Final answer:
Foliation in metamorphic rocks develops due to the combination of heat and pressure. Factors that promote the formation of foliation include increasing pressure, although not all metamorphic rocks exhibit foliation.
Step-by-step explanation:
Foliation in metamorphic rocks develops due to the combination of heat and pressure.
Heat works to 'soften' minerals, allowing ions to migrate in and out of crystal structures. Pressure then forces the minerals to re-orientate, resulting in the alignment of mineral crystals and the development of foliation.
Factors that promote the formation of foliation include increasing pressure, as the degree of foliation tends to increase with increasing pressure. However, not all metamorphic rocks exhibit foliation, as some minerals change very little under metamorphic conditions. These non-foliated or massive metamorphic rocks can be difficult to differentiate from igneous rocks.