Great batholiths such as those formed in western North America during the past 200 m.y. resulted from major additions of sialic rock to the crust and consequent volumetric and areal expansion of the sialic crust. Such crustal growth begins with the emplacement of rock of basaltic composition at the base of the crust. From this underplating, differentiated magmas rise as diapirs into the upper crust. Some of this material escapes upward into the zone of brittle deformation and forms shallow plutons and volcanic ejecta, but the greater portion comes to rest at depths of 5 to 15 km as essentially conformable plutons.
The volumetric additions caused by the emplacement of great volumes of rock within these depth limits are made possible by lateral compression and transport of the pre-existing upper crustal rocks. To accommodate lateral compressive transport at intermediate depths, supra-crustal rocks undergo lateral extension by block faulting, and deep crustal rocks extend by lateral flow. Zones of extension, lateral compression, and so-called load flattening—where they exist within belts of batholith emplacement—have probably formed simultaneously one above the other.