Structural controls on the morphology of Galapagos shields

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.