噴出物から探る西之島の新火山島形成プロセス

抄録

The island-forming eruption at Nishinoshima volcano, Ogasawara Islands, Japan, began its activity in November 2013 and once ended in late 2015 (1^st stage). The eruption restarted in April 2017 and finally the activity ceased in August 2017 (2^nd stage). In this paper we discuss the sequence of the eruption, and characteristics of magma, based on geological and petrological analyses. The eruptions were characterized by continuous Strombolian activity with lava effusion in both 1^st and 2^nd stages. One of the most intriguing characteristics of the products is a vast lava flow field, consisting of multiple lava lobes with clefts. The clefts are thought to be the products from lava inflation driven by an increase of internal pressure by successive injection of new lava into the lobes. During the landing survey in October 2016, we collected samples from lavas and fallout deposits. The petrological analyses were carried out together with other rock samples collected by different surveys at Nishinoshima. The 2013–2015 lava flows were andesite with <10 vol.% of phenocrysts. The whole-rock composition is 59.5–59.9 wt.% in SiO₂. The petrological features of the 2013–2015 lava flows are similar to those of products from the past eruptions; however, the whole-rock compositions are clearly distinguished from the 1973–1974 products and the pre-1702 products, and lies on the narrow range between these two products. Magma temperature was estimated to be 1050°C using a pyroxene thermometry. Viscosity was estimated to be ~10⁴–10⁶ Pa · s based on petrological data. The variation of phenocryst compositions suggests that andesite magma derived from a crystal-rich shallower reservoir and a crystal-poor deeper reservoir. The depth of shallower magma reservoir was estimated to be 1.5–2 km equivalent to water saturation pressure, which is estimated based on melt inclusion analysis and thermodynamic calculations.