During a 26 April visit to Santiago Crater, extremely weak emissions were observed from two or 3 small, quiet fumaroles at the base of the talus in the inner crater and up the W wall (toward Nindirí Crater). COSPEC measurements indicated an SO₂ flux of <10 metric tons/day (t/d), compared to 1500-2000 t/d during lava lake activity in 1980 (SEAN 05:12). Simultaneous use of SO₂ and HCl INTERSCAN instruments at the crater indicated HCl concentrations several times greater than SO₂. A drive on the WSW (downwind) ridge, the site of extensive acid gas deposition and acid rain during the early 1980's (SEAN 05:12, 06:12, and 07:08), showed that vegetation had recovered somewhat; the same stark deforested appearance was still evident, but low shrubs were healthier and larger.

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: S. Williams, Arizona State Univ; Martha Navarro C. and Silvia Arguello G., INETER.