Report on Cerro Negro (Nicaragua) — June 1994
Bulletin of the Global Volcanism Network, vol. 19, no. 6 (June 1994)
Managing Editor: Richard Wunderman.
Cerro Negro (Nicaragua) Temperature data and Rn, CO2, Hg, and He anomalies
Please cite this report as:
Global Volcanism Program, 1994. Report on Cerro Negro (Nicaragua). In: Wunderman, R. (ed.), Bulletin of the Global Volcanism Network, 19:6. Smithsonian Institution. https://doi.org/10.5479/si.GVP.BGVN199406-344070.
12.506°N, 86.702°W; summit elev. 728 m
All times are local (unless otherwise noted)
Beginning on 27 May, scientists from FIU and INETER spent 7 days at Cerro Negro as part of an ongoing project to determine the areal extent of passive degassing and the role of structural controls on degassing at the cone. During three field surveys, more than 65 stations were established over an 8 km2 area to determine the concentration of Rn, CO2, Hg, and He.
Two anomalies were discovered. The first fell along step-faults, fractures, and low-temperature (80-90°C) fumaroles on the N rim of Cerro Negro. The anomaly extended at least 1.5 km N into the Cerro La Mula cinder cone complex. Along the anomaly, Hg ranged from several hundred to >4,000 ppb; He ranged from 5,260 to 5,540 ppb; Rn ranged to 24 picoCurie/liter; and CO2 ranged to 3.1 volume %.
A second anomaly stretched from the ESE foot of the cone to about 1 km S to the Las Pilas-El Hoyo complex. Gas concentrations along this anomaly were somewhat less than those reported for the N-rim anomaly. The anomalies were collinear with an alignment of 12 cinder cones and maars, a progression of vents that includes Cerro Negro.
Ground temperatures measured in May along the N rim (70-90°C) and on the E flank of Cerro La Mula (45-65°C) were identical to temperatures measured in March. A low-temperature (75°C), low-flux fumarole was noticed for the first time, at the E base of Cerro La Mula, about 700 m N of Cerro Negro. This fumarole was probably ephemeral, it was apparently not fuming during the dry season.
Researchers further noted an apparent increase in flux from fumaroles on dike complexes in the crater of Cerro Negro. (Crater fumaroles at Telica behaved similarly in June, and nearby villagers reported that degassing always increases during the wet season). Compared to March, during May the concentrations of Rn and CO2 at Cerro Negro increased 2-6 fold. According to Wilfried Straunch and Helman Telano (Seismological and Volcanological section of INETER, respectively), seismicity at Cerro Negro remained at background levels. Thus, the recent increase in fumarole output and in gas concentrations appeared linked to the onset of the rainy season, and not to increased magmatic degassing.
Geologic Background. Nicaragua's youngest volcano, Cerro Negro, was created following an eruption that began in April 1850 about 2 km NW of the summit of Las Pilas volcano. It is the largest, southernmost, and most recent of a group of four youthful cinder cones constructed along a NNW-SSE-trending line in the central Marrabios Range. Strombolian-to-subplinian eruptions at intervals of a few years to several decades have constructed a roughly 250-m-high basaltic cone and an associated lava field constrained by topography to extend primarily NE and SW. Cone and crater morphology have varied significantly during its short eruptive history. Although it lies in a relatively unpopulated area, occasional heavy ashfalls have damaged crops and buildings.
Information Contacts: Michael Conway, Peter Lafemina, and Andrew Macfarlane, FIU; Christian Lugo, INETER.