Small off-axis volcanoes on the East Pacific Rise

A study of Sea Beam bathymetry and SeaMARC II side-scan sonar data allows a quantitative measure of the contribution of off-axis volcanism to the creation of abyssal hill topography on the East Pacific Rise (EPR) 9 degrees 15'N-9 degrees 50'N. In order to assess the role of off-axis volcanism, the distribution of volcanic edifices within 35 km of the ridge was determined. We measure the size and location of 55 edifices defined as local highs greater than or equal to 40 m with aspect ratios less than 2. The distribution of small volcanic edifices is notably different from that of larger volcanic constructions (seamounts) in that the former do not appear to be organized into discrete chains. The volcanic edifices form 5-10 km off-axis and are typically 40-70 m high. In contrast, the seamounts (greater than or equal to 200 m high) measured by Scheirer and Macdonald [1] first form between 5 and 15 km off-axis and then continue to grow in volume and size over a distance 2-4 times larger. The smaller edifices make a minute contribution to crustal volume (0.02-0.03%) but can cover 7-11% of the mature seafloor. Seamounts contribute similar to 0.3-1% to the volume of oceanic crust and cover only similar to 6% of the seafloor [1,2]. We propose that within 10 km of the ridge, seamounts and smaller volcanic edifices compete for a limited magma supply. The mechanism of melt delivery to seamounts at these distances is not yet fully mature, and some magma may leak in a random fashion onto the seafloor to form volcanic edifices typically less than 70 m high. Beyond 10 lan, the mechanism of melt delivery to seamounts is mature and no further formation or growth of small volcanic edifices occurs.