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
Classroom presentations Classroom presentations to accompany to accompany
Understanding EarthUnderstanding Earth, 3rd edition, 3rd edition
prepared by
Peter Copeland and William Dupré
University of Houston
Chapter 19Chapter 19Exploring Earth’s Interior
Exploring Earth’s Exploring Earth’s InteriorInterior
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.
Fig. 19.1
Refraction
Reflection
Refraction and
Reflection of a
Beam of Light
Fig. 19.2a
P-wave P-wave Shadow Shadow
ZoneZone
Fig. 19.2b
S-wave S-wave Shadow Shadow
ZoneZone
Fig. 19.3
P-and S-wave Pathways Through P-and S-wave Pathways Through EarthEarth
Seismograph Record of Seismograph Record of P, PP, S, and Surface WavesP, PP, S, and Surface Waves
Fig. 19.4
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
Structure of the EarthStructure of the EarthStudy of the behavior of seismic waves tells
us about the shape and composition of the
interior 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
Composition of the EarthComposition of the Earth
Seismology 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
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
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)
Crust as an Elastic SheetCrust as an Elastic Sheet
Continental ice loads the mantle
Ice causes isostatic subsidence
Melting of ice causes isostatic uplift
Return to isostatic equilibrium
Structure Structure of the of the
Crust and Crust and Upper Upper MantleMantle
Fig. 19.7
Earth’s internal heatEarth’s internal heat
• Original heat
• Subsequent radioactive decay
• Conduction
• Convection
Fig. 19.8
Upper Mantle Convection as a Upper Mantle Convection as a Possible Mechanism for Plate Possible Mechanism for Plate
TectonicsTectonics
Fig. 19.9
Seismic Tomography Scan of a Seismic Tomography Scan of a Section of the MantleSection of the Mantle
Subducted slab
Fig. 19.10
Temperature Temperature vsvs. Depth. Depth
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
Fig. 19.11
Magnetic Magnetic Field of Field of
the Earththe Earth
Fig. 19.11
Magnetic Magnetic Field of a Field of a
Bar Bar MagnetMagnet
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).
Earth's magnetic fieldEarth's magnetic field
The 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
Evidence of a Possible Reversal Evidence of a Possible Reversal of the Earth’s Magnetic fieldof the Earth’s Magnetic field
Fig. 19.12
Earth's magnetic fieldEarth's magnetic field
declination: horizontal angle between magnetic N and true N
inclination: angle made with horizontal
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).
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.
Fig. 19.11
Magnetic Magnetic Field of the Field of the
EarthEarth
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.
Self-exciting dynamoSelf-exciting dynamo
It 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!
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.
Self-exciting dynamoSelf-exciting dynamo
If the details seem vague, it is
because we have a poor
understanding of core dynamics.
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).
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.
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
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.
Fig. 19.13
Recording the Magnetic Field in Recording the Magnetic Field in Newly Deposited SedimentNewly Deposited Sediment
Lavas Recording Reversals in Lavas Recording Reversals in Earth’s Magnetic FieldEarth’s Magnetic Field
Fig. 19.14
Magnetic reversalsMagnetic reversals
We can now use the magnetic
properties of a sequence of rocks to
determine their age.
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.