GEOL 105 Test 4 Notes
GEOL 105 Test 4 Notes Geology 105
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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