Final answer:
Pressure applied during metamorphism causes mineral grains to align, developing foliation, a key feature in the formation of foliated metamorphic rocks.
Step-by-step explanation:
The application of pressure during metamorphism causes elongated grains to align parallel to each other; when this happens, minerals in the rock develop foliation.
Foliation is a planar arrangement of structural or textural features in metamorphic rocks along straight or wavy planes. This process is pivotal in the formation of foliated metamorphic rocks, where mineral crystals in the rock are aligned with each other, resulting in parallel planes or banded structures.
The degree of foliation increases with the pressure, contributing to the diverse textures and appearance of different metamorphic rocks such as slate, gneiss, and schist.