3:30 PM - 3:45 PM
[SVC31-05] The run-out distance of large-scale pyroclastic density currents in magmatic and phreatomagmatic eruptions: Numerical simulations of a two-layer depth-averaged model
Keywords:Pyroclastic density current, Two-layer model, Run-out distance, Magmatic eruption, Phreatomagmatic eruption, Ignimbrite
During large-scale explosive volcanic eruptions, a mixture of volcanic particles and gas is continuously ejected from the volcanic vent and can flow along the ground surface as a pyroclastic density current (PDC). Understanding the relationship between the run-out distance of PDCs (i.e., the extent of PDC deposits) and the eruption conditions (e.g., the magma discharge rate and the mass fraction of external surface water mixed with the magma) is desired to mitigate volcanic hazard associated with explosive eruptions. In particular, the effect of the surface-water mass fraction is a key issue, because it closely relates to the eruption style (i.e., magmatic and phreatomagmatic eruptions). This study aims to clarify the effect of surface water on the run-out distance of large-scale PDCs on the basis of numerical simulations of PDC dynamics.
PDC is a gravity current with strong stratification of particle concentrations; its run-out distance is determined by lift-off of the upper dilute current (particle concentrations <~1 vol.%) or deposition of the lower dense current (~10-50 vol.%). In order to reproduce this basic property, we have developed a two-layer depth-averaged model. This model can evaluate the effect of the temperature decrease due to surface water on the PDC dynamics through the suppression of the thermal expansion of the entrained air in the dilute current (e.g., Shimizu et al., 2019, JpGU Meeting, SVC37-09). In addition, this study considers the phase change between water vapor and liquid water and the latent heat associated with the phase change on the basis of the fact that the temperature of PDCs can become less than 100℃ by a large amount of surface water. We assume that a homogeneous mixture consisting of magma and surface water is steadily supplied during time >0 from the source vent and it spreads as a two-layer PDC radially from the source along the horizontal ground surface. We performed a parametric study for various mass fractions of surface water at the source (i.e., (mass of surface water)/(mass of magma and surface water)=0-0.5).
The numerical results show that as the dilute current spreads from the source, its flow density decreases through particle settling, air entrainment, and thermal expansion of the entrained air. When the frontal region of the dilute current becomes lighter than the ambient air to reverse buoyancy and lift off, the dilute current stops forward propagation and converges to a steady state (the distance between the front position and the source position is referred to as rU). Particles settling from the base of the dilute current form a thin dense current, which also spreads radially. When the radial mass flux of the dense current at the front is balanced with the basal deposition rate, the dense current stops forward propagation and converges to a steady state (the distance between the front position and the source position is referred to as rL). The results of the parametric study show that the relative magnitude between rU and rL depends on the surface-water mass fraction at the source; rU is shorter than rL when the surface-water mass fraction is small (<~0.2), whereas rU is longer than rL when the surface-water mass fraction is large (>~0.2). This variation comes from the fact that the lift-off distance of the dilute current (i.e., rU) significantly increases as the surface-water mass fraction at the source increases. The increase in rU results from the delay of lift-off caused by two factors: (1) the suppression of the thermal expansion of the entrained air and (2) the increase in flow density due to the condensation of water vapor. The results for large surface-water mass fractions (i.e., rU>rL) explain the observation that stratified surge deposits originated from dilute PDCs are widely observed in the deposits of phreatomagmatic eruptions.
PDC is a gravity current with strong stratification of particle concentrations; its run-out distance is determined by lift-off of the upper dilute current (particle concentrations <~1 vol.%) or deposition of the lower dense current (~10-50 vol.%). In order to reproduce this basic property, we have developed a two-layer depth-averaged model. This model can evaluate the effect of the temperature decrease due to surface water on the PDC dynamics through the suppression of the thermal expansion of the entrained air in the dilute current (e.g., Shimizu et al., 2019, JpGU Meeting, SVC37-09). In addition, this study considers the phase change between water vapor and liquid water and the latent heat associated with the phase change on the basis of the fact that the temperature of PDCs can become less than 100℃ by a large amount of surface water. We assume that a homogeneous mixture consisting of magma and surface water is steadily supplied during time >0 from the source vent and it spreads as a two-layer PDC radially from the source along the horizontal ground surface. We performed a parametric study for various mass fractions of surface water at the source (i.e., (mass of surface water)/(mass of magma and surface water)=0-0.5).
The numerical results show that as the dilute current spreads from the source, its flow density decreases through particle settling, air entrainment, and thermal expansion of the entrained air. When the frontal region of the dilute current becomes lighter than the ambient air to reverse buoyancy and lift off, the dilute current stops forward propagation and converges to a steady state (the distance between the front position and the source position is referred to as rU). Particles settling from the base of the dilute current form a thin dense current, which also spreads radially. When the radial mass flux of the dense current at the front is balanced with the basal deposition rate, the dense current stops forward propagation and converges to a steady state (the distance between the front position and the source position is referred to as rL). The results of the parametric study show that the relative magnitude between rU and rL depends on the surface-water mass fraction at the source; rU is shorter than rL when the surface-water mass fraction is small (<~0.2), whereas rU is longer than rL when the surface-water mass fraction is large (>~0.2). This variation comes from the fact that the lift-off distance of the dilute current (i.e., rU) significantly increases as the surface-water mass fraction at the source increases. The increase in rU results from the delay of lift-off caused by two factors: (1) the suppression of the thermal expansion of the entrained air and (2) the increase in flow density due to the condensation of water vapor. The results for large surface-water mass fractions (i.e., rU>rL) explain the observation that stratified surge deposits originated from dilute PDCs are widely observed in the deposits of phreatomagmatic eruptions.