The process of plate accretion at mid-ocean ridges, once thought to occur in a relatively simple, magmatic system, has been shown in recent years to possess unexpected layers of complexity. Particularly at lower spreading rates, the magma supply to some or all of the ridge decreases, with the plate spreading motion being taken up instead on faults. The balance between these magmatic and tectonic processes governs such features as the topography, seismic activity, location of hydrothermal vents, and degree of chemical exchange between crust and ocean at spreading axes. It therefore has important implications for the hydrothermal marine biosphere and global chemical budgets.