Final answer:
Magma composition changes from mafic to felsic mainly through geological processes at subduction zones and by melting different source rocks. Rocks are classified as mafic or felsic based on their mineral content and color, with mafic rocks being darker due to iron and magnesium minerals, and felsic rocks being lighter with more silica-rich minerals like quartz and feldspar.
Step-by-step explanation:
The composition of magma can change from mafic to felsic under different geological processes. Oceanic crust, for instance, is made from mafic magma which forms basalt and gabbro. At subduction zones, water released from the subducting plate lowers the melting temperature of the overlying mantle, which leads to the creation of felsic magma rich in minerals like quartz and feldspar. Furthermore, igneous rocks are classified based on their mineral content and color, with dark-colored rocks typically being mafic and light-colored rocks being felsic. The color of igneous rocks, along with the relative amount of light versus dark minerals present, helps geologists to identify them.
When it comes to classifying an igneous rock as mafic, felsic, intermediate, or ultramafic, mineral content is a key factor. A rock with a high percentage of minerals such as olivine and pyroxene is typically mafic or ultramafic, while a rock with a high percentage of quartz and feldspar is classified as felsic.