Petrology (Igneous, Sedimentary, Metamorphic)
Petrology tests the classification, texture, composition and origin of igneous, sedimentary and metamorphic rocks, and the processes that convert one type into another.
Igneous
- Texture: phaneritic (coarse, intrusive), aphanitic (fine, extrusive), glassy, vesicular, pyroclastic.
- Composition: felsic, intermediate, mafic, ultramafic. Granite is felsic plutonic; rhyolite is its volcanic equivalent; gabbro is mafic plutonic; basalt is its volcanic equivalent.
Sedimentary
- Clastic: conglomerate, sandstone, shale. Chemical: limestone, chert, evaporites. Organic: coal.
- Features: bedding, cross-bedding, graded bedding, ripple marks, mud cracks.
Metamorphic
- Foliated: slate, phyllite, schist, gneiss. Non-foliated: marble, quartzite, hornfels.
- Grade increases with temperature and pressure; index minerals mark zones.
Worked example
A coarse-grained rock contains quartz, orthoclase and biotite with no foliation; it is acidic and plutonic. Reasoning: coarse texture means intrusive; quartz plus feldspar means felsic; no foliation means igneous, not metamorphic. Answer: granite.
Traps
Marble is metamorphosed limestone; quartzite is metamorphosed sandstone, so do not swap them. Basalt and gabbro have the same composition but different texture. Foliation indicates directed pressure, not just heat.