THE VOLCANIC HISTORY OF VOLCAN-ALCEDO, GALAPAGOS ARCHIPELAGO - A CASE-STUDY OF RHYOLITIC OCEANIC VOLCANISM

Volcan Alcedo is one of the seven western Galapagos shields and is the only active Galapagos volcano known to have erupted rhyolite as well as basalt. The volcano stands 4 km above the sea floor and has a subae rial volume of 200 km3, nearly all of which is basalt. As Volcan Alced o grew, it built an elongate domal shield, which was partly truncated during repeated caldera-collapse and partial-filling episodes. An outw ard-dipping sequence of basalt flows at least 250 m thick forms the st eepest (to 33-degrees) flanks of the volcano and is not tilted; thus a constructional origin for the steep upper flanks is favored. About 1 km3 of rhyolite erupted late in the volcano's history from at least th ree vents and in 2-5 episodes. The most explosive of these produced a tephra blanket that covers the eastern half of the volcano. Homogeneou s rhyolitic pumice is overlain by dacite-rhyolite commingled pumice, w ith no stratigraphic 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 estimated to have been 1.2 x 10(8) kg/s . Rhyolitic lavas vented from the floor of the caldera and from fissur es along the rim overlie the tephra of the plinian phase. The age of t he rhyolitic eruptions is less-than-or-equal-to 120 ka, on the basis o f K-Ar ages. Between ten and 20 basaltic lava flows are younger than t he rhyolites. Recent faulting resulted in a moat around part of the ca ldera floor. Alcedo most resently erupted sometime between 1946 and 19 60 from its southern flank. Alcedo maintains an active, transient hydr othermal system. Acoustic and seismic activity in 1991 is attributed t o the disruption of the hydrothermal system by a regional-scale earthq uake.