Final answer:
The application of differential stress during metamorphism leads to the formation of a preferred orientation in minerals, a key feature in the identification of metamorphic rocks.
Step-by-step explanation:
The application of differential stress during metamorphism causes elongated grains to align parallel to each other; when this happens, minerals in the rock develop a preferred orientation. This process is part of the formation of metamorphic rocks, which occurs when a pre-existing rock (the protolith) is altered by heat, pressure, or contact with chemically reactive fluids. The presence of heat is crucial as it allows minerals to "soften" and reorient under pressure. During metamorphism, foliation can occur, which is the alignment of mineral grains within the rock resulting in a layered or banded appearance. The degree of foliation increases with pressure, and the resulting texture is important for identifying metamorphic rocks.