Scientists approached the incandescent window of the lava lake in Santiago's inner crater on 19 October to sample lava ejected during an episode of increased explosive activity at the beginning of October. The window was 15 m in diameter and 50 m deep with lava splashing every 10-15 seconds. Bright yellow incandescence was reported on 31 August and was first observed on 16 June of this year (BGVN 18:06, 18:07, and 18:09).

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: Alain Creusot, Instituto Nicaraguense de Energía, Managua, Nicaragua.