Petrology Study Notes

Petrology

• Deals with the study of rocks (Petro = rock, logos = study).

• Earth's crust (lithosphere) is made of different rock types.

• Petrology covers mode of formation, structure, texture, composition, occurrence, and types of rocks.

• Composition and texture affect rock strength and durability.

• Rocks are used as foundations for dams, tunneling, and construction materials based on suitability.

Classification of Rocks

• Igneous Rocks

• Sedimentary Rocks

• Metamorphic Rocks

Igneous Rocks

• Formed by cooling and solidifying magma or lava from the mantle or crust.

• Can be intrusive or extrusive.

• Crystallization leads to granular or crystalline rocks.

Geological Importance of Igneous Rocks

• Provide information about mantle composition and extraction conditions.

• Absolute ages from radiometric dating allow comparison to geological strata.

• Features indicate specific tectonic environments.

• Host mineral deposits (ores) like tungsten, tin, uranium, chromium, and platinum, associated with granites and diorites.

Forms of Igneous Rocks

• Intrusive Igneous Rocks

• Extrusive Igneous Rocks

Intrusive Igneous Rocks

• Formed when magma cools below the Earth's surface.

• Have large, well-formed crystals (e.g., granite, gabbro, diorite, dunite).

• Observed forms include dykes, sills, laccoliths, lopoliths, bysmaliths, phacoliths, chonoliths, volcanic necks/plugs, and batholiths.

Extrusive Igneous Rocks

• Formed during volcanic eruptions.

• Rapid solidification.

• Often fine-grained or glassy, with larger crystals in a fine-grained groundmass.

• Gas-rich lavas solidify into vesicular rocks like pumice and scoria.

• Fragmental pyroclastic rocks include tuff and volcanic breccia.

• Examples: basalt, andesite, rhyolite.

• Fluid basaltic lavas create shield volcanoes and volcanic plateaus.

• Viscous lavas and explosive activity create cinder cones and composite volcanoes.

• Bulbous lava domes form around vents.

• Calderas are vast depressions formed by volcanic collapse.

Forms of Extrusive Igneous Rocks

• Lava Flows

• Pyroclasts

Lava Flows

• Volcanic lava flows on the surface form lava flows.

• Block lava: rough, irregular surface.

• Ropy lava: smooth, shiny surface.

• Differences in vesicles occur due to ropy lava's hotter, less volatile nature.

Pyroclasts

• Rock fragments from volcanic eruptions.

• Categorized into volcanic blocks, volcanic bombs, and lapilli based on size and shape.

Classification of Igneous Rocks

• Based on silica percentage

• Based on silica saturation

• Based on depth of formation

Classification Based on Silica Percentage

• Rock chemical composition expressed in oxides (e.g., $SiO<em>2$, $AL</em>2O<em>3$, $Fe</em>2O<em>3$, $FeO$, $MgO$, $C</em>2O$).

• Silica percentage is crucial for mineral formation and classification.

  • Acidic: > 66% silica

  • Intermediate: 52-66% silica

    Basic: 45-52% silica

  • Ultrabasic: < 45% silica

Classification Based on Silica Saturation

• Oversaturated: more quartz and feldspars.

• Silica-saturated: less quartz and feldspars.

• Undersaturated: >60% feldspathoid minerals, no quartz.

• Specific mineral composition varies among rock types.

Classification Based on Depth of Formation

• Plutonic: coarse-grained texture.

• Volcanic: fine-grained texture.

• Hypabyssal: intermediate depths and transitional cooling rates.

• Depth determines cooling rate, crystal size, and texture.

Description of Igneous Rocks

• Granite

• Syenite

• Diorite

• Gabbro

• Pegmatite

• Dolerite

• Basalt

Pegmatite

• Very coarse-grained intrusive rock with large crystals (quartz, feldspar, mica, rare minerals).

• Forms from late-stage, water-rich magma.

• Variable color depends on minerals.

• Can contain valuable minerals.

• Often found as dikes associated with larger intrusions.

Granite

• Light-colored, coarse-grained intrusive rock (quartz, alkali feldspar dominant).

• Common in continental crust as large bodies (batholiths).

• Hard, durable, light-colored (pink, gray, white).

• Widely distributed globally.

Syenite

• Coarse-grained intrusive rock (alkali feldspar dominant, little to no quartz).

• Less common than granite, found in smaller intrusions.

• Often associated with alkaline rocks.

• Light to dark (gray, pink, red), hard, slightly denser than granite.

• More restricted distribution.

Diorite

• Coarse-grained intrusive rock (plagioclase feldspar, hornblende/biotite/pyroxene).

• Intermediate composition, often found in volcanic arcs.

• Salt-and-pepper appearance (dark minerals in light feldspar).

• Moderately hard, medium density.

• Distributed along continental margins.

Gabbro

• Dark-colored, coarse-grained intrusive rock (plagioclase feldspar, pyroxene dominant, may contain olivine).

• Mafic composition, often forms oceanic crust and layered intrusions.

• Dark gray to black, relatively dense, moderately hard.

• Significant in oceanic crust and large intrusions.

Dolerite (Diabase)

• Fine- to medium-grained intrusive rock (plagioclase feldspar, pyroxene).

• Often occurs as dikes and sills.

• Dark gray to black, medium density, moderately hard.

• Found globally in dikes, sills, and smaller intrusions.

Basalt

• Dark-colored, fine-grained extrusive rock (plagioclase feldspar, pyroxene, may contain olivine).

• Most common volcanic rock, forms lava flows and oceanic crust.

• Dark gray to black, relatively dense, moderately hard.

• Very widely distributed in oceanic crust and volcanic regions.

Sedimentary Rocks

• Formed by deposition and cementation of mineral and organic particles.

• Thin veneer over igneous and metamorphic rocks, deposited in layers as strata.

• Study provides information for civil engineering, construction of roads, houses, tunnels, canals, etc.

• Important sources of natural resources like coal, fossil fuels, drinking water, and ores.

Metamorphic Rocks

• Formed through metamorphism (significant physical and chemical changes).

• Classified by texture and chemical assemblage.

• Formed deep beneath the Earth's surface, from tectonic processes, or from magma intrusion.

• Study provides insights into temperatures and pressures at great depths.

Description of Sedimentary Rocks

• Sandstone

• Limestone

• Shale

• Conglomerate

• Breccia

Sandstone

• Sand-sized grains (mostly quartz), cemented.

• Forms in various sandy environments (rivers, deserts, beaches).

• Variable color (tan, red, gray), gritty.

• Variable hardness/porosity.

• Globally abundant.

Limestone

• Primarily calcium carbonate (shells/chemical precipitate).

• Forms in marine (reefs) and some freshwater environments.

• Light-colored, relatively soft, reacts with acid.

• Widespread globally.

Shale

• Very fine mud/clay particles, layered (fissile).

• Forms in quiet, low-energy environments (lakes, deep oceans).

• Variable color (black, gray, red), smooth, soft.

• Most abundant sedimentary rock globally.

Conglomerate

• Rounded gravel-sized fragments in a finer matrix.

• Forms in high-energy environments (fast rivers, beaches).

• Variable color, coarse/rough.

• Variable hardness.

• Found in various locations.

Breccia

• Angular gravel-sized fragments in a finer matrix.

• Forms in low-transport environments (talus slopes, fault zones, volcanoes).

• Variable color, coarse/sharp.

• Variable hardness.

• Found in specific tectonic/depositional settings.

Description of Metamorphic Rocks

• Quartzite

• Marble

• Slate

• Gneiss

• Schist

Quartzite

• Metamorphosed sandstone (mostly quartz), non-foliated.

• Very hard, resistant.

• Various colors.

• Found in ancient mountain ranges.

Marble

• Metamorphosed limestone/dolostone (calcite/dolomite), non-foliated.

• White or colored/veined, moderately soft.

• Reacts with acid (calcite).

• Found in metamorphosed carbonate regions.

Slate

• Metamorphosed shale/mudstone (clay minerals), fine-grained, foliated (slaty cleavage).

• Gray/black/colored, moderately hard.

• Splits into thin sheets.

• Found in low-grade metamorphic areas.

Gneiss

• High-grade metamorphosed igneous/sedimentary rocks, medium-coarse grained, foliated (gneissic banding - light/dark bands).

• Hard, strong.

• Found in cores of mountain ranges and ancient shields.

Schist

• Medium-grade metamorphosed shale/volcanic rocks, medium-coarse grained, foliated (schistosity - visible aligned platy minerals).

• Variable color/hardness, flaky appearance.

• Found in mountain belts.

Rock Cycle

• Describes transitions between sedimentary, metamorphic, and igneous rock types over time.

• Driven by plate tectonics and the water cycle.

• Rock change is a geologic and biogeochemical cycle.

Process of Rock Cycle

1. Magma

2. Crystallization (freezing of rock)

3. Igneous rocks

4. Erosion

5. Sedimentation

6. Sediments & sedimentary rocks

7. Tectonic burial and metamorphism

8. Metamorphic rocks

9. Melting

Dykes

• Vertical rock formation between older layers, cutting across flat wall structures.

• Magmatic dikes form when magma crystallizes.

• Clastic dikes form when sediment fills a crack.

Sills

• Sheet-like intrusion formed from magma injected along rock layering planes of weakness.

• Consolidates beneath the surface.

• Large horizontal extent.

• Distinct from discordant dykes.

Attitude of Beds

• Orientation in space, crucial for understanding subsurface geology and engineering construction.

• Described by strike (compass direction of horizontal plane intersection with dipping bed) and dip (angle of inclination from horizontal).

Outcrops

• Locations where geological beds are exposed at the Earth's surface.

• Allow direct visual and physical access to rocks.

• Used to measure the attitude of beds, identify rock types, assess properties, and observe geological structures.

• Patterns reveal the underlying three-dimensional structure.

Geological Maps

• Represents distribution of rock types and geological structures on a two-dimensional surface.

• Uses colors, symbols, and contour lines.

• Depicts rock types and ages, attitude of beds (strike and dip symbols), and locations of geological structures.

• Fundamental for site investigation and engineering design.

Study of Geological Structures

• Folds, faults, and joints influence stability, strength, permeability, and suitability of a site.

Folds

• Bends or curves in rock layers from compressional forces.

  • Complex subsurface geometry

  • Variation in rock strength and stability

  • Potential for increased factoring

  • Groundwater flow pathways

  • Excavation challenges

Faults

• Fractures with significant displacement.

  • Seismic activity

  • Weakened rock zones (fault gouge and breccia)

  • Groundwater barriers or conduits

  • Differential settlement

  • Landslide and fault stability

Joints

• Fractures with little to no significant movement.

  • Reduced rock mass strength

  • Increased permeability

  • Excavation instability

  • Foundation instability

Bearing on Engineering Construction

• Assess site sustainability.

• Design appropriate foundations.

• Plan and execute excavation safely and efficiently.

• Design stable slopes and retaining structures.

• Assess and mitigate seismic hazards.

• Manage groundwater fall.

Rock Mechanics

• Study of mechanical behavior of rocks and rock masses.

• Crucial in mining, civil engineering, and petroleum engineering.

• Understanding physical and mechanical properties is fundamental.

Key Physical and Mechanical Properties

Physical Properties

• Porosity (n)

• Permeability

• Density ($\rho$)

Porosity ($N$)

• Ratio of void space volume to total volume, expressed as a percentage.

Permeability

• Measure of rock's ability to transmit fluids.

• Depends on size, shape, and interconnectedness of pores.

• Crucial for groundwater movement, petroleum extraction, and dam foundation stability.

Density ($\rho$)

• Mass per unit volume.

  • Influenced by mineral composition and porosity.

Bulk Density ($\rho_b$)

• Mass of rock (including pores and fluids) divided by total volume.

• Note: Porosity influences density, strength, and permeability.

• High porosity means lower density and strength, higher fluid storage.

Grain Density ($\rho_g$)

• Mass of solid grains divided by volume of solid grains (excluding pores).

• Important for calculating stresses and geophysical interpretations.

Mechanical Properties

• Strength

• Hardness

• Elasticity

• Plasticity

Strength

• Ability to withstand stress before failure.

Comprehensive Strength ($\sigma_c$)

• Maximum compressive stress under uniaxial loading.

• Fundamental for foundation design and tunnel stability.

Tensile Strength ($\sigma_t$)

• Maximum tensile stress before pulling apart.

• Rocks are weaker in tension.

Shear Strength ($\tau$)

• Maximum shear stress before failure along a plane.

• Crucial for slope stability analysis and fault mechanics.

• Depends on mineral composition, grain size/shape, cementation, discontinuities, and confining pressure.

Hardness

• Resistance to scratching, abrasion, or indentation.

• Assessed using the Mohs Hardness Scale (1-10).

• Important for assessing durability in construction.

Elasticity

• Ability to deform under stress and return to original shape.

• Stress is proportional to strain (Hooke's Law).

Elastic Moduli

• Young's Modulus ($E$): Axial stress to axial strain (stiffness).

• Shear Modulus ($G$ or $μ$): Shear stress to shear strain (resistance to shear).

• Bulk Modulus ($K$): Hydrostatic pressure to volumetric strain (resistance to volume change).

• Poisson's Ratio ($ν$): Lateral strain to axial strain under uniaxial stress.

• Crucial for analyzing ground deformation and seismic wave propagation.

Plasticity

• Ability to undergo permanent deformation without fracturing.

• Retains new shape after stress removal.

• Common under high confining pressures and temperatures.

• Important in analyzing large-scale geological deformations and long-term behavior of underground structures.

Dynamic Properties of Rocks

• Wave Theory

• Factors Influencing Wave Velocity

• Static and Dynamic Moduli of Elasticity

• Grouting

Wave Theory

• Body Waves: Travel through rock (P-waves: compressional, faster; S-waves: shear, slower, only solids).

• Surface Waves: Travel along the surface (Love, Rayleigh), slower, more damaging.

Factors Influencing Wave Velocity

• Lithology, density, elastic properties (bulk, shear moduli), porosity, fluid saturation, pressure, temperature, fractures, anisotropy, weathering.

Static vs. Dynamic Moduli

• Static moduli (slow loading) represent sustained load response.

• Dynamic moduli (from wave velocities) are often higher due to rapid, small strains.

Grouting

• Injecting material into rock discontinuities to improve properties.

  • Increases strength/stiffness

  • Reduces permeability

  • Improves stability

  • Provides foundation

  • Seals against water inflow

• Effectiveness depends on grout type, pressure, fracture characteristics, rock permeability.

• Understanding rock properties (static & dynamic) is crucial for designing effective grouting programs.

Site Investigations

• Geological Methods

• Exploration Techniques

• Geophysical Methods

Geological Methods

• Desk Study: Reviewing existing data (maps, reports).

• Site Resonance: Visual site inspection.

• Geological Mapping: Mapping surface rocks and structures.

Exploration Techniques (Direct)

• Trial Pits/Trenches: Shallow excavations for direct observation and sampling.

• Borings (Drills): Creating boreholes for deeper sampling.

• Subsurface Soundings: Probing soil resistance (SPT, CPT).

Geophysical Methods (Non-Intrusive)

• Seismic Methods: Using wave propagation.

  • Refraction: Determining layer depths and velocities.

  • Reflection: Imaging subsurface structures.

• Electrical Methods: Using electrical properties.

  • ERT: Mapping subsurface resistivity (2D/3D).

  • Induced Polarization (IP): Measuring charge storage.

  • Self-Potential (SP): Measuring natural electrical potentials.

Direct Penetration

• Assessment and some sample recovery.

• Limited depth, primarily soil info.

Core Boring

• Drilling to retrieve cylindrical soil/rock cores for detailed examination and lab tests.

• Essential for rock mass quality (RQD) and undisturbed samples.

• Slower, more expensive.

Logging of Cores

• Systematic description of core characteristics (lithology, structure, discontinuities, weathering, etc.).

• Providing a visual and written subsurface record.

Geological Conditions for Construction

Dams

• Strong, impermeable foundation.

• Stable abutments.

• Watertightness.

• Seismic stability.

• Reservoir competence.

Tunnels

• Stable rock mass.

• Manageable groundwater.

• Excavatability.

• No swelling/squeezing ground or hazardous gases.

• Stable fault crossings.

Buildings

• Sufficient bearing capacity.

• Minimal settlement.

• Manageable groundwater.

• Stable slopes (if applicable).

• Seismic safety.

• Non-expansive soils.

Road Cutting

• Stable cut slopes.

• Easy excavatability.

• Proper drainage.

• Suitable material properties.

• Seismic stability.

• Environmental considerations.