Ocean ridges, where Earth’s tectonic plates are pulled apart, vary from more than 5- km depth in the Arctic to 750 m above sea level in Iceland. This huge relief is generally attributed to mantle plumes underlying mantle hotspots, areas of enormous volcanism marked by ocean islands. The plumes are thought to feed the mantle beneath adjacent ocean ridges. This results in thickened crust and ridge elevation to form ocean rises. The composition of mid-ocean ridge basalt, a direct function of mantle composition and temperature, varies systematically up ocean rises, but in a unique way for each rise. Here we present thermodynamic calculations of melt-evolution pathways to show that variations in both mantle temperature and source composition are required to explain rise basalts. Thus, lateral gradients in mantle temperature cannot be uniquely determined from basalt chemistry, and ocean rises can be supported by chemically buoyant mantle and/or by robust mantle plumes. Our calculations also indicate that melt is conserved and focused by percolative flow towards the overlying ridge, progressively interacting with the mantle to shallow depth. We conclude that most mantle melting occurs by an overlooked mechanism, focused melting, whereas fractional melting is a secondary process that is important largely at shallow depth.