Abstract
Numazawa volcano is a small caldera located in the southern part of the Northeast Japan Arc. Previous works have shown that this volcano has produced a pyroclastic flow deposit and a plinian pumice fall deposit, named the Numazawako Tephra (Nm-NM), at around 5ka. The existence of several tephra layers stratigraphically below the Nm-NM deposit has also been determined. The present authors have clarified that a series of eruptions from Numazawa volcano has produced tephra between 50-55ka preceding the Nm-NM eruption. This newly nominated tephra is named the Numazawa-Kanayama Tephra (Nm-KN). The purposes of this paper are (1) to present the petrographic properties, stratigraphic position, and distribution of this tephra, and (2) to discuss its eruptive age, volume of deposit, and relations with caldera formation history.
The Nm-KN around the caldera vent consists of several plinian pumice falls and a subsequent ignimbrite, which is partially welded. A volcanic clay fall deposit is found in the area close to the vent, suggesting that a steam eruption has occurred during the tephra eruption. The air fall deposit of the Nm-KN covers over 100km eastward of Numazawa volcano. In the Aizu Basin and on the flanks of Bandai, Azuma, and Adatara volcanoes, the Nm-KN is found as a plinian pumice deposit in most cases, and possible co-ignimbrite ash fall is observed. The Nm-KN is distinguished by its phenocrystic cummingtonite (n2=1.656-1.665) hornblende (n2=1.666-1.677), and biotite. The eruptive age of the Nm-KN is estimated at 50-55ka on the basis of its stratigraphic relationship with two well-dated tephras. These are Aso-4 (86-90ka) leveled below the Nm-KN, and DKP (50-52ka) just above. Calculated apparent eruption volumes of the Nm-KN are 1.4km3 for the plinian pumice deposit and 1.5km3 for the ignimbrite. It is most likely that the vent from which the Nm-KN originated was located at the present Lake Numazawa.
