Bull Volcano1 (1994) 56:243-260 Voli/igology 9 Springer-Verlag 1994 The volcanic history of Volcfin Alcedo, Galdpagos Archipelago: A case study of rhyolitic oceanic volcanism Dennis Geist 1, Keith A. Howard 2, A. Mark Jellinek 1, Scott Rayder 3 1 Department of Geology University of Idaho, Moscow, ID 83843 U.S.A. 2 U.S. Geological Survey, 345 Middlefield Rd. Menlo Park, CA 94025 3 5202 Fairlawn Dr. Fayetteville, NY 13066 Received: August 6, 1993/Accepted: January 25, 1994 Abstract. Volcfin Alcedo is one of the seven western Gal~pagos shields and is the only active Galfipagos vol- cano known to have erupted rhyolite as well as basalt. The volcano stands 4 km above the sea floor and has a subaerial volume of 200 km 3, nearly all of which is bas- alt. As Volc~in Alcedo grew, it built an elongate domal shield, which was partly truncated during repeated cal- dera-collapse and partial-filling episodes. An outward- dipping sequence of basalt flows at least 250 m thick forms the steepest (to 33~ ) flanks of the volcano and is not tilted; thus a constructional origin for the steep up- per flanks is favored. About 1 km 3 of rhyolite erupted late in the volcano's history from at least three vents and in 2-5 episodes. The most explosive of these pro- duced a tephra blanket that covers the eastern half of the volcano. Homogeneous rhyolitic pumice is overlain by dacite-rhyolite commingled pumice, with no strati- graphic break. The tephra is notable for its low density and coarse grain size. The calculated height of the eruption plume is 23-30 km, and the intensity is esti- mated to have been 1.2x108 kg/s. Rhyolitic lavas vented from the floor of the caldera and from fissures along the rim overlie the tephra of the plinian phase. The age of the rhyolitic eruptions is _< 120 ka, on the basis of K-Ar ages. Between ten and 20 basaltic lava flows are younger than the rhyolites. Recent faulting resulted in a moat around part of the caldera floor. A1- cedo most resently erupted sometime between 1946 and 1960 from its southern flank. Alcedo maintains an active, transient hydrothermal system. Acoustic and seismic activity in 1991 is attributed to the disruption of the hydrothermal system by a regional-scale earth- quake. Key words: caldera - Galapagos - ocean island - plin- ian - rhyolite - shield volcano - tephra fall Correspondence to: D. Geist Introduction Volc~in Alcedo is one of the seven coalesced shield vol- canoes that make up Isabela and Fernandina Islands in the western Gal@agos Archipelago (Fig. 1). The west- ern Gal@agos Islands are the type locality of the Gal- @agos shield volcanoes, characterized by gradual out- er flanks, steep upper flanks, flat summits, and large calderas (Fig. 2), resembling an inverted soup bowl (Williams and McBirney 1979). Volc~in Alcedo differs from the other western Gal@agos volcanoes because it has erupted rhyolite; so far as is known, the other west- ern volcanoes have erupted only tholeiitic basalt. ' ' | ' ' ' I ' ol 'HI I / I o 90~ . cocosl 80W il / -2 N ,'~ /Plate I Pacific U / GSC .rJ." r--a Ooeanjl / --"-~&"- dN~zoa / 1 / Lr~O ~ . ~ Plate r | | ? ........ ..... ] / Wolf Vol. ~archena V. Ecuador~ a ~ ' ~ _ ~ chena ~ ~ A Genovesa -0 ~ k,,'"*~ ~ Darwin Vol. ~,~ -- Fernandina~ "~ ,,,,~'~N~' j.tll~ Pinzon.,,~ Volcan'>~..,,.~ " ~ Santa Alcedo/,~ san'~a q' Fe .~San I ~ r .... ~Cristobal _lO.q Cerro~WSierra .... _ . v Azul ~ Negra 2 92 ~ I I 90~ Fig. 1. Location and tectonic setting of Volcfin Alcedo. Inset shows the location of the Galgpagos Islands in relation to the Gal@agos Spreading Center and South America. Alcedo lies at the center of Isabela Island. Contour is the 2000 m depth that outlines the Gal@agos Platform