Pre-caldera lateral activity at Somma – Vesuvius

S01.14 - Volcano geology and field observations aimed at validation of numerical models Pre-caldera lateral activity at Somma- Vesuvius Claudio Scarpati, Domenico Sparice, Annamaria Perrotta Dipartimento di Scienze della Terra, dell'Ambiente e delle Risorse (DiSTAR), Università di Napoli Federico II, Italy Five pre-caldera, scoria- and spatter-cones at Somma-Vesuvius were produced by alternative pulses of Strombolian and Hawaiian activity that emplaced scoria and welded spatter fall deposits, several meters thick. These vents cover a wide azimuth around the volcano. A persistent feature throughout the exposed stratigraphy of the studied parasitic cones is the abundance of coarse, juvenile material (97–100 wt%), a feature suggesting an overall magmatic style during these eruptive episodes. Most of the observed eruptive units show an alternation of unwelded scoria and welded spatter suggesting a repeated variation of the eruptive style. A dominant Strombolian style of fragmentation, with bubble coalescent and rhythmic outbursts, emplaced thick scoriaceous succession; episodically, lava fountaining activity ejected coarse clots of magma which fell near the vent producing spatter horizons. The fragmentary nature (spatter-fed) of lavalike facies strictly associated (transitional) to less welded to agglutinated facies, as well as the plastic deformation (flattening) of spatter fragments indicate the continuous fall deposition from Hawaiian firefountain episodes alternated with Strombolian phases emplacing loose scoria deposits. The remnants of two cones show a continuous sequence (no breaks) of pyroclastic beds emplaced as a result of a single coneforming eruptions. This allows us to define them “monogenetic”. Conversely, the presence of thick paleosols, reworked material, exotic tephras and deep erosional surfaces in the pyroclastic succession of the other three cones has to be regarded as clear, well developed and laterally traceable breaks in the pyroclastic sequence of these small parasitic volcanoes. In light of this, we define such edifices as the result of a polygenetic (multi-phase) evolution. These evidences indicate resumption of activity after a quite long, nonquantifiable period of quiescence. This behaviour should suggest more caution when considering the parasitic volcanoes that erupted in historical time, completely extinct. S01.14