Final answer:
As magma cools and crystals are separated, the residual melt becomes more felsic because mafic minerals like olivine and pyroxene crystallize first, leaving the magma enriched in silica and forming rocks like granite.
Step-by-step explanation:
As magma cools and the precipitating crystals are separated from the residual melt, the composition of the residual melt changes. The process described is known as fractional crystallization. During this process, the earliest crystals to form are typically mafic minerals like olivine and pyroxene, which crystallize at higher temperatures. These minerals contain a high proportion of magnesium and iron, hence the term 'mafic' from magnesium and 'ferric'. As these mafic minerals crystallize and are removed from the melt, the remaining liquid becomes enriched in silica, making it more felsic in composition.
Igneous rocks formed from this residual magma tend to be rich in minerals such as quartz and feldspar, leading to the formation of rocks such as granite. This granitic composition typically contains light-colored minerals and exhibits coarse-grained textures due to slow cooling deep below the Earth's surface. The separation of crystals from the melt essentially enriches the remaining liquid in silica and lighter elements such as aluminum, potassium, and sodium, which are characteristic of felsic rocks.
Therefore, as magma cools and precipitating crystals are removed, the composition of the residual melt moves toward the felsic end of the compositional spectrum. This gradual change in composition leads to a sequence of different rock types from a single body of magma, a process that plays a fundamental role in the diversity of igneous rocks found in the Earth's crust.