Report on Masaya (Nicaragua) — March 1997
Bulletin of the Global Volcanism Network, vol. 22, no. 3 (March 1997)
Managing Editor: Richard Wunderman.
Masaya (Nicaragua) Strombolian explosion; incandescent vent in Santiago crater; seismicity increases
Please cite this report as:
Global Volcanism Program, 1997. Report on Masaya (Nicaragua). In: Wunderman, R. (ed.), Bulletin of the Global Volcanism Network, 22:3. Smithsonian Institution. https://doi.org/10.5479/si.GVP.BGVN199703-344100.
11.984°N, 86.161°W; summit elev. 635 m
All times are local (unless otherwise noted)
A small Strombolian explosion on 5 December 1996 ejected blocks (<10 cm in diameter), ash, and some Pelee's hair. Some of the inner crater walls collapsed, partly closing the incandescent vent. Prior to this eruption the vent's gas temperature was 1,084°C; afterwards, it dropped to 360°C.
During three consecutive days in 1997, COSPEC SO2 fluxes varied as follows: on 12 February, 159 ± 73 metric tons/day (t/d) (1 sigma, n = 5); on 13 February, 363 ± 182 t/d (1 sigma, n = 6); on 14 February, 290 ± 65 t/d (1 sigma, n = 4). The 363 t/d figure is a minimum estimate since on the first 3 traverses the instrument went off the chosen recording scale indicating still larger values than reported.
A visit in March 1997 yielded COSPEC values of 300-400 t/d; these values were lower than those obtained during March 1996 (BGVN 21:04). Nightime observations of the active Santiago crater revealed that large amounts of incandescent gas were being released frequently through a conduit that had partially collapsed on 5 December 1996. As a result of the collapse, it was not possible to see incandescent magma during the night.
Seismicity increased since September 1996; in January 1997, 41 events (4 high- and 47 low-frequency) were recorded along with constant tremor. During 22 February-20 March, 18 events occurred, 15 of which were low-frequency and three high-frequency. Since November 1994 background levels of RSAM have varied between 12 and 16 RSAM units. Since mid-January, however, RSAM increased, fluctuating between 22 and 32 units.
In the crater area, gravity decreased steadily during 1993-95; it remained stable in 1996 and possibly increased a little in 1997.
A NE-trending fracture at the base of Comalito cone emitted gases reaching 68°C. In this same vicinity soil gas concentrations contained up to 25% CO2.
Geologic Background. Masaya is one of Nicaragua's most unusual and most active volcanoes. It lies within the massive Pleistocene Las Sierras pyroclastic shield volcano and is a broad, 6 x 11 km basaltic caldera with steep-sided walls up to 300 m high. The caldera is filled on its NW end by more than a dozen vents that erupted along a circular, 4-km-diameter fracture system. The twin volcanoes of Nindirí and Masaya, the source of historical eruptions, were constructed at the southern end of the fracture system and contain multiple summit craters, including the currently active Santiago crater. A major basaltic Plinian tephra erupted from Masaya about 6500 years ago. Historical lava flows cover much of the caldera floor and have confined a lake to the far eastern end of the caldera. A lava flow from the 1670 eruption overtopped the north caldera rim. Masaya has been frequently active since the time of the Spanish Conquistadors, when an active lava lake prompted attempts to extract the volcano's molten "gold." Periods of long-term vigorous gas emission at roughly quarter-century intervals cause health hazards and crop damage.
Information Contacts: Hazel Rymer and Mark Davies, Department of Earth Sciences, The Open University, Milton Keynes MK7 6AA, United Kingdom; John Stix, Dora Knez, Glyn Williams-Jones, and Alexandre Beaulieu, Departement de Geologie, Universite de Montreal, Montreal, Quebec H3C 3J7, Canada; Nicki Stevens, Department of Geography, University of Reading, Reading RG2 2AB, United Kingdom; Martha Navarro and Pedro Perez, INETER, Apartado Postal 2110, Managua, Nicaragua.