How do metamorphic rocks become foliated?
Foliation refers to the repeated layering of metamorphic rocks.
Each layer may be as thin as a sheet of paper but can also be up to a meter thick. The word "foliation" itself comes from the latin folium meaning leaf, expressing the planar like layering.
Foliation is caused by shearing, which are forces that push and pull sections of the rock in different directions. Foliation can also be caused by higher pressure from one area than the others. The layers of the rocks form parallel to the direction of the pushing and perpendicular the the pressure.
Foliation is common in rocks exposed to pressure in the Earth's crust, such as the formation of a new mountain belt.