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 front of a stationary lava flow. The front is considered to be a viscoelastic shell which deforms due to the pressure exerted on it by the inner fluid lava. The vent opens when the normal stress in the front overcomes the tensile strength of solid lava. Cooling of the front is considered 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. Considering stationary flow lengths of a few hundred meters, this timescale corresponds to an average viscosity of the crust of 10(12) Pas, assuming reasonable values of the crust strength and initial thickness. Observation of ephemeral vents on Mount Etna is found to be consistent with the model predictions.