Variation of volcanic gas composition during the eruptive period in 2014–2015 at Nakadake crater, Aso volcano, Japan

Abstract

Volcanic gas composition measurement by Multi-GAS was repeated during the eruptive period in 2014–2015 as well as the quiet period preceding the eruption at Nakadake crater, Aso volcano. The eruptive activity is characterized by continuous ash emission with intermittent Strombolian activity and temporal pauses. Volcanic gas composition measured during the eruptive period showed a rapid and large variation. In particular, the CO2/SO2 and SO2/H2S ratios varied in the rages of 1–8 and 3–300 during the ash eruption with a clear negative correlation. The large variation and the negative correlation of the compositions are attributed to two orders of magnitude difference of degassing pressure, such as 20 and 0.2 MPa; the gases with the large CO2/SO2 and the small SO2/H2S ratios are derived from the high pressure. The rapid and large composition variation suggests frequent ascent of bubbles formed at various depth during the eruption. The maximum CO2/SO2 ratio decreased with decreasing eruption intensity that suggests decrease in contribution of the bubbles derived from a large depth. With time, H2O/SO2 ratio of the gases increases from 30 to > 60, suggesting increase in a hydrothermal contribution.[Figure not available: see fulltext.].