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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 Dupr University of Houston Chapter 19 – PowerPoint PPT presentation

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Title: Classroom presentations to accompany Understanding Earth, 3rd edition


1
Classroom presentations to accompany
Understanding Earth, 3rd edition
  • prepared by
  • Peter Copeland and William Dupré
  • University of Houston

Chapter 19 Exploring Earths Interior
2
Exploring Earths Interior
3
Structure 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.

4
Refraction and Reflection of a Beam of Light
Refraction
Reflection
Fig. 19.1
5
P-wave Shadow Zone
Fig. 19.2a
6
S-wave Shadow Zone
Fig. 19.2b
7
P-and S-wave Pathways Through Earth
Fig. 19.3
8
Seismograph Record of P, PP, S, and Surface Waves
Fig. 19.4
9
Changes in P-and S- wave Velocity Reveal Earths
Internal Layers
Fig. 19.5
10
Structure of the Earth
  • Study of the behavior of seismic waves tells
  • us about the shape and composition of the
  • interior of the Earth
  • Crust 1070 km, intermediate composition
  • Mantle 2800 km, mafic composition
  • Outer core 2200 km, liquid iron
  • Inner core 1500 km, solid iron

11
Composition of the Earth
  • Seismology tells us about the density
  • of rocks
  • Continental crust 2.8 g/cm3
  • Oceanic crust 3.2 g/cm3
  • Asthenosphere 3.3 g/cm3

12
Isostasy
  • 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

13
The less dense crust floats on the less
buoyant, denser mantle
Mohorovicic Discontinuity (Moho)
Fig. 19.6
14
Crust as an Elastic Sheet
Continental ice loads the mantle
Ice causes isostatic subsidence
Melting of ice causes isostatic uplift
Return to isostatic equilibrium
15
Structure of the Crust and Upper Mantle
Fig. 19.7
16
Earths internal heat
  • Original heat
  • Subsequent radioactive decay
  • Conduction
  • Convection

17
Upper Mantle Convection as a Possible Mechanism
for Plate Tectonics
Fig. 19.8
18
Seismic Tomography Scan of a Section of the Mantle
Subducted slab
Fig. 19.9
19
Temperature vs. Depth
Fig. 19.10
20
Paleomagnetism
  • Use of the Earth's magnetic field to investigate
    past plate motions
  • Permanent record of the direction of the Earths
    magnetic field at the time the rock was formed
  • May not be the same as the present magnetic field

21
Magnetic Field of the Earth
Fig. 19.11
22
Magnetic Field of a Bar Magnet
Fig. 19.11
23
Use 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).

24
Earth'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

25
Evidence of a Possible Reversal of the Earths
Magnetic field
Fig. 19.12
26
Earth's magnetic field
  • declination horizontal angle between
    magnetic N and true N
  • inclination angle made with horizontal

27
Earth'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).

28
The Earth's magnetic field
  • Since the geothermal gradient in the Earth is
    25C/km, nothing can be permanently magnetized
    below about 30 km.
  • Another explanation is needed.

29
Magnetic Field of the Earth
Fig. 19.11
30
Self-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.

31
Self-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 fielda
    self-exciting dynamo!

32
Self-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.

33
Self-exciting dynamo
  • If the details seem vague, it is
  • because we have a poor
  • understanding of core dynamics.

34
Magnetic 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).

35
Magnetic 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.

36
Magnetic 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

37
Self-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 KAr)
    it has been shown that all rocks of a particular
    age have the same magnetic signature.

38
Recording the Magnetic Field in Newly Deposited
Sediment
Fig. 19.13
39
Lavas Recording Reversals in Earths Magnetic
Field
Fig. 19.14
40
Magnetic reversals
  • We can now use the magnetic
  • properties of a sequence of rocks to
  • determine their age.

41
The Geomagnetic Time 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.
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