Formation of the Aira Caldera, southern Kyushu, ∼22,000 years ago

Abstract

About 22,000 years ago a series of large‐scale pyroclastic eruptions produced the Aira caldera 20 km×20 km wide at the northern end of Kagoshima Bay in southern Kyushu. It started with a Plinian pumice eruption (Osumi pumice fall, 98 km3) followed by oxidized, fine‐grained Tsumaya pyroclastic flow (13 km3), both erupted from a vent located at the present site of Sakurajima volano, 8 km south of the caldera center. After a very short pause, violent explosive ejection of the basement rock fragments and pumiceous materials occurred at the central vent, gradually changing itself to a huge eruption column rapidly collapsing to form the Ito pyroclastic flow about 300 km3 in volume. The earliest phase produced up to 30‐m‐thick Kamewarizaka breccia developed along the caldera rim and charged with basement (lithic) fragments up to 2 m across. The breccia is a near‐vent variety of the bottom concentration zone of lithics in the Ito deposit. Various textural features and monotonous petrologic character indicate that the main part of the Ito pyroclastic flow was emplaced by a simple, short‐lived eruptive mechanism. The Aira‐Tn ash, a fine‐grained counterpart of the Ito pyroclastic flow, covered a wide area more than 1000 km from the vent. Evacuation of more than 110 km3 of rhyolitic magma produced a funnel‐shaped collapse structure with the center of the magma chamber about 10 km deep. Like many other Japanese Quaternary calderas, the Aira caldera is considered to heave formed not by a piston cylinder‐type subsidence utilizing a ring fracture but by coring and high‐angle slumping of the wall rocks into a funnel‐shaped central vent. The outline of the caldera was strongly controlled by the faults bounding the volcano‐tectonic graben forming Kagoshima Bay.