Effects of the crater on eruption column dynamics

Abstract

[1] During explosive eruptions, a mixture of pyroclasts and volcanic gas forms a buoyant eruption column or a pyroclastic flow. We systematically investigate how the condition that separates these two eruption styles (column collapse condition) depends on crater shape and magma chamber conditions by integrating the theoretical models for conduit flow, flow inside a crater, and eruption column dynamics. The results show that previous model predictions of column collapse condition based on the relationship between magma discharge rate ( _ m) and water content (n f ) strongly depend on crater shape (depth D and opening angle ). When a crater is present, the decompression and/or compression of gas-pyroclast mixture inside and just above the crater result in two distinct types of column collapse: collapse with increasing _ m (HM-side collapse) and that with decreasing _ m (LM-side collapse). HM-side collapse is caused by an increase in conduit radius during the waxing stage of an eruption. LM-side collapse is associated with a decrease in magma chamber pressure during the waning stage. The value of _ m for HM-side collapse varies by two orders of magnitude depending on crater shape for fixed n f . This estimate also depends on assumed models for decompression into the atmosphere. The value of _ m for LM-side collapse is <10 6 kg s −1 for a shallow crater with a small opening angle, whereas it can be >10 8 kg s −1 when D tan > 10 2 m. These results are consistent with the field observations from the St. Helens 1980 and Pinatubo 1991 eruptions.

3D numerical simulation of volcanic eruption clouds during the 2011 Shinmoe-dake eruptions

“…On the other hand, in reality, the eruption intensity fluctuates with time and the exit pressure can be higher than the atmospheric pressure. Under these conditions, the flow just above the vent becomes supersonic and transports large clasts up to higher levels (e.g., Koyaguchi et al, 2010). Further studies are needed to evaluate the effects of these processes on turbulent mixing and ash dispersal.…”

“…Here, we assume that the magma discharge rate was 1.5×10 6 kg s −1 . In general, the pressure of a gas-pyroclast mixture deviates from atmospheric pressure during explosive eruptions, with the mixture accelerated and/or decelerated during decompression and/or compression immediately above the vent (Woods and Bower, 1995;Ogden et al, 2008;Koyaguchi et al, 2010). For simplicity, we assume that the pressure at the vent is equal to atmospheric pressure, as the pressure of the mixture is considered to approach atmospheric values at a short distance from the vent (5-20 vent radii).…”

The dynamics of volcanic jets: Temporal evolution of particles exit velocity from shock‐tube experiments

“…Such shape changes are intrinsically related to eruption dynamics and may involve both widening (by, e.g., vent erosion and flaring) and narrowing (by, e.g., collapse, infill or accretion). At constant MER, an increase in the crater diameter will influence the flow dynamics such that column collapse becomes more likely [ Wilson et al ., ; Koyaguchi et al ., ]. Moreover, changes of vent geometry will also affect the flow dynamics in the underlying plumbing system.…”

ASHEE-1.0: a compressible, equilibrium–Eulerian model for volcanic ash plumes

“…In many cases, indeed, the eruptive mixture is injected into the atmosphere at pressure higher than atmospheric, so that the flow is initially driven by a rapid, transonic decompression stage. This is suggested by numerical models predicting choked flow conditions at the volcanic vent (Wilson, 1980;Wilson et al, 1980), implying a supersonic transition above the vent or in the crater (Kieffer, 1984;Woods and Bower, 1995;Koyaguchi et al, 2010) and it is supported by field evidences of the emission of shock waves during the initial stages of an eruptions (Morrissey, 1997). Despite the importance of the decompression stage for the subsequent development of the volcanic plume (Pelanti and LeVeque, 2006;Ogden et al, 2008b;Orescanin et al, 2010;Carcano et al, 2013) and for the stability of the eruptive column (Ogden et al, 2008a), our analysis is limited to the plume region where flow pressure is equilibrated to the atmospheric pressure.…”