Coupling of turbulent and non-turbulent flow regimes within pyroclastic density currents

Volcanic eruptions are at their most deadly when pyroclastic density currents sweep across landscapes to devastate everything in their path. The internal dynamics underpinning these hazards cannot be directly observed. Here we present a quantitative view inside pyroclastic density currents by synthesizing their natural flow behaviour in large-scale experiments. The experiments trace flow dynamics from initiation to deposition, and can explain the sequence and evolution of real-world deposits. We show that, inside pyroclastic density currents, the long-hypothesized non-turbulent underflow and fully turbulent ash-cloud regions are linked through a hitherto unrecognized middle zone of intermediate turbulence and concentration. Bounded by abrupt jumps in turbulence, the middle zone couples underflow and ash-cloud regions kinematically. Inside this zone, strong feedback between gas and particle phases leads to the formation of mesoscale turbulence clusters. These extremely fast-settling dendritic structures dictate the internal stratification and evolution of pyroclastic density currents and allow the underflows to grow significantly during runout. Our experiments reveal how the underflow and ash-cloud regions are dynamically related - insights that are relevant to the forecasting of pyroclastic density current behaviour in volcanic hazard models.