Kuju Volcano
1: Introduction - 2: Topography
3: Eruptive history of Kuju Volcano
4: Historic eruptions
5: Petrological characteristics of products
6: Hot springs and geothermal and ore deposits - 7: Current activities and observation system - 8: Characteristics of eruption activities and precautions taken in volcanic disaster prevention
Acknowledgements / References
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5: Petrological characteristics of products
The products of Kuju Volcano have a wide compositional range from basaltic andesite to dacite with SiO2 = 53–68 wt.% (
Table 2; Figure 10). Andesite with SiO2 = 57–63 wt.% is most common. Phenocrysts include olivine, clinopyroxene, and orthopyroxene, and in rocks with SiO2 of 59 wt.% or higher, hornblende phenocrysts are included. Biotite phenocrysts are common in the dacite rocks. A non-equilibrium combination of phenocrysts, such as quartz and olivine, are observed along with banded lava and banded scoria, indicating that mafic and felsic magmas were mixed. K2O relative to the whole-rock SiO2 indicates an increasing trend in K2O with time in the andesitic composition; however, dacite composition exceeding SiO2 of 65 wt.% was constant throughout the periods of activity.
Limited to the products of Stage 4, in which the stratigraphy is well known, each erupted units of Taisen and Kitataisen volcanoes have unique compositional ranges, and the identification of lava and tephra is relatively simple. The Taisen-higashi Lava of Taisen Volcano has a wide composition range of SiO2 = 59.8–66.2 wt.% and a high amount of K2O. Its unique compositional trend shows that K2O decreased with increased SiO2. The youngest magmatic eruption, Kurodake Volcano products, had little K2O in contrast. .