Ash eruption of the Naka-dake crater, Aso volcano, southwestern Japan

Abstract Naka-dake is the only active central cone in the Aso caldera, central Kyushu, Japan. During active periods it erupts characteristically black ash, and the ash has been the dominant eruptive product during the past 6,000 years at Naka-dake volcano. The ash is formed by brittle fracturing of the solid glassy top of the magma column, and is transported by a gas stream from under the solid top. Occasionally, frothy scoria, Pele' s hair and glass spheres are also ejected from the underlying liquid part. This type of ash eruption is not peculiar to Aso volcano but it is an important, fairly commonly occurring process in andesite volcanism.