The Earth’s earliest crust resembled today’s oceanic crust, therefore there were no continents. The early crustal rocks were recycled and returned to the mantle, carrying surface water. The water reacted with the mantle rock, which allowed for melting and the formation of new, less dense crustal rocks. Volcanic activity produced this crustal material, resembling today’s volcanic island arcs or granitic continental crust.
Between 4 and 2 billion years ago, high volcanic activity allowed many granite-rich crust fragments to develop. Near subduction zones, crustal fragments and volcanic arcs were slowly combined to form microcontinents. Most of them still exist today as the cores of all continents and are known as cratons, making up the oldest and most stable part of a continent. Cratons consist of continental lithosphere (crust and part of the upper mantle).
Oceanic crust is recycled through subduction at convergent boundaries and replenished by the formation of new crust at mid-ocean ridges (divergent boundaries). As continental crust is not subducted due to its buoyancy in the mantle, it is recycled at an exceedingly slower rate. The oldest oceanic crust is around 180 to 200 million years old, whereas the average age of continental crust is around two billion years old, with the oldest crustal rocks exceeding four billion years.