Aspects of pyroclastic flow movement and emplacement

Abstract

Pyroclastic flows are products of explosive volcanic eruptions. Composed of hot gas and particulates, they move as ground hugging density currents. This study utilized comparisons with similar geologic phenomena (e.g. debris avalanches), field data, and theoretical models to investigate aspects of the movement and emplacernent of pyroclastic flows. To constrain the factors influencing pyroclastic flow movement and emplacement, a field study of the Fogo A (Azores) intraplinian ignimbrites was conducted. Field relationships and granulometric and component analyses are used to interpret which parameters, and thus possible processes, were dominant. Velocity, as inferred from distance from the vent and underlying slope, had a large influence on the granulometric and component characteristics indicating deposition probably occurred continuously from the flow rather than by freezing of the flow en masse. Possible generation mechanisms are collapse of the eruption column circumference and partial asymmetrical column collapse. To investigate the problem of pyroclastic flow mobility, quantitative parameters of pyroclastic flow and debris avalanche mobility were collated. Based on a statistical comparison of the mobility of both deposit types, it is concluded that certain types of pyroclastic flows are no more mobile than debris avalanches of the same volume, suggesting fluidization is not an essential component in their mobility. To investigate the form of moving pyroclastic flows, a theoretical fluid dynamic model was formulated. From the governing equations, an advective-diffusive equation in the height of the flow is obtained and solved numerically. This equation treats the pyroclastic flow as a non-linear diffusive wave in height. The effect of various initial conditions on the form is investigated and possible effects on the depositional style are inferred. Varying the maximum height of the wave corresponds to varying the mass flux and indicates smaller mass flux flows are more likely to be deposited en masse while larger mass flux flows are more likely to be deposited continuously from the tail of the flow. Furthermore, deposits from larger mass flux flows are likely to display head deposits, while smaller mass flux flow deposits are likely to lack head deposits, which are due to overturn, air ingestion, and jetting at the flow front.