Pinkish ash layers as fingerprints of flank instability: Unveiling Stromboli’s collapse recurrence through tephrostratigraphy

Pyroclastic Density Currents (PDCs) can be generated by explosive eruptions or by gravitational flank collapses involving unstable volcanic material (e.g., lava fronts, crater rims, domes). While the distribution of block-and-ash flow deposits is topographically confined to the transport and accumulation basin, these phenomena are frequently associated with co-PDC ash clouds. These buoyant clouds, composed of fine particles elutriated from the flow, spread over wider areas and settle as thin fallout layers. The recognition of such widely distributed layers enables the tephrostratigraphic investigation of historical collapse events, which are often under-recorded in the geological record.

At Stromboli, a direct link between pinkish tephra layers and partial flank collapses was established through the observation and syn-emplacement sampling of the ash cloud generated by the 19 May 2021 crater rim collapse (Re et al., 2022). Analogous deposits, previously described in the stratigraphic record (Bertagnini et al., 2011; Rosi et al., 2019; Pistolesi et al., 2020), have been repeatedly observed in recent activity (e.g., July 2024), indicating that such collapses represent a recurrent phenomenon.

Here, we present the study, conducted in the framework of the REFLeCTS project (INGV), of a stratigraphic sequence found on the northern side of the San Bartolo lava flow, dating back to Greek-Roman times (360 BC - 7 AD; Speranza et al., 2008). This succession consists of alternating ash and lapilli fallout beds related to typical Strombolian activity, interspersed with several relatively thick (from few mm to 5 cm) pink ash layers. Given that the limited thickness of these layers and the highly dynamic environment of active volcano flanks usually lead to their rapid obliteration by erosion or burial, the exceptional preservation of these tephra layers offers a unique opportunity to assess the recurrence of flank collapse events during Stromboli's recent eruptive history.