The influence of pore texture in pyroclastic flow deposits on compaction and welding

In a pyroclastic flow deposit, pore space is formed by a combination of variably connected vesicles in individual pyroclasts and the intervening space between pyroclasts of a range of sizes from micrometers to centimeters. Processes of compaction and sintering alter these original structures, causing pore space collapse. We examine how this complex pore structure and the resultant texture of pyroclastic deposits are affected by compaction and welding. As a quantitative measure of pore geometry, we use both matrix permeability and electrical tortuosity measurements parallel and perpendicular to compaction foliation. In a broad sense, we find that permeability increases with porosity in a power law relationship with upper and lower bounds that are taken to represent lateral and vertical gas escape, respectively. The lateral case is taken as k=2.4*10^-21*φ^5.8; the vertical case is k=3.1*10^-26*φ^7.3, where k is permeability and φ is porosity. We combine calculated gas escape rates and observed deposit geometries with simplified models of cooling and compaction, to define regimes that favor welding, formation of gas escape structures, water resorption, and simple compaction without welding. Furthermore, although the matrix permeability will likely control large-scale compaction, heterogeneity in permeability between matrix and juvenile clasts will likely control local variations in deposit texture. In cases where the permeability of individual pumiceous clasts is higher than that of the surrounding matrix, the development of overpressure (and resultant water resorption) may be localized at clast boundaries. In the opposite case, where pumice clasts are less permeable than the matrix, water resorption preferentially occurs in pumice clasts and/or clasts retain heat longer than the matrix. Either process lowers the relative viscosity of clasts, resulting in greater deformation. Based on these complexities and their resultant affects on gas loss, compaction, and deposit texture, it is important to document the pore structure of both matrix and juvenile clasts in a pyroclastic deposit.