Granite may have more alkali feldspar, whereas a granite derived from partial melting of metaigneous rocks may be richer in plagioclase. It is on this basis that the modern "alphabet" classification schemes are based. The letter-based Chappell & White classification system was proposed initially to divide granites into I-type (igneous source) granite and S-type (sedimentary sources). Both types are produced by partial melting of crustal rocks, either metaigneous rocks or metasedimentary rocks. M-type granite was later proposed to cover those granites that were clearly sourced from crystallized mafic magmas, generally sourced from the mantle. However, this proposal has been rejected by
Granite dome on erosional offloading or pressure release. There is evidence that supports this hypothesis when looking at granitic landforms that have sheet fractures. Granite forms deep in the Earth’s crust under conditions of high ambient or lithostatic pressure. In order for the granite to be exposed at the Earth’s surface a considerable thickness of rock must be eroded. This unloading allows the granite to expand radially and sheet fractures form tangentially to the radial stress. This indicates that the shape of the pre-existing land surface determines the geometry of the sheet fractures. One hypothesis is that granite domes are uplifted blocks.