Recent eruptions of Sierra Negra, a basaltic shield volcano, have been along fissure zones circumferentially oriented around the northern side of the caldera near the volcano summit. Two fissure zones, the Caldera Rim fissure and the Volcan Chico fissure, have produced more than 85 percent of the lava exposed on the north side of the volcano. Radial fissures are not common on this portion of Sierra Negra, and they have contributed only minor amounts of lava and pyroclastic material.
Most of the recent activity has been localized along the broad zone of the Volcan Chico fissure, in areas of previous fumarolic activity. In 1963 an initial explosive phase produced a thin, widespread blanket of frothy pumice that was followed by the formation of agglutinate cones and voluminous lava flows. Late-stage slumping occurred and fumarolic activity continues.
The well-defined Caldera Rim fissure has been the locus of caldera collapse in the Volcan Chico area; to the west it forms an eruptive zone along the caldera rim. This fissure zone has been the source of surface lava flows between the Volcan Chico fissure and the present caldera. The area between the two major fissure zones has not been a source area for eruptions or fumarolic activity. This area does include several linearly arranged collapse depressions that imply subsurface magmatic activity. The exposures of thick, massive, widespread flows, at shallow depths in the depressions and in the caldera walls, have two important implications: (1) These earlier flows had sources that were located within the present caldera previous to its collapse along the Caldera Rim fissure, and (2) later activity has not contributed significantly to the construction of the rim of Sierra Negra.
The features examined in the Volcan Chico area are typical of Sierra Negra's recent activity and indicate the mechanisms of volcano growth and caldera development late in the history of a Galapagos volcano.