Gravity Anomalies in the Galapagos Islands Area

Abstract

In a recent report Case et al. (1) presented a free-air gravity anomaly map of the Galapagos Islands based on 32 gravity stations on the islands. On the basis of their data they stated that the Galapagos Islands are associated with an east-west trending "residual negative anomaly" which is superimposed on a "broader positive anomaly of unknown amplitude and extent." They concluded that "the gravity data can be most readily interpreted in terms of a low-density region related to a hot spot or plume" beneath the islands. We believe, however, that the data of Case et al. in no way support this interpretation. Their observations can, in fact, be explained simply if the Galapagos Islands are in some form of isostatic equilibrium. Any form of isostatic compensation will result in an "edge effect" in the free-air anomaly at the location of a large change in relief. For a relatively narrow feature, the edge effect anomalies over the two "edges" merge, resulting in a large positive anomaly. For a wider feature, the two edge effects become separated, resulting in an area of less positive anomalies over the center of the feature.