What controls the global ocean ridge axial depth variation?
We propose that plate tectonics dictates where ocean ridges begin and how they evolve, but that mantle density variation as a result of compositional (vs temperature) variation determines the axial depth of ridges. That is, mantle compositional variation results in mantle density variation that isostatically compensates the global ocean ridge axial relief of 5 km. Figures 15, 18 and 19 show explicitly that less depleted mantle peridotites with higher Al2O3 content favor the formation of a greater amount of garnet, making the bulk-rock denser beneath deep ridges than beneath shallow ridges. We elaborate the argument further below. The ridge axial depth varies from 6000 m at the deep end of the spectrum (e.g. the Cayman Rise) to 200 m at the shallow end (e.g. the Reykjanes Ridge) on a global scale. On local and regional scales, the ridge axial depth varies from <400 m along fast-spreading ridges (e.g. the East Pacific Rise, EPR) to as much as 1500 m within some segments of slow-spreading
