The inside of the Earth ESS 202 Earth p. 41 More pink shows less vegetation.
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Transcript of The inside of the Earth ESS 202 Earth p. 41 More pink shows less vegetation.
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The inside of the Earth
ESS
202
Earthp. 41
More pink shows less vegetation
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How to attract students to geoscience
• Just finished grad student recruiting• Eos article, March 19th, 2002• Emphasize financial rewards
– They don’t care if it’s interesting
• Don’t use too many words, more pictures
• Pictures of computers• Internet addresses• Shave off beards! Dress well!
– We’re perceived as looking like “nerds”
– And often like slobs
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Earth: Main ingredients
• Air• Oceans• Crust• Mantle• Core } This
lecture
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Radius:6371 km
Core radius:3470 km
Circumference:40,000 km
The Earth
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Masses
• Air 3 X 1019 kg
• Oceans 1000 X 1019 kg
• Crust 20,000 X 1019 kg
• Mantle 400,000 X 1019 kg
• Core 200,000 X 1019 kg
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What’s in the Earth?
• Quantities that we want to know– Forces, stresses, viscosity– Temperature, composition– History
• Quantities that we can measure– P & S wave velocities (seismology)– Density (seismology and gravity)– Surface rock, plate motions
(geodesy)
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How seismology looks at the Earth
• Travel times of direct waves– P waves– S waves– Surface waves, both Rayleigh and Love
• Reflected waves
• Trying to match entire seismograms
• Normal modes (Earth rings like bell)
• Plus gravity, magnetism, chemistry
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Waves P
S
Love
RayleighBolt, 1-9
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What controls size of waves?• Magnitude
– Bigger slip (offset) or fault area leads to bigger motions
• Distance• Wave type
– S larger than P because shearing motion of quake produces shear waves (S) preferentially to compressional waves (P)
– Surface waves larger than body waves because surface waves die away more slowly with distance
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Process
1. Identify many waves, each with a different path2. Measure either their amplitude and/or time of arrival3. Reconstruct the structure through which the waves must have traveled
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Surface waves, P, S, and PP paths
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Distance from earthquake epicenter (km)
Tim
e el
apse
d af
ter
eart
hqua
ke (
min
utes
) S waves travel more slowly than P waves
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Crust• Layer of lighter composition than mantle
– 2.7 g/cc in crust, 3.3 g/cc in mantle
• Mohorovicic seismic discontinuity (Moho) marks boundary between crust and mantle
• Thickness mapped by seismic work– Crust has P velocity 6 km/s, mantle 8 km/s– Crust has S velocity 3.5 km/s, mantle 4.5 km/s
• Thinner under oceans (4 to 6 km)• Thicker under continents (25 to 80 km)
– Causes most of topography on Earth
Andrija Mohorovicic
1857-1936
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Oil exploration• Mapping the upper few km of the
crust• Oil and gas seep upwards
– From buried, rotting and cooked organic stuff
• Gets trapped in pools in structures like faults and warped layers
• Looks almost entirely at sedimentary rock– Relatively young, not fully cooked rocks– Starts out laminated; sand, silt, pebbles ...
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Searching for oil traps
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Ship makes waves with air gun and tows seismometers that detect reflected waves
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Wave propagation
• Oil people generally only use P waves– S waves don’t travel through water
– Don’t travel as well through rock, either
• Most energy is transmitted through the water and rocks
• But a little is reflected back to make these images
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Oilexploration
on land
Flint, 21-14
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Dynamite
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Crust, Mantle, and Core
• Crust is thin veneer floating on mantle– 4 to 80 km thick– Upper part of rigid plates
• Mantle is most of Earth’s mass, dense rock– Slowly flowing in convection– Several “phase changes” in upper mantle
• Core’s radius is about half of Earth’s radius– Outer core is liquid iron, makes magnetic field– Inner core is solid iron
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Isostacy: Crust is less dense than mantle, like wood floating on water
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Moho is seismic jump that marks the base of the continental and oceanic crust
Moho
Grossly exaggerated vertical scale
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Example without vertical exaggeration
Moho
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Global crustal thickness
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Walter Mooney
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Granite- found in crust
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Olivine - found in mantle
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Some terms
• Lithosphere - strong layer composed of crust and uppermost mantle, 30-300 km thick (actually, lively debate about thickness)
• Aesthenosphere - underlying weak layer in the mantle
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Continent Ocean
Moho occurs within lithosphere.
Press 1-11
Moho
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(almost) truescale
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Details in the mantle
• But mantle is thought to be nearly uniform in composition
• Deeper rock is denser and stiffer due to increasing pressure, thus higher velocity
• Phase changes, 5% jumps in vel. & den. – Changes in molecular arrangement– At depths of 410 and 660 km
• 660 km depth separates upper and lower mantles
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Phasechanges
in theMantle
MoreUpper mantle
Lower mantle
Upper mantle
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Slab penetrating “410” and “660”
“410”
“660”
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Testing modelsby waveform match
Bad model
Good model
From Su & Dziewonski, 1994 paper
DataCalculation
seconds
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Record Section
of theEarth
Near side ofThe Earth
Far side ofThe EarthP
SSurfacewaves
Increasingtime
Half-wayaround
All the wayaround
P
S
Surfacewaves
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Listen for the tone of normal modes -
gives long-wavelength properties
Davidson, 5-11
Football mode
Balloon mode
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Quake BulletinIllinois M5.2
4:36am local time
Wabash Valley fault system
Felt up to 900 miles away
Little damage
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Midwest quakes
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Reflection:PcP
Beno Gutenberg(1889-1960)
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P&S wavesin the Earth
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Example of core reflections
Bullen & Bolt, 13-1
Echoes of anuclear explosion
10 minutes later
17 minutes later
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Interpreting Seismic Velocities
• Seismic wave velocity ~ Elastic stiffness Density
• Velocity increases with depth and so does density– Therefore, velocity is dominated by stiffness
• Stiffness controlled by– Pressure– Temperature– Composition– Water– Crystal structure
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P and S wave velocity vs.
depth
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P waveshadow
P waves bent downward (deflected) at core-mantle boundary, large velocity decrease there
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S waveshadow
No S waves pass through outer core, therefore it is fluid!
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Some real seismograms
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Outer Core
• Liquid, 84% iron + 8% sulphur + 8% oxygen?– Lower P velocity than mantle– No S waves allowed in liquid!– Presence inferred from P and S shadow zones
• Convection leads to magnetic field– In fact, magnetic field as important as inertia
• Complicated - magnetohydrodynamic!
– Magnetic field reverses from time to time– Keeps atmosphere from being blown away
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Magneticfield lines
Strength of field plus reversals imply that field generated by flow in conducting fluid - molten iron core.
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(Real science slide, Jon Aurnou, compatriot at UCLA)
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Inner core
• Solid, 92% iron 8% sulphur– hard to tell it exists, presence inferred from
normal mode analysis– recently discovered to slowly rotate
• About 0.2-0.3° every three years, still controversial
• Inner core grows as outer core “freezes”– because Earth is cooling, releases a lot of heat– eventually, outer core will all freeze– less protection from cosmic rays for us
Inge Lehmann(1888-1993)
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Innermost inner core• Remnant of earliest times?
• Georeactor?
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Other things that vary with depth
• Temperature
• Gravity
• Pressure
• Density
Jules Verne
Dante
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Temperature• Increases with greater depth
– Gets hot in mines at about 25°/km depth
• Generally near melting point inside mantle
• We know temp. at surface– 0° - 30° C in air, close to 0° at ocean bottom
• 0° to 1500° Celsius in crust
• 1500° to 3000° in most of mantle
• 3000° to 4000° in core
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GeothermTemperature
Air
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Gravity and Pressure
• Gravity– Roughly constant through mantle
– Diminishes to zero in the center of the Earth
• Pressure– Proportionate to weight of overlying material
– Increases enormously with depth
– Particularly in the iron core
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Gravityand
Pressurevs depth
Lowrie, 3-78
pressure
gravitySwiss
GeophysicistETH
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Density
• Density is mass per unit volume• Increases with depth
– Partly just due to compression from increasing pressure
– Partly from phase changes (small change)– Partly from compositional changes
• Crust to mantle (small change)• Mantle to core (big change)
– Partly from freezing (outer to inner core)
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Density
Lowrie, 3-77
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Lateral variation in the Earth
• Tomography– Buzzword for finding 3-D structure– Similar to CAT scans, which look inside people
• Wadati-Benioff zones– Cold, subducting material is stiffer than average– Subduction seems to extend down to core
• Hot spots– Warm, mushier material that is rising
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Cartoon view
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South American subductioncross-section
Wadati-Benioff zone
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Seismic tomography
• Like a CAT scan– reveals 3-D image of structure inside the Earth
• Shows where seismic waves travel faster or slower
• Colder material is stiffer (although denser)– Therefore has faster P and S velocities– But composition also affect wave speeds
Not a “cat scan”
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How CAT scan works
Repeat procedure fortransmitters all theway around target
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Medical CAT scanner
Preparing the ice manfor a CAT scan
Machinery
CAT scan of fourbaseball bat, two ofwhich are corked
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Tomography reveals the subducted Farallon plate. It is cold, so it has high seismic velocity
Surface
Mantle
Core
Pacificplate North American plate
Real data view
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Wild ASUCMB ideas
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Easter IslandPlume
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Convecting system
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Global heat flow pattern
• High at spreading ridges– Hot material is upwelling
• Cold on old continents– They have been cooling for billions of years
• Hot spots are also hot– but a minor feature
Bunsen burner
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Heat flow pattern
International Heat Flow Commission
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Mantle temperatures at 100 km depth
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The Earth:An ongoing project
• Connections– To what extent are the tectonic plates glued to the
underlying mantle?– How variable is the composition in the mantle?– What action is at the core-mantle boundary?
• What do plumes really look like?
• How does the core dynamo work?
• Why is there structure in the inner core?