Identification and modelling of variable convective regimes at Erta Ale lava lake

Abstract

Between February 17 and 19, 2002, we collected a combined thermal and seismic data set for studying the persistent lava lake activity at Erta Ale volcano, Ethiopia. These data indicate that the lake cycles between periods characterized by low (~0.05 m s -1 ) and high (~0.2 m s -1 ) rates of convection, respectively. We use our measurements to constrain two models to explain such convection cycles. The first model relates variable convection rates to pulses in the rate at which magma is supplied to the lake. This model requires supply rates to cycle between high convection rate phases fed by a magma volume flux of 0.2 m 3 s -1 with a viscosity of 140 Pa s, and low convection rate phases fed by a magma volume flux of 0.03 m 3 s -1 with a viscosity of 3100-3600 Pa s. The second model assumes that supply to the lake is steady and that cyclic convection is set up by the generation of convective instabilities within the lake. In this case, cooling of the surface layer generates a slow moving, viscous convection cell at the lake surface overlying a faster moving convection cell of lower viscosity. Recharge of the lower cell, at the expense of draining degassed magma, increases the buoyancy of the lower layer to eventually trigger an overturn event. At this point the surface of the low viscosity cell extends to the lake surface and the high viscosity cell sinks to be drained from the lake