A small, brief, explosive eruption from the bottom of the lava lake in Santiago Crater occurred at dawn on 7 October. Tephra, including blocks with volumes to 55 cm³, fell 300 m SE and covered an area of 150,000 m². The eruption killed a few trees and animals near the summit. Heat from the ejecta melted asphalt on a road, which was also slightly damaged by impact from larger tephra. Rumbling and explosion sounds were heard through the day. After the initial explosion, no additional tephra was ejected, but gas emission increased considerably, forming wide vapor columns that reached high altitudes.

The eruption was preceded by a change in the pattern of seismicity and accompanied by a magnitude 2.3 event lasting 3.7 seconds. After the eruption, small earthquakes occurred about every 6 minutes until 1100 on 8 October.

The 7 October eruption was larger than Masaya's previous explosion on 26 December 1981 (7:1). Strong vapor emission has made observations of the bottom of the crater difficult, obscuring any changes that may have occurred to the lava lake.

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: G. Hodgson V., INETER.