Temporal variations in discharge rate and component characteristics of tephra-fall deposits during the 2014–2015 eruption of Nakadake first crater, Aso Volcano, Japan

Abstract

The 2014–2015 eruption of the Nakadake first crater at Aso Volcano in southwestern Japan was characterized by continuous ash emissions and intermittent strombolian eruptions. In this paper, we present the distribution, discharged mass, and components of tephra-fall deposits to examine the sequence of activities. We installed 21 ash samplers around the crater (SW crater rim to 9 km in all directions) and calculated the mass of ash-fall deposits based on 28 isomass maps. From November 25, 2014, to the end of January 2015, the cumulative erupted mass increased at a high discharge rate (2.2 × 104 tons/day). After February 2015, the cumulative erupted mass decreased to a low rate of 0.6 × 104 tons/day, although this rate rose slightly in March and late April 2015. The 2014–2015 tephra-fall deposits consisted of glass shards, crystal, and lithic grains. In the November 25–27, 2014 ash-fall deposits, lithic fragments, which are interpreted to be derived from lavas or pyroclasts of previous eruptions, were dominant (59–68%). Thereafter, the proportion of glass shards, which are probably juvenile materials of newly ascending magma, gradually increased with time, and the December 21–23, 2014 ash contained abundant glass grains (63%). The proportions of glass shards ranged from 29 to 50% until February 25, 2015. Subsequently, they decreased with time and reached 14% on March 17. Afterward, the proportions increased again prior to April 27 and ranged between 20 and 30% in May 2015. The total erupted tephra mass from the November 2014–May 2015 activity of Nakadake first crater was 2.1 × 106 tons (1.2 × 104 tons/day), which was less than the tephra deposits of previous activities that have occurred within the past few decades.[Figure not available: see fulltext.].