Variation of the magma chamber decompression and scale of precursory eruption for caldera-forming eruptions

Nobuo Geshi¹

• Affiliations: ¹Department of Earth and Planetary Science, Kyushu University, Fukuoka, Japan 

• Presentation type: Poster

• Presentation time: Thursday 16:30 - 18:30, Room Poster Hall

• Poster Board Number: 57

• Programme No: 3.11.17

• Theme 3 > Session 11

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Abstract

The processes of magma chamber decompression leading to caldera collapse are studied using two VEI 7-class caldera-forming eruptions from Aira and Kikai calderas in southwestern Japan to test the thresholds of magma chamber decompression for caldera fault activation. Petrological evidence suggests that the depth to the magma chamber is deep (~6 km) for Aira and shallow (~3 km) for Kikai. Water contents in phenocryst glass embayments indicate that Aira experienced a large magmatic decompression before the onset of caldera collapse, whereas caldera collapse occurred at a relatively low decompression at Kikai. The volume of magma erupted during the precursory Plinian eruptive phase is large (~40 km³ DRE) for Aira and small (<10 km³ DRE) for Kikai. Our friction models for caldera faults show that the decompression required to collapse a magma chamber is proportional to the square of the depth to the magma chamber for calderas of the same horizontal size. Lower friction on the caldera faults also causes the caldera to collapse at smaller decompression. This model can explain the variations in magma chamber decompression for the onset of caldera collapse, and also the difference in volume ratio between the magma that erupted in the initial Plinian phase before collapse and the magma that erupted in the main ignimbrite phase after the onset of collapse.