"A small eruption occurred from the hole in Santiago Crater lava lake in the early evening of 16 December. No one witnessed the event, but people living S of the caldera reported hearing an explosion that was followed by ashfall several kilometers to the S. Highly vesiculated scoria fragments up to 20 cm in diameter fell as much as 200 m S of Santiago pit crater. As of late January, no subsequent explosive activity had been observed. A very large plume was still being continuously emitted. Incandescence was not readily visible during the day but was evident at night."

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: R. Stoiber, S. Williams, Dartmouth College; D. Fajardo B., Instituto de Investigaciones Sísmicas.