A previous report (BGVN 25:07) reviewed evidence for steam-and-ash emissions between November 1999 and January 2000, seismicity during April 1999-March 2000, and increased seismicity in the vicinity of both Masaya and Laguna de Apoyo in July 2000. Previously unreported observations and information from March-April 2000 regarding an ongoing international degassing study, and fumarole temperature measurements from INETER, are included below.

Degassing studies during March-April 2000. The current degassing crisis at Masaya began in mid-1993 with the brief formation of a lava pond and gradual increase in degassing (BGVN 18:04 and 18:07). Canadian, Belgian, British, and Nicaraguan scientists returned to Masaya caldera between March and April 2000 to continue the study of the ongoing degassing crisis (BGVN 23:09 and 24:04). Significant amounts of Pele's hair around the W and S rims of Santiago crater (first noted by Alvaro Aleman, Masaya Park guard) were likely the result of a gas-rich explosion one night either at the end of February or during the first week of March 2000. Two minor explosions, which produced small ash plumes, were witnessed at Santiago crater on 2 March at about 1545 and 1645.

A large gas plume was still being emitted from a vent (15-20 m in diameter) at the bottom of Santiago crater. Incandescence of the vent walls was visible only at night. Temperatures recorded at the vent with an infrared thermometer ranged between 200 and 380°C, and were highly dependent upon the opacity of the gas plume. COSPEC measurements of SO₂ revealed decreasing but nevertheless high emission rates, ranging from 740 ± 200 t/d to 1,850 ± 300 t/d. Remote sensing of the gas plume composition using an open-path Fourier transform infrared spectrometer (OP-FTIR) in a variety of modes revealed an average SO₂/HCl molar ratio of 1.7, comparable to that obtained in February-April 1998 and February-March 1999. The acid emissions continued to affect a vast area downwind of the volcano, and the rural population subsisting on soil cultivation has been severely impacted.

Microgravity measurements between March and April 2000 appeared to show a leveling off of the previous (1993-94 and 1997-99) decreasing gravity change immediately beneath the Santiago pit crater. These values are essentially the same (within error, ± 20 microgals) as those measured at Masaya in June 1999. This leveling off of gravity change and apparent decrease in gas flux is similar to a cycle of activity between 1994 and 1997 and may suggest that Masaya is entering the waning period of the current degassing crisis.

Fumarole temperatures during December 1999-April 2000. Fumaroles from the Cerro El Comalito area (table 3) showed uniform variations in their monthly average temperatures between December 1999 and April 2000. The fumaroles are close to one another, so this outcome was expected. Fumaroles in the Filete San Fernando area exhibited more variation, with some increasing in temperature and others decreasing.

Table 3. Average fumarole temperatures from the Cerro El Comalito (CEC) and Filete San Fernando (FSF) areas of Masaya during December 1999-April 2000. All the measurements were carried out with a thermocouple. Courtesy of INETER.

INETER also noted that there were no reports of landslides or incandescence from the lava lake in Santiago crater during March-April 2000. Seismic tremor was low throughout that period, and there were only six microearthquakes registered in March, followed by 12 in April.

Geological Summary. Masaya volcano in Nicaragua has erupted frequently since the time of the Spanish Conquistadors, when an active lava lake prompted attempts to extract the volcano's molten "gold" until it was found to be basalt rock upon cooling. It lies within the massive Pleistocene Las Sierras caldera and is itself 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 Nindirí and Masaya cones, the source of observed 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 6,500 years ago. Recent lava flows cover much of the caldera floor and there is a lake at the far eastern end. A lava flow from the 1670 eruption overtopped the north caldera rim. Periods of long-term vigorous gas emission at roughly quarter-century intervals have caused health hazards and crop damage.

Information Contacts: Glyn Williams-Jones, Dave Rothery, Hazel Rymer, Department of Earth Sciences, The Open University, Milton Keynes, United Kingdom; Pierre Delmelle, Unité des Sciences du Sol, Université Catholique de Louvain, Louvain-la-Neuve, Belgium; Clive Oppenheimer and Hayley Duffell, Dept. of Geography, University of Cambridge, Cambridge, United Kingdom; José Garcia Alavarez and Wilfried Strauch, INETER, Apartado Postal 2110, Managua, Nicaragua.