Geological and Petrological Study on the Debris Avalanche Deposits, Eastern Foot of Zao Volcano, Northeast Japans

Debris avalanche deposits related to the edifice collapse of the summit area of Zao Volcano have been identified for the first time at the volcano’s eastern foot. These deposits extend approximately 11–15 km from the summit. Based on their spatial distribution and clast petrology, the deposits are interpreted to have originated from the Umanose caldera. Deposit thickness ranges from 20–30 m in the western and northern parts to over 50 m in the eastern part, with an estimated volume of approximately 0.3 km³, comparable to that of the summit caldera depression. Matrix facies occur at most outcrops, whereas block facies are found at only three sites. The vertical drop-to-runout distance ratio (H/L) is less than 0.09, which falls within the typical range for debris avalanches but indicates relatively high mobility. Based on its volume and stratigraphic relationships, the collapse is interpreted to have been caused either by a phreatic eruption or by a non-eruptive large-scale failure of a hydrothermally altered zone beneath the summit area. The collapse is considered to mark the onset of the latest activity stage of Zao Volcano, and the petrological characteristics of the magma differ markedly from those of the preceding stage.