Final answer:
The Sierra batholith formed through the subduction of the Farallon Plate beneath the North American Plate, where magma generated in the subduction zone intruded into the continental crust and crystallized to form a large granite body.
Step-by-step explanation:
Formation of the Sierra Batholith
The Sierra batholith is a large mass of intrusive igneous rock that formed during the Mesozoic era, primarily in the Cretaceous period. The formation process of the Sierra batholith began with the subduction of the oceanic Farallon Plate beneath the North American Plate. This tectonic action occurred off the western coast of the Paleozoic North American continent.
As the Farallon Plate descended, it created a subduction zone, generating extreme temperatures and pressures which led to the melting of both the subducted oceanic crust and the overlying continental crust. This melting resulted in the formation of magma, which rose through the crust due to being less dense than the surrounding rocks. Over millions of years, successive intrusions of magma crystallized at depth to form the large body of granite known as the Sierra batholith.
The Sierra batholith is therefore an example of a plutonic or intrusive igneous rock body which never reached the surface as a volcanic eruption but instead slowly cooled and solidified below Earth's surface. Thus, the resulting rock type is more coarse-grained, as the slow cooling process allowed for larger crystal formation. Granite is the predominant rock type in the Sierra batholith, showcasing a composition rich in quartz, feldspar, and mica minerals.