When geologists visited Cerro Negro in June, fumaroles on the N side of the crater reached 320°C. A new, low-temperature, fumarolic area had formed 1 km N of Cerro Negro on the E flank of Cerro La Mula. A linear zone of recently-killed vegetation extended 200 m along a NNW trend. Cerro Negro last erupted in February 1971, ejecting about 4.5 x 10⁶ m³ of tephra.

Geological Summary. Nicaragua's youngest volcano, Cerro Negro, was created following an eruption that began in April 1850 about 2 km NW of the summit of Las Pilas volcano. It is the largest, southernmost, and most recent of a group of four youthful cinder cones constructed along a NNW-SSE-trending line in the central Marrabios Range. Strombolian-to-subplinian eruptions at intervals of a few years to several decades have constructed a roughly 250-m-high basaltic cone and an associated lava field constrained by topography to extend primarily NE and SW. Cone and crater morphology have varied significantly during its short eruptive history. Although it lies in a relatively unpopulated area, occasional heavy ashfalls have damaged crops and buildings.

Information Contacts: R. Stoiber, S. Williams, Dartmouth College; M. Carr and J. Walker, Rutgers Univ.; A. Creusot, Instituto Nicaraguense de Energía.