Basaltic pyroclastic eruptions : Influence of gas-release patterns and volume fluxes on fountain structure, and the formation of cinder cones, spatter cones, rootless flows, lava ponds and lava flows

Head, James W. and Wilson, Lionel (1989) Basaltic pyroclastic eruptions : Influence of gas-release patterns and volume fluxes on fountain structure, and the formation of cinder cones, spatter cones, rootless flows, lava ponds and lava flows. Journal of Volcanology and Geothermal Research, 37 (3-4). pp. 261-271. ISSN 0377-0273

Abstract

In basaltic pyroclastic eruptions, two variables - magma gas content and magma volume flux - determine the detailed dynamic structure of the fountain and the size distribution of clasts within it. The fountain structure and clast size distribution in turn determine the nature of the resulting deposits, whether these be stationary pyroclastic constructs or active lava flows. Although the physical relationships between gas content and clast size are not fully understood, empirical data are available for basaltic eruptions. Fountain dynamic structure is determined by the velocity profile at any given pressure level and the maximum spread angle of the fountain from the vertical. These two parameters completely determine the paths of pyroclasts in the fountain and their ultimate resting places. The combination of the pyroclast size and the spatial distribution determines the clast number density and thus the opacity of the fountain and the ability of the pyroclasts to cool in their local fountain environment. For a given set of conditions, two factors thus become important in determining the structure and morphology of pyroclastic deposits: local temperature and accumulation rate. For example, in typical basaltic pyroclastic eruptions, the majority of pyroclasts remain inside the optically thick central part of the fountain, undergo minimal cooling, and return to the surface to coalesce and contribute to a lava pond or lava flow. In the optically thinner outer parts of the fountain, clasts undergo relatively more cooling and return to the surface to contribute to the building of the pyroclastic cone (if the accumulation rate is low) or to form rootless flows (if the accumulation rate is high and minimal further cooling occurs). The relationships between these various parameters are investigated for Hawaiian-style eruptions in general and applied qualitatively to the interpretation of post-eruption deposits.