Petrology: Igneous, Sedimentary, Metamorphic Rocks

• Petrology is the study of rocks, their occurrences, composition, origin, and evolution, and is a fundamental part of geology
• The word "Petrology" comes from the Greek words "Petra" (rock) and "logos" (knowledge)
• Petrology is closely associated with mineralogy

Igneous Rocks

• Igneous rocks form from the cooling and solidification of magma, originating below the surface
• These rocks are composed of interlocking crystals that grew as the magma cooled
• Main minerals in igneous rocks are quartz, feldspar, mica, amphibole, pyroxene, and olivine

Sedimentary Rocks

• Sedimentary rocks form from accumulation and compaction
• These are fragments of pre-existing rocks disintegrated by erosion, organic debris like shell fragments or dead plants, or material dissolved in surface waters or groundwater precipitated in oversaturated conditions

Metamorphic Rocks

• Metamorphic rocks form from pre-existing rocks of any type
• Subjected to increased temperature (T) or pressure (P), or both

Plutonic Rocks

• Plutonic rocks involve the intrusion of magma from a deep-seated source

Hypabyssal Rocks

• Hypabyssal rocks involve the intrusion of magma from a moderate depth after solidification

Igneous Bodies

• Types of igneous bodies include batholiths, laccoliths, phacoliths, and lopoliths
• Dykes: Discordant bodies cutting across the bedding of country rock
• Sills: Concordant bodies occurring parallel to bedding planes
• Batholith: A very large, discordant igneous body, ranging from tens to hundreds of square kilometers
• Phacolith: A concordant igneous body that appears crescent-shaped
• Laccolith: An intrusive igneous body with a concordant relation to country rock, appearing dome-like
• Lopolith: An intrusive igneous body maintaining a concordant relationship with country rock and exhibits a saucer-shaped appearance

Grain Size in Igneous Rocks

• Coarse-grained: Average crystal size is greater than 2 mm, identified by the naked eye
• Medium-grained: Average size ranges between 0.25 mm and 2 mm, identified by hand lens
• Fine-grained: Crystal size is less than 0.25 mm, requires a petrological microscope

Pyroclastic Rocks

• Pyroclastic rocks are volcanic rocks erupted explosively, made of crystalline fragments or glass

Pegmatite

• Pegmatite possesses very large crystals, even with rapid magma cooling

Igneous Rock Classification

• Igneous rocks are classified as acidic (or felsic), intermediate, basic (or mafic), and ultrabasic (or ultramafic)
• Acidic: Contain over 66% silica, with minerals like alkali feldspar, quartz, and plagioclase feldspar, plus subordinate muscovite, biotite, and amphibole
  • Examples: Granite (coarse-grained), Rhyolite (fine-grained), and Rhyolite tuff (pyroclastic)
• Intermediate: Contain 52% to 66% silica, including plagioclase feldspars with biotite and amphibole in smaller amounts
  • Examples: Diorite (coarse-grained intermediate) and Andesite (fine-grained)
• Basic: Silica content is less than 52% (quartz is absent), with plagioclase feldspar and pyroxene plus olivine
  • Examples: Gabbro (coarse-grained basic) and Basalt (fine-grained)
• Ultrabasic: Silica content is less than 45%, containing olivine and pyroxene

Texture of Igneous Rock

• Refers to the pattern of grain arrangement
  • Pheneric: Rock exhibits grains in the form of large crystals
  • Aphanitic: Rock is fine-grained, and the ground mass is glassy
  • Ophitic: Arrangement of feldspar in a basaltic rock
  • Porphyry: Large crystals (phenocrysts) are embedded in a fine-grained matrix, found in hypabyssal rocks

Granite

• Granite is a medium-to-coarse-grained, light-colored rock with white or pink tint
• It contains quartz, feldspar (orthoclase, plagioclase, and microcline), and ferromagnesian minerals like hornblende and mica
• It is hard and durable with high strength, suitable for construction
  • Porphyritic granite: Contains large potash feldspar crystals embedded in a fine-grained groundmass
• Monzonite: Granitic rock with abundant feldspar
• Granodiorite: Plagioclase content is more than orthoclase

Pegmatite Rock

• Consists mainly of feldspar, quartz, and mica
• It occurs as a vein or dyke or as a large rock body intrusive into granite
  • Kaolin: This is a decomposition of feldspar in pegmatite
  • Aplite: Contains aggregates of sugary-grained quartz and feldspar

Syenite

• A medium-grained, light-colored, even-textured rock
• Consists of feldspar, hornblende, and mica or pyroxene
• Of deep-seated origin and high in alkali
  • Nepheline syenite: Contains a large amount of nepheline

Diorite

• Consists of plagioclase feldspar and ferromagnesian minerals like biotite and hornblende or pyroxene
• Used for foundation and construction purposes
  • Quartz-diorite: Contains sufficient quartz

Gabbro

• Consists of feldspar (plagioclase and oligoclase) and ferromagnesian minerals such as pyroxene (mainly augite)
  • Norite: Hypersthene content exceeds augite
  • Anorthosite: Mineral content consists of plagioclase and labradorite with very little augite

Dolerite

• A medium-grained basic igneous rock with the same composition as gabbro
• Originates from moderate depth and occurs as a dyke
• Used in engineering, especially as road metal
• Has capacity to hold bitumen coating as a binder

Basalt

• A fine-grained, dark-colored basic igneous rock
• Originates from cooling of volcanic lava flow
• Contains feldspars (plagioclase and oligoclase), pyroxene (augite), olivine, and iron oxide with ophitic texture
• Constitutes 70% of the Earth's ocean floor
  • Deccan basalt: Also called trap rock or simply trap
  • Amygdaloidal basalt: Contains secondary minerals such as chalcedony, opal, agate, jasper, and zeolite inside amygdular cavities
  • Laterite: Product of weathering of basalt

Sedimentary Rocks

• Derived from pre-existing igneous, sedimentary, or metamorphic rocks, and composed of clastic or non-clastic materials
• Denudation of rocks: The process where all existing rocks are subjected to water, wind, and ice, resulting in decay and disintegration
• Transportation and deposition: The processes that carry sediments to their final settlement in rivers, lakes, and oceans
• Bedding or stratification: A prominent primary structure of sedimentary rocks
• Primary structures: Graded bedding, ripple marks, mud cracks, and cross-laminations
• Secondary structures: Geodes, vugs, and solution concretions
• Initial dip of a bed: The maximum initial dip of a bed formed by deposition is about 45°

Vindhyan Formation

• Sedimentary rocks of the Vindhyan formation in India are famous for architectural works and used as construction material
• Common sedimentary rocks include sandstone and shale (clastic) and limestone (non-clastic)

Sandstones

• Sandstones are the most dominant type of sedimentary rock
• They occur as layered or bedded strata and vary in grain size from gravel to fine-grained
• Contain quartz in varied proportions (up to 95%), with other minerals like feldspar, iron oxide, mica, and chloride
• Siliceous matrix: The rock will be hard, compact, and possess high strength
• Vindhyan sandstones: Found in central India in various colors, consist of angular and sub-rounded quartz grains cemented in siliceous matrix, possess high strength and low porosity

Siwalik Sandstones

• Porous with low strength and occur in the foothills of the Himalayas
  • Sand-rock: Excessively porous and breaks down easily when saturated with water

Gondwana Sandstones

• Found in eastern and central India and are medium- to coarse-grained, generally of low strength

Tertiary Sandstones

• Low strength and high porosity, available in large parts of Assam

Cretaceous Sandstones

• Occur in the coastal belts of India and have an argillaceous and calcareous matrix that crumbles easily when saturated with water

Greywacke

• It resembles partially weathered basalt
• Consists of angular-to-subangular grains of quartz and feldspar and fragments of phyllite, slate, and shale
• Grains are bound together in a fine-grained matrix and originate from plutonic and metamorphic rocks

Orthoquartzite

• Contains 90% or more quartz sand, commonly called quartzose sandstone
• It resembles quartzite

Arkose

• Light grey to pale pink in colour and consists of coarse and angular grains of quartz and feldspar
  • Feldspathic sandstone: Feldspar content is more than 25%

Siltstone

• A light-colored rock consisting of sand particles between 2 and 75 microns
• Interbedded with shale and sandstone

Shale

• A soft sedimentary rock with thin layering
• It contains particles of silt and clay, with clay-size particles (2 microns or less) being the maximum
  • Fissility: Easily breakable characteristic of shale
  • Claystone or mudstone: When the clay percentage predominates

Breccia

• Majority of boulders in a sedimentary deposit are angular and uneven

Tillite

• A sedimentary rock where boulders in large numbers remain embedded in clay

Brockrams

• Breccia composed of limestone fragments in a red sandy mixture found in Cumbria

Fault-breccias

• Formed by crushing of rocks along a fault zone, fragments cemented by mineral matter

Volcanic breccias

• Results in the shattering of rocks at a volcanic vent

Limestone

• Major type of non-clastic sedimentary rock, composed of calcite with dolomite and aragonite, with occasional quartz

Oolite

• Variety of limestone consisting of spherical concretions

Chalk

• White variety comprising minute fossils of sea organisms

Marl

• Carbonate rock mixed with a large percentage of clay

Diatoms

• Unique forms of algae that grow silica shells, which are preserved in water under sediments
• Diatomaceous earth A stratified product of diatoms

Metamorphic Rocks

• Formed from the metamorphism of pre-existing rocks under high temperature and pressure conditions or by chemically-active fluids
• Metamorphism brings physical and chemical changes
• Xenolith: Metamorphic rock showing remnants of earlier rocks formed under pressure-temperature conditions
• Metamorphism occurs between 100°C and 900°C, generated from plutonic intrusions
• Contact metamorphism: Occurs when country rocks react with intrusive igneous bodies
• Regional metamorphism: Produces gneiss, schist, phyllite, and slate metamorphic rocks
• Foliation: The effect of pressure causes growth or reorientation of crystals to layers

Characteristic of Rocks

• Causes splits, wavy surfaces, or cleavage
  • Slate is fine-grained with closely packed foliation
  • Phyllite is fine grained with wavy surfaces caused by small mica or chlorite
  • Schist is coarse to medium coarse with undulating planes and abundant mica
  • Gneiss is coarse with widely spaced foliation
  • Migmatite is coarse with poor foliation showing signs of melting
• Hornfels: Metamorphic rocks of igneous origin do not develop foliation
• Metamorphic facies: Process of changes after metamorphism by heat and pressure Thermal metamorphism: Caused by proximity to hot intrusion Schistosity texture: Minerals are platy or columnar and have parallel orientation by regional metamorphism Foliation: Well-developed planes of schistosity alternate with less developed schistose planes Cleavage: A secondary foliation in metamorphic rocks with closely spaced weaknesses Slaty cleavage: Argillaceous rocks under stress Granular texture: Metamorphic rocks of quartz and feldspar under regional metamorphism Gneiss: Medium- to coarse-grained and banded or foliated, composed of quartz, feldspar, hornblende, micas, and accessory minerals such as garnet, tourmaline, apatite, zircon, sphene, and magnetite Archaean gneisses: Occur in many parts of India Schist: Derived from metamorphism of igneous and sedimentary rocks, consisting of quartz, feldspar, muscovite, and biotite Low strength and splits easily along schistosity planes Slate: Fine-grained rock with slaty cleavage that can be split into Sheets Quartzite: White to light grey, consists of re-crystallized quartz from sandstone Marble: Metamorphic equivalent of limestone resistant to meteoric weathering