A MODEL FOR THE OPENING OF EPHEMERAL VENTS IN A STATIONARY LAVA FLOW

A model is proposed for the opening of ephemeral vents in the solid fr ont of a stationary lava flow. The front is considered to be a viscoel astic shell which deforms due to the pressure exerted on it by the inn er fluid lava. The vent opens when the normal stress in the front over comes the tensile strength of solid lava. Cooling of the front is cons idered and the increase in crust thickness, as the isothermal surface at the solidus temperature deepens into the lava body, is calculated. The thermal effect is negligible, if the timescale for the opening of ephemeral vents is on the order of days, as is often observed. Conside ring stationary flow lengths of a few hundred meters, this timescale c orresponds to an average viscosity of the crust of 10(12) Pas, assumin g reasonable values of the crust strength and initial thickness. Obser vation of ephemeral vents on Mount Etna is found to be consistent with the model predictions.