Fig. 4: Evolution of magma decompression during the AT eruption.

Decompression of magma chamber and deformation of the host rock toward the caldera collapse and eruption of massive ignimbrite. A rhyolite magma chamber was subjected to lithostatic pressures of 140–260 MPa, corresponding to depths of 5.3–9.4 km below the surface of the Aira volcano before the eruption. Extraction of magmas from the magma chamber caused decompression in the magma chamber. Pressure in the magma chamber decreased to ~90–190 MPa due to the extraction of the magma by the eruption of the Osumi pumice fall during the Plinian phase. Down-sag deformation of the roof rock was initiated by the pressure difference between the lithostatic pressure acting on the host rock of the magma chamber and the decreasing magmatic pressure in the chamber during the Plinian phase. The extracted magmas were decompressed rapidly during their ascent through the conduit to the ground surface. Caldera collapse started when blocks of roof rock detached by the ring fracture subsided into the magma chamber. The subsidence of the blocks into the magma chamber accelerated further magma output, to form a massive ignimbrite.