Ch 21: Earth’s interior
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Transcript of Ch 21: Earth’s interior
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Ch 21: Ch 21: Earth’s interiorEarth’s interior
1. Probing Earth’s interior
2. Seismic waves and Earth’s interior
3. Discovering Earth’s major boundaries
4. Geodynamo
5. Earth’s internal heat engine
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Ch 21: Ch 21: Earth’s interior - Study guideEarth’s interior - Study guide
Know Earth’s major layers (and depths) based on compositional and mechanical differences.
Properties of P-and S waves, and which ones move through solid, which through liquid (molten) rock?
Difference between refraction and reflection What is the Moho and how was it discovered? How was the core-mantle boundary discovered? Variations of S-and P wave velocities with depth. How can
you explain the changes in velocity inside the asthenosphere? What does it tell you about the composition of the mantle?
Why does earth have a magnetic field? What is the source of heat inside earth and how it is
transferred through earth (conduction and convection)?
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earthquake happens (slip on a fault) seismic waves travel away from earthquake they carry info about material they travel through
earthquake happens (slip on a fault) seismic waves travel away from earthquake they carry info about material they travel through
1) Probing Earth’s interior1) Probing Earth’s interior
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1) Probing Earth’s interior1) Probing Earth’s interior
earthquakes make P-waves & S-waves seismologists use them to:
earthquakes make P-waves & S-waves seismologists use them to:
locate earthquakes
determine what deep Earth is made of
locate earthquakes
determine what deep Earth is made of
earlier lecture
today
earthquake happens (slip on a fault) seismic waves travel away from earthquake they carry info about material they travel through
earthquake happens (slip on a fault) seismic waves travel away from earthquake they carry info about material they travel through
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1) Probing Earth’s interior1) Probing Earth’s interior
The nature of seismic wavesThe nature of seismic waves
depend on material properties
are faster in more rigid materials
increase with increasing depth (higher pressure)
Seismic wave speeds:
P waves: compressional waves: are fastest
vibrate material back/forth in direction wave travels
S waves: shear waves: slower than P-waves
vibrate material side-to-side from direction wave travels
Don’t pass through liquids
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1) Probing Earth’s interior1) Probing Earth’s interior
The nature of seismic wavesThe nature of seismic waves
P waves always faster than S-waves
“primary” “secondary”
wave paths are “bent” when crossing from one material into another
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1) Probing Earth’s interior1) Probing Earth’s interior
The nature of seismic wavesThe nature of seismic waves
wave paths are “bent” when crossing from one wave paths are “bent” when crossing from one material into anothermaterial into another
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reflections
refractions
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1) Probing Earth’s interior1) Probing Earth’s interior
The nature of seismic wavesThe nature of seismic waves
wave paths are “bend” when going deeper in Earthwave paths are “bend” when going deeper in Earth
higher pressure = higher wave speedhigher pressure = higher wave speed
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Compositional layers
5 Physical/mechanical layers
2) Seismic waves & Earth’s Interior2) Seismic waves & Earth’s Interior
crust 3-70 km thick
mantle down to 2900 km depth
core 2900-6370 km depth
lithosphere stiff/strong, 0-100 km
asthenosphere soft/weak, 100-660 km
layer depth
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crust
mantle
core
3) Discovering Earth’s major boundaries3) Discovering Earth’s major boundaries
boundariesbetween
layers
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The Crust
3) Discovering Earth’s major boundaries3) Discovering Earth’s major boundaries
Thickness: ~ 30 km (continents, 70km under mountains) 3-15 km (oceanic)
Composition: Continents: felsic (granite)
and mafic (gabbro) rocks
Oceani: Basalt, Gabbro
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The “Moho”
3) Discovering Earth’s major boundaries3) Discovering Earth’s major boundaries
Boundary between the crust and mantle
Discovered in 1909 by Andrija Mohorovicic
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The Mantle
3) Discovering Earth’s major boundaries3) Discovering Earth’s major boundaries
Over 82% of Earth’s volume,mainly peridotite (mineralsOlivine and pyroxene)upper mantle 0 - 660 kmlower mantle 660-2900 kmD” region 2600-2900 km
mantle
D”D”
upper mantle
lowermantle
400 & 660 km depth“phase transitions”
400660
Minerals suddenly compress toa more compact form (phase change)
See Fig. 21.7
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“isostacy” = balance between gravitational force and buoyancy force, see Figure Story 16.16
Insert: Isostacy and crustal uplift/subsidenceInsert: Isostacy and crustal uplift/subsidence
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Less dense crust floats on top of the denser and deformable rocks of the mantle
Insert: Isostacy and crustal uplift/subsidenceInsert: Isostacy and crustal uplift/subsidence
“isostacy” = balance between gravitational force and buoyancy force, see Figure Story 16.16
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Isostatic rebound, adjustment: Readjustment of the isostatic equilibriumafter the ice-shield is removed, as happens still in Scandinaviaand Canada.
Insert: Isostacy and crustal uplift/subsidenceInsert: Isostacy and crustal uplift/subsidence
See 21.1: Isostacy and postglacial uplift
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crust
mantle
core
3) Discovering Earth’s major boundaries3) Discovering Earth’s major boundaries
about Mars sized
Nickel-iron alloy
Outer, liquid, spinning
Inner, solid
4 mio times atm. pressure at center
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The core-mantle boundary
3) Discovering Earth’s major boundaries3) Discovering Earth’s major boundaries
Boundary between the mantle and core
Discovered in 1914 by Beno Gutenberg
mantlemantle
corecore
Core-mantleCore-mantleboundaryboundary
…how?
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P-wave shadow zone
Fig. 21.2
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S-wave shadow zone
Core was discovered …from a “shadow zone”
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The inner core
3) Discovering Earth’s major boundaries3) Discovering Earth’s major boundaries
Boundary between the outer liquid and solid inner core
Discovered in 1936 by Inge Lehman
mantlemantle
corecore
Inner coreInner core
…how?
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The inner core
3) Discovering Earth’s major boundaries3) Discovering Earth’s major boundaries
She discovered reflections (‘echoes’) of seismic waves
‘LehmanDiscontinuity’
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The Core
4) Earth’s magnetic field- Geodynamo4) Earth’s magnetic field- Geodynamo
Core behaves like adynamo and thus sustainsEarth’s magnetic field.
Polarity of Earth’s magneticfield reverses about every million years
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4) Earth’s magnetic field- Geodynamo4) Earth’s magnetic field- Geodynamo
Geographic and magnetic poles do not coincide!Fig. 21.11
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4) Earth’s magnetic field4) Earth’s magnetic field
See time-line of magnetic field reversals (paleomagnetic time-scale), Fig. 21.15 . Remember that magnetic reversals recorded in seafloor basalts were a major confirmation of seafloor spreading (Fig. Story 2.11).
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3 reasons for internal heat:
5) Earth’s internal heat engine5) Earth’s internal heat engine
1) radioactive decay of uranium, thorium, potassium
2) heat released as inner core crystallized
3) from colliding particles during Earth formation
Ways to transfer heat:
1) conduction - molecular activity
2) convection - movement (circulation) of material
TSP 17.13
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5) Earth’s internal heat engine5) Earth’s internal heat engine
Fig. 17.14 Convective flow in the mantle
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5) Earth’s internal heat engine5) Earth’s internal heat engine
The geotherm: Increase of temperature with depth
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Ch 21: Ch 21: Earth’s interiorEarth’s interior
What is the approximate distance from the surface to the center of the Earth?
A. 700 km B. 2900 kmC. 6400 kmD. 24,000 km
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Ch 21: Ch 21: Earth’s interiorEarth’s interior
What type of seismic wave is depicted by the ray path in the diagram?
A. a P waveB. an S waveC. a surface waveD. all of the above
*earthquake
seismograph
4. The two kinks in the seismic wave path are examples of seismic __________.A. isostasyB. reflectionC. refractionD. tomographyAnswer = C (page 485)
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Ch 21: Ch 21: Earth’s interiorEarth’s interior
The two kinks in the seismic wave path are examples of seismic __________.A. isostasyB. reflectionC. refractionD. tomography
*earthquake
seismograph
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Ch 21: Ch 21: Earth’s interiorEarth’s interior
Continental crust beneath mountains can be up to ___ kilometers thick
A. 10B. 40C. 70D. 100
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Ch 21: Ch 21: Earth’s interiorEarth’s interior
Which of the following statements is false?A. P waves travel slower in the crust than in the mantle.B. The crust is denser than the mantle.C. The crust-mantle boundary is called the Mohorovicic discontinuity.D. The oceanic crust consists of basalt and gabbro.Answer = B (page 487)
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Ch 21: Ch 21: Earth’s interiorEarth’s interior
Which of the following regions in the Earth consists primarily of olivine and pyroxene?
A. the crustB. the upper mantleC. the lower mantleD. the inner core
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Ch 21: Ch 21: Earth’s interiorEarth’s interior
Which of the following statements about the Earth’s core is true?A. The inner core and the outer core are both liquid.B. The inner core and the outer core are both solid.C. The inner core is liquid and the outer core is solid.D. The inner core is solid and the outer core is liquid.
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Ch 21: Ch 21: Earth’s interiorEarth’s interior
What drives plate tectonics?
A. erosionB. solar energy C. thermal conductionD. thermal convection