Role of pyroclast recycling on style of small basaltic explosions at Stromboli Volcano, Italy, deduced from real-time collected ash samples.

Abstract:

Understandings of controlling mechanism on the style of volcanic explosion is one of the most important subject not only for volcanology but also for hazard mitigation. Vesiculation, outgassing and crystallization of magma, and interaction with external water have been considered as essential factors for the controlling mechanism (e.g., Houghton and Gonnerman, 2005). In contrast, recent studies suggested that vigorous recyclings of pyroclast into the vent occur at basaltic volcanoes with repeating small explosions (D'Oriano et al., 2014). The recycling of pyroclast that fills the shallow conduit can affect explosion dynamics such as shape and ejection speed of jet cloud (e.g., Ohba et al., 2002).

This study examined component, texture and chemical composition of ash samples collected during normal activity at Stromboli to discuss a controlling mechanism of small explosion styles in basaltic volcano. During studied term (14:26-18:29, May 21th, 2014), ash-rich explosion occurred in NE crater, and ash-poor explosion occurred in the central and SW craters. We performed real-time collection of falling ash every 4-18 minutes. The real-time collection permits direct comparison of the ash samples with explosion activity. We classify the ash particles into Juvenile (glassy particles having similar composition with HP magma), Recycled (similar texture with reheated basaltic ash; D'Oriano et al., 2013), Altered, and Crystal particles. Bulk component of recycled particle calculated from componentry variations with grain size and grain size distribution shows that the volume fraction of recycled particles increases with ash falling rate at sampling site. The ash falling rate at sampling site increased after the occurrence of ash rich explosions at NE crater.

Our ash observation indicates that burial of eruptive vent by recycled pyroclasts relates with occurrence of ash rich explosion at NE crater. Therefore, we suggest that combination of the amount of gas supplied from a deeper conduit and thickness of the buried sediment in the vent controls the style of small basaltic explosion in Stromboli (Burton et al., 2007; Patrick et al., 2007; Quane et al., 2009). Depositional structure of the vent determined by the pyroclast recycling and history of the explosions are important for understanding the explosion styles.