5. Evidence for Plate Tectonics from Magnetics William Wilcock

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5. Evidence for Plate Tectonics from
MagneticsWilliam Wilcock
OCEAN/ESS 410
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Lecture/Lab Learning Goals

• Understand the basic characteristics of the
  Earths magnetic field and how one measures its
  orientation
• Know the different kinds of rock magnetization
  and their use in paleomagnetism
• Be able to explain the historical concept of
  polar wander and its explanation in terms of
  continental drift
• Be able to explain patterns of marine magnetic
  anomalies in terms of plate spreading and
  magnetic field reversals
• Know how to interpret marine magnetic anomalies -
  LAB

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Earths Magnetic Field
Magnetic north
north pole
Geographic north pole
The Earth is surrounded by a magnetic field that
is strongest near the poles. The magnetic poles
are displaced 11.5 from the geographic poles
about which the Earth rotates.
Geodynamo Theory The magnetic field is generated
in the liquid metal region of the outer core.
The outer core is extremely hot and flows at a
rate of several km/yr in large convection
currents. Convecting metal (Fe) creates
electrical currents, which in turn create the
magnetic field.
Magnetic south
south pole
Geographic south Pole
south pole
After Plummer
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Earths Magnetic Field

• The Earths magnetic field close to a dipole. 
  The radial (vertical) and tangential
  (north-south) components a dipole field are given
  by

• ? - Colatitude (0º at south pole 90º at equator
  180º at north pole
• µ0 - magnetic permeability of a vacuum 4p x 10-7
  N A-2
• r - distance to the center of the earth (6.4 x
  106 m at the Earths surface)
• M - is the dipole moment which for the earth is
  7.95 x 1022 A m2
• B - is the magnetic field. It units are Teslas 1
  T 1 kg A-1 s-2. 1 nT 10-9 T 1 Gamma

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Earths Magnetic Field
From The way the Earth Works by P. J. Wyllie,
Wiley 1976
Field is twice as strong at the poles as at the
equator. About 60,000 ? at poles and 30,000 ? at
equator
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Measuring the Orientation of the Earths Magnetic
Field
D
I
D Declination (angle from geographic north) I
Inclination (dip angle)
From The way the Earth Works by P. J. Wyllie,
Wiley 1976
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Measurements of the Earths Magnetic Field in the
Oceans
Measurements of the Earths magnetic field in the
oceans were developed in the 2nd World War as a
way to detect submarines (and later mines)
Measurements of the magnetic field were first
made with a fluxgate magnetometer. Such
instruments are still in use today
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Rock Magnetization

• Most minerals either repel or concentrate the
  Earths magnetic field lines but do not
  themselves become magnetized.
• A few ferromagnetic minerals retain
  magnetization. In the oceanic crust the most
  important is magnetite (Fe3O4). Others include
  ilmenite (FeTiO3), hematite (Fe2O3), and
  pyrrhotite (FeS).
• Forms of rock magnetism
• Thermo remnant magnetism - rock becomes
  magnetized when it cools below the Currie
  temperature (580C) in a magnetic field
• Detrital remnant magnetism - sediments settle in
  a magnetic field
• Chemical remnant magnetism - Hematite
  precipitates from a fluid circulating through a
  rock.

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Paleomagnetism

• In the 1950s scientists learned how to measure
  the remnant magnetism of rock samples. If one
  can be sure that the rock has not been rotated by
  tectonic processes then
• The Declination of the remnant magnetism gives
  the apparent direction of the North Pole at the
  time the rock formed.
• The Inclination gives the latitude of the rock
  when it formed

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Geochronology
In the 1950s scientists also developed reliable
techniques of dating rocks using radioactive
isotopes The potassium isotope 40K decays to 40Ar
with a half-life of 1.3x109 years. As argon is a
gas any traces of that element will escape from
rocks when they are molten. Therefore, any argon
found in solid rocks must have been produced
since that molten state ended and the rock
solidified. The ratio of 40K to 40Ar can be
analyzed and a numerical date since the last
molten state can be assigned. By combining
paleomagnetic data from lava flows with the lava
ages, scientists were able to look at changes in
the apparent position of the Earths magnetic
pole with time.
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Polar Wander

1. Position of the North pole relative to Europe and
   Asia
2. Position of the North pole relative to Eurasia
   and North America

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Opening of the Atlantic
http//museum.gov.ns.ca/fossils/geol/globe.htm
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Polar Wander and Continental Drift
K - 100 Myr Tru - 200 Myr Cu - 300 Myr - 500
Myr

1. Polar wander for North America and Eurasia
2. Polar wander corrected for the opening of the
   Atlantic

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Evidence for Continental Drift - pre1960s

• Evidence
• Fit of the Atlantic Coastlines and Geology
• Paleontology (Fossils)
• Paleoclimate
• Paleomagnetism
• Why wasnt this evidence accepted?
• Physical impossibility of drift (the mantle is
  solid - it transmits seismic waves)
• Difficulties of magnetic measurements - scatter,
  reversals
• Conservatism

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Polarity Reversals
The mechanism of polarity reversals is poorly
understood but they happen quickly (within no
more than 1000 years)
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Using volcanic rocks to develop a polarity
timescale
Most geoscientists were initially skeptical of
magnetic reversals but interest increased once it
was realized that they provided a means to date
events
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Polarity timescale from magnetized lava flows
The first timescales were obtained in the early
1960s
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History of Polarity Reversals
Cretaceous Quiet Zone
Jurassic Quiet Zone (a period of very rapid
reversals?)
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Marine magnetic anomalies
The magnetization of the oceanic crust leads to
small variations in the intensity of the magnetic
field measured at the sea surface
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Marine Magnetic Anomalies
If we remove the background Earths magnetic
field from the total magnetic intensity, we
obtain the magnetic anomaly
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Relationship Between Magnetic Anomalies and the
Polarity of the Crust
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Magnetic Stripes
Raff and Mason, 1961
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Vine and Matthews Magnetic Tape Recorder
N
Normally magnetized crust
dikes
oceanic crust
Magma
Reversely magnetized crust
N
N
Magma
N
N
Normally magnetized crust
N
Magma
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Vine and Matthews magnetic tape recorder
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Global bathymetry, showing ocean ridge system
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Location of the Eltanin-19 profile

Ship track across the East Pacific Rise which
obtained the magnetic anomaly profile shown in
the next slide. The measurements were made in the
1960s by the Columbia University research vessel
Eltanin.
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Eltanin 19 Magnetic Anomaly Profile
The vertical scale for total intensity anomaly is
shown in gammas. This is the same as nanoTeslas
or nT. The horizontal lines are at zero anomaly
the scale is thus minus 500 to plus 500 nT.
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Symmetry of the Eltanin 19 profile
ESE
WNW
WNW
ESE
measured profile of total intensity anomalies
mirror image of measured profile to show symmetry
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Polarity Reversals and Spreading Rate
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Polarity Reversals and Sedimentation Rates
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Age of the Seafloor