Mod 7 Geologic Structure PDF

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Professor Aubrey Bonde, web-based...

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● Geologic structure: bending and breaking of rocks caused by plate tectonics- puts forces on rock that causes them to bend or break ○ Appalachians: 480-500 million years ago through collision of Earth’s plates, as plates moves apart they became flatter/ more rounded ○ Wichita: same time, collisional boundary with South American plate, as plates separated again the material eroded and was transported and formed areas like MI, LA- northern flatlands are oldest and most stable area ○ Rockies: Canada to Mexico, everything west is material that got stuck on- as small plates were pushed in from west to collide with North American plate they got stuck on and created mountains and rough area ■ Areas of marine sediments and oceanic environment ■ NV, AZ, UT, CA: small mountain ranges that are roughly parallel, range/basin environment- large flat U shaped area = Columbia river basalts, basalt lava flowed and followed valleys and filled it in with several thousand feet of flows ■ Coast range on West Coast, Sierra NV mtns, where plate subduction occurred and created igneous plutons along backbone of mountainscooled beneath surface and whole large block tilted over and brought to surface, material eroding off of them filled in central valley of CA (flat area between Sierra NV and Coast Range) ■ Cascades: Juan de Fuca plate under NA plate- volcanic mountains ○ Areas where rocks have been bent or broken within the US ● NV: generally small mountain ranges all trending in same direction and separated by

small valleys ● Basic types: ○ Faults: breaking with movement, have to show that there has been movement/ slippage along the breaks- layers were clearly connected but moved ○ Folds: rocks formed as horizontal layers, occurred after rocks were deposited (bending) ○ Joints: cracks/ fractures in rock along which no movement has occurred: sets of almost perpendicular cracks- multiple sets of cracks, cracked along well defined paths used to infer direction pressure ○ Foliation: when minerals recrystallize in a direction perpendicular to prevailing pressure- vertical banding shows horizontal pressure ● Force: the mass of an object times its acceleration (Newtons 2nd law) ○ Difference in mass = different acceleration- need to have same acceleration to reach same point, but more force may be needed for heavier thing than lighter (semi truck vs sports car) ○ Measuring force: pressure = Force/Area over which force is applied- halving area will double the pressure and vice versa- changing area will change pressure ● Confining pressure: pressure all around, uniform in all directions (being submerged in fluid) ○ Stacking books on top of hand: directional pressure → acting in a direction ● If there is no tectonic force, deep earth acts like a fluid to some extent, deeper you go = greater confining pressure ○ Tectonic forces add directional pressure

● Pressure vs stress: action of a force acting on a body = pressure, what’s going on inside = stress ○ Confining stress analogous to confining pressure, rock buried deeply under confining stress ○ Tensional stress: rock being pulled apart and stretched out ■ Elongation strain: longer parallel to stress and thinner perpendicular to stress ○ Compressions stress: rock being shortened ■ Shortening strain: shorter parallel to stress and thicker perpendicular to stress ○ Shear stress: when something is being twisted apart- forces acting in opposite directions ■ Shear strain: neither shortens nor elongates parallel to stress ○ All change size/ shape of rock- make it smaller, stretch it out, deform and twist it ● Deformation occurs in two forms, brittle and ductile ○ Brittle: breakage ○ Ductile: permanent bending ● Brittle deformation: increasing stress and strain (% change in length of sample) changes from elastic (strain will bounce back if stress is released) to brittle (brittle failure- will actually break) ○ Deformed under slight stress but it bounces back, after a certain point it becomes too stressed and breaks (lmao same)- strain and change in length is increasing while stress is decreasing

● Ductile deformation: increasing stress and strain, starts with elastic strain and then after a certain point becomes plastic/ ductile and won’t snap back to original shape- takes on permanent deformation ● Same rock could deform in either way- dependent on conditions ○ Stick of butter: cold butter breaks, soft butter bends- as it heats up it starts to become softer ○ Complicating factors: pressure (confining pressure makes them more ductilemore pressure = more likely to deform in plastic manner), speed- think silly putty, pull it apart slowly and its ductile, pull hard and fast and it rips ○ All make material more ductile- fast, low pressure, cold make it brittle ● Ductile materials undergo smooth plastic bends- look like this ~ vs brittle which looks like -_-`- y'know- much clearer breaks ○ Brittle: reverse faulting, normal faulting, strike slip faulting ● Key measurement in inclination of a plane: dip and strike, measure down steepest portion of slope ○ Dip: angle between horizontal and steepest slope line, downwards inclination from horizontal ○ Strike: direction in which it’s dipping- horizontal line across rock’s surface, compass measurement at direction line is pointing- intersection from plane you’re measuring and imaginary horizontal surface ○ Measured on top of sedimentary layer- use compass to measure first dip (inclination of block downwards from horizontal), degrees away from north (two strike directions in opposite directions, 180 degrees apart)

○ If perfectly horizontal- dip of zero, strike relative to dip (can’t be determined)circle with a cross- dip of 90 degrees, can’t determine which direction is strike ● Plunge: folded rock folded most tightly at hinge line- if horizontal it’s non plunging, if it’s diagonal/ at an angle = plunging (top of fold inclined from the horizontal, angle declined from horizontal) ○ Direction of hinge line = azimuth ● Joints: fractures or breaks in a rock unit that don’t show any evidence of sliding movement along the break (cracked windshield- stressed, cracked, but didn’t move) ○ Subjected to tensional force and pulled apart and cracked to relieve stress, angles, dip and strike can tell us about what happened- high amounts of joints is helpful for engineering purposes- rock is weak, groundwater will flow easily and quickly through it ● Faults differ from joints because they show evidence of movement ○ Offset: line should be connected but they're not -----/_____ like that ○ Scarp: surface exposed by vertical movement- will eventually disappear through weathering, often become softened and curved ○ Slickensides: scratches caused by two rocks sliding together ○ Fault gouge: rock was broke up and ground down by movement along fault plane ● Faults: ○ Dip-slip faults: primary direction of movement is vertical along dip of fault- fault plane dips downwards at some angle, moves up or down dip of fault ■ Footwall and hanging wall: hanging where they hung their lamp, footwallwhere they placed their feet- old mining term as they often mined along

faults ○ Normal: hanging wall goes down relative to footwall, occur when rock is under tension and being pulled apart ○ Reverse: hanging wall goes up relative to footwall, large angle > 45 degrees ○ Thrust: hanging wall up, footwall down, dip angle < 45 degrees, can be super cool because they shape landscape ■ Blocks pushed on top of other blocks, but over a long distance- can involve movement of 50-100 miles- messes with superposition because rocks wind up on top of… themselves? Layers get all mixed up ○ Strike slip: movement is horizontal, involve shearing (pushed in opposite directions), described as left lateral or right lateral ■ Left lateral: standing on block on right hand side, think the block would have moved to your left ■ Right lateral: vice versa ○ Transform faults: associated with plate boundaries (San Andreas occurs as Pacific plate moves northward relative to North American plate) ● Real faults are rarely simple: rarely just one or the other, often local changes with components of dip slip and strike slip ● Folds: bent rock, bends occur because rock is ductile when compressed- had to be warm under a lot of confining pressure, with slow deformation ○ Anticline: upfold in rock, has two “limbs” which dip away from center of fold, top of axial plane is fold axis/ hinge along top of surface- horizontal in horizontal fold, at an angle in plunging fold U

○ Syncline: downward fold in rock, literally just anticline but flipped (can be horizontal, plunging, etc) A ○ Often occur symmetrically: like a W- limb of anticline is often limb of adjacent syncline, etc, can have symmetrical or asymmetrical limbs around the hinge ■ Asymmetrical folds: axial planes aren't purely vertical but are at an angle, makes limbs angled too (steep on one side, shallow on the other) ■ Can be overturned: push em so hard they fall over, one of the limbs has been rotated from horizontal past 90 degrees ■ Recumbent: folds turned over and laying on their sides ○ When subjected to erosion, the surface is flattened and anticline is displayed with old rocks in center with other layers dipping away from center, syncline has youngest rocks in center with others dipping towards it ○ Monocline: sedimentary layers on top of metamorphic or igneous “basement”undergo dip slip and shift, rocks on top bend and stretch to cover resulting difference in basement, only have one limb and put rocks under tension ○ Dome: rock units all dip away from center- strike lines would form a circle, dip lines would point away from center, anticline ○ Basin: opposite of dome, piece of land sinking and stretching rocks above it, strike lines form a circle but dip lines point TOWARDS center, mostly syncline ● Foliation: recrystallization of minerals occuring perpendicular to stress- shows us compression direction because it grows in path of least resistance ○ Bedding can be bent into anticline, can demonstrate reverse faulting- all demonstrate compression

● Geologic forces change landscape: form basins, domes ○ Shear stress in a strike slip fault: stream going across the fault is moved so it no longer connects, leads to offsets, rocks along fault become damaged and subject to weathering (often form valleys because they weather more easily) ○ Compressional: often gets sets of synclines and anticlines, faults build up mountain ranges, unbalanced stress can lead to faults and minor shear stress that offsets folds- complicated landscape ■ Weathering attacks broken and tightly bent rocks, tops of anticlines may be eroded into valleys, topography may develop so there are valleys along anticlines but synclines are generally unweathered because rocks are less deformed ○ Tension: pulled apart, some rocks left untouched and form horsts (higher up), blocks between horsts drop between normal faults form grabens → have W shape where valleys are separated by ridges (can be very small or very large) ■ Covers most of NV- parallel mountain ranges separated by boundaries with normal faults where grabens have dropped downwards, sediment causes for sand and gravel filled basins between mountains Quiz: 1. Which statement best applies to the term anticline? erosion exposes the oldest rock units along the axial part of the fold 2. In the drawing shown, the house sits above: a monocline 3. Which of the following is not a dip-slip fault? a right-lateral fault 4. Left-lateral strike-slip faults result from: shear stress

5. Which of the following statements best describes the behavior of rocks during deformation? brittle materials deform by faulting, whereas ductile materials deform by folding 6. In the drawing shown, the house sits above: an anticline or dome 7. Older rocks are brought up in the foot wall of a ________ fault. Normal 8. A folded rock structure in which the rock units dip towards the axial plane in both directions is called a(n): syncline 9. A ridge lying in the footwalls of two bounding normal faults. Horst 10. This type of fault is often associated with divergent plate tectonics. normal...