White Paper Title: 
Changes in the formation of axial volcanic edifices in response to changes in magma supply

Julia Howell1, Scott White1, DelWayne Bohnenstiehl2

1. University of South Carolina, 2. North Carolina State University

The spatial distribution and abundance of volcanic edifices (>40 meters high) along normal mid-ocean ridge (MOR) systems (i.e., those where only spreading generates magma) have a well known variation correlated with axial morphology.  Along fast-spreading MORs, volcanic edifices tend to form near the ends of tectonic ridge segments where the lithosphere is brittle and magma can escape (Bohnenstiehl et al. 2008; White et al. 2000, 2002, 2006). In contrast, volcanic edifices along slow-spreading MORs tend to form around the center of a first or second-order axial valley segment (Cochran 2008; Smith and Cann, 1992).

However, spatial and size distributions of volcanic edifices along ridges with an elevated magma supply differ from distributions observed at normal ridges spreading at the same rate, suggesting that magma supply plays a fundamental role in controlling their formation.  Understanding how volcanic edifices form in response to changes in magma supply alone is possible along ridges of similar spreading rates, variable crustal thickness and mapped extent of elevated magma influence.  Recent geophysical and geochemical studies along the Galapagos Spreading Centers (GSC), Juan de Fuca Ridge (JdFR), Southeast Indian Ridge (SEIR) and the Valu Fa (VF) and Eastern Lau Spreading Centers (ELSC) constrain the extent and amount of extra magma supply. At these places, it is possible to control for the effects of spreading rate, making it possible to elucidate the formation of volcanic edifices from changes in magma supply and axial morphology.

To understand and identify the spatial and size distribution of volcanic edifices along ridges with an elevated magma supply, we applied a modified version of the closed contouring algorithm used in Bohnenstiehl et al. 2008 to the GSC, JdFR, SEIR and the VF and ELSC.  The new contouring algorithm includes a modified version of Hiller’s (2008) “Delimiting and isolating seamounts” method for picking the base of  a seamount by identifying slope breaks along the seamount elevation profile.  This method improves the ability of our algorithm to estimate an accurate volume and height. Volcanic edifices greater than 40 meters in height and a long to short axis ratio of < 2.5 were derived from shipboard multibeam bathymetry gridded at 100 meters. For direct age comparison, only volcanic edifices within 0.2 million years from the axis are studied.

Preliminary results from VF and ELSC (top half of figure) follow trends associated with spreading rate, not axial morphology.  These results seem to suggest magma supply is not the controlling factor on the formation of volcanic edifices along this back-arc spreading system.  It is interesting to note that volume distribution is not tied to abundance distribution, i.e. the same sized volcanic edifices are not forming in the same location along the segment.  The abundance percentage distributions along second order segments from the SEIR show trends we expect for axial high segments, even for segments with an axial valley morphology.  However, axial high segments directly influenced by the Amsterdam / St. Paul hotspot (AH-HS) have a distinctly different abundance distribution, similar to what we expect for normal axial valley segments (more randomly distributed).  In this case magma supply seems to be the controlling factor on the formation of volcanic edifices.