Earth & Planetary Science Part II

Absolute Age vs. Relative Age

Absolute: radioactive decay to measure age/sequences/cause & effect, Relative Ages: field relationships/fossil correlations to determine sequence of events

Radioactive Decay

Parent nuclide decays to daughter nuclide at constant rate (independent of physical/chemical conditions), accumulate daughter ions exponentially per rate of decay, need to know: 1/2 life/isotopes must be present & measurable (not gaseous)/initial concentration of parent or daughter/origin & geological behavior of material being dated

Rubidium & Strontium

St wants go into Ca/Plagioclase and Rb goes into non-Ca, as Rb decays more St accumulates (so rocks with more Rb initially will gain more St) and allows you date rocks bc how much Rb is left can be compared to how much St has built up

Isochrons

Slope that tells dating, you need: large parent to daughter ratio/parent and daughter should span a large range/close system/daughter are homogeneous isotope… some rocks want Ur some want Pb and large ratio Ur:Pb rock moves along geochron (specific isochron - Ur decays to Pb, allows older dating)… can use carbon dating for recent dating bc half life 5730 years (solar radiation causes N lose proton and become C-14 which mixes w/ O to become CO2)

Strike and Dip

Trend and plunge of a feature: Strike is intersection of planar feature with horizontal lines (North, South, East, West) relative to dip angle (maximum slope), strike isn’t constant when there’s deformation

Stress vs. Strain

Stress is applied field of force on a system (pressure: 𝜎) where pushing up/down is normal stress and horizontally is shear stress… Strain is accumulated change in length and angle of rock due to stress (can have a lot of stress before any strain) - length minus original length over original length

Elasticity

Rebound affect: linearly increases in pressure (𝜎) as rock pulling then snaps back to original point… stress = young’s modulus (how far will go relative to original position) times strain (low strain is a steep elasticity slope and high strain is a shallow elasticity slope function)… in elastic systems you apply stress to get an instant strain and deformation which is then reversible

Viscosity

Occurs in deep rocks where Rock flows and not relative to original position, stress is applied and strain grows with time, the longer stress is applied the rate of flow increases (doesn’t relax so when done flowing sits there)… stress = material property times strain rate per time 𝜎 = 𝜂 \* ė

Brittle Failure

Occurs in surface rocks with Amonton’s Law: more normal stress means need more shear stress, earthquakes occur when 𝜎 shear > 𝜎 normal \* 𝜇 (mu is friction property of rock), means deep earthquakes are big bc need more shear force to overcome,

Strength

Maximum shear stress a rock can take before brittle failure, if strength is x axis and depth is y, start increasing strength as depth increases until reach max and strength curves back down as depth continues until reach point transition between felsic and mafic (felsic ductile at 350 bc low melting temp and mafic ductile at 600 bc high melting point) at crust-mantle boundary so jumps up to high strength again then decreases slowly with depth

Anticline

Compression causes layers superimposed sediments to bulge up, then erodes top and get oldest rock in center with successively younger symmetrically surrounding, dip angle goes out

Syncline

Compression causes layers superimposed sediments to sink in, erosion doesn’t really affect top, you have youngest rock in center with successively older symmetrically surrounding, dip angle goes in (good for trapping oil)… can determine layers curve and connect under based on law lateral continuity

Folding in Domes and Basins

You can have tight/open/large/small folding, domes are plutonic/salt diapir (upward movement magma) and bulges center dips away all directions, or basin where pushes up on sides

Normal Fault

Dip slip fault extending (fault is on angle): Hanging wall (rock above fault) moves down relative to footwall (rock below fault), steep 60 degree dips, can create valley, think if drilled down/draw transverse the rock gets older but jumps to super old at fault

Thrust or Reverse Faults

Drip slip fault shortening: hanging wall move up relative to footwall, shallow 30 degree dips, think if drilled down/draw transverse the rock gets older then at fault resets to young (can get detached thrust zones where rock above surface curves and flows over)

Strike-Slip Fault

Transform boundaries, motion parallel to fault strike with no uplift, vertical 90 degree dip, moves laterally left or right, eg. San Andreas is right lateral (North American moves south and Pacific moves north)

Crustal Construction and Destruction

Divergent = construction, convergent = destruction, transform = conservation, types: Atlantic passive margins (570-350 Ma), Japanese subduction (400-250 Ma - island arc formed in ocean then shoved onto continent), Andean volcanic arcs (and forearc basins 200-65 Ma), Californian margin (20 Ma)… continental construction dependent on melting/cooling magma, destroyed through erosion (rare to subduct/detach from keel and reincorporate to magma)

Composition of North America

East coast continental transitions into Atlantic Ocean w/o tectonic boundaries, not a lot of older rock (younger rock easier to preserve/date)… interior shields and platforms (cratons) have had only vertical deformation since Precambrian and not much volcanism where western Rockies very active

Orogeny

Mountain building, magmatism/folding/faulting/erosion surrounding subduction/accretion zones, can create weakness points where future breakup can occur… narrow zones compressed folded rock at convergent boundaries

Epeirogeny

Vertical motions of flat lying rocks without folding or faulting - processes of rocks going up or down in the middle of continents not near tectonic boundaries… eg. sedimentary deposition or glaciers builds weight and mantle under flows away and land sinks, when weight leaves rises again, or lithospheric thinning by aesthenosphere upwelling

Shields

Old low elevation/flat rock (interior of continents) with basement of metamorphic/igneous rock in once tectonically active zones, eg. Canadian and Greenland shields

Platforms

Shields covered with series of horizontal sedimentary rocks, either transgressions (water washes over rock bringing sediments then subsides) or regressions (deposition through local sea level rise and fall)… stable platforms can create basins good for oil/gas

Formation of Appalachians

An oceanic island arc crashes into Laurentia (flooded now US) and accretes along with Baltica, then Eurasia’s Gondwana crashes in and forms Variscan Orogeny to complete Pangea and builds on the mountain,

Cordillera

Western third North America down to South 500 million years, includes: Sierra Nevadas, Cascades, Rocky Mountains, Snake River, Columbia Plateau, Basin and Range, Colorado Plateau, San Andreas Fault

Sierra Nevadas

Central valley foreland basin bulges into Sierra Batholiths then goes down in Great Basin, igneous but not eruptive, cretaceous 115-66 Ma

Cascades and Coast Ranges

Active subduction zones and volcanism, 35 Ma to present, Farallon plate subducts under North American until gone and Juan de Fuca replaces and moves up and under until Pacific replaces and forms CA

Rocky Mountains

Intraplate orogeny (far from subduction), minor deformation, 75-35 Ma, from shallow subduction plate subducted to form Sierras then leveled out right under continental because part of slab broke off and angle lessened, coupled rock as moved creating compression above that form Rockies and Colorado Plateau

Snake River and Columbia Plateau

Hot spot volcanism, plate moves west over fixed mantle plume (reaching from core-mantle boundary up to yellowstone)

San Andreas Fault

Strike slip fault from uplift of Coast Ranges, 28 Ma to today, Mendocino Triple Junction to Baja California, some shear motion east of Sierras

Basin and Range

Extension of faulting high plateau, east and west dips of normal faults, alternating high and low topography from lithospheric thinning causing uplift, still active, Cenozoic

Formation of Himalayas

India rammed into Asia 50 Ma and India subducts under to form thousands of km of mountains over 8000 m high, mountain actively getting higher so some offsets going into Tibetan Plateau

Parkfield Prediction Experiment

Tested idea that earthquakes cyclical periods bc stress builds as soon as dissipates, fault will produce same seismic waves every time fails so ruptures same way every time, San Andreas around 20-30 years so in 50s they predicted 90% M 6 in mid 80s but didn’t happen till 2004

Basics of Earthquakes: Stress vs. Strain

At a certain point you reach critical stress where force overcomes strength and you get failure, there will always be degree deformation from strain creating a bit of flexure/elasticity and after slip there’ll be small bit of rebound

Creep

Rock is continuously pulled (unlike stick-slip where jerky) and no earthquakes because plates aren’t locked together and dissipating stress field at all times

Epicenter & Focus

Focus is center of earthquake in crust’s interior, Epicenter is projection of focus on surface

P-Waves

Primary wave - arrives first, compressional wave (push and pull particles in direction of travel), passes through solids/liquids/gasses, 6-8 km/seconds

S-Waves

Secondary wave - arrives next, shear wave (displaces particles at right angles to direction), passes through solids but not liquids, 4-5 km/seconds

Surface Waves

Most destruction, Travel slowest (depending on rock rigidity/density/temperature/ viscosity), travel across surface, love waves flow like ridges (shakes ground sideways) and rayleigh’s waves oscillate (rolling motion in ground)

How We Locate Earthquakes

Take seismograms from three different stations and create wave field around (function of distance based on difference in travel times between P and S waves) and where the three fields intersect is epicenter

Earthquake Size and Magnitude

Intensity based on damage and human perception, factors: shaking amplitude/duration/frequency range/wave type… originally used Mercalli scale of not felt to destruction of buildings

Richter Scale

Logarithmic scale of ground motion, each increase in level magnitude (M) increases motion by factor of 10 and energy in system by factor of 30 (compare maximum ground shaking and maximum distance between P and S which also tells where/how far away - greater distance between P and S = farther)

Seismic Moment

How we actually measure earthquakes, amount of energy released in system… Moment Magnitude = 2/3 *log10 (rock rigidity* fault area \* dip) - 6

Gutenberg Richter Relationship

Smaller earthquakes far more complicated than larger, M 1 occurs 9000 times a day, M 8 occurs once a year, micro-earthquakes often undetected, frequency decreases by factor of 10 with each magnitude increase

Machine Learning

Computer marks when P then S arrive, trying to see if we can use AI to predict earthquakes, marking used to be done by hand and now we feed those hand maps to algorithms but is subject to extrapolation

First Motion of Earthquakes

Motion of different fault types creates different seismic wave patterns, right lateral strike-slip you’ll be pulled right (and up away) and left lateral strike-slip you’ll be pulled left (and up away), normal fault you’ll be pushed up and away if on the hanging wall and down on the footwall, reverse fault you’ll be pulled down and in if on the hanging wall and up on the footwall

Earthquake Relationship to Nuclear Bombs

Nuclear explosions send seismic waves in all directions equally and earthquakes send waves along the fault, government funds detection whether nukes are being tested underground or if they’re earthquakes

Shallow vs. Deep Earthquakes

Shallow are less than 50 km deep and around transform/divergent boundaries - lithosphere is thin at mid ocean ridges so plate contact is at surface, deeper occurs at subduction convergent boundaries - enough strength for earthquakes to occur so deep bc slab is still cold/brittle as goes down into ductile mantle

Left vs. Right Lateral

Left lateral indicated by bisected circle with top left/bottom right quadrants shaded, right lateral indicated by bisected circle with top right/bottom left quadrants shaded… if look at ridges separated by a strike slip fault think: which direction is rock flowing (in or away from ridge - moving left or right compared to ridge on other side)

Earthquake Hazards

Earthquakes are not killing mechanism but effects are: tsunamis, building/infrastructure failure (60 megacities near active faults), landslides and liquefaction, floods, wildfires… hazard depends on magnitude/distance/soil composition/topography (diffracts directions)

Tsnumanis

Thrust fault in ocean pushes wave that coalesces, can travel for hours and have backwelling, depend on bathymetry (local way underwater topography changes which correlates depth and speed)… you can date tsunami deposits/rings of trees killed by tsunamis/offshore canyon turbidite deposits

Landslides and Liquefaction

Stable bedrock is resistant to earthquakes (composition mountains) and mud/fill isn’t (foothills), unconsolidated soil sediment properties changes with shaking (light material buried in soils will become buoyant and stable heavy materials will sink)

Seismic Wave Refraction

When earthquake occurs waves shoot down at various angles into mantle then curve and come back up to surface bc speed is constantly changing (temp/pressure are instantaneously refracting waves), all S waves will be reflected if steep enough to reach core-mantle boundary (becomes ScS) and some P waves reflect (becomes PcP) and some go through and refract again (PKP) then come back out core with final refraction (PKIKP 142 degrees)… both S and P waves can reach a maximum angle of 105 degrees without interacting with core mantle boundary and follow their projection all the way to other side of earth creating shadow zone between 105 on either side