CROSS STRATIFICATIONS IN DEPOSITS OF EXPLOSIVE VOLCANIC ERUPTIONS

Pyroclastic density currents (PDC) are one of the most hazardous phenomena occurring during explosive volcanic eruptions, having caused thousands of fatalities. They are composed of a hot mixture of volcanic rock fragments supported by interstitial turbulent gas. Such density currents are produced during explosive eruptions and travel down the flanks of the volcanic edifice at high velocities (100s km/h) to distances of up to tens of kilometres. Due to their destructive nature, in situ observation by means of scientific instruments is impossible and our understanding is based on the study of the deposits.

Dune bedforms produced by dilute PDCs are a common feature in deposits of explosive volcanic eruptions. They show a wide range of shape, dimension, grain size distribution, and internal structure. These characteristics represent a key for the understanding of the flow dynamics of PDCs. We present a dataset of dune bedforms from Tungurahua (Ecuador), Laacher See (Germany), Ubehebe (USA), and Stromboli (Italy) volcanoes.

The internal structures usually show aggradation on both stoss (facing current) and lee (downstream) sides, indicating very strong sedimentation rates. Stoss and lee side angles can vary significantly, from less than 10º to 40º, probably being related to the current velocity. Low angle constructional bedforms resemble hummocky cross-stratifications (marine storm deposits) and structures deposited from turbidity currents (usually interpreted as antidunes). Lenses aggradated on the stoss side of obstacles or layers showing a local increase in thickness have usually been interpreted as chute and pool structures, related to sudden decrease in flow velocity (hydraulic jumps). An evolution of the size as well as outer shape of the bedforms is observed, with elongate, transverse, lunate, or periodic morphologies, providing insights on the flow dynamics.

The flow and depositional conditions during PDCs are still poorly constrained. From thorough field work, we inferred the qualitative evolution of flow behavior. Further field campaigns, laboratory experiments, and creation of a physical framework are needed to understand the genesis of dune bedforms produced by PDCs and access quantitative information on flow conditions for a better hazard assessment.