Abstract
Explosive volcanic super-eruptions of several hundred cubic kilometres or more generate long run-out pyroclastic density currents the dynamics of which are poorly understood and controversial. Deposits of one such event in the southwestern USA, the 18.8 Ma Peach Spring Tuff, were formed by pyroclastic flows that travelled >170 km from the eruptive centre and entrained blocks up to ∼70–90 cm diameter from the substrates along the flow paths. Here we combine these data with new experimental results to show that the flow's base had high-particle concentration and relatively modest speeds of ∼5–20 m s−1, fed by an eruption discharging magma at rates up to ∼107–108 m3 s−1 for a minimum of 2.5–10 h. We conclude that sustained high-eruption discharge and long-lived high-pore pressure in dense granular dispersion can be more important than large initial velocity and turbulent transport with dilute suspension in promoting long pyroclastic flow distance.
Highlights
Explosive volcanic super-eruptions of several hundred cubic kilometres or more generate long run-out pyroclastic density currents the dynamics of which are poorly understood and controversial
Explosive volcanic super-eruptions expel magma volumes of several hundred cubic kilometres or more and generate particle-gas flows called pyroclastic density currents[1,2]
Does long run out require high initial flow speeds and mass fluxes near the eruptive source? are the density currents fully turbulent and dilute or is much of their mass carried in concentrated, basal granular dispersions? It is commonly assumed that the long run out of large-volume pyroclastic density currents requires high initial flow speeds of 100–300 m s À 1, which might result from collapse of explosive eruption columns from heights of a few kilometres[3,7,8]
Summary
Explosive volcanic super-eruptions of several hundred cubic kilometres or more generate long run-out pyroclastic density currents the dynamics of which are poorly understood and controversial Deposits of one such event in the southwestern USA, the 18.8 Ma Peach Spring Tuff, were formed by pyroclastic flows that travelled 4170 km from the eruptive centre and entrained blocks up to B70–90 cm diameter from the substrates along the flow paths. We conclude that the long travel distances of such pyroclastic flows are related mainly to sustained high mass flux of material erupting from the volcano along with slow gas pore pressure diffusion within the flows due large amounts of ash, rather than from high initial flow velocities and/or transport via dilute turbulent suspension
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