Lecture notes 3


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Geology 11 subject lecture notes. Useful infos. Compiled concisely.

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Lecture notes 3

  1. 1. Lecture Notes # 3 10. Rock Deformation Stress: a measure of internal force applied to a deformable body Strain: material response (deformation) due to the stress applied; change in size and shape Types of stress: Confining stress/pressure: equal stress in all directions (in geology, lithostatic stress; in oceanography, hydrostatic stress) Differential stress: unequal stress Tensional stress: “stretching”, component perpendicular to given surface (Divergent Plate Boundaries) Compressional stress: “squeezing”, component perpendicular to given surface (Convergent Plate Boundaries) Shear stress: sidewards movement in opposite direction (Transform Plate Boundaries) Types of deformation (strains) Elastic: under stress, solid material deforms; however after stress is removed, solid material will return to its original shape (reversible) Ductile: under stress, solid material deforms and when stress is removed, solid material does not return to its original shape (irreversible); forms folds Brittle: under much stress, material breaks past its elastic and plastic (ductile) deformation ranges (irreversible); forms joints, fractures and faults Factors that affect deformation Type of force applied: the stronger the force, the higher the tendency to fail Pressure: higher confining pressure, less likely to fracture Temperature: higher temperature, less likely to fracture; material tends to behave like fluid Rock (mineral) composition: quartz, olivine, and feldspars are very brittle, while clay minerals, micas, and calcite are more ductile; water in chemical composition of minerals make it more ductile Measuring deformation in rocks Strike is the azimuth (degrees east of north) of the line formed by the intersection of a layer interface or bedding plane with the horizontal Dip is the angle between the layer interface or bedding plane and the horizontal measured perpendicular to the strike direction Notation: strike = N x E/W, dip = x N/S E/W; where x is a number (however, the dip direction should always be perpendicular [90 degrees] from the strike direction, i.e. if the strike direction is NW, the dip direction should be either NE or SW only) Parts of a fold Axial plane: imaginary surface that divides a fold as symmetrically as possible, one limb on each side. Fold axis: the line made by the length-wise intersection of the axial plane with beds in the fold Limbs: the two sides of an anticline or syncline
  2. 2. Anticline (“A”): oldest rocks at the core Syncline (“sink”): youngest rocks at the core Dome: Anticlines from all directions Basin: Synclines from all directions Joint: regular (same strikes and dips, regular spacing) breaks in rock with no movement Fracture: irregular breaks in rock with no movement Fault: breaks with movement Classification of faults: Strike-slip fault Right-lateral (Dextral) Fault: opposite block moves to the left Left-lateral (Sinistral) Fault: opposite block moves to the right Dip-slip fault Normal Fault: Hanging wall goes down, footwall goes up; tensional Reverse Fault: Hanging wall goes up; footwall goes down; compressional Thrust Fault: A reverse fault with an angle of depression less than 15 degrees Oblique-slip fault: faults with strike-slip and dip 11. Earthquakes Earthquake: any intense ground shaking caused by sudden release of energy and sudden slippage along faults Elastic Rebound Theory: As plates on opposite sides of a fault are subjected to forc deform until their internal strength is exceeded. At that time, a sudden movement occurs along the fault, releasing the accum energy, and the rocks snap back to their original undeformed shape. Earthquake nomenclature: Focus: where the earthquake originates Epicenter Fault Seismology Body Waves surface waves emitted by an ea P (primary) waves direction of propagation S (secondary) waves regular (same strikes and dips, regular spacing) breaks in rock with no movement : irregular breaks in rock with no movement : opposite block moves to the left : opposite block moves to the right n, footwall goes up; Hanging wall goes up; footwall goes down; A reverse fault with an angle of depression less slip and dip-slip component intense ground shaking caused by sudden release of energy; can be generated by bomb blasts, volcanic eruptions As plates on opposite sides of a fault are subjected to force and shift, they accumulate energy and slowly deform until their internal strength is exceeded. At that time, a sudden movement occurs along the fault, releasing the accum energy, and the rocks snap back to their original undeformed shape. Epicenter: projection of focus on the surface of the earth Fault: refer above Seismology – study of behavior of seismic waves Body Waves: traveling through the interior of the earth, body waves surface waves emitted by an earthquake; higher frequency than surface waves P (primary) waves: compressional/rarefaction waves; first to arrive; parallel to the direction of propagation S (secondary) waves: slower than P waves; does not travel in liquid; perpendicular to can be generated by bomb blasts, volcanic eruptions e and shift, they accumulate energy and slowly deform until their internal strength is exceeded. At that time, a sudden movement occurs along the fault, releasing the accumulated body waves arrive before the er frequency than surface waves : compressional/rarefaction waves; first to arrive; parallel to the l in liquid; perpendicular to
  3. 3. the direction of propagation Surface waves: travels in crust only; lower frequency Love waves: horizontal side-to-side movement Rayleigh waves: rolling movement How is an earthquake’s epicenter located? P waves arrive first, then S waves, then L and R Average speeds for all these waves is known After an earthquake, the difference in arrival times at a seismograph station can be used to calculate the distance from the seismograph to the epicenter. Time-distance graph showing the average travel times for P- and S-waves. The farther away a seismograph is from the focus of an earthquake, the longer the interval between the arrivals of the P- and S- waves Earthquakes and Plate Boundaries Intensity is the degree of ground shaking at a given locale based on the amount of damage (Modified Mercalli Intensity Scale Philippine Earthquake Intensity Scale) Magnitude is calculated from seismic records and estimates the amount of energy released at the source (Richter Scale) Structural damage based on wave amplitudes, duration of vibrations, nature of material upon which the structure rests, design of structure Secondary effects of earthquakes: tsunamis, landslides, ground subsidence, fire Earthquake prediction Monitor to look for patterns of recurrence Strange animal behavior Increase in seismic tremors (mini-quakes) Seismic gaps Gas emissions Electromagnetic signals Earth’s interior from earthquakes Crust Mantle – seismic wave velocities increase rapidly at the Moho Core – P wave velocities suddenly decrease and S wave velocities go to zero (outer core); at depth of ~4800 km, P wave velocities suddenly increase (inner core) Shadow Zone: part of the Earth where there are no P and S wave records, due to liquid outer core (between 104 and 105 degrees) 12. Mass Wasting Mass wasting: downslope movement of rock, regolith (unconsolidated material) and soil under the influence gravity Factors affecting mass wasting Slope Angle of repose: steepest angle that a cohesionless slope can maintain without losing its stability; function of particle size, particle shape and moisture content Type of rock Presence of joints, fractures or bedding planes Daylighting features: if the joints or bedding planes dip in the same direction as the slope On sediment: Friction, Presence of water (too much promotes mass wasting, enough amount of water hold material in place due to surface tension); Plants On rock: Natural cement, Crystals Triggering events Ground shaking, Slope modification, Excessive rainfall, Deforestation/devegetation
  4. 4. Classification Creep: slow downward movement of regolith as a result of gravitational force Slump: mass of regolith slides over or creates a concave surface (=scarp) Slide: material moves downhill in a fairly coherent mass along a flat or planar surface Flow: less uniform, or more chaotic, mass of material moves rapidly downslope Fall: at angles almost perpendicular to the ground, rock at the top of a slope is dislodged such that it either falls directly downward or bounces and rolls Talus: accumulation of debris at the foot of a mountain Mitigation Hazard maps provide information about proper land use in such areas “Hard” engineering measures: chicken wire, retaining net, concrete wall/shotcrete, coconet, weepholes, benching/terracing, rock barriers “Soft” measures: Information and education campaign, monitoring (e.g. slopes, rainfall conditions etc.), early warning system, hazard preparedness program, rehabilitation/coping mechanism program 13. Groundwater Groundwater: Fresh water located beneath the ground; stored in and transmitted through spaces between grains of (1) sediments, (2) clastic sedimentary rocks and (3) cracks or openings in all types of rocks Water table: Upper boundary of ground water Vadose zone: unsaturated zone; normally dry, where meteoric water leaches through Phreatic zone: saturated zone Porosity: Measures amount of water that can be held by rocks/sediments (volume of voids / total volume of material); affected by grain size, sorting and grain packing Permeability: ability to transmit fluids; degree of interconnection of voids in the material Aquifer: Stores and transmits sufficient amount of water Confining units: Aquitard – stores, but slowly transmits water Aquiclude – stores, but does not transmit water Aquifuge – does not store nor transmit water Unconfined aquifer: bounded at the bottom by a confining unit; water rises up to the water table Perched aquifer: unconfined aquifer defined by a discontinuous confining unit; local water table (usually above the main/regional water table) Confined aquifer: bounded at top and bottom by confining units; water rises up to the piezometric water level (also called potentiometric line/surface) Potentiometric surface - level to which water will rise in a well due to natural pressure in the rocks Darcy’s Law: Q = - KA (ΔH/L), where Q= discharge, K= hydraulic conductivity, A= area, H= hydraulic head, and L= length of path; (-) sign means direction of Q is from high to low hydraulic head *K: directly proportional to permeability Springs: when confined aquifers intersect ground surface Artesian wells: groundwater with sufficient hydrostatic pressure tend to rise above the aquifer containing it Aquifers are recharged by the infiltration of rainwater or snowmelt from the ground surface. Geologic work of groundwater Caverns: dissolution of soluble rocks (e.g., limestone) by acidic groundwater Speleothems: deposition of dissolved components in caverns; stalaCtite found in the Ceiling, while stalaGmite found on the Ground
  5. 5. Karst Topography: Type of topography that is formed over limestone, dolomite, or gypsum by dissolution, and that is characterized by sinkholes, caves, and underground drainage (Chocolate Hills, Hundred Islands) Sinkholes: collapsed due to emptied underground due to dissolution Problems Overdraft (or overuse) leading to subsidence; sinkhole formation Saltwater Intrusion Pollution 14. Historical Geology Historical Geology: deals with the origin of the Earth and its development through time; strives to establish an orderly chronological arrangement of the physical and biological changes and events that have occurred in the geologic past. “The present is the key to the past”: Former changes of the earth’s surface may be explained by reference to causes in operation Previous estimates of the age of the Earth: Cooling through conduction and radiation (Lord Kelvin, 1897): ~24 – 40 m.y. Rate of delivery of salt to the oceans (John Joly, 1899-1901): ~90 – 100 m.y. Thickness of total sedimentary record divided by average sedimentation rates (1910): ~1.6 b.y. Oldest rocks on Earth found so far: Acasta Gneisses in northwestern Canada near Great Slave Lake (4.03 Ga) Isua Supracrustal rocks in West Greenland (3.7 to 3.8 Ga) rocks found in the Minnesota River Valley and northern Michigan (3.5-3.7 billion years), in Swaziland (3.4-3.5 billion years), and in Western Australia (3.4-3.6 billion years) Oldest materials to be found on Earth: Zircon grains found in sedimentary rocks in west-central Australia = 4.4 b.y. 70 well-dated meteorites using different dating methods (e.g. Rb-Sr, Sm-Nd, Ar-Ar) = 4.4-4.6 b.y. Iron meteorite (Canyon Diablo meteorite) = 4.54 b.y. Most accepted age for the Earth and the rest of the solar system: ~4.55 b.y. old (+ ~1%) “Best” age of the Universe: 14 – 17 b.y. (Evidence: rate of evolution of stars and age of elements in the galaxy based on the production ratios of Os isotopes in supernovae) Relative Dating: putting rocks and events in their proper sequence of formation; dating of rocks and rock units with the use of fossils and correlation of different strata; does not require numerical ages of rocks or fossils or events Law of Uniformitarianism: in an undisturbed sequence of beds, the bed at the top is youngest while the bed at the bottom will be the oldest Steno’s Laws Law of Original Horizontality: Most layers are deposited horizontally (exceptions: pyroclastic deposits, reefs) Law of Lateral Continuity: Sediments would spread out until they thin out at the edge of the depositional basin, stop at a depositional barrier or grade into another type of sediment (indicative of a change in the depositional environment) Cross-cutting relationships: When a fault or intrusion cuts through another rock, the fault or intrusion is younger that the rocks which it cuts. Inclusions: The rock mass containing the inclusion is younger than the rock that provided the inclusion. Unconformity: Any significant break in time within a stratigraphic column. Angular unconformity: tilted or folded sedimentary rocks that are overlain by younger, more flat-lying strata (deposition – tilting – erosion – tilting) Disconformity: strata on either side of the unconformity are essentially parallel with a distinctly recognizable surface (deposition – erosion – deposition) Nonconformity: older metamorphic or igneous rocks are overlain by younger sedimentary strata, or intrusion/emplacement of an igneous body to a sedimentary sequence (igneous or metamorphic basement – deposition of sedimentary rocks OR sedimentary rocks – emplacement) Paraconformity: parallel deposition without any erosional surface, however break in age record is observed once further analysis is done (deposition – non-deposition – deposition) Faunal Succession: Fossil organisms succeed one another in a definite and determinable order. Thus, any time period can be recognized by its fossil content (unless fossil is reworked and transported) Correlation: To demonstrate correspondence between geographically separated parts of a geologic unit based on similarity of lithologic and paleontologic features
  6. 6. Fossils: Remains or traces of prehistoric life preserved in sedimentary rocks; important time indicators and play a key role in the correlation of rocks; include both the remains of organisms (bones or shells) (body fossils) and traces of organisms (trails, burrows or imprints) (trace fossils or ichnofossils); must be 10,000 years old *sorry, I mentioned in the class 100,000 years, my bad Requirements Rapid burial to prevent decomposition Presence of protective cover or preserving medium Possession of hard parts or durable tissues such as shells, bones, teeth and woody tissue 1. Preservation of unaltered body parts: a. Hard parts – usually shells, bone, teeth or pollen b. Soft tissue – by mummification or freezing. 2. Chemical alteration of hard parts: a. Carbonization – soft tissues preserved as thin carbon film b. Recrystallization – conversion of a mineral polymorph to another (e.g. aragonite → calcite) c. Replacement – dissolution of original material and precipitation of new mineral d. Permineralization – porous material filled with secondary materials e. Petrification – replacement of wood Uses: tracing the evolutionary history of extinct as well as living organisms reconstructing paleoclimates and paleoenvironments providing the source of energy resources (e.g. oil, gas, coal) Absolute dating: Numerical dating of rocks, minerals and fossils Utilizing radioactive isotopes Isotopes – variants of the same atom but with different mass numbers Half – life – the length of time required for one-half of the nuclei of a radioactive isotope to decay Radiocarbon dating-Carbon 14 and AMS-decay of carbon in organic materials. Potassium-argon (40K/40Ar)-decay of potassium into argon in volcanic materials. Fission-track dating-microscopic tracks in glassy material. Obsidian Hydration-hydration rim forms when stone tools are made from obsidian. Luminescence dating-heating of crystalline material. Magnetism-variation in the earth’s magnetic pole. Geologic Time: The history of the earth is broken up into a hierarchical set of divisions for describing geologic time. Units of time include eon, era, period, epoch, age. Paleozoic Era: Can Oscar See Down My Pants Pocket Periods: Cambrian, Ordivician, Silurian, Devonian, Mississippian, Pennsylvanian, Permian Mesozoic Era: Tom Jones Can Periods: Triassic, Jurassic, Cretaceous Cenozoic Era: Tom’s Queer! Periods: Tertiary, Quaternary Tertiary and Quaternary Periods, the Epochs are remembered by: Pretty Eager Old Men Play Poker Hard Epochs: Paleocene, Eocene, Oligocene, Miocene, Pliocene (for the Tertiary) and Pleistocene, Holocene (for the Quaternary)