Comprehensive Geology Study Guide (Chapters 8-14)
Chapter 8: Earthquakes
1. What are Earthquakes?
Earth shaking caused by a rapid release of energy, usually due to tectonic forces.
Occur frequently, with over a million detectable earthquakes per year.
Elastic Rebound Theory: Rocks bend elastically under stress; when stress exceeds strength, rocks rupture, releasing energy. Rocks then return to their original shape.
Hypocenter (Focus): The point within the Earth where fault slip initiates.
Epicenter: The point on the land surface directly above the hypocenter.
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2. Faults
Planar breaks in the crust where movement occurs. Most are sloped.
Footwall: Block below the fault plane.
Hanging Wall: Block above the fault plane.
Types:
Normal Fault: Hanging wall moves down relative to footwall; caused by extension.
Reverse Fault: Hanging wall moves up relative to footwall; caused by compression; steep slope (>30°).
Thrust Fault: Low-angle (<30°) reverse fault; common in compressional mountain belts.
Strike-Slip Fault: Blocks slide laterally past each other; no vertical motion across fault; fault plane usually near vertical.
Fault Trace: Where a fault intersects the ground surface.
Fault Scarp: A small cliff created by displacement at the surface.
Blind Faults: Faults that do not reach the surface.
Stick-Slip Behavior: Friction ("stick") prevents motion until strain builds up enough to overcome it, causing rapid movement ("slip").
3. Seismic Waves
Energy released during an earthquake travels as waves.
Body Waves (travel through Earth's interior):
P-waves (Primary): Fastest, arrive first. Compressional (push-pull) motion parallel to wave direction. Travel through solids, liquids, gases. Cause up-and-down motion.
S-waves (Secondary): Slower, arrive second. Shear motion perpendicular to wave direction. Travel only through solids. Cause back-and-forth shaking. Stronger than P-waves.
Surface Waves (travel along Earth's surface): Slowest and most destructive.
L-waves (Love waves): S-waves intersecting the surface. Side-to-side snake-like motion.
R-waves (Rayleigh waves): P-waves intersecting the surface. Rolling, ripple-like motion. Arrive last, often cause prolonged shaking.
4. Measuring Earthquakes
Seismograph: Instrument that records ground motion.
Seismogram: The data record produced, showing wave arrivals (P, then S, then Surface waves).
Locating the Epicenter: Time difference between P and S wave arrivals yields distance. Data from ≥3 stations pinpoints location via triangulation.
Intensity: Measures shaking/damage severity. Modified Mercalli Intensity (MMI) scale (I-XII) is subjective, based on observations. Decreases with distance.
Magnitude: Measures energy released based on seismogram amplitude. Logarithmic scales (M6 is 10x ground motion of M5, ~32x energy).
Richter Scale (ML): Older scale, best for nearby EQs.
Moment Magnitude (Mw): Most accurate, especially for large EQs.
5. Earthquake Locations & Plate Tectonics
Most EQs occur along plate boundaries.
Divergent: Shallow EQs (MORs, continental rifts).
Convergent: Shallow, intermediate, deep EQs along subducting slab (Wadati-Benioff zone). Large megathrust EQs possible. Thrust faults in collision zones.
Transform: Shallow EQs, often major. Example: San Andreas Fault.
Intraplate Earthquakes: Less common (~5%), occur away from boundaries, often on old rift zones. Example: New Madrid.
6. Earthquake Hazards
Ground Shaking & Building Damage: Causes collapse ("pancaking"), foundation failure, etc..
Landslides/Avalanches: Shaking destabilizes slopes.
Liquefaction: Shaking liquefies water-saturated sediments; ground loses strength, structures sink/topple. Sand volcanoes can form.
Fire: Broken gas lines, downed power lines, hindered firefighting efforts.
Tsunamis: Large waves generated by seafloor displacement (EQ, landslide, volcano). Travel fast in deep ocean, slow and grow tall in shallow water. Significant hazard, especially from megathrust EQs.
7. Prediction & Preparedness
Prediction: Long-term (decades/centuries) based on identifying seismic gaps/recurrence intervals is possible. Short-term (hours/weeks) is NOT reliable. Potential precursors (foreshocks, strain, well changes, gas) are inconsistent.
Preparedness: Map hazards (faults, liquefaction zones), develop building codes, regulate land use, educate public, conduct drills, maintain emergency supplies.
Chapter 9: Geologic Structures and Mountain Building
1. Orogenesis (Mountain Building)
Process of building mountains, occurring in belts (orogens).
Involves deformation, metamorphism, igneous activity, uplift, erosion. Driven by plate tectonics (convergence, collision, rifting).
Mountains reflect balance between uplift and erosion.
2. Deformation, Stress, and Strain
Deformation: Changes in rock location (displacement), orientation (rotation), or shape (distortion).
Strain: Change in shape due to deformation (stretching, shortening, shear).
Stress: Force applied per unit area. Types:
Compression: Squeezing (shortens/thickens).
Tension: Pulling apart (stretches/thins).
Shear: Sliding past (no thickness change).
Pressure: Equal stress on all sides.
Deformation Styles:
Brittle: Fracturing (faults, joints); shallow crust.
Ductile: Flowing/folding; deeper/hotter crust.
3. Measuring Structures
Strike: Compass direction of the intersection of a tilted plane with a horizontal plane.
Dip: Angle of tilt of the plane measured downward from the horizontal, perpendicular to strike.
4. Geologic Structures
Joints: Planar fractures with no offset; form from tensile stress. Often occur in sets.
Veins: Fractures filled with minerals.
Faults: Planar fractures with offset (displacement). See Chapter 8 for types (Normal, Reverse, Thrust, Strike-Slip). Oblique-slip faults combine dip-slip and strike-slip motion. Fault zones can contain fault breccia or gouge; slickensides indicate slip direction. Fault scarps mark surface displacement.
Folds: Wavelike bends in layered rock, usually from compression.
Geometry: Hinge (max curvature), Limbs (sides), Axial Plane (connects hinges).
Types: Anticline (arch-like, limbs dip away, older rocks in core after erosion), Syncline (trough-like, limbs dip toward, younger rocks in core after erosion).
Descriptions: Open vs. Tight (angle between limbs); Plunging (tilted hinge) vs. Non-plunging (horizontal hinge).
Large Folds: Dome (circular anticline, older in center), Basin (circular syncline, younger in center).
Formation: Flexural-slip (layers slide), Passive-flow (ductile rock).
Tectonic Foliation: Alignment of minerals due to compressional stress, often parallel to fold axial planes.
5. Cratons
Old (>1 Ga), stable continental crust.
Shields: Exposed Precambrian igneous/metamorphic rocks.
Platforms: Precambrian basement covered by younger sedimentary rocks. Often exhibit broad domes/basins.
Chapter 10: Geologic Time
1. Concepts
Deep Time: Immense span of geologic time (billions of years).
Relative Age: Order of events (older vs. younger).
Numerical Age: Age in specific number of years.
2. Principles of Relative Dating
Uniformitarianism: "The present is the key to the past".
Original Horizontality: Sediments deposited horizontally. Tilted layers = deformed.
Superposition: Oldest layers at bottom, youngest at top (undeformed sequence).
Lateral Continuity: Strata form laterally extensive sheets.
Cross-Cutting Relations: Younger features cut older features.
Baked Contacts: Intrusions bake older country rock.
Inclusions: Fragments within a rock are older than the host rock.
3. Fossils and Correlation
Principle of Fossil Succession: Fossils succeed one another in a known order; used for relative dating. Specific fossils found in limited time ranges.
Index Fossils: Diagnostic of a particular geologic time.
Fossil Range: Time between first and last appearance.
Correlation: Matching rock layers (strata) across areas.
Lithologic: Based on rock type.
Fossil: Based on fossils.
Formations: Mappable rock units.
Stratigraphic Columns: Depict strata sequences.
4. Unconformities (Gaps in Time)
Time gap in rock record due to non-deposition or erosion.
Angular Unconformity: Tilted/folded rocks overlain by younger, flat-lying rocks.
Nonconformity: Sedimentary rocks deposited on older eroded igneous/metamorphic rocks.
Disconformity: Gap between parallel sedimentary layers.
5. Geologic Time Scale
Calendar of Earth history, based on correlating rock layers (stratigraphic column) globally.
Hierarchy: Eons (largest) → Eras → Periods → Epochs (smallest).
Major Divisions: Precambrian (Hadean, Archean, Proterozoic Eons), Phanerozoic Eon (Paleozoic, Mesozoic, Cenozoic Eras).
6. Numerical (Absolute) Dating
Determines age in years.
Radiometric Dating: Uses decay of radioactive isotopes.
Isotopes: Atoms of same element, different neutrons. Radioactive isotopes decay spontaneously.
Half-life: Time for half of parent isotopes to decay to daughter isotopes; constant for each isotope.
Method: Measure parent/daughter ratio, calculate age using known half-life.
Other Methods: Dendrochronology (tree rings), Rhythmic layering (varves, ice cores).
Sedimentary rocks often bracketed by dating associated igneous rocks.
Chapter 11: Earth's Biography
1. Precambrian (4.54 Ga - 541 Ma)
Hadean Eon (~4.54 - 4.0 Ga): Earth/Moon formation, intense bombardment, early atmosphere/oceans formed.
Archean Eon (4.0 - 2.5 Ga): First continents form via plate tectonics. First life appears (prokaryotes); oldest evidence ~3.5-3.8 Ga. Stromatolites (cyanobacteria) begin producing oxygen (~3.2 Ga).
Proterozoic Eon (2.5 Ga - 541 Ma): Larger continents/plates. Cratons stabilize. Supercontinents form/break up (Rodinia, possibly Pannotia). Great Oxidation Event (~2.4-2.2 Ga); BIFs deposited. Eukaryotic cells evolve (~1.0 Ga). Ediacaran fauna (first multicellular animals) appear late. Possible "Snowball Earth" episodes.
2. Phanerozoic Eon (541 Ma - Present)
"Visible life"; abundant fossils with hard parts.
Paleozoic Era ("Ancient Life"):
Early (Cambrian/Ordovician): Pannotia rifts. Continents: Gondwana, Laurentia, Baltica, Siberia. Taconic Orogeny (Appalachians). Cambrian Explosion: rapid diversification. First vertebrates. Ordovician mass extinction.
Middle (Silurian/Devonian): Collisions build more of Appalachians (Acadian Orogeny). Antler Orogeny (west). Age of Fishes; land plants, insects, amphibians evolve (Tiktaalik).
Late (Carboniferous/Permian): Pangea assembled. Alleghanian Orogeny (final Appalachian pulse). Vast coal swamps. Reptiles evolve; amniote egg allows land colonization. Ends with largest mass extinction (Permian-Triassic).
Mesozoic Era ("Middle Life"; Age of Dinosaurs):
Early/Middle (Triassic/Jurassic): Pangea rifts apart. Atlantic opens. Western North America is active margin. First dinosaurs, mammals, flying reptiles appear.
Late (Cretaceous): Continued rifting. Sierran Arc active. Sevier and Laramide orogenies build Rockies. High sea levels (Western Interior Seaway). Warm climate. Angiosperms (flowering plants) evolve. Ends with K-Pg extinction.
K-Pg Boundary Event (~66 Ma): Mass extinction caused by meteorite impact (Chicxulub crater). Widespread devastation, climate change lead to extinction of dinosaurs (except birds).
Cenozoic Era ("Recent Life"; Age of Mammals):
Continents near modern positions. Alpine-Himalayan orogeny. Western North America tectonism continues (Andean/Cordilleran systems). Basin and Range extension. Mammals diversify. Pleistocene ice ages. Evolution of humans.
Anthropocene: Proposed recent interval marked by significant human impact.
Chapter 12: Energy and Mineral Resources
1. Energy Resources
Matter that can produce energy (heat, power, electricity).
Sources: Solar, gravity, photosynthesis, fossil fuels, nuclear, geothermal, chemical bonds.
Hydrocarbons (Oil & Gas): Formed from buried organic matter (plankton) transformed by heat/pressure into kerogen, then oil/gas.
Requirements (Hydrocarbon System): Source rock, migration pathway, reservoir rock (porous/permeable), trap (anticline, fault, salt dome, stratigraphic), seal rock.
Exploration: Geology, seismic surveys.
Production: Drilling, pumping (primary/secondary recovery). Refining separates components.
Unconventional Hydrocarbons: Tar Sands (bitumen), Oil Shale (kerogen). Require mining/processing.
Coal: Sedimentary rock from compacted plant remains. Formed in swamps. Mined via strip or underground methods. Major CO₂ source.
Nuclear Energy: From fission (splitting atoms, e.g., Uranium). Reactors control process to generate electricity.
Renewable Energy: Geothermal (Earth's heat), Hydroelectric (flowing water), Wind (turbines), Solar (collectors, PV cells).
2. Mineral Resources
Metals: Opaque, shiny, conductive, malleable. Native metals (pure) or extracted from ore minerals.
Ore Formation Processes: Magmatic, hydrothermal, secondary enrichment, groundwater transport (MVT), sedimentary (BIFs), residual weathering, placer deposits. Linked to tectonic settings.
Nonmetallic Resources: Dimension stone, crushed stone (aggregate), sand, gravel, salt, gypsum, clay, phosphate. Used in construction, industry, agriculture.
3. Environmental Issues & Future
Energy/Mineral extraction impacts: landscape scarring, spills, waste piles (tailings), acid mine drainage.
Fossil fuels are nonrenewable; reserves are finite. Peak oil production is a concern. Need for transition to sustainable energy mix. Recycling extends mineral resource lifetimes.
Chapter 13: Mass Wasting
1. Definition
Downslope movement of rock, regolith, snow, ice driven by gravity. Part of rock cycle.
2. Types (Slow to Fast)
Creep: Slowest; gradual downhill movement of regolith due to expansion/contraction. Tilts landscape features.
Solifluction: Slow flow of saturated tundra soil over permafrost.
Slumping: Coherent blocks slide on curved surface; creates head scarp, toe.
Flows (Mudflow, Debris Flow, Lahar): Material moves as viscous fluid. Follow channels, dangerous. Lahar = volcanic origin.
Slides (Rockslide, Debris Slide): Material moves suddenly down non-vertical plane. Can be very fast.
Avalanches: Turbulent cloud of debris/air. Snow avalanches are common, destructive. Dry avalanches faster than wet.
Falls (Rockfall, Debris Fall): Vertical freefall of material.
3. Submarine Mass Wasting
Occurs underwater: slumps, debris flows, turbidity currents (deposit graded beds).
4. Causes & Triggers
Instability Factors: Gravity on slopes, weak materials (weathered regolith), planes of weakness (joints, bedding, foliation), water saturation.
Triggers: Shocks (earthquakes, explosions), undercutting (natural or human), heavy rainfall/snowmelt, vegetation removal, overloading slope.
5. Mitigation
Assessment: Mapping hazards, identifying warning signs (cracks, tilts).
Prevention: Revegetation, regrading/terracing, drainage control, reducing undercutting, engineering structures (retaining walls, covers, bolts, sheds).
Chapter 14: Streams and Floods
1. Streams & Drainage
Flowing water in channels, draining watersheds (drainage basins).
Drainage Networks: Patterns (dendritic, radial, rectangular, trellis, parallel) reflect geology.
Drainage Divides: Separate basins.
Stream Types: Permanent (flow year-round) vs. Ephemeral (flow intermittently).
Discharge: Water volume per time (Width x Depth x Velocity).
2. Erosion & Transport
Streams erode by scouring, lifting, abrasion (forms potholes), dissolution.
Sediment Load: Dissolved, Suspended, Bed load.
Competence: Max particle size carried (depends on velocity). Capacity: Total load carried.
3. Profiles & Base Level
Longitudinal Profile: Concave-up elevation plot. Steeper near headwaters.
Base Level: Lowest point stream can erode to (ultimate = sea level). Affects erosion/deposition.
4. Channel Features & Landforms
Valleys/Canyons: Carved by stream erosion + mass wasting.
Rapids/Waterfalls: Steep gradients over obstructions/ledges. Waterfalls retreat upstream.
Stream Terraces: Abandoned former floodplains.
Depositional Features: Alluvium (stream sediment), Bars, Floodplains, Natural Levees, Alluvial Fans (at mountain front), Deltas (at standing water).
5. Channel Patterns
Braided Streams: Multiple channels choked with sediment. High sediment load.
Meandering Streams: Sinuous loops (meanders). Erosion on cut banks, deposition on point bars. Migrate over time. Form cutoffs and oxbow lakes.
6. Drainage Evolution
Processes: Base level change, stream piracy, formation of incised meanders, superposed/antecedent/diverted streams.
7. Floods
Water overflows channel banks.
Causes: Heavy/prolonged rain, snowmelt, dam/levee failure.
Types: Seasonal (gradual) vs. Flash (rapid, dangerous).
Mitigation: Dams, levees, flood walls (can fail/worsen downstream floods); flood risk mapping, land-use regulation.