Geology Essentials

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Chapter 1: The Origin, Scope, and Subdivisions of Geological Sciences 1. The Origin of Geological Sciences Early Roots: Ancient civilizations (Greek, Roman, Chinese, Indian) speculated about earthquakes, fossils, and erosion. Aristotle made early observations. The Great Debate (17th-18th Centuries): Neptunism (Abraham Werner): All rocks precipitated from a great primordial ocean. Plutonism (James Hutton): Rocks formed from cooling molten material. Hutton is "Father of Modern Geology" and proposed Uniformitarianism ("the present is the key to the past"). The 19th Century: Nicolas Steno: Established principles of stratigraphy. William Smith: Created first geologic map, used fossils to correlate rock layers. Charles Lyell: Firmly established Uniformitarianism. 2. The Scope of Geological Sciences Understanding the Earth: Origin, structure, composition, and history. Natural Resource Exploration: Locating groundwater, petroleum, natural gas, coal, metals, industrial minerals. Natural Hazard Mitigation: Studying earthquakes, volcanic eruptions, landslides, tsunamis, floods. Environmental Protection: Addressing climate change, soil erosion, pollution, remediation. Engineering Projects: Providing geological information for dams, bridges, tunnels, large buildings (Geotechnical Engineering). 3. Subdivisions of Geological Sciences A. Major Broad Disciplines: Physical Geology: Study of Earth's materials and processes (rivers, glaciers, volcanoes, earthquakes). Historical Geology: Study of Earth's origin and evolution, life forms (stratigraphy, paleontology). Mineralogy: Study of minerals (chemistry, crystal structure, properties). Petrology: Study of rocks (origin, formation, composition, classification). Structural Geology: Study of crust architecture, rock deformation (folds, faults). B. Applied & Interdisciplinary Fields: Economic Geology: Study of geologic materials for economic benefit (ore deposits, fossil fuels). Hydrogeology: Study of groundwater distribution and movement. Engineering Geology: Applying geologic knowledge to engineering problems. Environmental Geology: Applying geologic knowledge to solve environmental problems. Geophysics & Geochemistry: Using physics and chemistry to understand Earth's processes. Planetary Geology: Study of other celestial bodies. Geology as a Subdivision of Geography Geography: Broader science, "Why are things located where they are?" Geology: "How did the Earth get to be the way it is?" Physical Geography includes: Geomorphology, Climatology & Meteorology, Biogeography, Hydrology, Pedology. Geology is the bedrock. Earth's Internal Structure: Crust, Mantle, and Core Studied indirectly via seismology (seismic waves). Layers defined by: Chemical Composition: What they are made of. Physical Properties (Rheology): How they behave (solid vs. liquid, strong vs. weak). 1. The Crust What it is: Outermost, thinnest layer. Composition: Silicate minerals rich in oxygen and silicon. Types: Oceanic Crust: Thinner ($\sim 5-10 \text{ km}$), denser (basaltic). Continental Crust: Thicker ($\sim 30-50 \text{ km}$), less dense (granitic). Physical State: Rigid and brittle; causes earthquakes. 2. The Mantle What it is: Thick, solid layer, $\sim 84\%$ of Earth's volume. Composition: Ultramafic silicate rocks rich in iron and magnesium (e.g., peridotite). Physical State & Behavior: Solid, but upper part forms rigid lithosphere with crust. Below is asthenosphere (solid but plastic, flows slowly) driving plate tectonics. 3. The Core What it is: Innermost layer, primarily iron and nickel. Layers: Outer Core: Liquid. Movement generates Earth's magnetic field (geodynamo). Inner Core: Solid. Immense pressure keeps it solid despite higher temperature. Seismically Defined Layers Identified by analyzing seismic wave speed/path changes. Seismic Waves: P-waves (Primary/Compressional): Fastest, travel through solids, liquids, gases. S-waves (Secondary/Shear): Slower, travel only through solids. Key Seismic Discontinuities: Mohorovičić Discontinuity ("Moho"): Boundary between crust and mantle. Discovered by Andrija Mohorovičić (1909). Core-Mantle Boundary: Boundary between mantle and outer core. S-waves disappear (outer core is liquid). P-waves refract sharply. Inner Core Boundary: Boundary between liquid outer core and solid inner core. Discovered by Inge Lehmann (1936). 3. The Mechanical Layers (Defined by Behavior) Lithosphere: Rigid, brittle outer layer (crust + uppermost mantle). Breaks into tectonic plates. Asthenosphere: Beneath lithosphere, rock is solid but flows plastically. Low-velocity zone for seismic waves. Mesosphere (Lower Mantle): Stronger and more rigid than asthenosphere due to pressure. Geo-Internal Structures 1. The Engine: Mantle Convection What it is: Primary driver of geological activity. Solid mantle flows like viscous fluid over millions of years due to temperature differences. How it works: Hot material rises, cools, sinks, creating slow-moving conveyor belts. Surface Impact: Drives plate tectonics. 2. Key Seismically-Imaged Structures Subduction Slabs: Cold, dense oceanic plates sinking into mantle; show faster seismic wave speeds. Mantle Plumes: Narrow, upwelling columns of hot, buoyant rock; show slower seismic wave speeds. Can create hotspot volcanism. Large Low-Shear-Velocity Provinces (LLSVPs): Massive, mysterious structures on core-mantle boundary; very hot, possibly partially molten. Inner Core's Anisotropy: Seismic waves travel at different speeds depending on direction, indicating preferred crystalline orientation. 3. The Geodynamo: Structure of the Core What it is: Process in liquid outer core generating Earth's magnetic field. How it works: Convection: Liquid metal rises, cools, sinks. Rotation: Organizes convecting metal into spiraling patterns (Coriolis effect). Electrical Currents: Movement of conductive liquid iron generates currents. Self-Sustaining Cycle: Currents generate stronger magnetic field. Isostasy: The Principle of Gravitational Equilibrium Crust "floats" on denser, plastic mantle. Boat Analogy: Thicker, heavier blocks (crustal mountains) sit lower with a deeper "root." Thinner, lighter blocks (oceanic crust) sit higher with shallower "root." 1. The Two Main Models of Isostasy A. The Pratt Model (1854): Core Idea: Top of crust at uniform level; mountains are less dense rock. How it works: Columns of crust have same weight at base; higher elevations have lower density. Analogy: Tall, lightweight foam block vs. short, dense wood block. B. The Airy Model (1855): Core Idea: Crust has uniform density; mountains have thick, deep roots of light crustal material. How it works: Higher mountain = deeper root. Analogy: Iceberg model (90% below water). 2. The Mechanism: How Isostasy Works Mantle behaves in a ductile (plastic) manner. Loading: Crust forced downward by added weight (ice sheets, sediments, mountain building). Subsidence: Crust sinks to new equilibrium. Unloading: Weight removed (ice melt, erosion). Isostatic Rebound: Crust rises, mantle flows back in. 3. Evidence and Applications of Isostasy Post-Glacial Rebound: Regions covered by ice sheets still rising today. Mountain Roots: Seismic studies confirm thicker crust under mountain ranges. Erosion and Uplift: Erosion of mountains triggers rebound, causing the entire block to rise. Plate Tectonics: Isostasy is a consequence of plate tectonics (collisional tectonics thickens crust, extensional thins it). Chapter 2: Geologic Time 1. The Purpose and Structure of the Scale Purpose: Standardized, globally recognized timeline for Earth's history. Structure (Hierarchical): Eon: Largest subdivision. Hadean: Earth's formation. Archean: First continents, single-celled life. Proterozoic: Complex single-celled life, oxygen buildup. Phanerozoic: "Visible life," complex plants and animals. Era: Eons subdivided. Phanerozoic Eon has: Paleozoic: "Ancient life" (trilobites, fish, amphibians). Mesozoic: "Middle life" (dinosaurs, birds, mammals). Cenozoic: "Recent life" (mammals, humans). Period: Eras subdivided (e.g., Jurassic, Cretaceous). Epoch: Smallest subdivision (e.g., Pleistocene, Holocene). 2. The Basis for Boundaries: A Chronicle of Life and Catastrophe Boundaries defined by major, globally significant events. Mass Extinction Events: Most common marker. End of Paleozoic/beginning of Mesozoic: Permian-Triassic extinction (>$90\%$ marine species). End of Mesozoic/beginning of Cenozoic: Cretaceous-Paleogene (K-Pg) extinction (dinosaurs). Major Biological Innovations: Appearance of new life forms (e.g., Proterozoic Eon base defined by rise of cyanobacteria). 3. How the Scale Was Built: A Dual Approach A. Relative Dating: Established sequence of events. Superposition: Oldest layer at bottom, youngest at top. Cross-cutting relationships: Feature cutting another is younger. Index Fossils: Organisms with short, well-defined geologic time and widespread distribution. B. Absolute (Numerical) Dating: Assigned numerical ages. Radiometric dating: Uses decay of radioactive isotopes to calculate absolute age. Dendrochronology "The study of tree time." Dating based on tree rings. Core Principle: Uniform Annual Growth: One ring per year. Wide Ring: Favorable growing conditions. Narrow Ring: Stress or poor conditions. Scarring/Deformed Rings: Specific events (fires, landslides). Process: Building a Chronology: Sample Collection: Cores from living trees. Creating a Master Sequence: Overlapping ring patterns from living, dead, and archaeological wood. Applications: Absolute dating (archaeology, art history, geology), paleoclimatology, dating verification (calibrating radiocarbon dating). Limitations: Climate dependency, species dependency, geographical range, time limit ($\sim 13,000$ years). Fossils 1. Definition of a Fossil Any preserved evidence of ancient life, typically older than $10,000$ years. 2. Classification of Fossils A. Body Fossils: Preserved remains of actual body parts. Unaltered Remains: Original organic material preserved (e.g., mammoths in permafrost). Altered Remains: Original material changed or replaced. Permineralization/Petrification: Pores filled with minerals (e.g., petrified wood). Replacement: Original material dissolved and replaced by new mineral (e.g., silicified fossils). Carbonization (Distillation): Volatiles driven off, leaving thin carbon film (e.g., fossil ferns). B. Trace Fossils (Ichnofossils): Evidence of organism's activity (e.g., tracks, burrows, bite marks). 3. The Process of Fossilization (Taphonomy) Rare event, specific sequence of conditions. Stages: Death, Rapid Burial (most critical), Decomposition and Permineralization, Lithification, Exposure. 4. The Significance of Fossils Principles of Relative Dating: Basis for Faunal Succession. Evidence for Evolution: Direct evidence, shows change over time and transitional forms (e.g., Tiktaalik , Archaeopteryx ). Paleoenvironmental Reconstruction: Indicators of past climate, water depth. Recording Major Events: Diversification and mass extinctions. Evolution, and Extinction Part 1: Evolution Definition: Process of change in heritable characteristics of biological populations over generations. Evidence for Evolution: 1. The Fossil Record: Succession of forms, transitional fossils. 2. Anatomical Evidence: Homologous structures, vestigial structures. 3. Biochemical and Genetic Evidence: DNA/RNA similarities. Mechanism: Natural Selection (Darwin & Wallace): Variation: Heritable traits vary. Overproduction: More offspring than environment supports. Struggle for Existence: Competition for resources. Differential Survival and Reproduction: Better adapted individuals survive and reproduce. Descent with Modification: Advantageous traits become common. Part 2: Extinction Definition: Permanent termination of a biological lineage. 1. Background Extinction: Standard, low-level rate. Cause: Local environmental changes, competition, predation, disease. 2. Mass Extinction: Widespread, rapid decrease in biodiversity. Cause: Catastrophic, global-scale abiotic events. Bolide Impact: Asteroid/comet impact (e.g., K-Pg extinction). Flood Basalt Eruptions: Massive volcanic activity (e.g., Permian-Triassic extinction). Global Climate Change: Extreme warming or cooling. Sea Level Change: Major drops destroy habitats. Anoxic Events: Widespread ocean oxygen depletion. The Modern Cause: The Sixth Mass Extinction: Human Activity (habitat destruction, overexploitation, invasive species, pollution, human-induced climate change). Correlation Definition: Establishing equivalence of rocks or geological events from different locations (same age). Methods: 1. Lithostratigraphic Correlation (by Rock Type): Matching layers based on physical characteristics. 2. Biostratigraphic Correlation (by Fossils): Most powerful, uses Principle of Faunal Succession and index fossils. 3. Chronostratigraphic Correlation (by Absolute Age): Matching rocks formed at the exact same time using radiometric dating (e.g., volcanic ash layers). 4. Geophysical Correlation: Using physical and chemical properties in subsurface (well logging, seismic reflection). Practical Application: Reconstruct ancient environments, locate natural resources, understand geologic history. Unconformities Definition: Surface of erosion or non-deposition separating younger from older rocks. Represents a break in the geologic record. How They Form: Deposition $\rightarrow$ Uplift and Erosion $\rightarrow$ Subsidence and Renewed Deposition. Classification: 1. Angular Unconformity: Horizontal layers on top of tilted/folded layers. Signifies major tectonic event. 2. Disconformity: Parallel layers of sedimentary rock with an irregular erosional surface between them. 3. Nonconformity: Sedimentary rocks on top of eroded igneous or metamorphic rocks. Signifies profound time gap. Significance: Markers of major events, time gaps, structural traps (for oil/gas). Principles of Relative Dating 1. Principle of Superposition: Oldest at bottom, youngest at top in undisturbed sequence. 2. Principle of Original Horizontality: Sediments deposited horizontally; tilted/folded layers indicate disturbance after deposition. 3. Principle of Lateral Continuity: Sediments extend laterally until they thin out or encounter a barrier. 4. Principle of Cross-Cutting Relationships: Feature cutting another is younger. 5. Principle of Inclusions: Rock fragments (inclusions) are older than the rock containing them. 6. Principle of Faunal Succession (Biostratigraphy): Fossils succeed one another in a definite, recognizable order. Dating with Radioactivity: Significance Radiometric dating: Using decay of unstable radioactive isotopes to determine absolute age. Significance: 1. Provided first absolute dates for Geologic Time Scale. 2. Quantified immense age of Earth ($\sim 4.54$ billion years). 3. Calibrated fossil record and evolutionary rates. 4. Verified and refined Plate Tectonics. 5. Provides a clock for recent human history and climate change (Carbon-14 dating). 6. Applied to vast range of geological problems (mountain building, volcanic eruptions, ore deposits, glaciations). Basic Premise: Unstable Parent Isotope decays to Stable Daughter Isotope at fixed rate. The Half-Life: Time for half of parent atoms to decay. Constant, unaffected by external conditions. Radioactivity Core Definition: Spontaneous process where unstable atomic nucleus loses energy by emitting radiation. Cause: Nuclear Instability: Imbalance of protons/neutrons in nucleus. Main Types of Radioactive Decay: A. Alpha Decay ($\alpha$): Emits alpha particle (helium-4 nucleus). Parent loses 2 protons, 2 neutrons. B. Beta Decay ($\beta$): Emits beta particle (high-energy electron). Neutron transforms to proton + electron. Parent gains 1 proton. C. Electron Capture: Nucleus captures electron. Proton combines with electron to become neutron. Parent loses 1 proton. The Decay Series: Multiple decay steps for some isotopes (e.g., Uranium-238 to Lead-206). Significance for Geology: Spontaneous, random (statistically predictable), unaffected by conditions. Provides a reliable chronometer. Half-Life Core Definition: Time for half of parent atoms to decay into daughter atoms. Unique constant property for each isotope. Mathematical Pattern: $N = N_0 e^{-\lambda t}$, where $N$ is remaining parent atoms, $N_0$ is original, $\lambda$ is decay constant, $t$ is time. Range of Half-Lives: Different isotopes suitable for different time scales. Isotope System Parent Daughter Half-Life (Years) Effective Dating Range Useful For Carbon-14 $^{14}\text{C}$ $^{14}\text{N}$ $5,730$ Up to $\sim 50,000-60,000$ Organic materials, archaeology Potassium-Argon $^{40}\text{K}$ $^{40}\text{Ar}$ $1.25$ billion $100,000$ years to Earth's age Volcanic rocks, old rocks Uranium-Lead $^{238}\text{U}$ $^{206}\text{Pb}$ $4.47$ billion $\sim 1$ million years to Earth's age Zircon crystals, oldest rocks Rubidium-Strontium $^{87}\text{Rb}$ $^{87}\text{Sr}$ $48.8$ billion $\sim 10$ million years to Earth's age Igneous and metamorphic rocks "Reset" of the Clock: Mineral crystallization (igneous rocks), recrystallization (metamorphic rocks). Sedimentary rocks dated by interbedded volcanic ash layers. Radiometric Dating (Practical Application) Fundamental Principle: Measure parent/daughter ratio and use half-life. Formula: $t = [\ln(1 + D/N) / \ln(2)] * T$. Process: Sample Selection (fresh, unweathered rock), Laboratory Analysis (mass spectrometers), Calculation and Interpretation. Key Requirements: Closed System (no gain/loss of parent/daughter), Known Initial Daughter Composition. Carbon-14 Dating (Radiocarbon Dating) Core Principle: Used for once-living organic materials up to $\sim 50,000-60,000$ years old. Dates time of death. How the "Clock" is Set: Formation of C-14: Cosmic rays convert $^{14}\text{N}$ to $^{14}\text{C}$ in atmosphere. Uptake by Living Things: Organisms exchange carbon, maintaining constant $^{14}\text{C}/^{12}\text{C}$ ratio. Clock Starts at Death: Exchange stops, $^{14}\text{C}$ decays. Half-Life: $5,730$ years. Key Assumptions and Limitations: Constant Atmospheric C-14 Production (calibrated by dendrochronology, varved sediments), Contamination, Material Type (only organic). Significance and Applications: Archaeology, paleontology, paleoclimatology, geology. Chapter 3: Elements of Structural Geology Primary and Nontectonic Structures Core Concept: Primary structures form during rock origin, nontectonic. Secondary (tectonic) structures form after due to deformation. Definitions & Key Differences: Feature Primary / Nontectonic Structures Secondary / Tectonic Structures Definition Structures that form during the formation of the rock itself. Structures that form after the rock has formed , due to stress and deformation. Cause Processes of rock formation (cooling, sedimentation, compaction). Tectonic forces (compression, tension, shear). Timing Syn-genetic (same time as genesis). Post-genetic (after genesis). Analogy The swirls in a marble cake baked into it. Cracks or bends in the cake after it has cooled and been handled. Types of Primary (Nontectonic) Structures: A. In Igneous Rocks: Columnar Jointing: Polygonal cracks from cooling lava. Pillow Structures: Bulbous masses from underwater lava. Flow Banding: Layers from viscous lava flow. Vesicles: Holes from trapped gas bubbles. B. In Sedimentary Rocks: Bedding/Stratification: Layers of sediment. Cross-Bedding: Inclined layers within horizontal bed. Ripple Marks: Wave-like patterns from water/wind currents. Mudcracks: Polygonal cracks from drying mud. Graded Bedding: Grain size changes from coarse at bottom to fine at top. C. In Metamorphic Rocks (Special Case): Can preserve relict primary structures if metamorphism not too intense. Importance of Distinction: Interpret past environments, "way-up" indicators, avoiding misinterpretation. Force and Stress Core Concept: Force is a push/pull. Stress is force per unit area ($\sigma = F/A$). Stress ($\text{MPa}$) is more geologically relevant. Types of Stress: Type of Stress Effect on a Rock Volume Tectonic Setting Example Compression Shortens and flattens the rock. Convergent plate boundaries Tension Elongates and pulls apart the rock. Divergent plate boundaries Shear Causes parts of the rock to slide past one another. Transform plate boundaries The Stress Ellipsoid: A 3D Perspective: ($\sigma_1$ Maximum Principal, $\sigma_2$ Intermediate, $\sigma_3$ Minimum Principal). Differential Stress ($\sigma_1 - \sigma_3$): Key driver of deformation. High differential stress causes folding, faulting, flowing. Mean Stress: Average of three principal stresses. Responsible for compaction. Deformation and Strain Core Concept: Deformation is change in shape, volume, or position. Strain is quantitative measure of deformation. Types of Strain: Type of Strain Description Formula (Simplified) Geological Example Longitudinal Strain Change in length of a line compared to its original length. $e = (l - l_0) / l_0$ Fossil stretched from $10 \text{ cm}$ to $11 \text{ cm}$. Strain is $0.1$. Shear Strain Measures change in angle between two lines originally perpendicular. $\gamma = \tan(\psi)$ Cross-beds originally at $90^\circ$ smeared to $70^\circ$. Shear strain is $\tan(20^\circ)$. Volumetric Strain Change in volume compared to original volume. $\Delta = (V - V_0) / V_0$ Porous sandstone deeply buried, pores crushed out. Homogeneous vs. Heterogeneous Strain: Homogeneous: Uniform, straight lines remain straight (e.g., metamorphic rock foliation). Heterogeneous: Not uniform, lines become curved (e.g., folds). The Strain Ellipsoid: Visualizes 3D strain (X-axis: max elongation, Y-axis: intermediate, Z-axis: max shortening). How Strain is Manifested: Strain markers (deformed pebbles, fossils), distance between points. Rheology Core Concept: How rocks deform and flow under stress over different timescales. Fundamental Idea: Strength & Behavior: Depends on Confining Pressure (depth), Temperature (heat), Strain Rate (speed). Idealized Material Behaviors (Rheological Models): Rheological Model Response to Stress Equation (Conceptual) Real-World Analogy Geological Example Elastic (Spring) Deforms instantly, recovers instantly. No permanent strain. Stress $\propto$ Strain (Hooke's Law) Rubber band Seismic waves passing through rock. Viscous (Dashpot/Toothpaste) Flows continuously as long as stress is applied. Permanent deformation. Strain rate depends on stress. Stress $\propto$ Strain Rate (Newtonian Flow) Honey, toothpaste Salt domes, glaciers, mantle flow. Plastic (Cracking Point) Rigid until critical stress (yield strength) then deforms permanently. If Stress Yield Strength: Continuous deformation Credit card Fault formation (fractures). Elasto-Plastic: Elastic then plastic (most brittle rocks). Visco-Elastic: Elastic and viscous components (Earth's mantle). Brittle vs. Ductile Deformation: Characteristic Brittle Deformation Ductile Deformation Type of Strain Permanent loss of cohesion by fracturing. Permanent change in shape without fracturing. Mechanism Cracking, fracturing, frictional sliding. Flow via internal mechanisms: Crystal Plasticity, Diffusion Creep, Pressure Solution. Structures Formed Faults, Joints, Breccia. Folds, Foliation, Shear Zones. Analogies Breaking a piece of chalk. Bending a piece of soft clay. Favored Conditions Low Temperature, Low Confining Pressure, High Strain Rate (fast), Shallow Crust. High Temperature, High Confining Pressure, Low Strain Rate (slow), Deep Crust. The Brittle-Ductile Transition: Zone in crust ($\sim 10-15 \text{ km}$ depth) where conditions change from brittle to ductile. Chapter 4: Study of major Brittle and Ductile structures Fault and Fold- Geometry and displacement Core Concept: Faults (brittle) and Folds (ductile) are secondary structures, direct evidence of tectonic forces. Part A: FAULTS (Brittle Structures) Definition: Fracture in Earth's crust with measurable displacement. Fault Geometry: Key Elements: Fault Plane, Dip, Strike, Hanging Wall (above), Footwall (below). Classification of Faults (Based on Slip Direction): Fault Type Stress Responsible Hanging Wall Motion Effect on Crust Tectonic Setting Normal Fault Tension (pulling apart) Moves down relative to the footwall Extends and thins the crust Divergent boundaries Reverse Fault Compression (squeezing) Moves up relative to the footwall Shortens and thickens the crust Convergent boundaries Thrust Fault Compression A low-angle reverse fault (dip $ Shortens the crust over a large area. Convergent boundaries (e.g., Himalayas) Strike-Slip Fault Shear Blocks slide past each other horizontally. No vertical motion. Shears the crust. Transform boundaries (e.g., San Andreas Fault) Displacement on Faults: Described by slip vector . Net Slip: Total relative displacement. Heave: Horizontal component of dip slip. Throw: Vertical component of dip slip. Part B: FOLDS (Ductile Structures) Definition: Bend or warp in rock layer from permanent ductile deformation. Fold Geometry: Key Elements: Limb, Hinge/Hinge Line, Axial Plane, Crest & Trough. Classification of Folds: Fold Type Description Implied Stress Anticline Arch-like fold, oldest rocks in center, limbs dip away from hinge. Compression Syncline Trough-like fold, youngest rocks in center, limbs dip towards hinge. Compression Characteristics Part A: FAULT CHARACTERISTICS Fault Surface Features: Slickensides: Polished/scratched surfaces indicating movement direction. Fault Grooves & Striations: Deep scratches from abrasive rock fragments. Fault Steps: Small, steplike ridges indicating movement direction. Fault Rock Types: Fault Gouge: Soft, clay-rich material from grinding (shallow crust). Breccia: Coarse-grained rock with angular fragments (near surface). Cataclasite: Fine-grained, hard rock from mechanical crushing (deeper brittle). Mylonite: Fine-grained, streaky rock from ductile deformation (high P/T shear zones). Other Key Characteristics: Fault Scarp: Small cliff from vertical displacement. Fault Zone: Zone of numerous closely spaced fractures. Part B: FOLD CHARACTERISTICS 1. Fold Tightness (Based on Inter-limb Angle): Fold Type Inter-limb Angle Intensity of Deformation Gentle $180^\circ - 120^\circ$ Very mild Open $120^\circ - 70^\circ$ Mild Close $70^\circ - 30^\circ$ Moderate Tight $30^\circ - 0^\circ$ Severe Isoclinal $0^\circ$ (Limbs are parallel) Very severe 2. Fold Symmetry: Cylindrical (regular) vs. Non-cylindrical (changing shape, plunging). 3. Fold Style (Based on Layer Thickness): Parallel (constant thickness perpendicular to bedding) vs. Similar (constant thickness parallel to axial plane). 4. Minor Structures: Cleavage (planes of weakness perpendicular to shortening), Lineations (linear structures indicating tectonic transport). Classification (Faults) 1. Kinematic Classification (Slip Direction): Dip-Slip Faults: Normal, Reverse, Thrust. Strike-Slip Faults: Right-Lateral (Dextral), Left-Lateral (Sinistral). Oblique-Slip Faults: Combination of dip-slip and strike-slip. 2. Geometric Classification (Fault Plane Attitude): Strike Fault, Dip Fault, Oblique Fault, Bedding-Plane Fault (Detachment). 3. Tectonic Classification (Scale & Context): Thrust Fault/Overthrust, Growth Fault, Transform Fault, Listric Fault. Classification (Folds) 1. Classification Based on Fold Closure: Anticline, Syncline. 2. Classification Based on Inter-limb Angle: Gentle, Open, Close, Tight, Isoclinal. 3. Classification Based on Axial Plane Orientation: Upright/Symmetrical, Asymmetrical, Overturned, Recumbent. 4. Classification Based on plunge of the Hinge Line: Plunging Fold (Anticline, Syncline), Non-plunging Fold. 5. Classification Based on Layer Thickness: Parallel Folds, Similar Folds. Chapter 5: Mineralogy Definition of mineral Core Concept: Mineral is fundamental unit of geological materials, must meet 5 criteria. The Five-Part Definition: A mineral is: Naturally Occurring Solid Inorganic Definite Chemical Composition Ordered Internal Structure (crystal structure) Criterion Meaning Examples Counterexamples (Not Minerals) 1. Naturally Occurring Formed by natural geological processes. Quartz, Diamond, Gold Synthetic diamond, industrial corundum 2. Solid Has definite volume and shape. Ice (H$_2$O) Liquid water, mercury 3. Inorganic Not derived from living organisms or their remains. Calcite from limestone rock Coal, pearls 4. Definite Chemical Composition Expressed by a specific chemical formula. Quartz: SiO$_2$, Halite: NaCl, Gold: Au Obsidian (variable composition) 5. Ordered Internal Structure (Crystalline) Atoms arranged in a specific, repeating, 3D pattern (crystal lattice). Halite (salt) Opal (amorphous), a mineraloid Key Concepts and Exceptions: Solid Solution: Composition can vary within defined limits (e.g., Olivine (Mg,Fe)$_2$SiO$_4$). Mineraloids: Meet most criteria but lack crystalline structure (e.g., Opal, Obsidian). Biogenic Minerals: Modern gray area (e.g., aragonite in shells). Why Definition is Important: Systematic study, predicts properties, links to genesis. Compositions Core Concept: Mineral composition is its chemical recipe. Building Blocks: Elements and Ions: Minerals composed of cations (positive) and anions (negative) forming neutral compounds. Major Chemical Groups of Minerals: Classified by dominant anion/anionic complex. Mineral Group Defining Anion / Complex Example Minerals Importance / Notes Silicates [SiO$_4$]$^{4-}$ (Silicate tetrahedron) Quartz, Feldspar, Mica, Olivine Most important group (>90% of Earth's crust). Oxides O$^{2-}$ (Oxygen) Hematite, Magnetite, Corundum Important ore minerals for metals. Sulfides S$^{2-}$ (Sulfide) Pyrite, Galena, Chalcopyrite Primary ore minerals for most major metals. Sulfates [SO$_4$]$^{2-}$ (Sulfate) Gypsum, Anhydrite Form by evaporation of water (evaporite minerals). Carbonates [CO$_3$]$^{2-}$ (Carbonate) Calcite, Dolomite Main constituents of limestone and marble. Halides Cl$^-$, F$^-$ (Halogens) Halite, Fluorite Evaporite minerals, common table salt. Native Elements None (Pure elements) Gold, Diamond, Silver, Copper Minerals composed of a single element. Concept of Solid Solution: Atoms of similar size/charge substitute in crystal structure. Creates mineral series (e.g., Olivine Series: Forsterite (Mg-rich) to Fayalite (Fe-rich); Plagioclase Feldspar Series: Albite (Na) to Anorthite (Ca)). Role of Composition in Mineral Properties: Influences color, density, stability. Nature and properties Core Concept: Mineral's nature defined by chemical composition and crystalline structure. Dictates physical properties. Foundation: Crystalline Nature: Ordered internal structure (crystal lattice). Crystal Face: Flat surface from internal structure. Crystal Habit: Characteristic shape when growing freely (e.g., cubic, hexagonal). Physical Properties for Identification: Property Description & Cause Examples & Test Cleavage Tendency to break along planes of weak atomic bonds. Perfect (1 direction): Mica; Good (2 at $90^\circ$): Feldspar Fracture How a mineral breaks when not along a cleavage plane. Conchoidal (quartz), Irregular/Jagged (copper) Hardness Resistance to scratching (Mohs Scale 1-10). Talc (1) softest; Diamond (10) hardest. Luster Quality of light reflected from mineral's surface. Metallic (pyrite), Non-Metallic (vitreous, pearly) Color Most obvious but least reliable, often due to trace impurities. Quartz: clear, purple, pink, black. Streak Color of mineral in its powdered form. More reliable than color. Hematite: reddish-brown; Pyrite: greenish-black Other Properties Specific Gravity, Magnetism, Reaction to Acid, Taste. Galena (heavy), Magnetite (magnetic), Calcite (fizzes with HCl), Halite (salty). Relationship: Structure $\to$ Property: Mica (sheet structure) has perfect cleavage; Quartz (framework) has conchoidal fracture. Diamond (strong 3D structure) is hardest; Graphite (sheet structure) is softest. Polymorphism: Two minerals with same chemical composition but different crystal structure (e.g., Graphite vs. Diamond, Corundum vs. Diaspore, Quartz vs. Coesite/Stishovite). Rock forming minerals Core Concept: Rock-forming minerals make up majority of Earth's crust. Essential for classifying rocks. Major Rock-Forming Mineral Groups: Mostly silicates (based on SiO$_4$ tetrahedron arrangement). Mineral Group Structure of (SiO$_4$)$^{4-}$ Units Key Minerals Properties & Importance Olivine Group Isolated Tetrahedra Forsterite, Fayalite Olive green, granular, no cleavage, conchoidal fracture. High-temp. Mantle/mafic rocks. Pyroxene Group (Single-chain) Single Chains Augite Dark green to black, 2 cleavages at $\sim 90^\circ$. Mafic igneous rocks. Amphibole Group (Double-chain) Double Chains Hornblende Black/dark green, 2 cleavages at $\sim 60^\circ/120^\circ$. Intermediate igneous/metamorphic rocks. Mica Group (Sheet) Sheets Muscovite (light), Biotite (dark) Colorless/black, 1 perfect cleavage (peels into sheets). Igneous/metamorphic rocks. Feldspar Group (Framework) 3D Framework Plagioclase (Na-Ca), Orthoclase (K) White, pink, gray, 2 cleavages at $90^\circ$. Most common mineral. Quartz (Framework) 3D Framework Quartz Colorless/white, conchoidal fracture, no cleavage, H=7. Stable, all rock types. Important Non-Silicate Rock-Forming Minerals: Mineral Group Key Minerals Properties & Importance Carbonates Calcite, Dolomite White/colorless, rhombohedral cleavage, reacts with HCl. Limestone/Marble. Oxides Hematite, Magnetite Hematite: reddish streak; Magnetite: magnetic, black streak. Iron ores. Sulfates Gypsum Colorless/white, 1 perfect cleavage, H=2. Evaporite rocks. Halides Halite Colorless/white, cubic cleavage, salty taste. Rock salt. How to Approach Rock Identification: Light/dark (felsic/mafic), presence of key minerals (Quartz, Feldspar, black minerals). Mineral classification Core Concept: Hierarchical system based on dominant anion/anionic group. Dana and Strunz Systems: Two accepted systems. Dana System groups by chemistry, then structure. The Dana Classification System (Simplified Overview): Class Number Class Name Defining Anion / Complex Key Examples & Notes I Native Elements None Metals: Gold, Silver, Copper. Non-metals: Diamond, Graphite, Sulfur. II Sulfides S$^{2-}$ Pyrite, Galena, Chalcopyrite. Important ore minerals. IV Oxides O$^{2-}$ Simple Oxides: Hematite, Magnetite. Hydroxides: Brucite. For metal ores. V Halides F$^-$, Cl$^-$, Br$^-$, I$^-$ Halite, Fluorite. Often soft, soluble. VI Carbonates [CO$_3$]$^{2-}$ Calcite, Dolomite. Fizz with acid. Constituents of limestone. IX Silicates [SiO$_4$]$^{4-}$ (Silicate Tetrahedron) Most important class (>90% of Earth's crust). Sub-classified by structure. The Silicate Subclasses (Based on Structure): Silicate Subclass [SiO$_4$]$^{4-}$ Units Link Si:O Ratio Key Examples Geological Significance Nesosilicates (Island) Isolated Tetrahedra 1:4 Olivine, Garnet High-temperature minerals. Common in mantle and mafic rocks. Sorosilicates (Double Island) 2 tetrahedra share 1 O 2:7 Epidote Common in metamorphic rocks. Cyclosilicates (Ring) Tetrahedra form rings 1:3 Beryl, Tourmaline Often in pegmatites. Gemstones. Inosilicates (Chain) Single Chains 1:3 Pyroxene Group (e.g., Augite) Major minerals in mafic rocks. Double Chains 4:11 Amphibole Group (e.g., Hornblende) Major minerals in intermediate/metamorphic rocks. Phyllosilicates (Sheet) Sheets of tetrahedra 2:5 Mica Group, Clay Minerals Sheet structure $\to$ perfect cleavage. Primary weathering products. Tectosilicates (Framework) 3D Framework 1:2 Feldspar Group, Quartz, Zeolites Most abundant group. Form most of the crust. Mineral Series and Species: Mineral Series (solid solution, e.g., Olivine Series), Mineral Species (defined chemical formula, e.g., Forsterite), Mineral Varieties (sub-specific name based on color, e.g., Amethyst). Chapter 6: Petrology Definition of rocks Core Concept: Study of rocks. Rock is naturally occurring, cohesive solid of one or more minerals/mineraloids. Formal Definition: Naturally occurring, coherent aggregate of one or more minerals, or undifferentiated mineral matter. Criterion Meaning Examples Counterexamples (Not Rocks) Naturally Occurring Formed by natural geological processes. Granite, Basalt, Limestone Concrete, Brick Coherent Aggregate Grains/crystals locked together to form solid mass. Sandstone, Granite Pile of sand (sediment) One or more minerals Most are multi-mineralic, some mono-mineralic. Granite (Quartz + Feldspar + Mica), Limestone (Calcite) Amethyst crystal (mineral) Relationship Between Minerals and Rocks: Minerals are ingredients, rocks are recipes. Three Rock Types: The Rock Cycle: Igneous, Sedimentary, Metamorphic. Interconnected. Formation Core Concept: Formation (origin) defines rock type. Igneous Rocks: Formation from Molten Rock: Cooling and solidification of magma/lava. Key Processes: 1. Melting: Generation of magma (decompression, addition of volatiles, heat transfer). 2. Crystallization: Atoms arrange into orderly structures upon cooling. Rate of cooling determines texture: Slow cooling $\to$ large crystals (Intrusive/Igneous: Granite, Gabbro). Rapid cooling $\to$ tiny crystals (Extrusive/Volcanic: Rhyolite, Basalt). Very rapid cooling $\to$ no crystals (Obsidian). Sedimentary Rocks: Formation from Weathered Debris: From products of weathering. Key Processes (Sedimentary Cycle): 1. Weathering: Physical breakdown (frost wedging) and chemical alteration (feldspar to clay). 2. Erosion & Transport: Removal and movement of weathered material. 3. Deposition: Settling of transported sediment. 4. Lithification: Loose sediment to solid rock (compaction, cementation). Types of Sedimentary Rocks: Clastic: From cemented fragments of other rocks (Sandstone, Shale). Chemical/Biochemical: From minerals precipitated from water (Limestone, Rock Salt). Metamorphic Rocks: Formation by Change: Pre-existing rocks subjected to high T, P, fluids, changing in solid state. Key Agents of Metamorphism: 1. Heat (Temperature): Drives chemical reactions, recrystallization. 2. Pressure (Stress): Confining pressure (equal, compacts), differential stress (unequal, bends, flattens, foliates). 3. Chemically Active Fluids: Dissolve and transport ions, facilitate recrystallization. Types of Metamorphism: Regional (mountain building, foliated rocks), Contact (intruding magma, non-foliated rocks). Compositions Core Concept: Rock composition = chemical + mineralogical makeup. Key to classification, origin, history. Two Ways to Describe Composition: Chemical (bulk percentage of elements/oxides) and Mineralogical (identity/abundance of minerals). Chemical Composition: Major Elements: Earth's crust dominated by 8 elements (SiO$_2$, Al$_2$O$_3$, FeO/Fe$_2$O$_3$, MgO, CaO, Na$_2$O, K$_2$O, H$_2$O). SiO$_2$ content determines felsic/mafic. Mineralogical Composition: Major Players: Minerals combine in specific ways. Rock Type Felsic (Silicic) Composition Intermediate Composition Mafic Composition Ultramafic Composition General Silica-Rich (>65% SiO$_2$), Light-colored, Less dense Medium Silica (55-65% SiO$_2$), Salt & pepper appearance Silica-Poor (45-55% SiO$_2$), Dark-colored, Dense Very Silica-Poor ( Key Minerals Quartz, Potassium Feldspar, Sodium-rich Plagioclase, Muscovite Mica Amphibole (Hornblende), Calcium-rich Plagioclase, Biotite Mica, Some Quartz Calcium-rich Plagioclase, Pyroxene (Augite), Olivine, Amphibole Olivine, Pyroxene, No Feldspar Igneous Example (Intrusive) Granite Diorite Gabbro Peridotite (Mantle Rock) Igneous Example (Extrusive) Rhyolite Andesite Basalt Komatiite (Rare, ancient) Sedimentary Equivalent Sandstone (Arkose), Shale (some) Graywacke (Dirty Sandstone) Shale (some) Very Rare Metamorphic Equivalent Gneiss, Quartzite Amphibolite Amphibolite, Greenschist Serpentinite Composition Across Rock Types: Same chemical composition can form different rocks (e.g., CaCO$_3$ forms Limestone (sedimentary) or Marble (metamorphic)). Importance of Silica Content (SiO$_2$): Controls melting temperature, magma viscosity, mineral stability. Nature and properties Core Concept: Rock nature defined by mineral composition and texture. Basis for classification. Properties of Igneous Rocks: Dominated by cooling history. Property Description & Significance Examples Texture Most diagnostic property. Determined by cooling rate. Phaneritic (slow cooling, large crystals): Granite, Gabbro, Diorite. Aphanitic (rapid cooling, microscopic crystals): Rhyolite, Basalt, Andesite. Porphyritic (large crystals in fine-grained groundmass, two-stage cooling): Porphyritic Andesite. Glassy (extremely rapid cooling, no crystals): Obsidian. Vesicular (gas bubbles trapped): Scoria, Pumice. Composition Ranges from Felsic (light, high SiO$_2$) to Ultramafic (dark, low SiO$_2$). Felsic: Granite; Mafic: Gabbro; Ultramafic: Peridotite. Properties of Sedimentary Rocks: Dominated by depositional environment and lithification. Property Description & Significance Examples Texture Most diagnostic property. Clastic Texture (cemented fragments): Sandstone, Shale, Conglomerate. Crystalline Texture (interlocking crystals): Rock Salt, Crystalline Limestone. Grain Size & Rounding (reveals energy/transport distance): Conglomerate (high energy, rounded clasts) vs. Breccia (angular clasts). Bedding/Stratification (primary sedimentary structure): Visible in sandstone/shale. Composition Clastic: Minerals stable at surface (Quartz, Clay). Chemical/Biochemical: Calcite, Dolomite, Halite, Gypsum. Quartz Sandstone, Arkose, Limestone. Properties of Metamorphic Rocks: Dominated by heat, pressure, stress. Property Description & Significance Examples Texture Most diagnostic property. Determined by differential stress. Foliated (minerals aligned in parallel planes/bands due to directed pressure): Slate, Schist, Gneiss. Non-Foliated (lack of mineral alignment, from confining pressure/monomineralic protoliths): Marble, Quartzite, Hornfels. Composition Depends on parent rock (protolith) and grade of metamorphism. Pelitic (from shale), Calcareous (from limestone). Other Diagnostic Physical Properties Hardness/Durability, Reaction with Acid, Density, Color. Quartzite/Metaconglomerate (tough), Limestone/Marble (fizz with HCl). Classification Core Concept: Systematic way to name/categorize rocks based on observable properties (texture, mineral composition). Igneous Rock Classification: Double-axis system (texture and composition). Texture: Intrusive (Phaneritic), Extrusive (Aphanitic/Glassy). Composition: Felsic, Intermediate, Mafic, Ultramafic. Special Textures: Porphyritic, Glassy, Vesicular. Sedimentary Rock Classification: By origin, then by texture/composition. A. Clastic (Detrital) Rocks: Classified by particle size . Particle Size Rock Name Typical Composition Boulder, Cobble, Pebble ($>2 \text{ mm}$) Conglomerate (rounded), Breccia (angular) Rock fragments, Quartz Sand ($1/16 - 2 \text{ mm}$) Sandstone Quartz Sandstone, Arkose, Graywacke Silt ($1/256 - 1/16 \text{ mm}$) Siltstone Quartz, Feldspar Clay ($ Shale (fissile), Mudstone (blocky) Clay Minerals, Quartz B. Chemical & Biochemical Rocks: Classified by composition . Composition Rock Name How it Forms Calcite (CaCO$_3$) Limestone Biochemical (shells), Chemical (precipitation) Calcite & Dolomite Dolostone Alteration of limestone by Mg-rich water. Microcrystalline Quartz (SiO$_2$) Chert (Flint, Jasper) Biochemical (silica shells), Chemical (precipitation). Halite (NaCl) Rock Salt Evaporation of saline water. Gypsum (CaSO$_4 \cdot 2\text{H}_2\text{O}$) Rock Gypsum Evaporation of saline water. Plant Fragments (C) Coal Compression of organic matter. Metamorphic Rock Classification: By texture (foliated/non-foliated), then by mineral composition/grain size. A. Foliated Rocks: Foliation Type Grain Size Rock Name Parent Rock Slaty Cleavage Very Fine Slate Shale, Mudstone Phyllitic Sheen Fine Phyllite Shale, Mudstone Schistosity Medium to Coarse Schist Various (often shale or basalt) Gneissic Banding Coarse Gneiss Various (Granite or Shale) B. Non-Foliated Rocks: Composition Rock Name Parent Rock Calcite Marble Limestone Quartz Quartzite Quartz Sandstone Various Hornfels Various (baked by magma) Pyroxene, etc. Amphibolite Mafic Igneous Rocks (e.g., Basalt) Serpentine Serpentinite Peridotite (ultramafic) Role of Protolith and Grade: Protolith (parent rock) controls possible minerals. Metamorphic Grade (intensity) controls texture/minerals (Low $\to$ Slate, Intermediate $\to$ Phyllite/Schist, High $\to$ Gneiss). Chapter 7: Geology of Bengal Basin Basin History Core Concept: Bengal Basin is one of world's largest sedimentary basins. Complex history of tectonic collision, mountain building, sediment deposition. What is a Basin?: Sedimentary basin is region of Earth's crust with long-term subsidence, filled by eroded sediments. Bengal Basin is a foreland basin . Stages of Bengal Basin History: Phase 1: Pre-Collision Stage (Cretaceous - $\sim 50$ Ma): Indian Plate racing north towards Asia. Passive continental margin. Shallow marine sediments deposited. Phase 2: Collision and Basin Initiation (Oligocene - $\sim 30$ Ma): Indian Plate collided with Eurasian Plate, beginning Himalayan mountain belt formation. Immense weight caused flexure and downwarp, creating Bengal Foredeep. Phase 3: Progradation of the Bengal Fan (Miocene - Present): Himalayas supplied massive sediment. Ganges and Brahmaputra carried sediment south. Bengal Fan (Delta) prograded seaward, world's largest submarine fan. Phase 4: Modern Deltaic Stage (Pleistocene - Present): Sea-level changes and continuous sedimentation shaped modern landscape. Ganges-Brahmaputra-Meghna (GBM) river system formed world's largest active delta. Surface geology of Bangladesh is very young, unconsolidated sediments. Tectonic Framework: The Three-Way Collision: Complex setting with Indian Plate, Eurasian Plate, Burma Microplate. Creates active plate boundaries: Northern Boundary: Main Frontal Thrust (Himalayas, convergent). Eastern Boundary: Indo-Burman Ranges and Sagaing Fault (transpressional). Summary of the Basin Fill: Pre-Collision Rocks: Cretaceous shelf sediments (deep basement). Syn-Collision Rocks: Oligocene-Miocene marine sediments (e.g., Surma Group), major hydrocarbon reservoirs. Post-Collision Rocks: Pliocene-Pleistocene deltaic/fluvial sands/clays (e.g., Dupi Tila Formation), major aquifers. Modern Deposits: Holocene deltaic/floodplain deposits. Seismic Background Core Concept: Bengal Basin is tectonically active, high seismic hazard. Source of Seismicity: Tectonic earthquakes from strain release along active faults. Driving force: Indian Plate movement ($\sim 45 \text{ mm/year}$) subducting beneath Burma Plate. Major Seismic Sources (Fault Systems): Seismic Source Type of Fault Location & Impact Plate Boundary Fault (PBF) Megathrust (Main subduction interface) Primary fault where Indian Plate subducts beneath Burma Plate. Capable of generating megathrust earthquakes (M8.0+). Significant seismic hazard. Blind Thrusts (e.g., Madhupur Fault) Reverse/Thrust (Hidden) Faults within basement rock, don't break surface. Deform overlying sediments into anticlines. Hazardous, close to population centers. Strike-Slip Faults (e.g., Sagaing Fault) Strike-Slip (Right-lateral) Runs north-south through Myanmar. Major source of large earthquakes. Felt in eastern Bangladesh. Faults in the Indo-Burman Fold Belt Reverse/Thrust Many faults in Chittagong Hill Tracts. Source of frequent, moderate to strong local earthquakes. Himalayan Frontal Thrust (HFT) Megathrust Main fault at base of Himalayas. Capable of generating great earthquakes (e.g., 2015 Gorkha). Felt in northern Bangladesh. Historical Seismicity: History of destructive earthquakes (1762 Arakan, 1885 Bengal, 1897 Great Indian, 1918 Srimangal, 1997 Bandarban). Seismic Hazard and Vulnerability: High hazard due to multiple active faults. Extreme vulnerability due to: Population Density Unconsolidated Sediments: Amplify seismic shaking. Liquefaction Potential: Saturated sands lose strength, behave like liquid. Building Construction: Widespread non-engineered construction. Key Concepts for the Region: Seismic Gap: Segment of active fault likely to rupture next (PBF south of 1762 rupture). Liquefaction: Strong ground shaking causes water-saturated soil to lose strength, act like viscous fluid. Primary hazard for deltaic parts of Bangladesh.