A new hazard scenario at Vesuvius: deadly thermal impact of detached ash cloud surges in 79CE at Herculaneum

Abstract

Diluted pyroclastic density currents are capable to cause huge devastation and mortality around volcanoes, and temperature is a crucial parameter in assessing their lethal power. Reflectance analysis on carbonized wood from ancient Herculaneum allowed a new reconstruction of the thermal events that affected buildings and humans during the 79CE Vesuvius eruption. Here we show that the first PDC entered the town was a short-lived, ash cloud surge, with temperatures of 555–495 °C, capable of causing instant death of people, while leaving only a few decimeters of ash on ground, which we interpret as detached from high concentration currents. The subsequent pyroclastic currents that progressively buried the town were mostly higher concentration PDCs at lower temperatures, between 465 and 390 and 350–315 °C. Charcoal proved to be the only proxy capable of recording multiple, ephemeral extreme thermal events, thus revealing for the first time the real thermal impact of the 79CE eruption. The lethal impact documented for diluted PDC produced during ancient and recent volcanic eruptions suggests that such hazard deserves greater consideration at Vesuvius and elsewhere, especially the underestimated hazard associated with hot detached ash cloud surges, which, though short lived, may expose buildings to severe heat damages and people to death.

The April 2015 Calbuco eruption pyroclastic density currents: deposition, impacts on woody vegetation, and cooling on the northern flank of the cone

“…Abrasion by PDCs imposed some damage to upright and tipped stems on their upstream sides, as indicated by asymmetrical removal of bark and wood, but in some sites subsequent bedload transport of cobbles during fluvial erosion appears to have caused more abrasion than the low-density, gascharged PDCs, as indicated by the bark present on upright trees still partially buried in PDC deposits. These lower-temperature impacts contradict recent findings in Herculaneum during the 79 CE eruption of Vesuvius, where dilute PDCs left a lethal thermal overprint on the residents (Pensa et al, 2023). Despite the differences leading to heterogeneous impacts produced by the Calbuco 2015 PDCs compared to those in Vesuvius require an in-deep investigation, one possible explanation is related to ash-cloud decoupling.…”