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Although Galapagos shield volcanoes exhibit a variety of morphologies, their steep slopes and deep, wide calderas distinguish them from shields of other provinces. We suggest that the differences in these morphologies may reflect differences in the geometry of subjacent magma chambers and loading conditions on the margins of those chambers. We present the results of calculations using an elastic solution for stresses and displacements along a free surface interacting with a magma chamber at depth. For relatively shallow magma chambers (2 to 4 km deep) calculations that employ a ratio of approximately 10:1 for elastic shear modulus to magma pressure, produce steep surface slopes comparable to those found on Volcan Wolf and Darwin. Broad, gentle slopes, similar to those found on Sierra Negra and Alcedo are developed for elastic shear modulus to magma pressure ratios of 20:1 to 50:1 and for magma chamber depths greater than 3 km.

Basalts containing as much as 50% plagioclase phenocrysts that attain lengths of several centimeters are common on the northern islands of the Galapagos Archipelago. Mineralogic and geochemical evidence requires that the large anorthitic phenocrysts nucleated and grew from a less evolved magma than that in which they were erupted. We suggest that plagioclase-ultraphyric basalts (PUBs) form when a buoyant phenocryst-laden magma enters a chamber containing a denser, more evolved magma. As the new magma mixes with the old, anorthitic plagioclase phenocrysts may accumulate near the roof in a hybrid liquid. Thinner crust under the northern islands increases the probability that viscous plagioclase-rich magmas can ascend to shallow magma chambers and may account for the fact that PUBs are more common on the northern islands, than in the rest of the Archipelago.