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Co-ignimbrite plumes: insights from the deposits of the VEI 6 Pozzolane Rosse co-ignimbrite deposits (Colli Albani, Italy)

Giordano G.1, G. Di Camillo1, B. Giaccio2, F. Pardini3, M. de Michieli Vitturi3, T. Esposti Ongaro3, P. Gabellini4, R. Cioni4, L. Calabrò5

  • Affiliations:  1 Università Roma Tre, Italy, 2 CNR-IGAG, Italy, 3 INGV, Italy, 4 Università di Firenze, Italy, 5 CNR-ISSMC, Italy 

  • Presentation type: Poster

  • Presentation time: Friday 16:30 - 18:00, Room Poster Hall

  • Poster Board Number: 184

  • Programme No: 3.9.17

  • Theme 3 > Session 9

Abstract

Co-ignimbrites ash plumes commonly form as trailing ash clouds detached from ground hugging high concentration pyroclastic flows. The associated deposits are usually made of fine ash and may be dispersed over very vast areas. The co-ignimbrite deposits of the 460 ka, VEI6-7 Pozzolane Rosse ignimbrite from Colli Albani (Central Italy) are dispersed towards the east as far as Albania, covering more than 200,000 km2. Unexpectedly, the co-ignimbrite fall deposits at 80-100 km distance from vent are lapilli-sized, made of scoria, accessory lithics and crystals as the parent ignimbrite. Reconstruction of the isopachs and isopleths of the co-ignimbrite suggests a minimum deposit volume of 16.5 km3 and a column height >30 km. Based on previous numerical modeling of the pyroclastic flows generating the Pozzolane Rosse ignimbrite (Calabrò et al. 2022, JGR), we suggest that the associated co-ignimbrite plume formed largely at maximum runout, where buoyancy becomes positive and the pyroclastic flow lifts off, turning from horizontal to vertical at absolute velocities values up to >100 m s-1, hence able to easily entrain dense lapilli-sized clasts, at mass fluxes of the same order of magnitude than those that fed the deposition of the ignimbrites. Such conditions are conducive to the generation of "rootless" buoyant columns at the front of the pyroclastic flow, comparable to Plinian in size and dynamics. The suggested mechanism is distinctly different from trailing co-ignimbrite ash clouds and may be modeled to infer source parameters of the parent pyroclastic flow based on co-ignimbrite deposits dispersal and grain size.