Foliation is the alignment and regrowth of minerals when a rock is subjected to pressure. The pressure can come from deep burial, or the lateral squeezing caused by plate collisions and subduction. Mineral crystals change their shape and orientation in response to pressure by elongating at right angles (or ‘normal to’) to the direction of pressure. Molecules in crystals migrate from the pressurized surface of a mineral to a zone of less pressure within the same mineral. Crystals reform into flattened shapes. The reoriented flattened and elongated minerals are now aligned in planar arrangements with each other, parallel along their long axes. This alignment can impart a layered or banded appearance to the rock, always on the microscopic and often on the visual scale. This is not sedimentary bedding, which is typically destroyed during this metamorphism. The thin layering is called ‘foliation’ from the Latin term folium, a leaf.
Examples of foliated metamorphic rocks are slate, phyllite, schists of various kinds, and gneiss. All these can be found in the northern Cascades. Some guides on this blog to foliated rocks:
–phyllite in the South Fork Nooksack;
-greenschist and phyllite near Burlington
The text and drawings are from Geology Underfoot in Western Washington by Dave Tucker.