E¡ect of the Gala¤pagos hotspot on sea£oor volcanism along the Gala¤pagos Spreading Center (90.9^97.6‡W)

Studies along the Mid-Atlantic Ridge (MAR) and East Pacific Rise (EPR) show that seamount abundance is a strong function of spreading rate. At the MAR, small axial seamounts are a dominant morphologic feature of the inner valley floor, while at the EPR seamounts are rarely observed within the neovolcanic zone. The Gala¤pagos Spreading Center (GSC) provides an excellent location to test the influence of hotspot-related magma supply variations on seamount formation at a relatively constant spreading rate. In this study, we use multibeam bathymetry data to examine the distribution of axial seamounts with distance from the hotspot along the GSC. A numerical algorithm is developed to identify isolated volcanic edifices by searching bathymetry for closed, concentric contours protruding above the surrounding seafloor. Seamount populations are fit with a maximum likelihood model to estimate the total number of seamounts per unit area and the characteristic seamount height. The number of seamounts in the axial zone decreases significantly as the Gala¤pagos hotspot is approached, varying from MAR-like abundances west of 95.5‡W to EPR-like abundances east of 92.7‡W. The along-axis variation in seamount density corresponds to a change in lava flow morphology from dominantly pillow flows in the west to a combination of pillows and sheet/lobate flows in the east. These changes in volcanic style suggest a transition from point-source to fissure-fed eruptions as magma supply increases. We find no evidence for along-axis variations in lava viscosity and, therefore, interpret the differences in seamount abundance and flow morphology to indicate an increase in effusion rates approaching the hotspot. Comparing seamount abundance with axial morphology, crustal thickness, and the presence and depth of an axial magma chamber (AMC), we find that the transition from point-source to fissure-fed eruptions is most sensitive to the presence of a steady-state AMC. In summary, the correlation between seamount abundance, lava morphology, and magma supply at the GSC suggests that effusion rate is directly related to the presence of a steady-state crustal magma chamber. = 2003 Elsevier B.V. All rights reserved.