Gravity modeling of a volcanically active site on the East Pacific Rise axis

Seafloor and shipboard gravity observations are used to study the East Pacific Else (EPR) axis at 9 degrees 48'N, a site of recent volcanic activity. A constant crustal density model fits the seafloor and shipboard gravity data within expected errors. A layered density model, consistent with rise axis seismic constraints, also fits the gravity data. These constant and layered density models show no evidence for a volumetrically large magma chamber and indicate the importance of the shallow crustal density structure to the marine gravity field. Density variations in the shallow crust may be explained by porosity and thermal variations. The average shallow (0-450 m) EPR 9 degrees 48'N crust has a density of 2420 +/- 80 kg m(-3), significantly lower than other rise axis sites. This density is less than molten basalt, suggesting that magma buoyancy will not contribute to ascent through the uppermost crust, enhancing the formation of volcanic dikes and sills below the layer 2A/2B boundary.