Ejecta stratigraphy and eruptive sequence of the Eruptive Episode M in Towada Volcano, Northeast Japan

Abstract

Towada Volcano is an active volcano located in the northern part of the Northeast Japan Arc. This study describes the stratigraphy, field characteristics and eruptive sequence of Eruptive Episode M, which occurred at 21 cal kyr BP during the caldera-forming stage. The proximal eruptive products consists of the Kumoi Pyroclastic Flow Deposits and Karatamazawa Pumice; the distal facies consists of the Maita Tephra (subdivided into the Maita 1 Pumice and Maita 2 Ash). The Karatamazawa Pumice and Maita Tephra are pyroclastic fall deposits probably derived from Plinian and phreatoplinian eruptions. All these deposits are composed of phenocryst-poor vesiculated rhyolite pumice. Their stratigraphic relationship remains unknown because they have not been observed in the same outcrop. The Kumoi Pyroclastic Flow Deposits and the Karatamazawa Pumice are estimated to be contemporaneous with the Maita 2 Ash based on the similarity of petrological features. During Eruptive Episode M, a Plinian eruption first formed the Maita 1 Pumice; shortly thereafter, intermittent phreatoplinian eruptions under the influence of lake water formed the Maita 2 Ash and Karatamazawa Pumice. During these eruptions, the eruption column partially collapsed and the resulting pyroclastic flows emplaced the Kumoi Pyroclastic Flow Deposits. The apparent and dense rock equivalent eruptive volumes of Eruptive Episode M are estimated to be 9.0 km³ and 2.8 km³, respectively.