Morphology and Structure of the Western Galápagos Volcanoes and a Model for Their Origin

Abstract

The two westernmost Galapagos Islands, Isabela and Fernandina, comprise six large, active, basaltic shield volcanoes, which are closely related in space and in the time of their formation. The volcanoes have marked morphological and structural similarities, but gradational differences exist which show relative volcano ages. These features also indicate the mechanisms of volcano origin and development. The unusual subaerial profiles are typified by steep upper slopes and a broad, flat top. Profiles, including submarine portions, show that all six volcanoes have four slope segments: two submarine and two subaerial. Distinct profile differences provide evidence for various formational processes; the subaerial upper segments are sensitive indicators of these processes. Superimposed profiles show that erupted lavas move far down the volcano slopes to be deposited on the subaerial lower segment and especially on the submarine segments. Relative slope maturities on a volcano can be determined that depend upon the volume of lava added to different profile directions. Different ranges of slope maturities indicate the relative volcano ages. Subaerial profiles are influenced by the submarine base which formed prior to subaerial accumulation. The “flattish” volcano tops are mostly large calderas, although some calderas are surrounded by a wide, flat rim area. Caldera characteristics, such as diameter, circularity, and depth, show trends which agree with the age sequence determined from the profiles. Volcano elevations decrease regularly with increasing caldera diameter. A radial fissure pattern is well developed on the younger volcanoes but becomes less well defined with age. When present, this pattern is restricted to the subaerial upper segment of the volcano. Fissures on subaerial lower segments have a linear nature; they extend upslope to the calderas and downslope to link adjacent volcanoes. A circumferential fissure pattern around the caldera is well displayed by some volcanoes. The diameter of the circumferential fissure pattern increases with age; on the youngest volcano, circumferential fissures have a strong linear nature. Coalescence of circumferential and linear fissures, caldera elongation, and eruption locations define dominant, throughgoing lineaments which cross each volcano and link the six volcanic centers. Subordinate lineaments can also be defined which intersect dominant lineaments at each caldera. Two lineament orientations are present which correspond to previously proposed trends for the Galapagos Islands. A model for the origin and development of the volcanoes outlines five main periods: (1) submarine base construction, (2) subaerial construction, (3) tumescence and radial fracturing, (4) caldera development, and (5) morphological decline. A key concept in the model is the formation of a dome by tumescence and associated radial fracturing. The dome is supported by radial dikes so that a steep upper slope segment is formed and maintained during caldera development. Circumferential fissures are viewed as “passive” features which form during collapse of the central part of the dome. Calderas are initially grabenlike because of the control of the throughgoing dominant lineament. Caldera growth is accompanied by increasing circumferential fissure diameter; radial fissures become less distinct as tumescence decreases in importance. During morphological decline, the caldera consumes the upper portion of the volcano while eruptions broaden and flatten the profile. The resulting landform is similar to shield volcanoes of other areas and to the older Galapagos Islands.