A newly recognized 7.5 ka dome-forming eruption of Towada volcano, Northeast Japan Arc

The Towada volcano is an active caldera volcano located in the northern part of the Northeast Japan Arc. The Towada volcano has erupted repeatedly with a magnitude of VEI=4-5 at intervals of several hundreds to thousands years during the last 10,000 years. Therefore, long-term prediction of the eruptive activity of the Towada volcano is needed to avoid volcanic hazards. The Towada volcano consists of a caldera (10 km in diameter) formed at 15.5 ka, a small stratovolcano formed within the caldera after 15.5 ka, the Nakanoumi Crater (NC) on the summit of the stratovolcano, and the Ogurayama Lava Dome (OLD) as a flank volcano on the slope of the stratovolcano. In previous works, the OLD was regarded as the latest eruptive product of the 915 AD eruption. However, there is no definite geological evidence substantiating this opinion. The present study shows that the OLD was formed at 7.5 ka eruption, reconstructs its eruption sequence, and indicates the significance from the viewpoint of long-term prediction of eruptive activity. The OLD overlies pyroclastic deposits of 9.2 ka and is overlain by pyroclastic deposits of 6.1 ka. This shows that the OLD was formed between 9.2 and 6.2 ka. In this time interval, two pyroclastic eruptions occurred at 8.2 ka and 7.5 ka, and they are recognized as tephra fall deposits in the distal area. The distribution of the tephra fall deposits from the 7.5 ka eruption shows that the source vent was located in the vicinity of the OLD. Furthermore, the petrological features of the OLD closely resemble those of the 7.5 ka pumices. These observations indicate that the OLD was a product of the 7.5 ka eruption. The probable eruption sequence of the 7.5 ka eruption is as follows. Initially, intermittent phreatomagmatic eruptions occurred. These eruptions produced the lowermost part of the tephra fall deposits. Subsequent lava eruptions formed the OLD and accompanied intermittent vulcanian eruptions produced the main part of the tephra fall deposits. Previous studies have considered that the source vent of the 915 AD eruption was directly under the OLD. However, the source vent was not under the OLD, because the OLD had been formed at the 7.5 ka eruption. Since the only crater topography currently recognized in the Towada volcano is the NC, the source vent of the 915 AD eruption is considered to be the NC. The 915 AD eruption was the largest eruption in Japan during the last 5,000 years. Therefore, a re-examination of the eruptive style and eruption sequence of the 915 AD eruption should be conducted in the future. Since the NC has been the main crater throughout the post-caldera stage of the Towada volcano, future eruptions will probably occur in the NC. Currently, the NC is located at the bottom of a lake with a depth of 320 m. This suggests that the next eruption will probably be subaqueous eruptions under deep lake. If explosive eruptions occur in such a condition, the eruption plumes may collapse and not reach the air. Therefore, a detailed examination of the probable eruption style in the future will be required for predicting the volcanic activity of the Towada volcano.