Violent Strombolian to Subplinian Eruption of a Monogenetic Cone Complex, Chaîne Des Puys, France

Abstract:

The Chaîne des Puys, central France, is a highly variable monogenetic lineament, with eruption styles that vary from Hawaiian to Subplinian and compositions from alkali basaltic to tachytic. The chain is an excellent natural laboratory to explore the variability of monogenetic volcanoes, and improve eruption scenarios. The eruption of the La Vache and Lassolas cone complex ca. 9 ka cal BP is the most recent large basaltic eruption. It formed two separate large scoria cones, a 14 km long lava flow and a well-defined fall out deposit. The wide distribution (18 km²) and thickness deacay rate of the tephra deposit suggests violent strombolian to subplinian activity. This study aims to determine the processes that produced this widespread deposit by combining detailed tephra studies, including grain size and componentry analysis with textural studies of clast shape, density and permeability. Componentry analysis in the 32 mm-180 µm size range shows that the juvenile fraction comprises mainly very vesicular, ragged juvenile clasts and minor amounts of dense sheet like juvenile clasts and round juvenile clasts. Except for an accidental lithic rich basal layer, lithic clasts are an accessory fraction that contains predominantly clasts from the granitic basement and rarely basaltic clasts from the underlying older lava flows. Both componentry and grain size are invariable within the main proximal section, while the density of juvenile clasts increases slightly with stratigraphic height. However, in the top part of the section the grain size decreases and the componentry changes to containing predominantly round juvenile clasts. This data indicates constant eruption conditions during most of the activity, with slightly decreasing vesiculation, and thus increasing juvenile clast density. This increase may be coherent with changes in the eruption style, documented by changes in grain size and componentry seen at the top of the sequence. This work provides important information on violent monogenetic eruptions and the possible impact of similar activities on populated areas.