classroom presentations to accompany understanding earth , 3rd edition
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Classroom presentations to accompany Understanding Earth , 3rd edition. prepared by Peter Copeland and William Dupré University of Houston. Chapter 19 Exploring Earth’s Interior. Exploring Earth’s Interior. Structure of the Earth. - PowerPoint PPT PresentationTRANSCRIPT
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Classroom presentations Classroom presentations to accompany to accompany
Understanding EarthUnderstanding Earth, 3rd edition, 3rd edition
prepared byPeter Copeland and William Dupré
University of Houston
Chapter 19Chapter 19Exploring Earth’s Interior
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Exploring Earth’s Exploring Earth’s InteriorInterior
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Structure of the EarthStructure of the Earth• Seismic velocity depends on the
composition of material and pressure.
• We can use the behavior of seismic waves to tell us about the interior of the Earth.
• When waves move from one material to another they change speed and direction.
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Fig. 19.1
Refraction
Reflection
Refraction and
Reflection of a
Beam of Light
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Fig. 19.2a
P-wave P-wave Shadow Shadow
ZoneZone
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Fig. 19.2b
S-wave S-wave Shadow Shadow
ZoneZone
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Fig. 19.3
P-and S-wave Pathways Through P-and S-wave Pathways Through EarthEarth
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Seismograph Record of Seismograph Record of P, PP, S, and Surface WavesP, PP, S, and Surface Waves
Fig. 19.4
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Changes Changes in P-and S- in P-and S-
wave wave Velocity Velocity Reveal Reveal Earth’s Earth’s Internal Internal LayersLayers
Fig. 19.5
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Structure of the EarthStructure of the EarthStudy of the behavior of seismic waves tellsus about the shape and composition of theinterior of the Earth:
• CrustCrust: ~10–70 km, intermediate composition
• MantleMantle: ~2800 km, mafic composition
• Outer coreOuter core: ~2200 km, liquid iron
• Inner coreInner core: ~1500 km, solid iron
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Composition of the EarthComposition of the EarthSeismology tells us about the density
of rocks:
• Continental crustContinental crust: ~2.8 g/cm3
• Oceanic crustOceanic crust: ~3.2 g/cm3
• AsthenosphereAsthenosphere: ~3.3 g/cm3
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IsostasyIsostasy
• Buoyancy of low-density rock masses “floating on” high-density rocks; accounts for “roots” of mountain belts
• First noted during a survey of India
• Himalayas seemed to affect plumb
• Two hypotheses: Pratt and Airy
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The less dense crust “floats” on The less dense crust “floats” on the less buoyant, denser mantlethe less buoyant, denser mantle
Fig. 19.6
MohorovicicDiscontinuity
(Moho)
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Crust as an Elastic SheetCrust as an Elastic SheetContinental ice loads the mantle
Ice causes isostatic subsidence
Melting of ice causes isostatic uplift
Return to isostatic equilibrium
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Structure Structure of the of the
Crust and Crust and Upper Upper MantleMantle
Fig. 19.7
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Earth’s internal heatEarth’s internal heat
• Original heat
• Subsequent radioactive decay
• Conduction
• Convection
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Fig. 19.8
Upper Mantle Convection as a Upper Mantle Convection as a Possible Mechanism for Plate Possible Mechanism for Plate
TectonicsTectonics
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Fig. 19.9
Seismic Tomography Scan of a Seismic Tomography Scan of a Section of the MantleSection of the Mantle
Subducted slab
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Fig. 19.10
Temperature Temperature vsvs. Depth. Depth
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PaleomagnetismPaleomagnetism• Use of the Earth's magnetic field to
investigate past plate motions
• Permanent record of the direction of the Earth’s magnetic field at the time the rock was formed
• May not be the same as the present magnetic field
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Fig. 19.11
Magnetic Magnetic Field of Field of
the Earththe Earth
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Fig. 19.11
Magnetic Magnetic Field of a Field of a
Bar Bar MagnetMagnet
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Use of magnetism in geologyUse of magnetism in geology
Elements that have unpaired electrons (e.g., Fe, Mn, Cr, Co) are effected by a magnetic field. If a mineral containing these minerals cools below its Currie temperature in the presence of a magnetic field, the minerals align in the direction of the north pole (also true for sediments).
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Earth's magnetic fieldEarth's magnetic fieldThe Earth behaves as a magnet whose
poles are nearly coincident with the spin axis (i.e., the geographic poles).
Magnetic lines of force emanate from the magnetic poles such that a freely suspended magnet is inclined upward in the southern hemisphere, horizontal at the equator, and downward in the northern hemisphere
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Evidence of a Possible Reversal Evidence of a Possible Reversal of the Earth’s Magnetic fieldof the Earth’s Magnetic field
Fig. 19.12
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Earth's magnetic fieldEarth's magnetic field
declination: horizontal angle between magnetic N and true N
inclination: angle made with horizontal
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Earth's magnetic fieldEarth's magnetic field
It was first thought that the Earth's magnetic field was caused by a large, permanently magnetized material deep in the Earth's interior.
In 1900, Pierre Currie recognized that permanent magnetism is lost from magnetizable materials at temperatures from 500 to 700 °C (Currie point).
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The Earth's magnetic fieldThe Earth's magnetic field
Since the geothermal gradient in the Earth is ≈ 25°C/km, nothing can be permanently magnetized below about 30 km.
Another explanation is needed.
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Fig. 19.11
Magnetic Magnetic Field of the Field of the
EarthEarth
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Self-exciting dynamoSelf-exciting dynamo
A dynamo produces electric current by moving a conductor in a magnetic field and vise versa. (i.e., an electric current in a conductor produces a magnetic field.
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Self-exciting dynamoSelf-exciting dynamoIt is believed that the outer core is in
convective motion (because it is liquid and in a temperature gradient).
A "stray" magnetic field (probably from the Sun) interacts with the moving iron in the core to produce an electric current that is moving about the Earth's spin axis yielding a magnetic field—a self-exciting dynamo!
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Self-exciting dynamoSelf-exciting dynamo
The theory has this going for it:
• It is plausible.
• It predicts that the magnetic and geographic poles should be nearly coincident.
• The polarity is arbitrary.
• The magnetic poles move slowly.
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Self-exciting dynamoSelf-exciting dynamo
If the details seem vague, it is
because we have a poor
understanding of core dynamics.
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Magnetic reversalsMagnetic reversals• The polarity of the Earth's magnetic
field has changed thousands of times in the Phanerozoic (the last reversal was about 700,000 years ago).
• These reversals appear to be abrupt (probably last 1000 years or so).
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Magnetic reversalsMagnetic reversals
• A period of time in which magnetism is dominantly of one polarity is called a magnetic epoch.
• We call north polarity normal and south polarity reversed.
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Magnetic reversalsMagnetic reversals
• Discovered by looking at magnetic signature of the seafloor as well as young (0-2 Ma) lavas in France, Iceland, Oregon and Japan.
• When first reported, these data were viewed with great skepticism
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Self-reversal theorySelf-reversal theory
• First suggested that it was the rocks that had changed, not the magnetic field
• By dating the age of the rocks (usually by K–Ar) it has been shown that all rocks of a particular age have the same magnetic signature.
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Fig. 19.13
Recording the Magnetic Field in Recording the Magnetic Field in Newly Deposited SedimentNewly Deposited Sediment
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Lavas Recording Reversals in Lavas Recording Reversals in Earth’s Magnetic FieldEarth’s Magnetic Field
Fig. 19.14
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Magnetic reversalsMagnetic reversals
We can now use the magnetic
properties of a sequence of rocks to
determine their age.
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The GeomagneticThe GeomagneticTime ScaleTime Scale
Based on determining the magnetic characteristics of rocks of known age (from both the oceans and the continents).
We have a good record of geomagnetic reversals back to about 60 Ma.