Granites and their associated extrusive rocks are formed in large volumes whenever the continental crust is heated by rising hot mantle, or thickened by collision processes. The complexity of rocks of the granite family is related to the complexity of the continental crust itself and the complexity of processes which lead to thermal perturbations. The light continental crust acts as a density filter which screens out heavy mantle magmas and leads to complex underplating and magma mixing processes. Perhaps the primary cause of crustal melting is the deep recycling of volatiles which are fixed in the oceanic crust before subduction. Modern studies of subduction and collision processes show the large scale and complexity of processes which modify old continental crust.
