5:15 PM - 7:15 PM
[MIS01-P05] Reconstruction of the Pinatubo 1991 eruption based on a semi-two-dimensional two-layer model for pyroclastic density currents
Keywords:Pyroclastic Density Current, Co-Ignimbrite Ash Cloud, Ignimbrite, Pinatubo 1991 Eruption, Two-Layer Model, Shallow-Water Equations
During the June 15, 1991 eruption of Mt. Pinatubo, a large-scale eruption column partially collapsed to generate a pyroclastic density current (PDC) and an umbrella cloud, which formed a large amount of pyroclastic deposits. Pyroclastic materials from both the Plinian eruption column and the co-ignimbrite ash cloud were supplied to the umbrella cloud (Figure). For such an eruption style, the relationship between the dynamics of umbrella cloud and its deposits has been well studied using physics-based models. On the other hand, the relationship between the dynamics of PDC and its deposits and the effect of the co-ignimbrite ash cloud on the umbrella cloud dynamics have not been fully understood. We attempt to reconstruct the entire Pinatubo 1991 eruption using a physics-based PDC model (Shimizu & Koyaguchi 2024 JpGU).
Shimizu & Koyaguchi (2024 JpGU) developed a two-layer PDC model consisting of upper dilute and lower dense currents to describe the dynamics and deposits of PDCs in a unified way. The dilute current is generated from the column, flows on the ground, and settles particles at the bottom to become lighter than the ambient air and form a co-ignimbrite ash cloud. The dense current is generated by the particles settling from the dilute current and flows along topographic valleys under the influence of deposition and erosion at the bottom. The whole PDC stops forward propagation and converges to a steady state where the sum of the sedimentation rate at the bottom Qsed and the particle supply rate from the PDC to the co-ignimbrite ash cloud Qco is balanced by the particle supply rate from the column to the PDC QPDC (Figure). The dynamics of large-scale PDCs can be approximated by a semi-2-dimensional (D) two-layer model where the dilute and dense currents are described by steady-state 1-D axisymmetric and time-dependent 2-D shallow-water equations, respectively. In this case the relationship of QPDC=Qsed+Qco is approximately satisfied.
We compared the numerical results of the semi-2-D two-layer model with observation data to reconstruct the dynamics and deposits of the PDC for the Pinatubo 1991 eruption as follows.
(1) The value of Qsed is estimated to be 3×108 kg/s from the observation data by Scott et al. (1996): i.e., the amount of PDC deposits (6.48×1012 kg) and the eruption duration (5 h).
(2) The value of QPDC is estimated to be 1.27×109 kg/s by searching for input conditions that satisfy the above Qsed. This estimate yields Qco=9.7×108 kg/s, which roughly coincides with the previous estimate of mass discharge rate based on a 3-D eruption cloud simulation (109 kg/s; Suzuki & Koyaguchi 2009). The co-ignimbrite ash mass (1.7×1013 kg), obtained by the product of Qco and eruption duration, is significantly larger than the previously estimated mass of pure Plinian pyroclasts in the umbrella cloud (3×1012 kg; Koyaguchi & Ohno 2001). These suggest that the co-ignimbrite ash cloud was the primary heat source driving the umbrella cloud during the Pinatubo 1991 eruption.
(3) The run-out area of the dilute current in the semi-2-D two-layer PDC model using the above QPDC coincides with the distribution of stratified deposits near the source in the field. By tuning model parameters related to the deposition and erosion speeds of the dense current, the numerical results reproduce the distribution of the PDC deposits in the field (e.g., thick valley-filling massive deposits far from the source) (cf. Shimizu & Koyaguchi 2022 JpGU).
These results indicate that our semi-2-D two-layer PDC model together with the existing eruption cloud models allows us to understand the dynamics of partially collapsed eruption column, PDC, umbrella cloud, and their pyroclastic deposits for complex explosive eruptions.
Shimizu & Koyaguchi (2024 JpGU) developed a two-layer PDC model consisting of upper dilute and lower dense currents to describe the dynamics and deposits of PDCs in a unified way. The dilute current is generated from the column, flows on the ground, and settles particles at the bottom to become lighter than the ambient air and form a co-ignimbrite ash cloud. The dense current is generated by the particles settling from the dilute current and flows along topographic valleys under the influence of deposition and erosion at the bottom. The whole PDC stops forward propagation and converges to a steady state where the sum of the sedimentation rate at the bottom Qsed and the particle supply rate from the PDC to the co-ignimbrite ash cloud Qco is balanced by the particle supply rate from the column to the PDC QPDC (Figure). The dynamics of large-scale PDCs can be approximated by a semi-2-dimensional (D) two-layer model where the dilute and dense currents are described by steady-state 1-D axisymmetric and time-dependent 2-D shallow-water equations, respectively. In this case the relationship of QPDC=Qsed+Qco is approximately satisfied.
We compared the numerical results of the semi-2-D two-layer model with observation data to reconstruct the dynamics and deposits of the PDC for the Pinatubo 1991 eruption as follows.
(1) The value of Qsed is estimated to be 3×108 kg/s from the observation data by Scott et al. (1996): i.e., the amount of PDC deposits (6.48×1012 kg) and the eruption duration (5 h).
(2) The value of QPDC is estimated to be 1.27×109 kg/s by searching for input conditions that satisfy the above Qsed. This estimate yields Qco=9.7×108 kg/s, which roughly coincides with the previous estimate of mass discharge rate based on a 3-D eruption cloud simulation (109 kg/s; Suzuki & Koyaguchi 2009). The co-ignimbrite ash mass (1.7×1013 kg), obtained by the product of Qco and eruption duration, is significantly larger than the previously estimated mass of pure Plinian pyroclasts in the umbrella cloud (3×1012 kg; Koyaguchi & Ohno 2001). These suggest that the co-ignimbrite ash cloud was the primary heat source driving the umbrella cloud during the Pinatubo 1991 eruption.
(3) The run-out area of the dilute current in the semi-2-D two-layer PDC model using the above QPDC coincides with the distribution of stratified deposits near the source in the field. By tuning model parameters related to the deposition and erosion speeds of the dense current, the numerical results reproduce the distribution of the PDC deposits in the field (e.g., thick valley-filling massive deposits far from the source) (cf. Shimizu & Koyaguchi 2022 JpGU).
These results indicate that our semi-2-D two-layer PDC model together with the existing eruption cloud models allows us to understand the dynamics of partially collapsed eruption column, PDC, umbrella cloud, and their pyroclastic deposits for complex explosive eruptions.