From Proximal Accumulation to Collapse: Mechanisms of Deposit-Derived Pyroclastic Density Currents

The collapse of high-temperature volcanic material is a widespread process affecting lava domes, lava flows and proximal volcaniclastic accumulations, including spatter agglutinates and crater-rim deposits. Such collapses can generate small-volume pyroclastic density currents (PDCs; 10³–10⁷ m³), capable of travelling several kilometres while maintaining temperatures of up to ~700°C. Failure of volcaniclastic material and the generation of deposit-derived PDCs represent a major hazard, particularly during effusive to violent Strombolian activity. These events commonly occur with limited or no clear precursory signals, posing a threat to both local communities and visitors. Two end-member mechanisms are identified: (i) gravitational instability of hot volcaniclastic deposits dominated by rapid proximal accumulation during fire-fountaining and lava flow emplacement on steep slopes (Fuego-type), with basal undercutting acting as a secondary, facilitating process; and (ii) enhanced magmatic thrust exerted by dense, degassed magma ascending within the conduit, which may destabilise crater rims and proximal structures (Arenal-type). Comparable processes operate during gravitational lava dome collapses, driven either by gravitational loading alone (Merapi-type) or by internal overpressure (Peléan-type).

Robust hazard assessment requires constraining both the long-term preconditioning factors that control volcanic slope instability and the short-lived processes capable of triggering collapse. This study integrates field-based stratigraphic and geomechanical observations with numerical modelling of slope instability, supported by a comprehensive database of historical deposit-derived PDC events. Geophysical monitoring data are incorporated within these databases to provide contextual constraints, while the primary focus of the analysis remains on field evidence and physics-based modelling approaches. Slope stability is analysed through two-dimensional limit-equilibrium methods adopting multiple shear-strength criteria, informed by site-specific stratigraphic constraints and mechanical characterisation of proximal deposits.

Sensitivity analyses highlight the key role of slope geometry, deposit thickness, mechanical properties and structural discontinuities in controlling failure conditions. The consistency between modelled unstable sectors and observed collapse areas supports the robustness of the proposed framework and its applicability to other volcanic systems characterised by similar morphologies and depositional environments. The approach can be readily extended to lava dome instability by accounting for dome lithology, mechanical heterogeneity and the properties of surrounding talus, as well as for the influence of endogenous and exogenous growth phases and the presence of hydrothermally altered material near conduits and crater rims.