Tall Volcanic Edifices on MOR Spreading Boundaries-Characteristics and Causes

Some volcanic edifices straddling the axis of the Mid-Oceanic Ridge (MOR) exceed 1 km in relief (maximum known heights reach 3 km) relative to the adjacent rift valley floor along some slow/ultraslow to intermediate- spreading ridges. Referred to here as Tall Axial Edifices (TAEs), such volcanoes largely fill the rift valley, making the MOR axis locally appear 'intermediate' on cross-axis topographic profiles. Although some TAEs are associated with regional 'hotspots', there is no systematic correspondence. At least four TAEs are associated with paired off-axis 'half-edifices' evidently representing former TAEs split by rifting, their original along-axis relief reduced by 1/2 or more, and rafted off-axis. The few known examples of TAEs with off-axis expressions suggest sub-axial magma chambers may persist for 1 Ma or longer, may be episodically productive on 0.2-1 Ma time scales, and may lack fixity in 'absolute' reference frames. Based on finite edifice widths and local opening rates, the non-zero average TAE crustal ages place them in the same age vs. height distribution as off-axis edifices (Vogt, EPSL, 1974), all adjusted to remove the buoyancy effect of the water column. Volcano heights increase as the square root of crustal age, supporting Pratt isostacy as the primary limit on edifice height. TAE heights also increase with decreasing opening rate. We attribute this primarily also to Pratt isostacy: The magma column is in balance with nearby average axial crustal and upper mantle density structure. We replicate the observed relationship using the axial lithosphere thickness vs. opening rate model result of Phipps Morgan and Chen (Nature,1993). Available bathymetry has not revealed flexural moats around TAEs- a result compatible with local (Pratt) compensation. However, lava and debris flows may have buried any moats. Other factors with may promote higher axial volcanoes at lower opening rates include: 1) better magma focussing; 2) more time for axial volcanoes to grow; 3) less magma accommodation space in axial fissures; 4) locally normal speading segments along oblique MOR portions more typical on slow and ultra- slow spreading ridges; and 5) lower magma eruption rates, resulting in steeper edifices. A review and synthesis of TAE rock sample geochemistry--with new results from the TAE on the MAR near 48N, which we plan to explore on a research expedition--suggests that on average TAEs are at least transitional to slightly LREE-enriched. Although this result is consistent with somewhat more fertile mantle sources being responsible for the robust TAE magmatism, the pattern may also be an effect of this magmatism.