Abstract
Kumanodake agglutinate of the Zao volcano in NE Japan comprises pyroclastic surge layers accumulated during the early newest stage. Rocks are mixed calc-alkaline olv-cpx-opx andesite (55.2-56.2% SiO2). The magma feeding system comprises a shallow felsic magma (63-67% SiO2, 940-970 deg C, cpx-opx-plg) injected from depth by mafic magmas. The main mafic magma was basalt (ca. 50% SiO2, 1110-1150 deg C, olv). The mafic magmas are stored occasionally at 3-6 km depth and are differentiated to basalt (1080-1110 deg C, olv-plg). The forced injection of mafic magmas withdrew felsic magma before their mixing during eruption. Their explosivity decreased over time because the scale of the scoriae decreased and the erupted magma composition became more mafic. These features reflect the increased percentage of mafic magma involved in mixing. At the beginning of activity, the mafic magma also acted as a heat source for activation of the felsic magma chamber, thereby suppressing the volume percentage of mafic magma in the mixing. End-member magmas mixed more easily as the activity proceeded.
