PHYSICAL PROPERTIES of the EARTHS INTERIOR: MAGNETIC FIELD

1
PHYSICAL PROPERTIES of the EARTHS INTERIOR
MAGNETIC FIELD

• Magnetic theory
• Classification of materials based on magnetic
  properties
• Magnetic properties of minerals
• Magnetic properties of rocks
• Natural remanant magnetism
• Magnetism of the Earth
• Nature of the geomagnetic field
• The external magnetic field
• Local magnetic anomalies

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Magnetic theory

• Coulombs Law
• Magnetic Force (F)
• F Magnetic Force (dynes)
• p1 p2 The poles of strength
• ? Magnetic permeability (a property of the
  medium)
• r Distance (cm)
• r1 Unit vector

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• Magnetizing field (H) Magnetic Field Strength
• H Oersted (dynes/unit pole) cgs unit
• 1? 10-5 Oersted

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• Ampères Law (Biot-Savart Law)
• ?H Magnetizing field
• I Electrical current
• ? l Length of conductor
• r Distance
• r1 Unit vector
• H Ampere/meter- SI unit (4?x10-3 Oersted- cgs
  unit)

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• Magnetic dipole moment
• ?????? 2l ??????
• p () p(-)
• m2lpr1
• m A vector in the direction of r1
• Magnetic polarization (M) Magnetization
  intensity
• MkH
• k Magnetic susceptibility (magnetic response of
  rocks and minerals)

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• Magnetic induction (B) total field
• B?o(HM)
• B?oH (the Earths magnetic induction)?oM (the
  rocks magnetic induction)
• ?o Permeability of free space (4?x10-7 Wb/A-m)
• B ? Tesla1 Newton/ampere-meter1 weber/m2 - SI
  unit
• Gauss10-4 Tesla
• 1 ?10-9 Tesla1nT (nanoTesla) emu-unit

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• Magnetic flux or magnetic lines of force ?
• ? B.A
• B The density of magnetic flux
• A A vector area
• ? Weber (T-m2) - SI unit
• Maxwell10-8 Weber - emu-unit

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• Classification of materials based on magnetic
  properties
• Magnetic phenomena arises from the motion of
  electrically charged particles
• (electrons and molecules) within the substance.
• MkH
• M Strength of induced magnetization (dipole
  moment/unit volume)
• H Strength of applied magnetic field
• k Magnetic susceptibility of applied magnetic
  field (dimensionless)
• F Ferromagnetic
• P Paramagnetic
• D Diamagnetic

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• Diamagnetic (complete electron shells)
• H0 ? Magnetic moment0 k lt 0 (Temp.
  independent)
• Graphite, marble, qtz
• For rock forming minerals k? -10-5
• Paramagnetic (incomplete electron shells)
• H0 ? Magnetic moment?0 k gt 0 (Temp. dependent)
  10-4-10-2
• For rock forming minerals k? 10-4-10-2
• Ferromagnetic (coupled electron shells)
• H0 ? Magnetic moment?0 k gtgt 0
• Iron, Nickel, Cobalt

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• Orientation of magnetic moments
• External moments Macro Zero
  Some
• (Schön, 1998)

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• Magnetic hysteresis curve
• The shape of curve and magnitude of parameters
  intrinsic properties
• of ferri magnetic components, grain size and
  internal stress
• Schematic Volcanic rock
• (Schön, 1998)
• Ms Saturation magnetization
• Mr Remanent magnetization
• Hc Coercitive field strength

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• TiO2 Rutile FeO Wüstite Fe2O3 Hematite
• a Pseudobrookite (igneous and metamorphic)
• b Ilmenite-hematite (igneous metamorphic
  rocks)
• c Titanomagnetite (igneous rocks)
• d Titanomaghemite (basaltic oceanic basement,
  continental igneous
• rocks)
• Tcruie ? ? Oxidation ratio ?
• (Schön, 1998)

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• Magnetic properties of minerals
• Paramagnetic minerals
• k ? large Fe2 Fe3

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• Diagmagnetic minerals
• k ? - small
• (Schön, 1998)

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• Ferro Ferri magnetic minerals
• (Schön, 1998)

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• Magnetic properties of rocks
• Magnetic properties of rocks (variable within a
  rock type)
• Chemical inhomogenity
• Depositional and/or crystallization
• Post formational conditions
• Mineral constituents and their fractions
• Most abundant minerals in common rocks are
  paramagnetic or diamagnetic.
• Magnetic rock properties are controlled by the
  ferrimagnetic minerals (10 vol).
• Igneous Rocks ? Fe-Ti system (dominant)
• Sedimentary Rocks ? Fe-hydroxides (dominant)

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• k of magmatic rocks ? from acid to basic rocks.
• k of sedimentary ? clay content ?
• k of igneous rocks is influenced by specific
  genetic conditions.
• k is affected by alteration.
• k magnetic minerals and their contents
• k grain size shape
• k stress
• k temperature

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• The mineral contribution to the susceptibility of
  a rock
• (Schön, 1998)

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• (Schön, 1998)

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• (Schön, 1998)

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• Natural Remanant Magnetism (NRM)
• The total magnetization M of any rock is the sum
  of two vectors
• Induced magnetization ( external field)
• Remanent magnetization
• Types of NRM
• Chemical Remanent Magnetization (CRM)
• Depositional Remanent Magnetization (DRM)
• Thermo Remanent Magnetization (TRM)
• Others
• Isothermal Remanent Magnetization (IRM)
  lightining
• Viscous Remanent Magnetization (VRM) time
• Piezo Remanent Magnetization (PRM) pressure

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• Chemical Remanent Magnetization (CRM)
• CRM occurs during the formation of a magnetic
  mineral
• (origin growing process)
• oxidation of magnetite to hematite or maghemite
• oxidation of titanomagnetic to titanomaghemite
• dehydration of iron hydroxide to hematite
• precipitation of ferrimagnesian minerals
• recrytallisation of ferrimagnetic minerals below
  Tc
• Ocean floor basalts CRM

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• Depositional Remanent Magnetization (DRM)
• DRM in sediments
• DRM depositional environment sediment type
• (Schön, 1998)

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• Thermo Remanent Magnetization (TRM)
• TRM grain size ? ? magnitude of TRM ?
• applied field
• TRM has an important application in the study of
  paleomagnetism.
• (Cox, 1986)

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• Magnetism of the Earth
• Disk Dynamo As a rotating of copper moves
  across magnetic field
• lines, a voltage is induced in the disk which
  drives current through
• the system. This current generates a magnetic
  field which reinforces
• the initial weak field.
• (Cox, 1986)

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• Treversals ? 500 000 years
• Presently, 700 000 years
• (Plummer McGeary, 1991)

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• Origin of the main field
• Convection currents in the liquid outer core
  (2800 - 5000 km)
• Iron, nickel (good electrical conductors)
• Self-excited dynamo
• Changes in convection currents in the core ?
  Secular variations of the main field.
• For 100s of years I 75º ? 65
• D 10ºE ? 25W
• Changes in the direction of the Earths
  magnetic field.

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• Elements of the Earths Magnetic Field
• (Telford, et al., 1990)

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• Fe2He2Ze2Xe2Ye2Ze2
• HeFeCosI
• ZeFeSinI
• XeHeCosD
• YeHeSinD
• D Declination I Inclination (dip)

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• (Cox, 1986)

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• (Cox, 1986)

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• (Smith, 1981)

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• (Smith, 1981)

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• The red contours are lines of equal field
  strength of 10-9 tesla.
• The blue contours are lines of equal rate of
  change of field strength
• in units ?10-9 tesla per year (Smith, 1981)

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• Nature of the geomagnetic field
• Secular variation of the declination and
  inclination at London since 1570
• (Bott, 1982)

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• The changes in length of day changes in the
  geomagnetic secular variation
• electromagnetic coupling between the outermost
  core and mantle and core
• mantle boundary

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• The external magnetic field
• The geomagnetic field of the Earth
• The main field (internal origin) 99
• A small field (external origin)
• Very small field (near surface crust)
• The external field electric currents in the
  ionized layers of the upper
• atmosphere
• Time variations of the external field
• T11 years sunspot activity
• T24 h, 30nT solar diurnal variations and
  solar wind
• T25 h, 2nT Lunar variations
• (moon-ionosphere interactions )
• Transient disturbances 1000nT Magnetic
  storms, Aurora sunspot activity
• Magnetic storms affect the magnetic prospecting.

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• (Bott, 1982)

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(Bott, 1982)
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(Bott, 1982)
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(Smith, 1981)
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• There is an interaction between the geomagnetic
  field and the solar
• wind.
• Solar wind compresses the geomagnetic field on
  the side of the Earth
• towards the Sun.
• The field is confined to a zone known as
  magnetosphere.
• The boundary of magnetosphere is called
  magnetopause.

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Polar wandering

• The study of ancient magnetic fields is called
  paleomagnetism.
• Apparent Polar wandering
• Assume that the Earths axis of rotation has
  remained fixed and
• plot the post position of the continent assuming
  values of
• the longitude.
• Or
• Assume that the continent has fixed in position
  and estimate position
• of poles from the ancient dip and declination.

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• (Plummer and McGeary, 1991)

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• (Plummer and McGeary, 1991)

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(Smith, 1981)
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Marine magnetic anomalies

• (Plummer and McGeary,1991)

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(Bott, 1982)
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Local magnetic anomalies

• (Plummer and McGeary, 1991)

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• Local magnetic anomalies magnetic mineral
  content of near surface
• rocks
• The sources of local magnetic anomalies can not
  be very deep
• because the temperature below 40 km should be
  above the Curie
• point (550º C) at which rocks lose their magnetic
  properties.
• The local anomalies features in the upper
  crust.

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REFERENCES

• Bott, M.H.P., 1982, The Interior of the Earth
  its structure, constitution and
• evolution, Elsevier, p 175-178, 248-265 (ITU
  Mustafa Inan Library, QE 28.2.B68).
• Cox, A. and Hart, R.B., 1926, Plate tectonics
  How it works, Blackwell Scientific
• pub. P. 263-292.
• Plummer C.C. and McGeary, D. 1991, Physical
  Geology, Wm.C. Brown Pub.,
• p 385-387 (ITU Mustafa Inan Library, QE 28.2
  .P58).
• Press and Siever, 1997, Understanding Earth, W.H.
  Freeman Company, p 498-503
• (ITU Mustafa Inan Library, QE 28.P74 ).
• Schön, J.H., 1998, Handbook of Geophysical
  Exploration, Seismic Exploration, V 18
• Physical Properties of rocks Fundamentals and
  Principles of Petrophysics,
• Pergamon press, p 79, 82, 83, 85, 89, 90 (ITU
  Mustafa Inan Library,
• 431.6 P5 S34 1998).
• Smith, D.G., 1981, The Cambridge Encyclopedia of
  Earth Sciences,Cambridge Univ
• Press, p 110, 111, 112, 113, 114, 118, 119, 123.