Continent

Whether directly or indirectly, the source of all Earth's crust is the mantle. Radioactive decay in Earth's interior produces heat, which warms regions of the mantle. This causes mantle rock, although solid, to convect upward where pressure is inevitably lower. Pressure and melting temperature are directly related, so decreasing pressure eventually causes the rock to begin melting, a process called pressure-relief melting. Every mineral, due to its composition and atomic structure, has its own melting temperature, so not all the minerals in the convecting rock melt. Instead, the first minerals to melt are the ones with the lowest melting temperatures. Generally, the higher the silica content of a mineral, the lower its melting temperature.

The mantle is composed of the ultramafic rock peridotite. Partial melting of peridotite produces molten rock, or magma, with a mafic, or basaltic, composition. This magma, now less dense due to melting, continues convecting upwards until it arrives below an oceanic ridge where it crystallizes to form new ocean crust. Over time, the crust slowly moves away from the oceanic ridge allowing more new crust to form, a process called sea floor spreading. For its first 100 million years or so, the older oceanic crust is, the cooler it becomes. This increases its density and, therefore, its likelihood of subducting. By the time subduction occurs, the crust is rarely more than 150-170 million years old.