Volumetric observations during paroxysmal eruptions at Mount Etna: pressurized drainage of a shallow chamber or pulsed supply?

The October 17 to November 5, 1999, eruption of Mount Etna’s Bocca Nuova crater emplaced a ∼15×106 m3 flow field. The eruption was characterized by 11 paroxysmal events during which intense Strombolian and lava fountain activity fed vigorous channelized ′a′a flows at eruption rates of up to 120 m3 s−1. Each paroxysm lasted between 75 and 450 min, and was separated by periods of less intense Strombolian activity and less vigorous (<10 m3 s−1) effusion. Ground-based, satellite- and model-derived volumetric data show that the eruption was characterized by two periods during which eruption rates and cumulative volume showed exponential decay. This is consistent with a scenario whereby the system was depressurized during the first eruptive period (October 17–23), repressurized during an October 24 pause, and then depressurized again during the second period (October 25–28). The imbalance between the erupted and supplied volumes mean that the two periods involved the collection of 1.5–5.7×106 m3 and 1.2–3.6×106 m3, respectively, or an increase in the time-averaged supply to 11.6–13.6 m3 s−1 and 12.5–14.9 m3 s−1. Two models are consistent with the observed episodic fountaining, derived volumetric trends and calculated volume imbalance: a magma collection model and a pulsed supply model. In the former case, depressurization of a shallow reservoir cause the observed volumetric trends and foam collapse at the reservoir roof powers fountaining. In the pulsing case, variations in magma flux account for pressurization–depressurization and supply the excess volume. Increases in rise rate and volatile flux, coupled with rapid exsolution during ascent, trigger fountaining. Limiting equations that define critical foam layer volumes and magma rise rates necessary for Hawaiian-style fountaining favor the latter model.