The mid-ocean ridges together generate around 2.5 square kilometres of new ocean crust each year produced by partial melting of the mantle beneath. How does this happen? When a rock melts, some of its chemical constituents tend to stay in the solid rock – the residue – and some collect in the melt, depending on the temperature and pressure. The minerals that form when this melt in turn solidifies again depend on the physical conditions and the composition of the original solid. Lavas formed at Mid Ocean Ridges worldwide have very similar compositions, indicating that they formed through a similar process; ocean island basalts have similarly uniform, although distinct, patterns.