SMALL OFF-AXIS VOLCANOS ON THE EAST PACIFIC RISE

A study of Sea Beam bathymetry and SeaMARC II side-scan sonar data all ows a quantitative measure of the contribution of off-axis volcanism t o the creation of abyssal hill topography on the East Pacific Rise (EP R) 9 degrees 15'N-9 degrees 50'N. In order to assess the role of off-a xis volcanism, the distribution of volcanic edifices within 35 km of t he ridge was determined. We measure the size and location of 55 edific es defined as local highs greater than or equal to 40 m with aspect ra tios less than 2. The distribution of small volcanic edifices is notab ly different from that of larger volcanic constructions (seamounts) in that the former do not appear to be organized into discrete chains. T he volcanic edifices form 5-10 km off-axis and are typically 40-70 m h igh. 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 c rustal 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 w ithin 10 km of the ridge, seamounts and smaller volcanic edifices comp ete for a limited magma supply. The mechanism of melt delivery to seam ounts at these distances is not yet fully mature, and some magma may l eak in a random fashion onto the seafloor to form volcanic edifices ty pically less than 70 m high. Beyond 10 lan, the mechanism of melt deli very to seamounts is mature and no further formation or growth of smal l volcanic edifices occurs.