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[SCG57-01] Mechanism of preparation, trigger, and sequence of massive pyroclastic eruptions
★Invited Papers
Keywords:massive pyroclastic eruption, collapse caldera, large-volume ignimbrite, silicic magma chamber
Massive pyroclastic eruptions that produce large-volume ignimbrites and form a collapse caldera, produce 10-100 km3 of silicic magma within a short period, possibly within days. Eruptions of this magnitude occur several times per 1000 years on Earth. The Kikai-Akahoya eruption about 7,300 years ago is one of the youngest examples in Japan.
Massive pyroclastic eruptions need a preparation process for the accumulation of large-volume magmas in the shallow level in the earth’s crust, and a mechanism to eject voluminous magmas in a short period. We understand little about the accumulation process of magma preceding a massive pyroclastic eruption and the structure of the magma storage zone.
Eruptions from a silicic magma chamber in relatively shallow areas (several kilometers) can be triggered in various ways. Magma eruption from a magma chamber decompresses the magma chamber. If the extraction rate is relatively low, the decompression of the magma chamber is offset by the volume reduction of the magma chamber due to the deformation of the wall rock. However, rapid magma eruption from the magma chamber, which exceeds the deformation of the wall rock, causes decompression of the magma chamber and accumulation of shear stress in the wall rock of the magma chamber, especially in the roof rock. If it exceeds the shear strength of the roof, the roof rock fails and the caldera collapse starts. The timing of the collapse depends on the strength of the rock and the amount of decompression of the magma chamber. If the depth of the magma chamber is shallow, i.e., if the roof rock is thin, a small amount of decompression is required to rupture the roof rock of the magma chamber, whereas if the magma chamber is relatively deep, a large amount of decompression is required to rupture the roof rock of the magma chamber. Furthermore, when the depth of the magma chamber is deeper than a certain depth, the rupture of the roof rock of the magma chamber rock does not occur even if the magma chamber is completely depressurized. In other words, a caldera collapse cannot occur. The rupture of the roof rock of the magma chamber rock and its subsidence into the magma chamber promotes magma ejection from the magma chamber through the fracture system in the roof rock, which results in the eruption of a large-volume pyroclastic flow. En-mass collapse of the roof rock at a time would produce a single pulse of a large-scale pyroclastic flow, while repeated staged collapses would produce a large-scale pyroclastic flow divided into multiple flow pulses. This collapse pattern may be affected by the amount of decompression of the magma chamber. Ultimately, the magma will continue to be ejected until the roof rock of the magma chamber sinks to the "bottom" of the magma chamber. Therefore, the volume of the eruption of a caldera-forming eruption depends on the volume of magma prepared in the crust and the architecture of the magma chamber. To predict the size of a possible future giant eruption, it is necessary to estimate the volume of magma accumulated in the crust.
Massive pyroclastic eruptions need a preparation process for the accumulation of large-volume magmas in the shallow level in the earth’s crust, and a mechanism to eject voluminous magmas in a short period. We understand little about the accumulation process of magma preceding a massive pyroclastic eruption and the structure of the magma storage zone.
Eruptions from a silicic magma chamber in relatively shallow areas (several kilometers) can be triggered in various ways. Magma eruption from a magma chamber decompresses the magma chamber. If the extraction rate is relatively low, the decompression of the magma chamber is offset by the volume reduction of the magma chamber due to the deformation of the wall rock. However, rapid magma eruption from the magma chamber, which exceeds the deformation of the wall rock, causes decompression of the magma chamber and accumulation of shear stress in the wall rock of the magma chamber, especially in the roof rock. If it exceeds the shear strength of the roof, the roof rock fails and the caldera collapse starts. The timing of the collapse depends on the strength of the rock and the amount of decompression of the magma chamber. If the depth of the magma chamber is shallow, i.e., if the roof rock is thin, a small amount of decompression is required to rupture the roof rock of the magma chamber, whereas if the magma chamber is relatively deep, a large amount of decompression is required to rupture the roof rock of the magma chamber. Furthermore, when the depth of the magma chamber is deeper than a certain depth, the rupture of the roof rock of the magma chamber rock does not occur even if the magma chamber is completely depressurized. In other words, a caldera collapse cannot occur. The rupture of the roof rock of the magma chamber rock and its subsidence into the magma chamber promotes magma ejection from the magma chamber through the fracture system in the roof rock, which results in the eruption of a large-volume pyroclastic flow. En-mass collapse of the roof rock at a time would produce a single pulse of a large-scale pyroclastic flow, while repeated staged collapses would produce a large-scale pyroclastic flow divided into multiple flow pulses. This collapse pattern may be affected by the amount of decompression of the magma chamber. Ultimately, the magma will continue to be ejected until the roof rock of the magma chamber sinks to the "bottom" of the magma chamber. Therefore, the volume of the eruption of a caldera-forming eruption depends on the volume of magma prepared in the crust and the architecture of the magma chamber. To predict the size of a possible future giant eruption, it is necessary to estimate the volume of magma accumulated in the crust.