Viscosity of andesitic lava and its implications for possible drain-back processes in the 2011 eruption of the Shinmoedake volcano, Japan

Abstract

The 850 m diameter crater of the Shinmoedake volcano was filled by andesitic lava after three subplinian eruptions on 26-27 January 2011. We analyzed blocks thrown from the lava-filled crater by subsequent Vulcanian explosions to estimate the lava’s viscosity and evaluate the possibility of drain-back processes in the crater. Petrographic work on the ejecta, including bulk and glass chemistry, phenocryst and microlite modes, and the water content of the glass enabled us to estimate the bulk viscosity of the lava to be 109.8(+15 − 12) Pa s. The conduit radius is constrained to 4.5 to 6 m by the eruption rate of preceding subplinian eruptions (450–740 m3/s dense rock equivalent). We estimate the simple drain-back rate of the lava to be 3 × 10−2 ~ 2 × 10−5 m3/s. At this rate, less than 1 percent of the total amount of the effused lava could drain back within 100 days. Synthetic aperture radar (SAR) observations did not reveal evidence of drain-back after the eruption, possibly because the chamber was sustained, at least in part by repressurization and refilling as observed by global navigation satellite system (GNSS) measurements of the volcano. This study showed that degassing and crystallization of the andesitic magma during emplacement increased magma viscosity by more than five orders of magnitude, prohibiting drain-back of the lava that filled the crater after the emplacement.