A discriminatory diagram of massive vs stratified deposits based on the sedimentation and bedload transportation rates. Experimental investigation and application to pyroclastic density currents.

Dense gas-particle jets similar to collapsing eruption columns were generatedby large-scale experiments. The column collapse resulted in a ground-hug-ging current forming stratified layers with bedding similar to natural pyro-clastic density current deposits. At the impact of the collapsing column onthe ground, a thick, massive bed was formed due to a high sedimentationrate that dumped turbulence due to high clast concentration. Down-current,flow expansion favoured turbulence and dilute gas-particle current thatformed thin rippled layers deposited under traction. Experiments fed withfine ash (median size 0066 mm) formed deposits without tractional struc-tures, because fine particles, as other sedimentary fine material, is cohesiveand exposes a limited surface to the shear stress. Experimental outcomesshow that massive beds are formed where the sedimentation rate per unitwidthSrwexceeds the bedload transportation rateQbby two orders of mag-nitude. A lower ratio generates traction at the base of the flow and formationof shear structures that increase in wavelength and height with a decreasingflux. This study presents a diagram that provides a useful addition for faciesanalysis of pyroclastic density currents, provided that deposits representingsustained sedimentation can be identified in the field. In the diagram adecrease in theSrw/Qbratio corresponds to an increase in bedform size.Application of the diagram for hazard assessment purposes allows the recon-struction of the mass eruption rate of the Agnano–Monte Spina eruption atCampi Flegrei, which is the main variable defining the intensity of pasteruptions, and of the Bingham rheology of the massive underflow of theMercato pyroclastic density current at Vesuvius.