Some remarks on volcanic vent evolution during plinian eruptions

Abstract

The magma mass discharge rate governs the eruption magnitude of plinian eruptions, and depends strongly on conduit geometry. The presence of lithic clasts in pumice deposits results largely from conduit erosion, and the magma discharge rate and the lithic abundance in the resulting deposit are parameters linked by the overall vent widening rate. Many pumice fall deposits have lithic-enriched basal and top sections; pumice flow deposits tend to carry more lithics than the preceding fall layer. Theoretical models of vent evolution for a cylindrical conduit with radius R indicate an R 4 dependency of the magma discharge rate, which would lead to rapidly decreasing lithic abundances from bottom to top in plinian fall deposits, and from fall to flow deposits, contrary to observations. To simulate observed lithic abundance patterns, a model of conduit evolution with a stepped geometry was developed, in which the lower conduit section remains at approximately constant radius, while the upper conduit section widens. The mass discharge rate during an eruption then increases with R 2 top , and predicted lithic abundance patterns agree qualitatively with field data. Field evidence for conduit widening in the upper conduit section only is presented from plinian pumice beds from Nisyros, Greece. The change in eruption style from plinian fall to surge and ash flow is sometimes accompanied by a dramatic change in lithic abundance and lithic type. In these cases, this transition in deposition style is not the result of a gradual increase in mass discharge rate because of overall vent widening, but is caused by catastrophic rupture of the lower vent section during incipient caldera collapse, with resulting changes in magma discharge rates, lithic abundance and possibly water/magma interaction.