Question 2 of 9: Rock Identification

Question 2: Rock Identification (10 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) Vesicular volcanic igneous rock → Scoria (mafic) or Pumice (felsic) – both are extrusive rocks whose rounded gas-bubble cavities (vesicles) froze in as lava depressurized and exsolved dissolved gas at the surface; scoria is the standard single answer for a generic vesicular volcanic rock, with pumice as the felsic/glassy alternative.

b) Sandstone dominated by feldspar sand → Arkose. Ordinary quartz arenite is >90% quartz; when feldspar makes up >25% of the framework grains (usually signalling rapid erosion/short transport from a granitic source before the chemically less stable feldspar could weather away), the sandstone is classified as arkose.

c) Cemented, rounded gravel and pebbles → Conglomerate. Rounding of the clasts indicates significant transport/abrasion (as opposed to breccia, whose angular clasts indicate little transport); once buried and cemented (typically by silica or calcite), the rounded-gravel framework becomes conglomerate.

d) Strongly foliated, mica-scale, scaly rock → Schist. Slate has foliation too fine-grained to see individual grains; phyllite has a silky sheen but grains still barely visible; schist is the term for rock coarse enough that individual mica flakes are visible and impart the described "scaly" appearance (schistosity) – a higher metamorphic grade than slate/phyllite.

e) Hard, non-foliated, contact-aureole metamorphic rock → Hornfels. Contact metamorphism around an igneous intrusion "bakes" the surrounding country rock with heat but little directed stress, recrystallizing it into a fine-grained, very hard, splintery, non-foliated rock – the classic contact-aureole product.