Final answer:
Most magmas that rise slowly through the crust become more felsic due to fractional crystallization and assimilation. This gradual cooling process allows larger crystals to form, leading to a more felsic composition, which is typical of continental crust.
Step-by-step explanation:
Due to fractional crystallization, which involves some minerals forming first at lower temperatures and sinking, and assimilation of surrounding continental crust rocks, most magmas that rise slowly through the crust evolve chemically to become more felsic. This process relates to the cooling of magma below the Earth's surface, often at significant depths where it cools gradually and slowly. When magma intrudes into the surrounding rocks, the individual crystals have more time and space to grow large, contributing to a coarser texture in the resulting igneous rock. Key minerals such as feldspar and quartz play an important role in this evolution—feldspar crystallizes at high temperatures early in the cooling process and forms straight crystal sides, while quartz crystallizes later and fills spaces. As a result, continental crust, typically composed of granite and other felsic materials, is associated with less dense, light-colored minerals, which contrasts with the darker, denser, mafic composition of oceanic crust.