Trachyandesite scoria-flow and associated trachyte pyroclastic flow and surge at Roccamonfina Volcano (Roman Region, Italy)

Abstract

The pyroclastic deposits (6–7 km 2 in area) of Garofali (Roccamonfina Volcano, Italy) consist of the 239 ± 8 ka Grey Trachyandesite Scoriae of Riella (RTS) conformably capped by the Trachyte Tuff of Garofali (GTT), the latter consisting of pyroclastic flows, surges and overlying lapilli tuff. Locally, the uppermost part of the RTS consists of finely bedded surge deposits. Both RTS and GTT contain RTS-derived debris flows. Grain-size features suggest that the RTS can be interpreted as proximal scoria and lithic flow deposits. The flow origin is suggested by absence of bedding, emplacement of scoria clasts in a still plastic state, attitude to excavating channels in underlying pyroclastics, and grain size features. A surge origin for the transitional RTS is suggested by fine layering, presence of dune, pinch-and-swell structures and accretionary lapilli, and grain size. These characteristics become dominant in the surge component of the GTT, where they are associated with chute-and-pool structures. Grain size distributions show that the GTT deposits — both pyroclastic flows and surges — have a sorting similar to the RTS but a smaller median diameter. Gas-escape structures occur largely in the upper and lower levels of RTS and GTT, respectively. They result from strong gas-fluidification of the RTS during deposition of GTT. Isopach and isograde maps locate the source vent of the RTS between Tuorisichi and S. Lorenzo, but indicate that three source vents were active during eruption of the GTT. Measurements of dunes gave good directional vectors for the GTT. Absence of a depositional break between RTS and GTT deposits suggest that these represent two different episodes of the eruption of a compositionally zoned magma source, through one (RTS) and several (GTT) conduits. The eruption of the RTS was driven essentially by magmatic volatiles partially affected by preeruptive involvement of water. Eruption of the transitional RTS-GTT and mainly of GTT deposits developed entirely because of magma-water interaction. This phenomenon decreased during the lapilli-tuff eruption causing a lesser fragmentation of GTT magma.