Petrology (Igneous, Sedimentary, Metamorphic)
What the exam tests
Petrology covers the origin, classification and textures of igneous, sedimentary and metamorphic rocks, plus Kerala's Precambrian charnockite–khondalite–migmatite assemblage and its Tertiary sedimentary beds.
Core rules
- Igneous: plutonic rocks are coarse (slow cooling), volcanic rocks fine (fast cooling); glassy and vesicular textures record rapid quenching or gas escape. Silica-rich (acidic) magmas are pale and viscous; basic magmas are dark and fluid.
- Sedimentary: clastic (conglomerate, sandstone, shale) versus non-clastic (limestone, coal). Grain size, sorting, roundness, matrix and cement describe clastics; cross-bedding, ripple marks and graded bedding are structures.
- Metamorphic: foliated (slate, phyllite, schist, gneiss) versus non-foliated (quartzite, marble). Grade rises with temperature and pressure; index minerals define zones.
- Kerala: charnockite is an orthopyroxene-bearing granulite; khondalite is a garnet–sillimanite–graphite gneiss; migmatites record partial melting. Warkalli beds (sandstone, clay, lignite) and Quilon beds (limestone, marl) are Tertiary.
Worked example
Shale buried and heated gives, in order, slate → phyllite → schist → gneiss. Reasoning: rising grade first produces slaty cleavage, then a phyllitic sheen, then visible schistosity with micas, then compositional banding. Marble and quartzite, derived from limestone and quartz sandstone, remain non-foliated because their minerals are equidimensional.
Traps
Calling charnockite sedimentary because it is dark. Describing gneiss as non-foliated. Classifying quartzite and marble as sedimentary. Confusing texture (grain size and arrangement) with composition.