Precursors and transitions in explosive activity at Stromboli (Aeolian Islands) and implications for shallow vent processes

Abstract:

Explosive activity at Stromboli (Italy), believed to be caused by the burst of gas pockets through a shallow lava surface in the volcanic conduit, can be classified by the frequency and the amount and size of pyroclasts ejected into the atmosphere. While frequent (>1000 per hour), weak gas bursts (usually without the ejection of pyroclasts) define puffing activity, classical Strombolian explosions (1-10 per hour) can either erupt dominantly lapilli- to bomb-sized pyroclasts at initial velocities up to 400 m/s or be ash-rich. The latter are usually less energetic, and occur at less active vents.

To better constrain the controls on the different eruption styles, we collected a large database of thermal infrared and visible-light highspeed videos representative of twelve vents over three field campaigns in 2012, 2013 and 2014. Puffing occurred in eight cases, at rates between 0.4 and 1.5 Hz, ejection velocities of ~10 m/s, and temperature anomalies from 1 to 500K. In two cases, the puffing activity was associated with the ejection of pyroclasts at intermediate velocities (15–30 m/s). Strombolian explosions occurred in six cases, two of them simultaneously presenting a puffing activity, at rates never exceeding 10 per hour and ejection velocities up to 200 m/s.

In some cases, transient phenomena preceded the classical Strombolian explosions by 1 to 10 seconds. Before ash-rich explosions, both inflation of the visible vent surface and the release of hot gas at the center or the edge of the crater can be observed. Ash-poor classical explosions that occur on persistently puffing vents are preceded by a significant increase in the volume and the apparent temperature of the gas pockets. We relate these observations in both cases to the presence of a high-viscosity plug at the top of the magma column, which modulates the rise, accumulation, and release of the gas pockets. We hypothesize that the contrasts between the various kinds of Strombolian explosions and their associated precursors may arise by the thickness and nature of the plug.