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Magma supply likely exerts primary control on seafloor morphology of oceanic crust, but most studies have related morphology to spreading rate. Here we examine global patterns of morphology on mid‐ocean ridge (MOR) flanks in relation to magma supply derived from residual mantle Bouguer gravity anomaly (proxy for relative crustal thickness) and spreading rate. We use multibeam bathymetry to characterize morphology using both qualitative (descriptive) and quantitative approaches, and we compare results to both magma supply and spreading rate. Morphology becomes more isotropic and abyssal hills are more irregular and discontinuous as magma supply decreases, while roughness, area of steeper slopes, and anomalous fabric orientation increase. We interpret these changes to reflect changing magma distribution along‐axis, from large‐volume and spatially extensive to progressively reduced, increasingly localized, and more irregularly emplaced. Observed relations between crustal thickness and morphology imply that average thickness of purely magmatic crust in the Atlantic and parts of the Indian ridge system is significantly less than average seismically determined crust. Thus seismically defined crustal thickness in those regions likely includes significant non‐magmatic components such as serpentinized mantle. Excepting regions of extensive mantle exposure, most morphologic parameters that we examined are sensitive to estimated magma supply but not necessarily to spreading rate alone. We summarize our results in schematic models that relate morphologic variations to changes in magma supply and mantle serpentinization throughout the global MOR system. Finally, we note that combined qualitative and quantitative results of our study may be useful for developing automated morphologic classification schemes. Plain Language Summary: Mid‐ocean ridges encircle the globe and spread apart along their axes to form ocean crust. Seafloor morphology of crust on the flanks of different ridges can be highly variable. The variations classically have been attributed to differences in spreading rate but there are notable exceptions to this correlation. Here we use changes in gravity over different mid‐ocean ridges (MORs) together with spreading rate to infer magma supply to the ridges, and we examine how morphology varies with changes in the magma supply. We find that features such as long, linear, and structurally continuous abyssal hills correlate well with high magma supply while progressively rougher, steeper, and more irregular morphology forms as magma supply is reduced; this is observed irrespective of spreading rate, indicating that magma supply better predicts morphologic style. Observed relations between crustal thickness and morphology imply that seismically determined crustal thickness in the Atlantic and parts of the Indian Ocean MOR system includes significant components of non‐magmatic crust (e.g., altered mantle). Our study characterizes crustal morphology both qualitatively (visually) and quantitatively; correlations between these characterizations may be useful for developing automated morphologic classification schemes. Key Points: Variable morphology of global mid‐ocean ridge flanks is examined qualitatively and quantitatively using multibeam bathymetric dataChange in magma supply as derived from gravity and spreading rate, plus mantle serpentinization, explains observed morphologic variationsMorphologic changes indicate thinner magmatic crust than seismically determined crust in the Atlantic and parts of the Indian Ocean [ABSTRACT FROM AUTHOR]

We studied long‐term evolution of nontransform discontinuities (NTDs) on the Mid‐Atlantic Ridge from 0‐ to ~20‐ to 25‐Ma crust using plate reconstructions of multibeam bathymetry, long‐range HMR1 sidescan sonar, residual mantle Bouguer gravity anomaly (RMBA), and gravity‐derived crustal thickness. NTDs have propagated north and south with respect to flowlines of relative plate motion and both rapidly and slowly compared to the half spreading rate; at times they have been quasi‐stable. Fast, short‐term (<2 Myr) propagation is driven by reduced magma supply (increased tectonic extension) in the propagating ridge tip when NTD ridge‐axis offsets are small (≲5 km). Propagation at larger offsets generally is slower and longer term. These NTDs can show classic structures of rift propagation including inner and outer pseudofaults and crustal blocks transferred between ridge flanks by discontinuous jumps of the propagating ridge tip. In all cases crustal transfer occurs within the NTD valley. Aside from ridge‐axis offset, the evolution of NTDs appears to be controlled by three factors: (1) gross volume and distribution of magma supplied to ridge segments as controlled by 3‐D heterogeneities in mantle fertility and/or dynamic upwelling; this controls fundamental ridge segmentation. (2) The lithospheric plumbing system through which magma is delivered to the crust. (3) The consequent focusing of tectonic extension in magma‐poor parts of spreading segments, typically at segment ends, which can drive propagation. We also observe long‐wavelength (5‐10 Myr) RMBA asymmetry between the conjugate ridge flanks, and we attribute this to asymmetric distribution of density anomalies in the upper mantle. Key Points: Propagation of non‐transform discontinuities on the Mid‐Atlantic Ridge was analyzed in reconstructions of bathymetric and gravity dataRapid and slow propagation occurs and is controlled by magma distribution within the mantle and crust of spreading segmentsWe attribute long‐wavelength cross‐axis asymmetry in gravity anomalies to asymmetric distribution of density anomalies in the upper mantle [ABSTRACT FROM AUTHOR]