Merapi Volcano: From Volcanic Gases to Magma Degassing

Volcanic activity at Merapi is characterised by continuous summit degassing, sustaining a volcanic plume both between and during eruptions. A considerable amount of data was obtained on these volatile emissions which, in complement to geophysical surveys, provides a sensitive understanding of the volcano dynamics. Direct sampling and analysis of high-temperature (400-870 degrees C) gases issued from the long-lived fumarolic fields of Gendol and Woro was initiated in the late 1970s, then operated for routine monitoring until the 2010 eruption. At the same time, the SO2 plume output became regularly surveyed using remote UVspectroscopy (COSPEC then DOAS). Both types of measurements, plus isotopic investigations and trace metal analysis, have provided important insight into the magmatic source of Merapi, the gas emission rates and their relationship with volcanic activity, and the compositional gas changes precursory to major eruptive events (e.g. the 2010 Plinian eruption). Moreover, the initial abundances and pressure-related behavior of volatile components in Merapi magma were determined from both microprobe analysis of natural samples (melt inclusions, crystal zoning) and petrologic experiments. Combined with volcanic gas data, such information allows constraining the magma supply rate and degassing budget, as well as the depth and dynamics of magma degassing processes controlling the volcanic activity. Finally, a specificity of Merapi is that its upper plumbing system intrudes thick carbonate sediments whose fragments are commonly entrained by the magma and whose metamorphism or even melting may contribute significant amounts of carbon dioxide to the volcano degassing budget. After reviewing this information, two synthetic models for Merapi volatile sources and emissions are proposed.