds to small volume domes and spines, and is produced by shallow, weakly overpressured chambers, wide conduits and high viscosity contrasts. Near the transitional domain, small changes of parameters are sufficient to cause major changes of eruption regime. The model provides explanations for the variation in gross morphology of lava extrusions, transitions between flow, dome and spine eruptions and for long-lived episodic extrusion of lava. Model predictions are consistent with features of the eruptions of Montagne Pelée (1902–1903), Mount St. Helens (1980–1986), and Montserrat (1995–1997 period). The model is used to estimate the magnitude of overpressure driving eruptions and suggests that magma buoyancy is important in driving many terrestrial lava dome eruptions. The model further suggests that thick extrusions on Venus were driven primarily by buoyancy sourced at great depth.