The historical eruption records of Ontake Volcano were reexamined by this study. Previous study pointed out that there were historical eruption records of Ontake Volcano in 774 AD and 1892 AD; however, there were no description of the eruptions in the original records. Therefore, the historical eruption records of Ontake Volcano before 1979 AD phreatic eruption are undiscovered now. Probably, the Ontake Volcano did not erupt from the 18th century to 1979 eruption; however the fumarolic activities in the Jigokudani at the summit area have existed since the middle of 18th century. After 1979 eruption, the fumarolic activities are recognized in Jigokudani and Hachotarumi, which is the summit area of Ontake Volcano. In 1991 and 2007 AD, there was emission of the volcanic ash (micro eruption) from the fumaroles in Hachotarumi. The volcanic activity of Ontake volcano after 1979 eruption is the most active in 250 years recently.
The Monzen Formation is defined to occur between the Akashima Formation and the Nomuragawa and Daijima Formations and the type locality is specified along the coast from Monzen to Akashima, western Oga Peninsula. The lower contact with the Akashima Formation is faulted and the upper contact with the Nomuragawa and Daijima Formations is unconformable. The Monzen Formation is newly divided into the following members in ascending stratigraphic order: Butaijima Basalt, Ryugashima Dacite, Chorakuji Sandstone and Conglomerate, Chorakuji Basalt, Nagasaki Dacite, Kenashiyama Andesite, and Shinzan Rhyolite. The Chorakuji Basalt is locally interbedded with Chorakuji Sandstone and Conglomerate. The volcanic rocks dominated in the Monzen Formation erupted in a short period of time most likely from 36 Ma to 34 Ma to constitute volcanic edifices in a terrestrial to shallowwater area slowly rifting under extensional stresses. The geologic processes of the Monzen Formation are likely to represent precursors of the Early Miocene rapid opening of the Japan Sea. However, there is a large time gap over 107 years between the Monzen Formation and the overlying Daijima Formation, and it is necessary to extend the survey area to find the missing link between the two units.
Magnetite-series Paleogene granitoids of Mo-mineralized region of eastern Shimane Prefecture were studied chemically and compared with ilmenite-series granitoids of the Ryoke belt of the Chubu district. The eastern Shimane granitoids are divided into coarse-grained granodiorite and granite of batholithic bodies, and fine-grained granitoids occurring close to the roof-pendant. The fine-grained ones, varying in composition from quartz diorite to aplite are further subdivided into: Zakka and Kawai types, Rengeji type, leucogranites in the ore horizon, Yamasa type, Shimokuno type, and Ouchidani type. These granitoids are poor in A/CNK, Ga, Ga x 10000/A, K2O, Rb, Ba, Pb, CaO, Fe2O3, Zn, Y, La and Ce, and rich in Na2O, MgO, V and U, as compared with those of the Ryoke granitoids in the Chubu district. The chemical data indicate that the eastern Shimane granitoids are originated in source rocks with poor continental components, such as Al2O3, K2O, REE, and organic carbon. Some gabbroids which are high in Sr content and Sr/Y ratio may have been derived from slab-melting. Rengeji Older Granite is potassic being K2O>Na2O, and rich in La, Ce, Y, Nb, Th, U; thus continental in the source rocks.
There occur small metamorphic bodies containing locally spinel and andalusite along the northwestern margin of the Rengeji Granodiorite. Kanenari hornfels is considered mainly psammitic with peraluminous layers. Togiishiyama hornfels has high A/CNK ratio but the K2O contents are normal as to the SiO2 contents. This rock contains rock-forming mineral of magnetite and has unusually high amounts of S, Cu, Pb, Zn and MnO. Therefore, the original rocks are considered intermediate to felsic tuffaceous sediments containing very little organic carbon. Mo-contents are high as 2.0 to 6.3 ppm Mo in average of the granitoids that host major molybdenite deposits, such as Kawai mingled rocks (Daito mine), Rengejileucogranites (Seikyu and Higashiyama mines) and Yamasa leucogranite (Yamasa mine). Therefore, trace amounts of Mo of fresh granitoids can be used as an exploration indicator.
Kabuki Iwa (Rock) at the northwestern shore of Oga Peninsula is composed of Late Eocene basaltic andesite aa lava flows and pillowed lava flows, dacitic pyroclastic flows, debris flows and other epiclastic rocks. This close association of the subaerial and subaqueous volcanic products demonstrates a transitional environment between land and shallow water. NE-SW-trending parallel dikes and normal faults are also associated with these rocks in the surrounding areas, and the volcanic succession at Kabuki Iwa is interpreted to have accumulated in an extensional basin which slowly subsided with volcanism before the rapid opening of the Japan Sea.
In order to clarify the change in the geochemical characteristics of water during hydrological cycle in mountainous and forested region, we studied chemical compositions of precipitation, streamwater, spring water and groundwater in the Kanamaru area, northeastern Japan, since 2002 to 2005. The groundwater was taken from 50 meters deep nine wells in a foot part of mountainous region where the Cretaceous granites and overlying Miocene sedimentary rocks are distributed. Each well was drilled through both the sedimentary rocks and granitic basement. The precipitation, streamwater and spring water were collected from the area surrounding the wells. The precipitation, streamwater and spring water show seasonal change in electric conductivity (EC) and concentrations of Cl－ and Na+; they are higher in the winter season and lower in the summer and snow-melt seasons. The geochemistry of groundwater is characterized by increase of pH, EC and concentrations of Ca2+ and HCO3 － and decrease of dissolved oxygen and Eh with increasing depth.