The I-type and S-type granites are formed by the melting of metamorphic rocks at extremely high temperatures and pressures. Both form when sediment and other rocks are buried in layers of the Earth's crust. Through volcanic activity and movement of tectonic plates, the Earth's crust puts pressure and extreme heat on the rocks, causing them to melt. The rocks eventually cool off and form granite. The conditions of the granite's formation and types of materials that are within the layer of sediment dictate what the granite looks like when it cools.
Chappell and White differentiate between the two types of granite by looking at their chemical and molecular compositions. S-type granite has low sodium content, with a sodium content of 3.2 percent or lower. S-types also have a higher molecular density than I-types. S-type granite also has particularly high levels of silicon oxide, although I-type may contain small traces of silicon oxide as well.
I-type granite has an extremely high sodium content, which measures 3.2 percent or higher. The contents of I-type granite are extremely varied, and they are usually described as very malefic and felsic, which are terms that describe the mineral composition of the rock. I-types typically have an abundance of xenoliths, which are small rock formations within the stone that belong to a different category of rocks. These xenoliths become trapped in the granite during its formation.
Whereas S-type and I-type are generally considered to be the only two types of granite, some geologists offer sub-categories of I-type rocks: M-type and A-type. M-type granite comes from magma in the Earth's mantle, and this type of granite is crystalized in the Earth's core. A-type granite, also called anorogenic granite, comes from the dry regions of Earth's lower crust. These dry areas are called hot spots, and they yield A-type granites as well as other types.