Petrology (Igneous, Sedimentary, Metamorphic Rocks)
Petrology tests how rocks form, how they are classified, and how textures record their history. The Karnataka shield provides igneous, sedimentary and metamorphic examples: Peninsular Gneiss, Dharwar schists, Closepet granite, charnockite, banded iron formation, quartzite and limestone.
Core rules
- Igneous rocks crystallise from magma or lava; classify by mineralogy, silica content and texture. Slow cooling gives coarse grains; rapid cooling gives fine grains; very rapid cooling gives glass.
- Bowen's reaction series explains crystallisation order and mineral associations.
- Sedimentary rocks form by weathering, transport, deposition, compaction and cementation; bedding, cross-bedding, ripples and fossils are diagnostic structures.
- Metamorphic rocks form in the solid state under changed temperature, pressure or fluid activity; foliation and porphyroblasts are common.
- Metamorphic facies indicate pressure-temperature conditions.
Worked example
A coarse, equigranular specimen contains quartz, K-feldspar, plagioclase and biotite, with no preferred orientation. Identify it. Reasoning: coarse crystalline texture indicates slow cooling; quartz plus both feldspars and biotite gives granite. If the same minerals were arranged in alternating light and dark bands, the rock would be a gneiss, not granite.
Traps
- Grain size alone does not distinguish igneous from metamorphic rocks; look for foliation and fabric.
- Banding in banded iron formation is sedimentary, not necessarily metamorphic foliation.
- "Basalt" is fine-grained and mafic; "gabbro" is its coarse-grained equivalent.
Karnataka link
Dharwar greenstones include metabasalts; Closepet granite is a large granitic batholith; charnockites are high-grade rocks of the Eastern Ghats and southern Karnataka.