Mount Fuji

One of the Earth's most famous composite cones, Japan's Mount Fuji is composed of a group of 3 overlapping cones and 100 peripheral cones. Most of these vents are located along a north-northwest-south-southeast-trending line that is parallel to regional structures (Kuno, 1962; Tsuya et al ., 1981). The 1.5- × 0.7-km summit crater is 750 m deep. Older lavas at Fuji are olivine basalts, whereas the younger cones are composed of andesite and peripheral cones of olivine basalt. The cones comprise interbedded ashfall deposits, lahar deposits, ignimbrites, and lava flows. The complex is ~80,000 years old, and the most recent activity occurred in 1707 AD (Kuno, 1962).

All thermal anomalies on Fuji are masked by movement of shallow groundwater. Yuhara (1974) reported that each year 77% of the precipitation flows out at the foot of the cone and another ~20% is lost by evaporation. Tritium analyses have revealed that water emerging near the base is not old. Shallow aquifers follow scoria-lapilli beds and fractures in lava flows. Impermeable units are mudflow deposits and older basement rocks. Very little of the recharge water enters the volcano's interior to be heated. Mount Fuji has not been drilled for geothermal resource evaluation.

From Yuhara's (1974) observations at Fuji and Mount Yotei (also in Japan), it was determined that recharge into the hydrothermal system of a composite cone may not be adequate to maintain a hydrothermal system. Conditions necessary for adequate recharge will most likely depend upon the structural framework of the individual volcano, its maturity, fault patterns, and the degree of intrusion and hydraulic fracturing associated with dike-sill systems and small plutons. As described earlier, eroded, mature volcanoes in areas with thick continental crust expose large volumes of hydrothermally altered lava and pyroclastic rock.