96 ANALYTICAL METHODS/Gravity Figure Marine free air gravity anomaly map derived from satellite altimetry (Sandwell and Smith (1997)) Warm colours indicate positive gravity anomalies The gravity anomaly primarily indicates the shape of the seafloor, due to the strong density contrast from seawater to oceanic crust Oceanic island chains, subduction zone trenches, and mid ocean ridges form features visible on this world map Locations of Figures 4, and 13 indicated (Image courtesy of NGDC.)