Physical volcanology and structural development of Cerro Azul Volcano, Isabela Island, Galapagos: implications for the development of Galapagos-type shield volcanoes

Cerro Azul is an active basaltic shield volcano forming the southwestern end of Isabela Island in the western Galapagos Archipelago. Ten eruptions have been witnessed between 1932 and 1998, an average of one eruption every 6.6 years. Although Cerro Azul has been constructed primarily by effusive Hawaiian-style eruptions, explosive hydrovolcanic eruptions have occurred intermittently from vents on the caldera floor and southern summit rim. The unique "overturned soup plate" morphology of Galapagos shield volcanoes is well displayed at Cerro Azul. Gently dipping lower flanks with a slope of less than 4 degrees steepen to over 25 degrees toward the summit at an average elevation of 600 m. The steep upper flanks are surmounted by a wide flat summit rim and a 450-m-deep caldera. A nested caldera geometry not seen at the other western Galapagos shields has produced a scalloped, irregular outline with excellent three-dimensional exposures of the summit structure, providing an opportunity to evaluate various models proposed for the formation of the Galapagos shields. The nesting also indicates several stages of caldera collapse with different centers of collapse, followed by partial refilling. The steep upper flanks are constructed of short, stubby pahoehoe lava flows (each 0.01-0.02 km(3)) erupted from circumferential vents near the summit. These flows are nearly an order of magnitude smaller in volume than the predominantly aa flows erupted from radial eruptive fissures near the break in slope (0.06-0.1 km(3)). The differences in volume and flow morphology with altitude are due to slower eruption rates from summit vents than from flank vents, which, in turn, are attributable to the different heights the magmas must ascend from shallow reservoirs. These observations support the contention that the steep upper flanks were constructed by the buildup of short lava flows rather than by the structural deformation of originally gently dipping flanks. In addition to the higher eruption rates, a subdued lower flank geometry is promoted by the deposition of lava deltas onto the shallow Galapagos platform on the western, northern, and eastern flanks of the volcano. 40Ar/39Ar geochronology and volume estimates show that, despite their morphologic differences, the growth of the western Galapagos shields has been nearly synchronous, precluding an evolutionary model for their development. The wide variations in elevation, volume, area, and the distribution of slope angles among the western volcanoes can be linked instead to different long-term eruption rates and, to a lesser degree, the position of each volcano relative to the edge of the Galapagos platform.