Question 6 of 21: Question 2, Part 4: Metamorphic Rock Identification

Question 2, Part 4: Metamorphic Rock Identification (3 marks)

Question text not reproduced: the examination questions are © Engineers and Geoscientists BC. Open the official past paper (linked at the top of this page) to read the question, then follow the worked solution below.

a) Foliated. The photo shows distinct, parallel, alternating light- and dark-mineral bands – the textbook definition of foliation (planar alignment of platy/prismatic minerals or compositional banding).

b) Gneiss. Coarse, well-segregated light (quartzo-feldspathic) and dark (mafic-mineral) bands at this scale are characteristic of gneiss, a high-grade regional metamorphic rock (as opposed to the finer, more continuous foliation of schist or slate).

c) Compressed perpendicular to the banding (i.e. "top to bottom" relative to the photographed face). Foliation develops perpendicular to the direction of maximum compressive (principal) stress, as platy/prismatic minerals recrystallize and rotate into the plane of least stress; since the visible bands run roughly across the sample, the shortening direction that produced them acted perpendicular to those bands.