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GEOL 105 Test 4 Notes

by: Alaina Notetaker

GEOL 105 Test 4 Notes Geology 105

Alaina Notetaker
University of Louisiana at Lafayette
GPA 3.5

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About this Document

Covers: Chapter 8 - Metamorphism and Metamorphic Rocks Chapter 9 - Earthquakes and Earth's Interior Chapter 10
Geology and man
Elisabeth Boudreaux
Metamorphism, Metamorphic Rocks, Earthquakes, Earth's Layers, Geology, Deformation, mountains, continents
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This 8 page Bundle was uploaded by Alaina Notetaker on Tuesday September 20, 2016. The Bundle belongs to Geology 105 at University of Louisiana at Lafayette taught by Elisabeth Boudreaux in Spring 2016. Since its upload, it has received 7 views. For similar materials see Geology and man in Geology at University of Louisiana at Lafayette.

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Date Created: 09/20/16
METAMORPHISM AND METAMORPHIC ROCKS • Metamorphism ⿞Transformation of pre existing rock into texturally or mineralogically distinct new rock as a result of high temps, high pressure, or both... But without the rock melting in the process (then it would be igneous) ‣ Solid state= rock does not melt ‣ Between 200degreesC and melting of rock ⿞Characteristics of metamorphic rocks are controlled by: ‣ Composition of parent rock • Usually no new elements or compounds are added to the parent rock during metamorphism (exception: with water) ‣ Heat • Geothermal gradient (burial due to subduction) • Lava • Migrating magmas (intense heat will affect surrounding rocks) • Increases the rate of chemical reactions that produce different minerals ‣ Pressure • Confining pressure ⿞Lithostatic (under land) and Hydrostatic (under water) pressure ‣ Mineral grains become closely packed ‣ Recrystallization may occur, producing smaller and more dense minerals ‣ Equal on all sides • Differential pressure- stress NOT equal in all directions ⿞Deforms rocks ⿞Creates foliation • Pressure gradient ‣ Fluid activity • Water and carbon dioxide are almost always present in metamorphic regions • Fluids enhance metamorphism by increasing the rate of chemical reactions • Fluid Sources ⿞Water trapped in the pore space of sedimentary rocks ⿞Magma ⿞Dehydration of water-bearing minerals from heat and pressure ‣ Time • The longer rocks are exposed to heat and pressure, the more metamorphism will occur ⿞Types of Metamorphism ‣ Contact Metamorphism • Hydrothermal alteration- hot, watery solutions can be released from cooling magmas ⿞Usually occurs near the Earth's surface ⿞May result in valuable mineral deposits ‣ Migration of metallic ions in hydrothermal solutions ‣ Copper, gold, iron ores, tin, zinc • Factors in contact metamorphism ⿞Initial temps and size of the intrusion ⿞Presence and chemistry of fluids ‣ Dynamic Metamorphism • Associated with faults • High pressures • Form narrow bands of metamorphic rocks song fault zones (mylonites) ⿞Hard ⿞Dense ⿞Fine grained ⿞Thinly laminated ‣ Regional Metamorphism • Produces more metamorphic rocks • Covers large geographic area • Shows a gradation of deformation corresponding to areas of the most inte ‣ Metamorphic aureoles • Zones of mineral assemblages surrounding intrusion ⿞Types of metamorphism ‣ Metamorphic index minerals ⿞Metamorphic Rock Names ‣ Rock texture (grain size and fabric) • Foliated ⿞Parallel layers of platy, flat minerals ⿞Shale => Slate ‣ about 200degreesC ‣ Minerals align ‣ Shiny surface ‣ Exhibits slaty cleavage ⿞Slate => Phyllite ‣ about 300degreesC ‣ Larger mica ⿞Phyllite =>Schist ‣ about 400degreesC ‣ Crystals can be seen with unaided eyes ‣ Most commonly formed by regional metamorphism ‣ Most form from clay-rich sedimentary rocks ⿞Schist => Gneiss ‣ about 500-700degreeC ‣ Felsic and mafic minerals start to separate ‣ Recrystallization Of clay-rich sedimentary rocks, igneous rocks, or older metamorphic rocks ⿞Gneiss => Migmatite ‣ about 600-800degreesC ‣ Felsic minerals melt ‣ Mafic minerals stay metamorphic ‣ Both igneous and metamorphic rock ("mixed rocks") • Non-Foliated ⿞No platy minerals ⿞Fine-grained (micro granular) ⿞Equal sized visible (crystalline) ⿞Limestone => Marble ‣ Marble is composed of calcite or dolomite ⿞Sandstone => Quartzite ‣ Quartzite is formed from sandstone • Intermediate to Hugh grade metamorphism ‣ Parent material (composition) ‣ Metamorphic minerals ‣ Any appropriate special name ⿞What are Metamorphic Zones and Facies? ‣ Metamorphic zones- belt of rocks showing the same general degree of metamorphism ‣ Metamorphic Facies- groups of rocks characterized by mineral assemblages formed under the same broad temperature-pressure conditions • Each Facies is named after the most characteristic rock or mineral ⿞How does Metamorphism Relate to Plate Tectonics? ‣ Metamorphism associated with all 3 types of plate boundaries • Convergent plate boundaries ⿞Temp and pressure increases ‣ Several Facies are defined by the temp/pressure conditions ⿞Pressure-plates collide ⿞Pressure and temp increase with subduction ⿞Contact metamorphism • Divergent Plate Boundaries ⿞Contact metamorphism ⿞Sea water increases reactions ‣ Hydrothermal solutions can precipitate economically important minerals • Ex: copper • Transform Plate Boundaries ⿞Dynamic Metamorphism ⿞Metamorphism and Natural Resources ‣ Marble and slate have been produced for centuries ‣ Ores of tin, tungsten, galena, and pyrite are produced by contact metamorphism EARTHQUAKES AND EARTH'S INTERIOR • EARTHQUAKES ⿞Earthquakes: Trembling or shaking of the ground caused by a sudden release of energy in the rocks beneath the Earth's surface ⿞Main cause of earthquakes- faulting ‣ Faulting: displacement of rocks along fractures ‣ Energy released along plate boundaries ⿞Sequence of Earthquake generation ‣ Foreshocks: smaller ‣ Main Earthquake ‣ Aftershock: smaller, can continue up for 1-2 years ⿞Origin of Earthquakes: Elastic Rebound Theory ‣ Explains how energy is stored and released in rocks ⿞Seismic Waves ‣ Rocks can deform only so much before they break ‣ When rocks break, energy is released in the form of seismic waves ‣ Seismology: the study of earthquakes ‣ Seismograph (seismometer): instruments that detect, record, and measure vibrations produced by an earthquake ‣ Seismogram: the record made by a seismograph ‣ Seismic Waves: radiate out from focus ‣ Focus: usually >100km deep- the point at which energy is first released ‣ Epicenter: point on Earth's surface directly above focus ‣ TWO TYPES: • Body waves: travel through the Earth's interior, outward from focus ⿞P waves (Primary waves) ‣ Push-pull waves (contract-expand) ‣ Fastest ‣ Compressional waves ‣ Travel through solids, liquids, and gases- parallel to wave propagation ⿞S waves (Secondary waves) ‣ Shake/shear waves ‣ Slower ‣ Travel only through solids- perpendicular to wave propagation • Surface Waves: travel along the Earth's surface from the epicenter; slowest waves ⿞L (Love waves) ‣ Move side to side ‣ Like horizontal S waves ‣ Travel through solids- perpendicular to wave propagation ⿞R (Rayleigh waves) ‣ Rolling motion, similar to ocean waves ‣ Travel only through solids ⿞Wave behavior ‣ P waves arrive first, then S waves, then L and R ‣ Avg. speed for all waves are known ‣ Difference in arrival times at a seismograph station can be used to calculate the distance from the seismograph to the epicenter • Because rocks are elastic (exhibit elasticity), they will return to their original shape when the force is no longer present ‣ Refraction • Waves are bent as they pass through different material (density, rigidity) ‣ Reflection • Waves are reflected from interface between two materials of different density or elasticity • Seismic reflection used in oil exploration ‣ Useful for: • Mapping the Earth's interior • Exploration of resources ⿞Where do earthquakes occur and how often? ‣ More than 150,000 quakes strong enough to be felt are recorded each year ⿞Earthquake Zones ‣ 80% occur along the Pacific Rim ‣ Convergent plate Boundaries • Benioff Zone ⿞Dipping seismic zones ⿞Indicate angle of plate descent along boundary ‣ Transform Plate Boundaries • San Andreas Fault ‣ Divergent Plate Boundaries ‣ Intraplate Earthquakes ⿞Earthquake Scales ‣ Mercalli Scale- Guiseppe Mercalli 1902 • Based on the amount of damage caused= intensity • Scale from 1-12 • Shortcomings: ⿞Somebody had to be there to witness the damage ‣ Modified Mercalli Intensity Map • 1994 Northridge, CA earthquake, magnitude 6.7 ‣ Richter Scale- Charles Richter 1935 • Based on magnitude • Open-ended scale, beginning with 0 • Largest recorded: 9.5 • Amplitude of the largest wave • P-S time interval • Shortcomings; ⿞Log scale ⿞Difficult to measure quakes >7 ⿞Magnitude is based on one instant during the earthquake ‣ Seismic-Moment Magnitude Scale • Scale used currently • Considered TOTAL amount of energy released • Strength of the rocks, area of fault rupture, and amount of movement on the faulty ⿞What are the destructive effects of earthquakes? ‣ Ground shaking • How consolidated are sediments/rocks? ‣ Liquefaction • Water saturated soil or sediment tends to turn to slurry during earthquakes (quick sand) • Causes buildings to sink into the ground • Alaska 1964 ‣ Fire • 80-90% of damage in the 1906 San Fransisco Earthquake ‣ Landslides • Triggered by ground shaking earthquakes • Cause extensive damage ‣ Permanent Displacement of land surface • Relative movement of rock bodies on opposite sides of fault ‣ Tsunamis • Caused by submarine quakes • Move at 800km/hr • Travel across oceans • 1960 tsunami generated off Chili • 7hr travel to Hawaii: killed 61 • 22hr travel travel to Japan: killed 180 ⿞Sumatra Earthquake ‣ Most powerful earthquake in 40 years (since Alaska 1964)- 9.1 moment magnitude ‣ Focus was 18.6 miles below sea level ‣ 30-45ft. Displacement along the fault ‣ Slightly affected Earth's rotation • Earth wobbled on its axis by about an inch • Shortened length of a day by 2.68 microseconds ⿞Minimize the Effects of Earthquakes ‣ Building codes: • San Fransisco quake in 1986 with a Richter magnitude of 7.1 killed 40 • 10 months later a quake in Armenia (no earthquake proof building code) with a magnitude of 6.9 that killed 25,000 ⿞Earthquake prediction ‣ Monitoring of foreshocks ‣ Establishing network of seismographs ‣ Landform studies (uplift, subsidence, or movement along faults) ‣ Animal behavior ‣ Can we predict... • Location? For the most part • Magnitude? For the most part • Time? No! • THE EARTH'S INTERIOR ⿞How do we know? ‣ Geophysics • Application of physics to the study of Earth • Studies: ⿞Seismic waves ⿞Earth's magnetic field ⿞Gravity ⿞Heat ⿞Earth's interior ‣ Crust • Oceanic crust ⿞Mafic ⿞5-10cm thick • Continental crust ⿞Granitic ⿞Avg 35km thick ⿞Thicker under mountains ‣ Mantle • Solid, perhaps with some small pockets of magma in the upper mantle • Asthenosphere ⿞Low velocity zone ⿞Rocks closer to their melting point, possibly partially melted • Partial melting ⿞May generate magma ⿞May allow rocks to flow, which can drive plate tectonics ‣ Core • Composition of the Core • Reflection/ refraction of P and S waves indicated liquid outer, solid inner core • Core is iron mixed with nickel and possibly other lighter elements DEFORMATION, MOUNTAINS, CONTINENTS • Deformation ⿞Deformation of rocks ‣ Deformation: changes in volume or shape of a material • Remember Lithostatic vs differential pressure ‣ Strain • Changes in size, shape, or volume in response to stress • Types of strain ⿞Elastic Strain: when deformed rocks return to their original shape when deforming forces are relaxed ⿞Plastic Strain (ductile): folds ⿞Brittle Strain: Faults and fractures ‣ Stress • Results from the force applied to a given rock • Caused by tectonic forces or Lithostatic pressure • Force per unit area (usually kg/cm2) • Types of stress ⿞Compression: Rock layers are shortened in the direction of stress ⿞Tension: Lengthens rocks or pulls them apart ⿞Shear ‣ Material behavior • Tempurature ⿞Low temp = brittle ⿞High temp = plastic • Pressure ⿞Low pressure = brittle ⿞High pressure = plastic ‣ Strain rate = deformation/time • Fast = brittle/breaks • Slow = plastic/flows ‣ Composition • Clay, mica, calcite = plastic • Quartz, olivine, feldspar = brittle ⿞Strike and Dip ‣ Strike: a line representing the intersection of the feature with a horizontal plane ‣ Dip: measure of an inclined plane's deviation from horizontal • Measured at right angles to the strike direction ⿞Deformation and geologic structures ‣ Remember strain = deformation (caused by stress) ‣ Geologic structure: any features resulting from deformation • Folded • Fractures • Combination of both ‣ Folds • Permanent/plastic deformation • Ductile deformation • Compressional stress • Stress over a long time • Rate of deformation low • Most occur deep within the crust where rocks are more ductile like • 3 basic types ⿞Monoclines ⿞Anticlines - upside down "U" shaped fold in rocks ‣ The oldest rocks are in the middle ‣ The youngest layers are on the outside ⿞Synclines - U shaped fold in rocks ‣ The oldest rocks are on the outside ‣ The youngest rocks are in the middle • Axis • Axial plane - divides the fold in half • Limbs - each half • Geometry of folds ⿞Symmetric ⿞Asymmetric ⿞Overturned ⿞Recumbent ⿞Plunging Folds ‣ Fold axes dip down or plunge below the surface ⿞Non plunging • Domes and basins ⿞Circular or oval folds ‣ Joints • Fractures along which no movement has taken place • Most rocks are jointed • Very little stress is required • Other examples: columnar joints, sheet jointing (pressure release) ‣ Faults • Fractures along which differential movement HAS taken place • Types of faults (depends on type of stress) ⿞Dip-slip faults ‣ Movement is vertical (up and Dow) ‣ Normal faults- hanging wall block moves down ‣ Reverse faults- hanging wall moves up ‣ Thrust fault - fault dip <45degrees ⿞Strike-slip faults ⿞Oblique-slip faults


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