TEN YEARS OF DEGASSING AT MASAYA VOLCANO, NICARAGUA : WHAT HAVE WE LEARNED ?
Delmelle1 P, Allen2 AG, Delfosse3 T, Gauthier4 P-J, Lewicki5 JL, Mather6 TA, McGonigle7 AJS,Oppenheimer8 C, Pyle6 DM, Stix9 J, Williams-Jones10 G
1Environment Dept., University of York, York, UK, email : pd512@york.ac.uk
School of Geography, Earth and Environmental Sciences, University of Birmingham – 3Unité des Sciences du sol,
Université catholique de Louvain, Louvain, Belgium 4 – Laboratoire Magmas et Volcans, Observatoire de Physique duGlobe, Clermont-Ferrand, France – 5Earth Sciences Division, Lawrence Berkeley National Laboratory, Berkeley, USA– 6Dept. of Earth Sciences, University of Cambridge, Cambridge, UK – 7Dept. of Geography, University of Cambridge,Cambridge, UK – 8Dept. of Geography, University of Sheffield, Sheffield, UK – 9Dept. of Earth & Planetary Sciences,
McGill University, Montreal, Canada – 10Dept. of Earth Sciences, Simon Fraser University, Vancouver, Canada

2In mid-1993, appearance of an ephemeral lava lake accompanied by emission of a persistent gas
plume indicated reactivation of Masaya volcano, a large basaltic shield edifice in central Nicaragua.
Almost 13 years later, the gas crisis continues with average SO2 emission rates of ~800 Mg/day andpersistent incandescence of the degassing pit vent. Historical accounts reveal that several episodesof intense gas emissions have occurred at Masaya since the time of the Spanish Conquistadors. The
ongoing gas crisis has provided an unprecedented stream of geophysical, geochemical andenvironmental data, acquired on a more or less regularly basis for the last 10 years. These havegiven new insights into magma degassing processes, plume chemistry and environmental impacts atMasaya volcano.
Integration of micro-gravity and gas flux measurements reveals a pattern of repeated fluctuations,which cannot be explained by intrusion of new magma batches at shallow depths. Rather, it has
been surmised that a gas-rich vesiculated zone forms and oscillates near the surface immediately
beneath the active crater. In parallel with conventional plume sampling techniques, Fourier
Transform InfraRed (FTIR) spectroscopy has been intensively used to monitor Masaya’s plume. Astriking result is that in terms of SO2, HCl and HF, the Masaya gas composition has been relatively
stable since 1998, suggesting an open-degassing system with deep release of volatiles from themagma. In contrast to the major gas species, recent time-series data on radioactive disequilibria in
the gas plume of Masaya indicate significant variations. The working hypothesis is that input ofdeep pristine magma in a large magma reservoir controls the gas flux at the surface. Soil diffuse
degassing also occurs on the flanks of Masaya. Fluxes of CO2 (with a deep origin) are among thehighest reported for volcanoes worldwide.
The plume of Masaya has also been measured for the near-source chemistry of its particulate phase.
This work has highlighted the presence of "primary" sulphate aerosols that may have been formed
by SO2 oxidation at high temperature. Another interesting discovery is linked to the possibility thatthe hot vent allows thermal fixation of atmospheric N2 and concomitant formation of NO, NO2 andHNO3. In addition to the near-source studies, a few experiments have been carried out with the aimof understanding how the plume chemistry evolves during transport in the atmospheric boundary
layer. Apparently, there is little variation in the ratios of the plume components between the crater
rim and 15 km downwind, suggesting that, over this length scale, atmospheric processes do notaffect the plume composition significantly.
Atmospheric dispersion of the low-altitude gas plume of Masaya results in profound environmental
and agricultural impacts downwind. These include SO2 concentration levels well above backgroundvalues, and moderate to extensive damage to vegetation in areas directly exposed to the volcanic
emissions. Large deposition fluxes of acidic species (SO2, HCl and HF) affect a 22-km2 area southwest of the volcano, and concerns have been raised over the possibility of soil acidification.
Laboratory and field experiments involving the volcanic ash soils found in this area demonstrate thestrong capacity of the soils to buffer acid inputs. However, continuous deposition of volcanic acids
has led to spectacular accumulation of fluoride in the soils. It is anticipated that both short- andlong-term negative health effects occur as a result of chronic exposure of people to Masaya’s gases
and particles. High level of fluoride in soils also may pose a risk to grazing animals.