Block and Bomb Field of the 1888–1890 CE Eruption at La Fossa Cone (Vulcano, Aeolian Islands): Constraints on Conduit Stratigraphy and Eruptive Dynamics

Abstract

Vulcanian eruptions are classically associated with the launch of ballistic ejecta and particularly with the production of breadcrust bombs. Morpho-textural analysis of these ejecta provides critical insight into the poorly constrained processes controlling the complex dynamics of Vulcanian activity. Here, we present results from extensive fieldwork describing lithology, grain size, and spatial dispersal of ballistic products from the 1888–1890 CE eruption at La Fossa cone (Vulcano, Aeolian Islands). A dedicated strategy adopted during the field survey allowed us to investigate ballistics across the whole crater terrace and recognize a marked lithological diversity in the erupted products. Six types of ballistics were distinguished based on lithological, structural and textural features: Dense blocks [D]; Breccias-bearing Dense blocks [D-Br]; Black Scoria bombs [S]; and breadcrust bombs [BcB] with three sub-categories based on external rind features: thick crusted [BcB-Tk], thin crusted [BcB-Tn] and vesicular rind [BcB-Vr]. Dense blocks are the dominant ballistic product, representing up to 70 wt% of the total deposit, while breadcrust bombs account for only 20–25 wt%. The spatial dispersal of different bomb types is highly variable across the crater terrace in terms of both size distribution (modes around −7 Φ) and textural features. By combining measured data with direct observations from historical chronicles, we reconstruct the syn-eruptive conduit stratigraphy at Vulcano for this eruption. The eruptive dynamics of this archetypal Vulcanian activity was interpreted as mostly controlled by gas accumulation and fragmentation of an intensely fractured and slowly ascending viscous magma plug, which was only occasionally remobilized by the arrival of lower viscosity, volatile-richer magma from the deeper system.