Physical volcanology and structural development of Sierra Negra volcano, Isabela Island, Gal´apagos archipelago

Abstract

Sierra Negra is the most voluminous of the western Galapagos shields and exhibits many unusual morphologic features: extensive lower flank apron, steep upper flanks, a broad summit plateau, a large complex caldera, ubiquitous occurrence of aa, and radial and circumferential fissures. A field-based investigation of the volcano was conducted in order to establish the distribution and characteristics of eruptive zones and associated flow fields and to construct a volcano-wide surface flow stratigraphy. Geochronologic data were used to determine the age of the units and, combined with flow-volume calculations, to estimate both the historic and total volcano eruption rates. Ninety-one kilometers of radial and circumferential fissures were mapped and grouped into four eruptive zones which form a volcano-wide east-northeast–trending rift system that deflects around the caldera. The eruptive zones are restricted spatially and temporally and document long-term changes in the volcano's development. Long, fluid lava flows are broadly distributed among five ages, resulting in a subdued flank profile by the development of an extensive lower flank apron around the volcano. Ten historical eruptions have occurred, with eruption rates of 44 to 248 m3/s and an average growth rate of 12 × 106 m3/yr. Age determinations of lava flows by 14C and cosmogenic 3He range from 400 to 6900 yr. The total volcano growth rate is estimated to be 1 × 106 m3/yr. These results confirm the young age of Sierra Negra lavas and the validity of the relative ages determined from stratigraphic study. In total, the volcano has undergone over 90% resurfacing in the past 4500 yr. The summit of Sierra Negra is entirely occupied by a shallow, elliptical (7 × 10 km) caldera. Near-vertical ring faults circumscribe the summit and expose 100 to 140 m of lava flows. Subparallel sets of caldera growth faults cut the western and southern caldera walls and are not associated with eruptive activity. The inclination of remnant magnetism and the attitudes of the flows exposed in the caldera walls indicate that the lavas have not been appreciably deformed nor have they been intruded by dikes or sills. The caldera interior is structurally complex and contains a 14-km-long, C-shaped sinuous ridge, composed of a complex set of normally faulted blocks with inward dipping (20°–60°) flow tops. The sinuous ridge is interpreted to have formed gradually as the caldera floor pivoted in a trap door fashion, jammed its western edge, and then fractured in several places, concomitant with the long-term vertical and lateral growth of the volcano. A regional tectonic stress field superimposed upon the Galapagos platform appears to have exerted primary control on the location of Sierra Negra, including the early development of the east-northeast rift system. Intervolcano stresses have controlled the distribution of eruptive zones only where significant subaerial overlap of adjacent volcanoes occurs. Large volume, high discharge rate eruptions from fissures along the east-northeast rift system are responsible for the development of an extensive coastal apron, flow stacking at mid-flank elevations, and generally subdued slopes in comparison to other western Galapagos volcanoes. The summit caldera and underlying magma chamber are subordinate in influencing the distribution of lower flank stress but dominate the development of the summit stress field and orientation of eruptive fissures. The absence of summit deformation, together with the ubiquitous occurrence of short stubby summit flows, indicates that the steep upper flanks are constructional in origin.