Earth Structure (Ch 3) 3(1)
Internal Structure of Earth / Evidence for Layering (Lec
5)
1. Internal structure of Earth
• Layered (concentric shells) - looks like an onion (Fig
3.1 T011 3.1) How do we Know
R s layered?
-Earth's `*total density (mass/volume) = 5.5 g/cc (known from planetary data li!e size of orbit)
-Rx on surface have lower densities like 2.7 g/cc (granite) & 2.9 g/cc (basalt).
' -Higher density materials must exist at depths that are even greater Cian 5.5 g/cc, since the lighter rx on surface must be compensated for by denser rx at depth.
-So there must be at least 2 layers in Earth - an outer, light one concentrically wrapped around an inner, heavy one. Actually, diff lines of evidence suggest there are several layers.
,Observations about layering
-Layering controlled by density (measure of relative heaviness:
Mass per unit Volume or grams/cubic centimeter ("glee")
-Gravity sorts layers of different density (most dense pulled closer to carver of Earth) - known as donelly stratification stratified means layered)
-Layering is different depending on whether classification is chemical or physical, although density is still driving force

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A. Layering from chem composition standpt-3 layers (FIn 3.2)
(classical viewpt until advent of plate tectonics theory)
1. Crust -outer layer granite =02,SI,Al density 2.7g/cc basalt =02,Si,Mg,Fe density 2.9g/cc
2. Mantle - middle layer peridotite=02,Si,Mg density 4.5 g/cc
3. Core- inner layer
Fe (90°/) and Ni density +/- WHAT? look it up
B Layering from a phys properties standpt-5 layers
This classification important because actual tectonic processes mountain-building, for example) are controlled more by diff phys props than by chem props
(this is why there is confusing overlap in the classic schemes mantle for, instance, is 1 entity from chem viewpt, but 3 entities from phys pr )ps viewpt, Fia WWM 3.7
Rx have diff physical properties depending upon temperature and pressure conditions
Example: Si02 (quartz)
As rigid cold material it deforms permanently by brittle breakage (wind,)w glass)
As plastic,warm material it deforms permanently by flow
(when cooling from glass-blower's pipe)
As liquid, very hot material it deforms temporarily under stress, but assumes diff shape when stress released (water, oil are like this too, even when cold )

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5 physical layers are (from outer to inner): Fig 3.2. WWM 3.1
1. Lithosphere (cool,rigid)
2. Asthenosphere (warm,plastic)
3. Mesosphere (warm,rigid due to high pressure)
4. Outer core (hot,liquid)
5. Inner core (hot,rigid due to high pressure)
C. How phys & Chem layers relate to each other (WWM Fig. 3.1)
-Rigid lithosphere (crust & upper mantel in chem. classif. ) floats upon dense but plastic asthenosohere (middle mantle)
-Rigid bodies have diff thicknesses
-Oceanic crust=4 mi thick
-Cont crust = 22-44 mi thick
Why such a difference in thickness?
D. Phenomena of buoyancy & isostatic equilibrium explain why lithosphere floats on asthenosphere, why continental crust is thicker than oceanic crust
Definition of buoyancy: "the ability of an object to float in a liquid by displacing a volume of that fluid that is equal in mass to the floating object's mass" - remember that mass is the controlling factor here; the displaced fluid volume is whatever volume it takes to equal the mass of the floating object
Example: Fg 3.4 –ship floating in water displaces a volume of water that has same mass as ship
If mass of ship increases by adding cargo, it sinks lower in water because it displaces more water- it is in isostatic equilibrium
("equal standing") w/ the water

3(4) Example:
Peridotite slab diplaces 100 cc ... pretend we can cut it out of the mantle
Fig WWM 3.2
What volume of this piece of granite would it take to equal the amount of mass in the peridotite slab? (We know the mass of peridotite and the density of the granite) Fig WWM 3.2
The granite slab would have twice the volume of the peridotite slab; thus it would be twice as high as the peridotite slab if the base were the same dimensions; it would appear to be floating in a "sea" of peridotite like an iceberg floats in the ocean
Tupperware experiment again if there's time ....... shows low density of continental crust, higher density of oceanic crust, how both float on the asthenosphere, but higher mass of oceanic crus ^auses more mass & volume displacement of asthenosphere than does continental crust
E. Observations about mountain - building and heat
•Orig thinking was that Earth had been cooling ever since formation ....
-Lord WHO? thought Earth to be 80 million yr old from calculations of how long it would take Earth to cool from molten state
-Mts were believed to be due to "raisin" effect - wrinkling of outer skin while interior cooled & contracted
-Later data (fossils & radioactive dating) showed Earth is 4.6 billion yrs old
-Key point : If 4.6 billion yr age is correct, Earth should be cold by now according to heat flow calculations
?his not the case ...see some evidence that Earth has not entirely cooled off ...WHAT Evidence??
-Postulated source of internal Earth heat is radioactive decay think)
-Most decay occurring in crust & upper mantle now (we

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•Types of heat flow
1. Conduction -transmission of heat through a conductor
(skillet handle)
2. Convection - mass mvmt of heat through fluid: heating reduces density, reduced density causes hot fluid to rise (eg, a rolling boil in water when you're cooking spaghetti)
111. Seismic evidence for layering
A. Seismic waves due to eqks indicate layered struc because of diff waves and their diff abilities to propagate through solids &/or liquids
• 2 main types of waves (FI
3.6
1. Primary or
IrpII waves (compressional) - -energy transmitted along axis of wave propagation
2. Secondary or "s" waves (shear) - energy transmitted transverse to axis of wave propagation
Characterstics: p waves arrive first s waves arrive later travel through WHAT travel through WHAT
B. Sequence of arrivals of waves suggests Earth gets faster as you go deeper (Fig WWM 3.3)
1. First seis evidence of layering
*Richard Oldham in 1900 discovered that waves at certain places arrive sooner than at other, closer places (Fig WWM 3.3)
This shouldn't happen if Earth were homogeneous ...waves would just travel same velocity, and the closer the eqk, the sooner the wave would arrive
-What really happens - Waves go down into faster layers, arrive earlier than waves staying closer to surface (Fig
WWM 3.3)

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?. First seis evidence of fluid-filled core
-Oldham (1906) found some p-waves arriving more slowly than expected, and s-waves weren't arriving at all
-This indicated presence of a slower, fluid-filled portion of Earth somewhere at depth slowing down the -waves and completely stopping the s-waves (Fig .7
-He also predicted existence of a "shadow zone" for each eqk where there were no direct arrivals (Fig 3.7)
-Shadow zone does exist as a belt of no direct arrivals from
103°-143° away from eqk epicenter (Fig 3.7)
- No swaves make it past 103° from eqk due to fluid outer core -
- p-waves slow down & refract back toward cents, of
Earth until 143° from eqk, where they re-emerge
3. First seis evidence of solid inner core w/in fluid-filled outer core
-In 1936, Lehmann discovered that faint p waves that made it directly across the globe (central area on (Fig 3.7)had sped up while going through the innermost part of Earth
-This showed that innermost core of Earth is solid
(p-waves had to go through a solid to speedup from the slow liquid)
Summary- current theory of Earth structure- combining chem & phys props.
1. Rigid Inner core (mostly iron) that speeds up p-waves
2. Liquid outer core (iron & nickel) that refracts & slows down p-waves while it stops s-waves
3. Rigid lower mantle (WHAT?sphere)
4. Plastic middle mantle (WHAT? sphere)
5. Rigid upper mantle and crust (WHAT?sphere)

•I. Plate Tectonics - Observations & Hypotheses
A. Age & Evolution of Earth - early theories
-Earth young
- Bishop Ussher & catastrophrism
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-Earth old
- Hutton & Darwin - uniformitarianism & natural selection time for processes to work.
–takes
R Initial ideas of plate tectonics Fig 3.9,
-Da Vinci noticed fit of of continents if oceans were removed
-Frances Bacon (1620) wrote of fit across S. Atlantic
-Suess (1885) (Glossapteris fern distribution)
-Wegner (1912 )
-Proposed breakup of Panqea
"continental drift" 200 million yrs ago
-Proposed evidence by
Wegener
-Shoreline fit
-Glossopteris distribution
-Glacial distribution
-Coal (formed in warm swamps) in
Antarctica
-Proposed mechanism -Centrifuge effect slung continents toward equator
Result?
“Dismissed as a crank" by colleagues -Biggest problem was driving mechanism -Central view of Earth was as rigid body, nothing could
"drift" on rigid body -Some argued for "plastic mantle" which could allow drift
-Conventional Wisdom said:
"No Way "
-Theory in decline by Wegener's death in 1930

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C. Theory continued to faintly glow as evidence mounted
-Wadati (1935) speculated about volcannos near Japan
-Benloff (1940) - saw range of eqk epicenter depths at trenches World Seismicity Map
-Radiometric dating showed ocean to be very young
(less than 200 million yrs old)
-Mid-Atlantic Ridge accurately profiled by Lament
Doherty scientists
-Bullard fit - big breakthrough - showed incredible fit at edges of continental shelves Fig 3-13
-Standardized reporting o asks resulted in discovery of low velocity plastic layer in mantle
D. Theory of Sea Floor Spreading - Hess (1960), Princeton Fig. 3.14
-Seafloor created at ridges -Seafloor consumed of subduction zones
-Driving mechanism proposed - convection currents in mantle -Eqks found at ridges transform faults, trenches ie plate boundaries
)
E. Theory of Plate Tectonics - Wilson (1965) U. Toronto
-Integrated previous theories of continental drift & seafloor spreading -Proposed dozen rigid plates floating on plastic asthenosphere Fig 3.14
-Convection currents in asthenosphere drag the lithosphere, as if on a conveyor belt
-Almost all Earth deformation occurs at edges of these plates where they hit each other. Fig. 3.15.

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III- Plate Tectonics - Dynamics, Evidence, Confirmation
-
A. General dynamics
-Most Earth deformation occurs at plate boundaries/~ 3.1r
-Happening for millions or billions of yrs
-Latest breakup - Pangea ("supercontinent")
+/- 200 million yrs ago Fig 3.76
B. Types of plate boundaries
1. Divergent ("moving away") boundary
,Driving mechanism Fie 3.15
- Upwelling due to convective heating ............. ... r~h.s,~
,Dynamics Fig 3.77 &°~w
-Crust created
-Extensional ("pull-apart") stress thins & "necks" crust (like pulling taffy)
,Geologic features
-Ocean ridges (eg, Mid-Atlantic)
-Continental rifts (eg, E Africa)
2. Convergent ("moving toward") boundary
,Driving mechanism Fir 3.75
-Subduction due to gravity pulling down J~ccr~
*Dynamics -Crust destroyed (or subducted)
-Compressional ("push together") stresses
,Geologic features
-Trenches
-Island arcs (or continental mountain belts)

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3. Transform ("slide past") boundary ,Driving mechanism
Fig 3.74 - Shearing due to plates moving past each other on way from ridge to subduction zone ,Dynamics
-Crust neither cmated nor destroyed -Shear ("Sliding") stresses ,Features -Transform faults (eg, San
Andreas) -Fracture zones ,Unwanted side effects -
San Franciscans will have to deal with their noisy neighbors in S.F.'s new western suburb of Los
Angeles
B. Evidence - plate tectonics provides unifying theory for many observed phenomena, among them. Hot spots -Atolls & guyots -Young seafloor sediments -Ocean ridge locations
(vs looking like a bathtub) -Heal flow patterns -"Terranes"
-Fossil distribution (eg, Glossopleris) -Sea turtle migration
(OK, this is a stretch!)
J~.
LGP•~h~>,~~

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C. Confirmation - paleomagnetic bands -Paleomagnetic stripes on ocean floor clearly indicate spreading Fig 3.30 -Earth periodically "flips" N & S magnetic poles (is, N becomes S & S becomes N) -Iron-bearing minerals orient themselves in whatever magnetic field is present when they're floating in liquid magma
-Their orientation is "frozen" in place when magma solidifies -As spreading proceeds, stripes of alternating "polarity' are emplaced symmetrically on either side of spreading center Fig 3.31