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- 1I Metamorphic Rocks and Environments A. Formed from existing rock (parent, protolith). Oldest rocks—old mountain cores B. Subject to high temperature, high pressure, and/or chemically active fluids 1. High temperature due to adjacency to magma or geothermal gradient (~ 30 degrees per km depth) 2. Pressure is confining pressure due to overburden/depth OR differential pressure due to tectonic forces 3. Hot fluids heated groundwater carrying dissolved ions—precious metals C. Recrystallization occurs—crystal size and/or chemical composition changes D. NO MELTING E. Classify as foliated (subjected to differential pressure) or non-foliated F. Classify by degree of metamorphism (higher T & P = higher grade) G. Metamorphic environments 1. Burial metamorphism—deep sedimentary basins High T High P 2. Contact metamorphism—near magma High T / low P 3. Regional metamorphism – tectonic activity/compressive forces High T High P 4. Dynamic metamorphism—localized around fault zones High P Low T H. MM rock classification by type of recrystallization and metamorphic grade 1. Foliated versus non-foliated 2. Size of crystals/time for recrystallization indicates metamorphic grade 3. Index minerals help determine metamorphic grade (indentifies specific T and P conditions) 4. Sequence of increasing metamorphic grade: Slate, Phyllite, Schist, Gneiss, Migmatite 5. From parent rock: basaltgreenstoneschist 6. Limestone marble, granite gneiss, sandstonequartzite I. Metamorphic facies are pathways through metamorphism 1. Moderate pressure + low temperature: greenschist facies 2. High pressure + low temperature: blueschist facies 3. Moderate pressure + high temperature: amphibolite facies 4. High temperature + high pressure: ecologite facies. 5. Geothermal/contact T: hornsfelsfacies J. Plate Tectonic environments 1. Divergent: hydrothermal fluids present—low pressure and high temperatures 2. Convergent: directed stress= foliation. High pressure. Greenschist, amphiboliate, granulite facies. 3. Hotspots: contact metamorphism 4. Intraplate: deep burial—confining pressureII Mountain Building/Geologic Structures
A. Stresses applied to rock produce strain in rock
1. Compressional (convergent margins)
2. Tensional (divergent margins)
3. Shear (transform margins)
4. Result of strain is brittle OR ductile deformation (faults OR folds), depends on
pressure and rock strength
5. Stages for all rock: Elastic Deformation, Plastic Deformation, Fracture/Failure
6. Generally higher pressure = plastic deformation (folds)
B. Permanent deformation = structure (joint, fault, fold)
C. Joints brittle deformation with no movement (e.g. cooling joints)
D. Dip is angle made with horizontal (for fault or fold), strike is direction perpendicular to dip
(N, S, E, W)
E. Folds have hinges, limbs, axis
1. Anticline (“A” shaped fold)
2. Syncline (“U” shaped fold)
3. Monocline (one sided fold)
4. Folds result of compression
5. Folds can be overturned, recumbant
F. Appearance of folds at surface (after erosion)
1. Plunging folds not parallel to horizontal—produce concentric parabolas
2. Non plunging folds appear as parallel “stripes” at surface, symmetric around axis
3. Anticlines = oldest rock at the center
4. Synclines= youngest rock at the center
G. Domes and basins
1. Dome is raised bowl (anticline on anticline) oldest rock in middle of concentric
circles
2. Basin is sunken bowl (syncline on syncline) youngest rock in middle of concentric
circles
H. Faults are brittle deformation (fracture)
1. Plane of movement
2. Parts: dip, foot wall, hanging wall
3. Dip slip (vertical movement) or strike-slip (horizontal movement)
4. Normal: tensional forces—hanging wall down
5. Reverse: compressional forces, foot wall down
6. Thrust: low angle reverse faults (less than 45 degrees)
7. Strike-slip: shear forces, defined as left-lateral and right-lateral
I. Surface evidence of faults
a. Scarps
b. Lateral offset c. Stratigraphic displacement
d. Topography
J. Find faults through seismic activity
K. Mountains created by volcanism (not deformation)
1. Several volcanoes in a line
2. Over subduction zones
3. Result of volcanic products
4. Cascades
L. Mountains created by faulting (fault-block mountains)
1. Divergence
2. Crustal extension
3. Creates horsts and grabens
4. Basin and Range
M. Mountains created by folding (fold and thrust mountains)
1. Collisional mountains
2. Highest and most complex (highly metamorphosed)
3. Large compressive force and crustal thickening
4. Appalacians
5. Orogenies are phases of mountain bulding
6. Three orogenies for appalacians: ends when Pangea begins to rift/Atlantic Ocean
begins to form - 2III. Earthquakes A. Elastic Rebound Theory and Fault Creep 1. Stresses regionally from plate tectonics and locally (compressional, tensional, shear) 2. Sudden release of energy from built up strain = seismic waves (elastic rebound) 3. Movement along fractured plane==fault 4. Fault creep = slow continual motion as a results of stresses, no seismic waves 5. Earthquakes also caused by explosions, volcanoes, meteor impacts (rare) B. Parts 1. Focus 2. Epicenter 3. Fault 4. fault scarp 5. seismic waves C. Earthquake waves (body waves versus surface waves) 1. Body wave--P waves: compressional, fastest 2. Body wave--S waves: shear (up and down), second arrival (half as fast) does not travel through liquid 3. Surface waves—Rayleigh wave, wave vertical displacement on Earth’s surface 4. Surface wave—Love wave horizontal ground motion, arrives last 5. Travel times of different waves D. EQ measuring devices=seismometers, seismograms, accelerometers 1. Seismometers record seismic waves in three directions 2. Triangulate epicenter with 3 seismograms a. Determine S-P arrival time lag b. Determine distance from epicenter using S-P lag and ground material type c. Trangulate with radial distances around three seismograms to locate epicenter d. Determine Richter magnitude by examining largest amplitude wave and distance (using nomogram) E. Intensity versus Magnitude 1. Magnitude is energy released--logarithmic 2. Richter magnitude for local EQs—based on maximum seismic wave amplitude 3. Moment Magnitude (for large EQs) 4. A 1.0 increase in magnitude is 32 times energy released 5. Intensity is amount of shaking—varied by location 6. Modified Mercalli intensity ranges I to XII 7. Intensity based on shaking strength, type of ground material, distance from epicenter, type of structures 8. Many earthquakes are happening all the time, but they are small 9. There are very few large earthquakes F. EQs and stresses (review plate tectonics) 1. Most EQs on plate boundaries 2. Divergent boundaries, shallow EQs on boundary 3. Deeper earthquakes along subduction zone—get deeper as move away from boundary on overriding plate (Watati-Benioff zone) 4. Transform boundaries, strike-slip motion 5. Interplate EQs: less common, poorly understood G. Destruction from earthquake shaking depends on: 1. Intensity of shaking 2. Duration of shaking 3. Design of structure 4. Foundation material H. Other Earthquake Hazards 1. Building collapse 2. Infrastructure damage 3. Liquefaction: brings groundwater to surface and reduces sediment cohesion (behaves like quicksand) 4. Landslides 5. Tsunami: movement on ocean floor generates energy wave in ocean 6. Fire—causes most damage I. San Andreas Fault 1. Relative motion of Pacific Plate northward is 2 cm/yr 2. Most is only on 2/3 of SAF—some portions have not moved in a while 3. Compressional forces build depending on local geometry—Big Beng J. Recent Earthquake activity 1. 8.2 in Chile week of April 1st on convergent subduction boundary—tsunami 2. 5.1 on Puente Hills thrust—over 200 aftershocks, blind thrust (not seen at surface) 3. 4.8 in Yellowstone—interplate location 4. A large earthquake will disrupt/interrupt power, water supply, sewers, telecommunications, transportation, and structural damage 5. How to prepare for an earthquake: secure your space, make a plan, create disaster kits, retrofit your home 6. DROP, COVER, and HOLD ON during an earthquake. Do not run outside during an earthquake. Stay away from large objects that can fall on you. K. Using seismic waves to explore interior of Earth 1. Seismic waves reflect and refract when encounter material of different density 2. Change in material type called discontinuity 3. Higher density material = higher velocity seismic waves 4. P-waves have small “shadow” zones created by refraction traveling through lower density outer core 5. S-waves have large “shadow” zone created by absorption of P-waves by liquid outer core 6. See Mohorovicic discontinuity at 8 km beneath ocean, 20-79 km beneath continent = discontinuity between crust and lower part of lithosphere (upper mantle that is part of lithosphere)IV. Mass Wasting
A. Large landslide in Oso, WA on March 23. 36 dead, 10 still missing
B. Mass wasting is downward movement of earth materials under influence of gravity
1. Resisted by friction
2. Produced by combination of tectonics/uplift and gravity
3. Usually earth materials weakened by weathering, fractures, tectonic activity
4. Depends on material type, cohesion, slope gradient, water content
C. Classification
1. By material type (rock, debris, coarse soil, mud)
2. By movement type (fall, topple, slide, flow/creep)
3. By velocity (ranges from 1 cm/yr to 5 m/s)
D. Creep
1. Slow motion – cm/yr
2. Related to freeze/thaw wet/dry expansion and contraction of soils
3. See ripples on surface, trees growing with bent trunks
4. Medium water content
E. Slump
1. Moves as single mass over short distance as cohesive block
2. Leaves scarp faces (upslope failure surface)
3. Rotational movement along failure plane
4. Medium speeds: mm/day to 10s of meters per minute.
5. Medium water content
F. Solifluction
1. See ripples on land
2. Slow movement
3. Medium water content
4. Upper layers of permafrost that defrost and move with water along frozen lower layer
G. Flows (debris flows, mudflows, lahar)
1. Entire mass moves
2. Moving slurry—speed depends on slope and water content
3. Can be rock (dry), debris (moderate), mud (wet)
4. Can be fast—up to 110 km/hr
5. E.G. Thistle Utah
H. Slides (rockslides, debris slides)
1. Sudden motion downslope, leaving scar and debris pile
2. Medium to fast speed—up to 300 km/hr
3. Translational movement along failure plane
4. Debris slide high water 5. Rock slide dry
6. E.G. La Conchita, CA
I. Falls (rock falls, debris falls, topples)
1. Mass falls from near vertical slope (cliff)
2. Rotational fall = topple
3. Forms debris pile (talus) at base
4. Fast movement, up to 300 km/hr
5. Dry
6. E.G. Yosemite National Park, CA
J. Avalanches, lahars, submarine landslides are special types
K. Slope failure caused by gravity
1. Material has angle of repose—angle for stable slope for dry unconsolidated material
2. Larger particles generally have steeper angles, smaller particles less steep angles
3. Increased water content decreases material strength
4. Earthquakes can lead to failure (eg liquefaction) through vibration
5. Weathering and removal of vegetation can also lead to more movement
L. Oversteepening of slopes increases mass movement
1. increasing weight on slope
2. cutting into base of slope
3. addition of water to material
4. wave action cutting into base of slope (undercutting)
M. Prevention methods
1. Barriers
2. Removing groundwater
3. Grading slope
4. Removing weight from upper surfaces
5. Add vegetation to surfaces
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- FlashCards sobre Sem título, criado por michellai15 em 17-04-2014.
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