Petrology: Igneous, Sedimentary and Metamorphic Rocks
What this domain tests
Petrology questions ask you to name a rock, interpret its texture, deduce its origin and, for metamorphic rocks, identify the likely protolith and grade. You need the IUGS classification for igneous rocks, the Wentworth grain-size scale and maturity concepts for sedimentary rocks, and metamorphic facies and foliation terminology.
Core rules and concepts
- Igneous rocks are classified by mineral mode (QAPF for quartz-bearing rocks) and by SiO2: ultramafic <45%, mafic 45-52%, intermediate 52-63%, felsic >63%. Texture records cooling: phaneritic = slow, plutonic; aphanitic = rapid, volcanic; porphyritic = two-stage; glassy/vesicular = very rapid or gas-rich.
- Colour index (mafic minerals %) supports but does not replace mineralogical classification.
- Sedimentary rocks: clastic, chemical and biochemical. Wentworth scale: clay <1/256 mm, silt 1/256-1/16, sand 1/16-2, gravel >2. Maturity increases with quartz content, rounding and sorting. Matrix >15% makes a wacke; well-sorted quartz arenite is supermature. Carbonates follow Dunham (mudstone, wackestone, packstone, grainstone, boundstone) or Folk.
- Metamorphic rocks: foliated sequence slate to phyllite to schist to gneiss with increasing grade; non-foliated hornfels, marble, quartzite. Facies: zeolite, greenschist, amphibolite, granulite, eclogite, and hornblende-hornfels for contact metamorphism. Index minerals (chlorite, biotite, garnet, staurolite, kyanite, sillimanite) map the grade.
Worked example
A coarse-grained rock contains 30% quartz, 45% alkali feldspar, 20% plagioclase and 5% biotite, with a weak preferred orientation of biotite. Ignore the biotite for the mode: Q = 30/(30+45+20) = 31.6%; A = 47.4%; P = 21.1%. A/(A+P) = 47.4/68.5 = 69%. On the QAPF diagram Q between 20 and 60 and A/(A+P) between 35 and 90 is the granite field. The weak foliation makes it a granite gneiss, not a schist, because the felsic minerals dominate and the fabric is not schistose.
Traps that cost marks
Naming a rock from colour; calling a porphyritic rock volcanic when the groundmass is actually coarse; using 'marble' for any carbonate; confusing shale and mudstone; forgetting that eclogite is a facies defined by garnet + omphacite, not by foliation; ignoring the difference between contact and regional facies series.