Abstract
Decompression of a magma chamber is a fundamental condition of caldera collapse. Although theoretical models have predicted the decompression of magma chambers before caldera collapse, few previous studies have demonstrated the amount of magma chamber decompression. Here, we determine water content in quartz glass embayments and inclusions from pyroclastic deposits of a caldera-forming eruption at Aira volcano approximately 30,000 years ago and apply this data to calculate decompression inside the magma chamber. We identify a pressure drop from 140–260 MPa to 20–90 MPa during the extraction of around 50 km3 of magma prior to the caldera collapse. The magma extraction may have caused down-sag subsidence at the caldera center before the onset of catastrophic caldera collapse. We propose that this deformation resulted in the fracturing and collapse of the roof rock into the magma chamber, leading to the eruption of massive ignimbrite.
Highlights
Decompression of a magma chamber is a fundamental condition of caldera collapse
Based on the variation in water content within volcanic glasses trapped in phenocrysts, we show that decompression of the magma chamber preceded caldera collapse, which resulted in a VEI 7 eruption at Aira volcano[18] in southern Kyushu Island, Japan
A glass embayment is a glass-filled channel found inside a phenocryst, one side of which is open to the surface of the phenocryst and connected to the external groundmass glass, and the other side is closed inside the phenocryst
Summary
Decompression of a magma chamber is a fundamental condition of caldera collapse. theoretical models have predicted the decompression of magma chambers before caldera collapse, few previous studies have demonstrated the amount of magma chamber decompression. One of the typical sequences involves a preceding Plinian eruption phase and/or an eruption of minor ignimbrite from a subsidiary vent, followed by caldera collapse and a catastrophic pyroclastic eruption from a ring-vent system[7,8,9,10,11,12]. Based on the variation in water content within volcanic glasses trapped in phenocrysts, we show that decompression of the magma chamber preceded caldera collapse, which resulted in a VEI 7 eruption at Aira volcano[18] in southern Kyushu Island, Japan. Though several minor ignimbrites older than the Ito pyroclastic flow (mainly erupted in the middle Pleistocene) are distributed around the Aira caldera, there is no clear evidence for the existence of a collapse caldera at the present location of the Aira caldera before the AT eruption[24]
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