The Yakeyama Volcano, Niigata Prefecture

Abstract

The Yakeyama volcano is accessible from Kajiyashiki, a small town along the Hokuriku main line, passing through Otozaka and Yunokochi. It is about 16 kilo-meters from Yunokochi to the summit.
The volcano is an isolated cone, rising up to 2, 400.3 meters above the sea-level. It is underlain by Tertiary sediments assigned to the Miocene and Pliocene. These formations are mainly composed of shale and sandstone with thin layers of limestone, being intruded by liparite, propylite, etc. They are well bedded and highly cuffed along the Yake-kawa and Ogura-gawa. Recent fluviatile deposits mostly accumulate along the Haya-kawa, and talus deposits rest upon the steep slope of the volcano.
Mt. Yake is a Massive volcano built up of lava and agglomeratic lava partly covered by talus deposits and mud flow. These two kinds of lava are similarly represented by olivine-hornblende-bearing two pyroxene andesite containing the small fragments of liparite and basaltic rock. The mud flow begins to be exposed on the northern flank, from where it is traceable about 12 kilometers down to Otozaka along the Yake-gawa, Ogura-gawa, and Haya-kawa. It is characterized by the abundance of andesite blocks petrographically similar to the lava mentioned above in association with angular pieces of base rocks.
The volcano has three explosion crater and many fumaroles. One of these-explosion craters is found on the summit and seems to have been formed in 1361. Other craters are situated on the northwestern flank just above Jigokudani and also at the northern foot close to the upper end, of Sainokawara. The former is said to have been exploded in 1852, whereas the latter is of unknown age and the mud flow is supposed to have poured out from here.
Fumaroles were mostly opened in the recent explosion which suddenly took place on the fifth of February in 1949 after a long interval of quiescence elapsed since 1854.. Several fissures, on which these fumaroles were formed, are traceable from northeast to southwest on the summit (No. 1) and northeastern flank (No. 2-No. 7). Such fumaroles are bowl-like pits on the slope covered by talus deposits, but pass into irregular crevasses when the lava is exposed. The smoke is almost composed of steam with a small amount of H₂S and SO₄. Its temperature is 90, 2°-94, 5°C at the mouth of fumaroles.The surface close to smoking pits and crevasses is thinly veiled with sulphur and halotrichite. The seismological investigation carried out by Dr. Minakami, suggests that the source of this activity is probably situated three kilometers below the surface.