Geology and petrology of Moriyoshi volcano

Abstract

Moriyoshi volcano is situated between the inner volcanic zone (the Chokai zone) andthe outer one (the Nasu zone). The erupted materials cover an area of about 80 km² and their total volume is estimated to be about 21 km³.
The volcanic activity is divided into two main stages, earlier and later stages. In the earlier stage, pyroclastics with. small amount of lava flows erupted to form a cone-shaped stratovolcano. Rocks are augite-hypersthene andesite (SiO₂ 57-62%) accompanied with olivine-bearing augite-hypersthene andesite (SiO₂ 55-57%). The volcanism of the earlier stage terminated with a dacitic lava (SiO₂ 66%) flow eruption, thereafter an ellipse-shaped caldera (3.5×3km in size) was formed. In the later stage, lava flows effused and lava domes extruded nearby the caldera wall. At the eastern portion of the volcano, lava dome was accompanied by the formation of explosion crater. Rocks of the later stage include quartz-bearing hornblende-augite-hypersthene-olivine basalt (SiO₂ 52%), quartz-bearing olivine-aguite-hornblende-hypersthene andesite (Si0₂ 55-61%) and quartz-augite-bearing hypersthene-hornblende dacite (SiO₂ 65%). Olivine+quartz phenocrysts assemdlage is common in the basalt and andesite.
Major element analyses of 23 rocks indicate that the FeO^*/MgO of the rocks from the earlier stage increases with increasing SiO₂, while that from the later stage does a little. In the view of lateral variation of geochemical data, alkali contents of the rocks from Moriyoshi volcano show transitional values between those of the inner volcanic zone and the outer one.