Evolution of a large-scale phreatoplinian eruption: Constraints from the 40 ka caldera-forming eruption of Kutcharo volcano, eastern Hokkaido, Japan

Abstract

“Phreatoplinian” is an explosive phreatomagmatic eruption style that is defined by the fragmentation of magma and widespread dispersal of the resulting fine ash and accretionary lapilli. These eruptions pose significant future risks at caldera volcanoes that host lakes and abundant groundwater. There have been no direct observations of a phreatoplinian eruption, therefore, constraining the detailed mechanisms and sequences of such events relies on studying the deposits of previous eruptions. In order to advance our understanding of these hazardous phenomena we conducted a case study of the 40 ka caldera-forming eruption (Kp I) from Kutcharo volcano in eastern Hokkaido, Japan. We subdivided Kp I eruption deposits into 7 units (Units 1 to 7 in ascending order). Units 1 to 6 are air fall deposits consisting of alternating thin pumice and thick silty ash layers with abundant spherical accretionary lapilli. Stratigraphically higher ash fall units are thicker, finer in grain-size, and more widely distributed. The maximum eruption column height and mass-discharge rate were calculated to be 40 km and 1.4 × 10⁹ kg/s, respectively. Unit 7 is a climactic ignimbrite (76 km³), which is distributed widely over the area north of Kutcharo caldera.

Unit 6 is the largest air fall unit and can be considered to have been deposited by a phreatoplinian eruption, given its abundant accretionary lapilli, wide dispersion, and high degree of fragmentation. Unit 6 had the highest mass discharge rate (1.4 × 10⁹ kg/s), suggesting the interaction between magma and external water was most intense, and it is thought that a large eruption column covered eastern Hokkaido. In addition, Kp I eruption deposits commonly contain glass shards derived from fragmentation via both magma degassing and Molten Fuel Coolant Interaction (MFCI). To account for this observation, we infer that the conduit penetrated a large aquifer, and the margin of the ascending magma came into contact with this external water source. Due to repeated caldera-forming eruptions, intra-caldera filled deposits (hosting a large aquifer) likely played a key role in supplying external caldera lake water to a level near the fragmentation depth of H₂O-saturated felsic magma. The occurrence of these intra-caldera conduit and caldera-lake systems may provide the required conditions for phreatoplinian eruptions at continental arc caldera volcanoes in Japan and globally.