Merapi volcano in Central Java is an archetype of persistently degassing and erupting andesitic arc volcanoes with extruding lava domes. Over the past three decades the Merapi Volcano Observatory (BPPTKG, Yogyakarta) sampled and analyzed its hot gas emissions (500 to 900°C) and surveyed its SO2 plume flux. In November 2010, however, a centennial paroxysmal eruption (VEI = 4; Volcanic Explosivity Index) interrupted standard dome-building activity, blasting out the volcano summit and the fumarolic fields previously accessible to gas survey. Although one million people were successfully evacuated, the 17-km long pyroclastic flows claimed approximately 400 victims. Since then no new magma has extruded, but in 2012 to 2014 the volcano produced several discrete explosions (with no warning signal at all), with columns reaching up to eight km in height and ballistic blocks one km distance. As no more gas surveying has been possible at Merapi since October 2010, it is impossible to know whether these explosions had precursory geochemical signals.
In August to September 2014 a team of volcanologists from IPGP (Institut de Physique du Globe de Paris, France, P. Allard, PI), Palermo University (Italy, A. Aiuppa and R. Di Napoli), ISTO (Institut des Sciences de la Terre, Orléans, France, Y. Moussalam), and IRD (Bandung, Indonesia, P. Bani), collaborating with BPPTKG, performed the first measurements of Merapi volcanic gases since the 2010 eruption. A mini-grant from DCO’s DECADE (Deep Carbon Degassing) initiative and funding from the French ANR project “DOMERAPI” (Dynamics of an arc volcano with extruding lava domes, Merapi (Indonesia): from the magma reservoir to eruptive processes) supported this work. The researchers measured the overall chemical composition and mass flux of current post-paroxysmal degassing by combining OP-FTIR and UV remote sensing (dual UV cameras and scanning DOAS), as well as in situ MultiGAS analysis of the volcanic plume. DECADE scientists focused on quantifying magma-derived CO2 emissions from Merapi in its post-paroxysmal stage—observations of particular importance to understanding possible precursor geochemical signals of eruptions.
Preliminary results reveal an extremely low magma degassing rate in the current stage of activity when compared to previous periods. Low vent temperatures (<200°C) made OP-FTIR remote sensing from the crater rim impossible, but MultiGAS analysis of the air-diluted volcanic plume allowed the team to determine the molar composition of current gas emissions from the 2010 lava dome: 97.6 to 98.1% H2O, 1.5 to 2% CO2, 0.28 to 0.32% SO2, and 0.014 to 0.018% H2S. The SO2 plume flux, measured at close distance with dual UV cameras, averaged about 80±10 tons/day which, combined with the gas composition, implies current mass fluxes of about 7,000 t/d H2O and 300 t/d CO2. When compared with data for the time-averaged composition and flux of pre-2010 gas emissions, these values demonstrate a strong water-enrichment (or hydrothermal dilution) and low emission rate of present-day Merapi volcanic gases, consistent with low levels of seismicity and progressive cooling of the lava dome (thermal infrared imaging). Put altogether, these observations indicate no shallow magma refilling until now.
To monitor the future reawakening of the volcano and to detect possible precursory gas signals of the discrete explosions, our Indonesian colleagues in BPPTKG propose to establish an automated survey of volcanic gas emissions, using both MultiGAS and UV camera (with radio transmission of data). Thanks to the DECADE project and the team’s experience with Merapi, such a step ahead could be extremely promising for both research (CO2 flux in relation to eruptive activity changes) and risk assessment on one of the most active and hazardous volcanoes in the world.
Report and images provided by Patrick Allard, IPGP.