Evolution and Eruption Processes of a Highly Porphyritic Silicic Magma System: Petrology of the Historical Eruptive Stage of Hokkaido-Komagatake Volcano, Japan

After about 5000 years of dormancy, the Hokkaido-Komagatake volcano started its historical eruptive stage with a plinian eruption in ad 1640. During this eruption, a zoned magma chamber was formed with upper mafic and lower silicic layers. This study focuses on the temporal evolution and eruption processes of the magma chamber based on petrological investigations of subsequent eruptions in ad 1694, 1856, 1929 and 1942. In all the eruptive products the majority of the clasts are strongly porphyritic white pumice (silicic andesite), which was derived from the silicic end-member magma. These are associated with small amounts of mafic ejecta (mafic to intermediate andesite). Based on density calculations for the magmas, the layered structure of the zoned magma chamber is likely to have been stable throughout the historical eruptive stage. In each eruption since ad 1694, relatively mafic magma erupted initially followed by more silicic magma. In addition, although there is petrological evidence for a mafic magma injection into a resident silicic magma chamber prior to the ad 1640 eruption, all eruptions from ad 1694 onwards appear to have taken place without new mafic injections. This suggests that since the ad 1694 eruption magmas have been erupted sequentially from the upper mafic part of the magma chamber. The composition of scoria, the most mafic ejecta, changed from relatively mafic magma in the ad 1694 eruption to gray pumice in later eruptions, which represent a hybrid magma. This temporal change is interpreted to indicate that mafic magma mixed gradually with more silicic magmas during repeated eruptions. In addition, the volume of the mafic and hybrid magmas decreased because of repeated eruptions without new mafic magma injections, so that the ad 1942 eruption was dominated by the eruption of silicic magma. We consider the present chamber to be filled with highly porphyritic, viscous silicic magma.