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Physical Geography — Lithosphere

Types Of Rocks

Study notes, physical mechanisms, regional distribution patterns, and exam questions on Types Of Rocks for UP TGT & PGT Geography.

UP PGT Geography UP TGT Social Science B.Sc. / M.Sc. Level

The lithosphere is fundamentally composed of rocks—naturally occurring solid aggregates of minerals or mineraloids. Lithospheric dynamics are governed by the continuous transformation of these lithic units through the Rock Cycle, driven by internal (endogenic) forces like plate tectonics, magmatism, and metamorphism, as well as external (exogenic) forces of weathering, erosion, and sedimentation. Rocks are fundamentally categorized into Igneous, Sedimentary, and Metamorphic classes based on their mode of genesis. Igneous rocks, formed via the cooling and solidification of molten magma or lava, serve as the primary or 'parent' rocks of the Earth's crust. Sedimentary rocks accumulate layer by layer through lithification of weathered detrital fragments, chemical precipitates, or organic remains, forming the principal archives of paleontological and climatic history. Metamorphic rocks emerge from pre-existing rocks subjected to intense heat, pressure, and chemically active fluids without melting. Spatial distribution of these rock types dictates regional geomorphology, soil fertility, hydrological regimes, and economic mineral belts globally and across the Indian subcontinent.

1. Igneous Rocks: Origin, Intrusive vs Extrusive Forms, and Bowens Reaction Series

Crystallization of silicate melts under varying cooling regimes dictates mineralogical composition and structural morphology.

  • Originates from cooling of magma (intrusive) or lava (extrusive).
  • Divided into Plutonic (deep-seated, slow cooling, coarse-grained like granite), Hypabyssal (shallow depth, intermediate like dolerite), and Volcanic (surface, rapid cooling, fine-grained/glassy like basalt).
  • Bowen's Reaction Series explains temperature-dependent mineral crystallization sequence from ultramafic/mafic to felsic minerals.
  • Silica content classifies them into Felsic (>66% SiO2, light-colored, e.g., granite), Intermediate (52-66%, e.g., diorite), Mafic (45-52%, dark-colored, e.g., basalt), and Ultramafic (<45%, e.g., peridotite).
  • Key structures include batholiths, laccoliths, sills, dykes, and pahoehoe/aa lava flows.
Comparative Analysis: Unlike layered sedimentary rocks, igneous rocks are massive, non-fossiliferous, and crystalline. Intrusive plutonic rocks cool slowly under high pressure yielding phaneritic textures, whereas volcanic extrusives cool rapidly yielding aphanitic or glassy textures.
  • Granite and basalt form the fundamental sialic and simatic layers of the Earth's crust.
  • Bowen's continuous and discontinuous branches dictate mineral stability under surface weathering.

2. Sedimentary Rocks: Diagenesis, Lithification, and Classification

Denudation, transport, deposition, and diagenetic compaction/cementation of detrital, chemical, or organic materials.

  • Formed via exogenic agents through weathering, mass wasting, transportation, deposition, and lithification.
  • Lithification involves compaction (reduction of pore space) and cementation (binding via silica, calcite, or iron oxide).
  • Mechanically/Clastic sedimentary rocks: Breccia, conglomerate, sandstone, shale (classified by grain size via Wentworth scale).
  • Chemically precipitated sedimentary rocks: Limestones, dolomites, rock salt, gypsum, and chert.
  • Organically/Biologically derived sedimentary rocks: Coal (peat, lignite, bituminous, anthracite), chalk, and coral limestone.
  • Characteristic structural features include stratification (bedding planes), ripple marks, mud cracks, and fossil inclusions.
Comparative Analysis: Unlike primary igneous rocks, sedimentary rocks cover 75% of the Earth's land surface exposed area but constitute only 5% of the crust's volume. They act as stratigraphic markers and hydrocarbon reservoirs.
  • Shale is the most abundant sedimentary rock, accounting for over 50% of the total sedimentary volume.
  • Fossils and paleoclimatic indicators are exclusively found in stratified sedimentary formations.

3. Metamorphic Rocks: Agents, Types, and Facies Concepts

Solid-state mineralogical and structural readjustment of parent protoliths under elevated temperature, pressure, and fluid activity.

  • Thermal (Contact) Metamorphism: Caused by high heat from magma intrusion; creates non-foliated rocks like marble (from limestone) and quartzite (from sandstone).
  • Dynamic (Cataclastic) Metamorphism: Caused by mechanical shearing along fault zones; produces fault breccia and mylonite.
  • Regional (Dynamothermal) Metamorphism: Caused by simultaneous high temperature and directed tectonic pressure; produces foliated textures like slate, phyllite, schist, and gneiss.
  • Metasomatism: Chemical alteration of rocks by hydrothermal fluids introducing new mineral components.
  • Metamorphic facies concept (Eskola): Groups of rocks formed under specific P-T stability fields (e.g., greenschist, amphibolite, granulite, eclogite).
Comparative Analysis: Igneous rocks form from melts and sedimentary rocks from surface debris, whereas metamorphic rocks retain solid-state characteristics while undergoing complete textural and mineralogical phase changes.
  • Gneissification represents extreme regional metamorphism where mineral segregation forms distinct light and dark bands.
  • Limestone converts to marble, sandstone to quartzite, and granite to gneiss.

4. The Rock Cycle and Lithospheric Dynamics

James Hutton's principle of uniformitarianism embodied in the continuous recycling of Earth's crustal materials through geological time.

  • Magma crystallizes into Igneous rocks via cooling.
  • Igneous, sedimentary, or metamorphic rocks undergo weathering and erosion to form sedimentary deposits.
  • Burial and lithification transform sediments into Sedimentary rocks.
  • Heat and pressure transform sedimentary or igneous rocks into Metamorphic rocks.
  • Extreme subduction and melting return all rock types to magma reservoirs within the asthenosphere.
  • Non-linear pathways exist, such as igneous rocks transforming directly into metamorphic rocks or sedimentary rocks melting back into magma.
Comparative Analysis: The rock cycle bridges endogenic tectonics with exogenic geomorphology, demonstrating that no rock is permanent on geological timescales.
  • The rock cycle is powered by Earth's internal thermal engine and external solar energy.
  • Coined fundamentally by James Hutton, the 'Father of Geology'.

Spatial Patterns & Regional Distribution

🌐 Global Geographic Belts & Spatial Variation

Igneous and metamorphic rocks dominate Precambrian continental shields and cratons (e.g., Canadian Shield, Baltic Shield, Brazilian Shield, Australian Shield), forming the stable continental cores. Volcanic igneous belts strictly align with global plate boundaries—specifically the Circum-Pacific 'Ring of Fire' (convergent boundaries), Mid-Atlantic Ridge (divergent boundaries), and East African Rift Valley. Extensive sedimentary basins occupy downwarped geosynclinal or cratonic basins such as the Gulf Coast Basin, Mesopotamian Basin, and the West Siberian Basin.

🇮🇳 Indian Subcontinent Context & Regional Corridors

Peninsular India is a stable Precambrian shield dominated by Archean gneissic complexes, Dharwarian schistose metasedimentary belts (rich in metallic minerals), and Purana sedimentary basins (Cuddapah and Vindhyan). The Deccan Trap region (Maharashtra, Madhya Pradesh, Gujarat) features over 500,000 sq km of horizontal Cretaceous-Eocene basaltic lava flows. The Extra-Peninsular region (Himalayas) consists of complexly folded and thrust-faulted sedimentary, metamorphic, and intrusive igneous rocks. The Indo-Gangetic alluvial plains represent a massive foredeep filled with quaternary unlithified sedimentary deposits derived from Himalayan erosion.

📐 Key Geographic Formulas, Scales & Classifications
Wentworth Sedimentary Grain Size Scale
phi (\phi) = -\log_2 (d_mm)
Parameters: phi is the Wentworth phi scale unit; d_mm is the grain diameter in millimeters. Boulders (>256mm), Cobbles (64-256mm), Pebbles (2-64mm), Sand (1/16-2mm), Silt (1/256-1/16mm), Clay (<1/256mm).
Exam Focus: Frequently tested in UGC-NET and UP PGT for matching clastic sedimentary class names with precise metric size ranges and phi values.
Geothermal Gradient Calculation
dT/dz \approx 25^\circ\text{C to } 30^\circ\text{C per km of depth}
Parameters: T is temperature in Celsius; z is depth in kilometers. Drives thermal metamorphism and melting parameters in crustal rocks.
Exam Focus: Crucial for questions addressing the depth thresholds required for greenschist to amphibolite metamorphic facies transitions in orogenic belts.

Mnemonics, Memory Shortcuts & Exam Traps

💡 Bowen's Reaction Series Mineral Crystallization Order
O-P-A-B-P-M-Q (Only Poor Archaeologists Buy Pots Made Quietly)

Stands for Olivine, Pyroxene, Amphibole, Biotite, Plagioclase Feldspar, Muscovite, Quartz in the discontinuous and continuous crystallization sequence.

⚠️ Exam Trap: Examiners often trick candidates by stating Quartz crystallizes first; remember quartz is the last mineral to crystallize (lowest temperature, most stable at Earth's surface).
💡 Metamorphic Protolith Transformation (Parent to Daughter)
LS-Q, L-M, S-S, G-G (Lush Swamps Quickly, Leave Many Soft Shells, Give Great Gems)

Sandstone becomes Quartzite; Limestone becomes Marble; Shale becomes Slate/Schist; Granite becomes Gneiss.

⚠️ Exam Trap: Do not confuse thermal contact metamorphism (non-foliated like marble/quartzite) with regional dynamothermal metamorphism (foliated like slate/gneiss/schist).