A Mouthful of Dirt: Feedbacks Between the Presence and Nature of In-Vent Debris and the Dynamics of Strombolian Explosions

Abstract:

Textural, modelling and observational evidences indicate the presence of loose to molten debris capping the magma column in the conduit of Strombolian-type volcanoes. This debris is thought to have a role in controlling the release of gas during one explosion and the amount and grain size of the eruption products. Here we report direct observations on the behaviour of debris covering the eruptive vents during Strombolian explosions at Stromboli volcano, Italy. During several field campaigns from 2008 onwards, the eruptive vents were in direct view from our observation point, and volcanic activity was recorded using a variety of video equipment, including high-speed and thermal cameras. Debris-covered vents were observed in several cases. In such cases the explosion was preceded by the gradual, time-exponential bulging of the debris cover, followed, in some cases, by a deflation and in-vent collapse of the debris. The amount and grain size distribution of the debris control the eruptive behaviour. Often, blocks and lapilli may not be entirely removed by explosions, which may occur through the debris and around the larger blocks. In such cases the explosions tend to erupt relatively cold (non-plastic) bombs and lapilli, and minor amounts of ash. Finer-grained debris covers are entirely removed by explosions, and can be entrained in the explosions and collapse separately to form small pyroclastic density currents. The ensuing explosions are invariably ash-rich. Both fine and coarse debris is observed to fall back in the vent after each explosion but also to gravitationally accumulate in between the explosions from the seep crater flanks. In contrast, open-vent eruptions tend to erupt hotter (plastically deforming) pyroclasts at a higher exit velocity.