caldera that was by . The Aira Caldera is one of a series of volcanic complexs in the Kagoshima Graben which has been postulated to extend northward from the undersea Kikai Caldera to the Ata South Caldera, Ata North Caldera (see Ata Caldera), the Aira Caldera associated with Kagoshima Bay and through past to the Kirishima Volcano Group. This alignment was first noted in the 1940s. The tectonic processes are rather complex in this region where the Okinawa Plate is colliding with the Amur Plate and the Pacific Plate is subducting under both. The formation of Aira Caldera started with a Plinian pumice eruption of the Osumi pumice fall from a vent near where Sakurajima is now and was quickly followed by an oxidised Tsumaya pyroclastic flow. It is likely subsequent eruptions in this series were at vents in what has been termed the Wakamiko caldera to the north west. Basement rock fragments and pumiceous materials from a massive explosion formed the Ito pyroclastic flow which deposited more than of Ito Ignimbrite (known as “Shirasu” locally) and of Aira-Tn Tephra in volume. Within the constraints that much of the caldera is under the sea, the reason for the large vent area is because the caldera erupted well over earlier estimates of of magma in a short amount of time. The caldera is known for its gravitational anomalies which is associated with a funnel-like shape in the strata. The structure of the caldera seemed unique in early work as it was different from the then typical Valles-type Caldera whose defining characteristics include a Valles-type ring fracture which acts as a channel for such large-scale pyroclastic flows. Such diffuse non directional pyroclastic flows, overwhelming the local landscape, have now also been described in New Zealand, for example in the Hatepe eruption. Local Impact Ito eruption Before the initial eruption of 25,000 years ago there was a wide and shallow basin of nearly the same size as the present Aira Caldera occupying the northern end of Kagoshima Bay with an east-west orientation. The basin is separated from the rest of the bay by a ridge with heights to above sea level. The topography encompasses the outline of an older caldera so suggesting there were pyroclastic flows that pre-dated the formation of present day Aira Caldera. The first phase of activity resulted from injection of mafic magmas that destabilized the stored rhyolite magma and