CALDERA MORPHOLOGY IN THE WESTERN GALAPAGOS AND IMPLICATIONS FOR VOLCANO ERUPTIVE BEHAVIOR AND MECHANISMS OF CALDERA FORMATION

Caldera morphology on the six historically active shield volcanoes tha t comprise Isabela and Fernandina islands, the two westernmost islands in the Galapagos archipelago, is linked to the dynamics of magma supp ly to, and withdrawal from, the magma chamber beneath each volcano. Ca ldera size (e.g., volumes 2-9 times that of the caldera of Kilauea, Ha wai'i), the absence of well-developed rift zones and the inability to sustain prolonged low-volumetric-flow-rate flank eruptions suggest tha t magma storage occurs predominantly within centrally located chambers (at the expense of storage within the flanks). The calderas play an i mportant role in the formation of distinctive arcuate fissures in the central part of the volcano: repeated inward collapse of the caldera w alls along with floor subsidence provide mechanisms for sustaining rad ially oriented least-compressive stresses that favor the formation of arcuate fissures within 1-2 km outboard of the caldera rim. Variations in caldera shape, depth-to-diameter ratio, intra-caldera bench locati on and the extent of talus slope development provide insight into the most recent events of caldera modification, which may be modulated by the episodic supply of magma to each volcano. A lack of correlation be tween the volume of the single historical collapse event and its assoc iated volume of erupted lava precludes a model of caldera formation li nked directly to magma withdrawal. Rather, caldera collapse is probabl y the result of accumulated loss from the central storage system witho ut sufficient recharge and (as has been suggested for Kilauea) may be aided by the downward drag of dense cumulates and intrusives.