The upper part of the Hanno Formation, distributed on the western part of the Kanto Plain, consists of a pebble~boulder, subrounded~wellrounded, clast-supported conglomerate, and the gravel species are sandstone, mudstone, chert and include a small amount of plutonic rock.

About the supply route of gravels in the upper part of the Hanno Formation and source of the plutonic rock gravels, two hypotheses have been proposed, the Tama River hypothesis and the Chichibu Depression Zone hypothesis. The former refers to the source as the Kofu and/or the Mitosan plutons. On the other hand, the latter uses the river flowing through the Chichibu Depression Zone as a supply route, and seeks the origin of plutonic rock gravels in the Chichibu and/or the Kofu plutons. Both hypotheses remain to this day without definitive geological evidence. Therefore, this study aimed to directly identify the source of the plutonic rock gravels in the upper part of the Hanno Formation from the geochemical data.

Plutonic rock gravels were collected from two points on the banks of the Iruma River, Azu and Yaoroshi. In addition, the Kofu pluton Tokuwa type, the Mitosan pluton, and the Chichibu pluton, which are considered to be the source of plutonic rock gravels, were subjected to petrographical observations, analysis of major components, trace elements, and rare earth elements.

This study focuses on the gravels of the Hanno Formation,which were determined to be fresh rock using Chemical Index of Alteration, and classified into three types based on observations: Azu MA (Major)type, Yaoroshi MA (Major) type, and Yaoroshi LE (Leucocratic) type. It was revealed that Azu MA type and Yaoroshi MA type have characteristics in common with the Kofu pluton Tokuwa type. Both MA types include samples that are thought to have advanced crystal differentiation than the Kofu pluton Tokuwa type. The Yaoroshi LE type is often inconsistent with any pluton in classification diagrams, and since it has a high Sr/Y ratio, it is presumed to be adakite, and it was concluded that the source may be an unknown pluton.

In the "Earth Science Experiment" class at the Tokyo University of Agriculture, field observations are conducted for one day in Jogashima, Miura City, Kanagawa Prefecture. In the field training conducted in May 2022, volcanic block presumed to be a breadcrust volcanic bomb was discovered in the scoria layer of the Misaki Formation. In general, volcanic bombs erupt as masses of magma larger than 64 mm in diameter, and volcanic bomb, volcanic block and lapilli with ballistic trajectories have an average reach of less than 2 km, and reach more than 5 km from the volcanic crater is considered to be rare (Blong 1984). According to Ui (1997), cinder and volcanic bomb cause disasters within a radius of 2 to 3 km from the volcanic crater. Volcanic block presumed to be a breadcrust volcanic bomb with a diameter of more than 64 mm discovered this time indicate that a volcano with a crater above sea level was within a radius of about 10 km from Jogashima. This suggests that it has important implications for estimating the location of source volcano.