Petrology & Rock Classification
Petrology Basics · Igneous & Sedimentary Rocks · Metamorphic Rocks (28 Micro-Realms)
The Science of Rocks (Petrology)
Petrology is the science of rocks—studying their mineral composition, texture, structure, origin, and alteration. Based on their mode of formation, all crustal rocks belong to three great families: primary Igneous Rocks solidified from magma, stratified Sedimentary Rocks lithified from denuded fragments, and recrystallized Metamorphic Rocks altered under Pressure, Volume, and Temperature (PVT).
I. Petrology Basics & Igneous Rocks ("Ignis" — Primary Rocks)
What is Petrology?
- Science of rocks ("Petra")
- Aggregate of 1+ minerals
- Studies origin & history
Igneous Primary Rocks
- Latin "Ignis" means Fire
- Solidified from magma/lava
- Unstratified & unfossiliferous
Intrusive (Plutonic) Rocks
- Slow cooling at great depth
- Large coarse crystals
- Granite, Gabbro, Diorite
Extrusive (Volcanic) Rocks
- Sudden surface lava cooling
- Small fine/glassy grains
- Basalt (Deccan Traps), Obsidian
Hypabyssal Rocks
- Shallow intermediate depth
- Mixed porphyritic texture
- Dolerite in dykes & sills
Acidic vs Basic Igneous
- Acidic (>65% $\text{SiO}_2$): Granite
- Basic (45–52% $\text{SiO}_2$): Basalt
- Ultramafic (<45%): Peridotite
Pegmatite, Pumice & Tuff
- Pegmatite: Giant crystals
- Pumice: Vesicular floating rock
- Tuff: Consolidated volcanic ash
II. Sedimentary Rocks (Lithification & 3 Modes of Formation)
Denudation & Lithification
- Latin "Sedimentum" (settling)
- Compaction + cementation
- Covers 75% of Earth's surface
Strata & Fossil Archive
- Distinct bedding planes
- Contains fossil life records
- Porous & holds groundwater/oil
1. Mechanically Formed
- Clastic physical fragments
- Sandstone, Conglomerate, Shale
- Limestone, Clay & Loess (NCERT)
2. Organically Formed
- Biogenic shell & plant remains
- Geyserite, Chalk, Limestone
- Coal (Peat to Bituminous)
3. Chemically Formed
- Precipitated from solutions
- Chert, Limestone, Halite
- Potash & Gypsum evaporites
Arenaceous vs Argillaceous
- Arenaceous: Coarse Sandstone
- Argillaceous: Fine Shale/Clay
- Rudaceous: Pebble Conglomerate
Sedimentary Structures
- Cross-bedding wind/water dunes
- Wave ripple marks
- Desiccation mud cracks
III. Metamorphic Processes (PVT) & Structural Foliation
PVT Metamorphism
- "Metamorphic" = Change of form
- Pressure, Volume, Temperature
- Solid-state recrystallization
Dynamic Metamorphism
- Mechanical disruption & shear
- Without appreciable chemical change
- Crushes rock into Mylonite
Thermal Metamorphism
- Chemical alteration & recrystallization
- Driven by intense heat
- Divided: Contact & Regional
Contact Metamorphism
- Rocks touch hot magma/lava
- Local baked aureole halo
- Recrystallizes into Hornfels/Marble
Regional Metamorphism
- Tectonic mountain-building shear
- High Pressure + High Temp
- Deforms vast orogenic belts
Foliation (Lineation)
- Grains arranged in layers/lines
- Perpendicular to stress
- Slaty cleavage & Schistosity
Metamorphic Banding
- Alternating light & dark layers
- Different mineral groups segregate
- Characteristic of Gneiss
IV. Foliated vs Non-Foliated & Parent-Rock Transformations
Foliated vs Non-Foliated
- Foliated: Gneissoid, Slate, Schist
- Non-Foliated: Marble, Quartzite
- Syenite & Serpentine (NCERT)
Shale → Slate → Schist
- Shale $\to$ Slate (low grade)
- Slate $\to$ Phyllite (silky sheen)
- Phyllite $\to$ Schist $\to$ Gneiss
Granite → Gneiss
- Igneous Granite parent rock
- High-grade regional P–T
- Coarse banded Gneiss
Sandstone → Quartzite
- Sandstone quartz grains fuse
- Non-foliated granoblastic rock
- Breaks across quartz grains
Limestone → Marble
- Limestone/Dolomite parent rock
- Interlocking Calcite crystals
- Makrana (RJ) Taj Mahal stone
Basalt → Amphibolite
- Basalt/Gabbro mafic parent
- Forms Greenstone & Amphibolite
- Ultra-HP subduction Eclogite
Coal → Graphite / Diamond
- Bituminous Coal $\to$ Anthracite
- Extreme heat $\to$ Graphite
- Mantle pressure $\to$ Diamond
Core Distinctions
Igneous + Metamorphic rocks make up 95% of the total volume of the Earth's crust, whereas Sedimentary rocks make up only 5% by volume—yet cover 75% of the Earth's land surface area as a thin veneer.
2. NCERT 3-Way Sedimentary Classification• Mechanically formed: Sandstone, Conglomerate, Limestone, Shale, Loess.
• Organically formed: Geyserite, Chalk, Limestone, Coal.
• Chemically formed: Chert, Limestone, Halite, Potash.
Foliation (Lineation): Arrangement of mineral grains in parallel layers or lines during metamorphism. Banding: Arrangement of materials of different chemical/mineral groups into alternating light and dark layers (e.g., Gneiss).
Protolith → Metamorphic Matrix
High-Yield UPSC Prelims Transformations