Mount Aso, a volcanic caldera on central Kyūshū Island, Japan, erupted at about 11.45 am local time on Wednesday 20 October 2021, according to the Japan Meteorological Agency. The eruption produced a column of gas and ash about 3.5 km high from its No. 1 Nakadake Crater, a pyroclastic flow was observed on its western flank, and cinder falls recorded as far as 900 m from the crater. The eruption occurred without warning, and 16 climbers were on the volcano when it happened, although all have been accounted for and are uninjured.
The Aso Caldera is the largest in Japan, and one of the largest in the world, being approximately 25 km in diameter. The caldera is thought to be the result of a series of four massive eruptions, the first of which took place around 300 000 years ago, and the last around 90 000 years ago. The caldera contains five smaller summits in a complex at its centre, the highest of which Mount Taka, rises 1592 m above sea-level. These are also a series of hot springs within the caldera, which is a popular tourist resort and part of the Japanese Geoparks Network. Despite the site's dramatic history modern eruptions tend to be quite small, and there are no records of any historical fatalities connected with the volcano.
Japan has a complex tectonic environment with four plates underlying parts of the Islands; in addition to the Pacific in the east and the Othorsk in the North, there are the Philipine Plate to the south and the Eurasian Plate to the West. Kyūshū Island lies at the northeast end of the Ryukyu Island Arc, which sits on top of the boundary between the Eurasian and Philippine Plates. The Philippine Plate is being subducted beneath the Eurasian Plate, in the Ryukyo Trench, to the Southeast of the Islands. This is not a smooth process, with the two plates continuously sticking together then breaking apart as the pressure builds up, leading to frequent Earthquakes in the region.
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