Tying textures of breadcrust bombs to their transport regime and cooling history

Abstract

The thermal evolution of explosive eruptive events such as volcanic plumes and pyroclastic density currents (PDCs) is reflected in the textures of the material they deposit. Here we evaluate how the rinds of breadcrust bombs can be used as a unique thermometer to examine mafic to intermediate explosive eruptions. These eruptions can produce breadcrust bombs in either PDCs or as projectiles following nearly ballistic trajectories. We develop an integrated model to examine bubble growth, pyroclast cooling, and dynamics of PDCs and projectiles from buoyant plumes. We examine rind development as a function of transport regime (PDC and projectile), transport properties (initial current temperature and current density), and pyroclast properties (initial water content and radius). The model reveals that: 1) rinds of projectile pyroclasts are in general thicker and less vesicular than those of PDC pyroclasts; 2) as the initial current temperature decreases due to initial air entrainment, the rinds on PDC pyroclasts progressively increase in thickness; and 3) rind thickness increases with decreasing water concentration and decreasing clast radius. Therefore, the modeled pyroclast's morphology is dependent not only on initial water concentration but also on the cooling rate, which is determined by the transport regime.