Emplacement of radial pyroclastic density currents over irregular topography: the chemically-zoned, low aspect-ratio Green Tuff ignimbrite, Pantelleria, Italy

Low aspect-ratio ignimbrites are thought to be emplaced by particularly hazardous, radial, high-velocity pyroclastic density currents from caldera-forming eruptions. Their circular distribution has been inferred to record simultaneous flow in all directions from source, overtopping hills, rather than passively flowing down valleys. This study aims to understand how such currents behave and evolve during an eruption by mapping out the internal chemical-architecture of a zoned, low-aspect ratio ignimbrite sheet on the island of Pantelleria, Italy. The pristine, welded Green Tuff Formation (aspect ratio >1:1,000) was deposited during the most recent (c. 50 ka) explosive eruption on the island. The extensive flow-unit is zoned from pantellerite to trachyte, recording changes in the composition of the erupting magma with time. Detailed logging with very close-spaced sampling for chemical and petrographic analysis has distinguished an internal chemical stratigraphy that allows the brief history of the sustained current to be divided into successive time-periods. The compositional zones have been mapped around the island enabling the reconstruction of how the footprint of the sustained current shifted during the eruption as the current waxed and then waned. Furthermore, the mapped compositional zones can be used to assess how the current and its resultant deposit encountered and overtopped barriers, such as cone-shaped hills and transverse ridges. This study has revealed that the ignimbrite was not emplaced entirely radially: rather, it flowed into certain sectors before others, and that the leading edge of the current advanced and then retreated, and shifted laterally with time. Deposition was diachronous, and previously proposed lithofacies correlations within the ignimbrite are demonstrated to be incorrect. As the pyroclastic density current encroached upon topographic barriers, it was initially blocked, reflected, or deflected around the lower flanks of the barrier. As the mass-flux of the eruption increased, the current waxed and was progressively able to overtop topographic barriers.