The evolution of magma and the 2000AD eruption and caldera subsidence event of Miyakejima Volcano, Izu arc, Japan

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In the course of this research project,1. Thirty-eight aerial inspection flights for caldera wall and growth of caldera from a helicopter were carried out during September 2000 to January 2003.2. Field survey and sample collection of the 2000 products were carried out in December 2001.3. Whole-rock and mineral composition of the juvenile materials during the last 10ka and borehole samples were analyzed to depict the evolution of magma.During the period from 2001 to 2002, enlargement of the subsidence 2000 Caldera proceeded very little, but for occasional collapse of unstable wall. Observation of the caldera wall revealed that the older pyroclastic products are exposed on the northern half in contrast to the younger lavas are piled up on the southern wall.Growth history and evolution of magma during the last 10ka was divided into following four eruptive stages ;1) Ofunato Stage of 10-4ka, characterized by porphyritic basalts, 2) Tsubota Stage of 4-2.5ka, distinguished by andesitic products, 3) Oyama Stage of 2.5ka to the 12^<th> century, characterized by basalt to basaltic andesite. This stage began with discharge of ca. 0.37km^3 and subsequent formation of the Hatchodaira Caldera. 4) Shinmio stage, from 1469AD to 1983AD eruption. The variation of Mg# (=Mg/(Mg+Fe)*100) versus erupted age during the last 4ka show saw-teeth like change ; abrupt increase of the ratio at 2.5ka, 1.3ka, and 0.5ka followed by gentle decrease. This pattern suggests occasional input of relatively undifferentiated magma into the magma plumbing system, which was slowly differentiating. The bulk composition of bombs ejected on August, 18, 2000 shows clear increase in Mg#. This implies that the magma plumbing system having been working since 15^<th> century was failed by the 2000 event and shifted to a new eruptive system.