GEOLOGY OF VOLCÁN DARWIN, ISABELA ISLAND, GALÁPAGOS ARCHIPELAGO

New field studies at Volcán Darwin document that it is unique among the western shields of the Galápagos Archipelago. The caldera is nearly circular with an average diameter of 4.5 km and although it is similar in dimension to the other neighboring shields, it is filled to within 250 m of the summit rim. The eastern caldera floor is composed of a complex vent system that includes multiple rifts, lavas, spatter cones, and a 250 m-high cinder cone complex. These deposits slope westward where they are covered by an extensive pahoehoe flow that comprises the western caldera floor. Over much of its area, the western caldera floor is broken into a unique pattern of evenly spaced, quasi-circular blocky ridges. These ridges are composed of foundered lava blocks up to 3 m in diameter. These blocky ridges are up to 7 meters high and clearly pre-date the final episode of flows as they are lapped onto by the youngest flow sequence that covers the western two-thirds of the caldera floor. In addition, these rings of block ridges have isolated central floors that are covered with the same pahoehoe lava seen elsewhere on the western caldera floor. This indicates a local, restricted eruptive source within each isolated domain. This curious pattern suggests that both expansion and contraction mechanisms effected the caldera floor as it transitioned from molten to solid in the waning periods of the eruption. Another unique discovery is that two populations of flows are intermixed throughout the volcano. The first type is very crystal rich (up to 40%) and contains rounded, embayed and zoned plagioclase phenocrysts. The other type contains few crystals or is aphyric. These populations suggest that two very different conditions alternate within the magma system at Darwin; either long periods of storage with high degrees of cooling, or, short residence times and little cooling. The strongly phyric lavas are found as both aa and pahoehoe flows indicating that phenocryst type and abundance had little controlling influence on lava type. Explosive hydro-volcanism has also produced extensive fragmental deposits that cover much of the volcano. The most striking features of these explosive episodes are the large tuff cones of Tagus and Beagle craters on the western coastline. At 400 meters high and 4 km in diameter, these cones are the largest littoral cones in the archipelago.