Geo,paleomagnetism
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  • 1. MAGNETICS Introduction Geo & Paleo Magnetism
  • 2. Role of Geo & Paleo in Geophysics & Geology Like other geophysical methods, magnetism is also divided into Applied & Paleo areas. • Geomagnetism deals with the exploration of minerals, basement under sedimentary column (oil industry), salt domes, igneous bodies in the subsurface, groundwater in igneous terrain. • Paleomagnetism deals with the history of magnetic poles/polarity, history of rocks, and plate tectonics.
  • 3. Features of a magnetic Bar • Magnetic bar has two magnetic poles: S N North pole or +ve pole, and South pole or –ve pole. • The poles are located 1/12 of bar length inside from the end. • Magnetic bar is surrounded by a magnetic field produced by magnetic lines of force which flow from north to south pole. • If an iron needle goes in the magnetic field or touches the pole, the needle is magnetized and starts behaving as a tiny magnet. • Bar magnet behaves as a Line magnet.
  • 4. Magnetic Parameters • Magnetic moment(M)=2L*m, • • • • • where ‘L’ is half the distance between two poles, ‘m’ is the magnetic poles. If ‘L’ is reduced to infinitesimal ds, then bar is converted into Magnetic Particle, its magnetic moment is M= m*ds, If magnetic bars are placed end-on or side by side, then M=n*m*2L Unit magnetic pole when is placed 1 cm away from similar pole, is acted upon by a force of 1 dyne. Unit pole is also equivalent to 4π (12.5667) lines of force. A magnetic pole of strength ‘m’ will generate 4π*m lines of force.
  • 5. Magnetic Permeability • Lord Kelvin defined Permeability as the ease with which a magnetic flux can be established in a body. • Permeability is a ratio between the magnetic flux through a unit cross sectional area of a body and the flux through a like unit cross sectional area of the air. µ=(φ/A)/H= B/H --------- µ for space (air) is 4π*10‾7 and is indexed as 1.
  • 6. Magnetic Susceptibility • This is an ability of a magnetic material to be magnetized. The intensity of magnetization ‘I’ depends also on magnetizing field ‘H’. • The intensity of magnetization, I, is related to the strength of the inducing magnetic field, H, through a constant of proportionality, k, known as the magnetic susceptibility.
  • 7. Magnetic Lines of Force Associated with Magnetic Dipoles • The force associated with this fundamental element of magnetism, the magnetic dipole, now looks more complicated than the simple force associated with gravity. Notice how the arrows describing the magnetic force appear to come out of the monopole labeled N and into the monopole labeled S
  • 8. Magnetic Lines of Force Associated with Magnetic Dipoles • Mag Bar
  • 9. Earth’s magnetism: due to electric currents in liquid outer core 11.5º magnetic dipole [the source producing the magnetic field is far from where were are measuring its field; within the core] Magnetic field is measured (units): in Teslas [T, too large]  nanoTesla (nT, 10-9 T) 9
  • 10. Earth’s Mag field 10
  • 11. 11
  • 12. Magnetic Field Nomenclature • At any point on the Earth's surface, the magnetic field, F*, has some strength and points in some direction. The following terms are used to describe the direction of the magnetic field. • Declination - The angle between north and the horizontal projection of F. This value is measured positive through east and varies from 0 to 360 degrees. • Inclination - The angle between the surface of the earth and F. Positive inclinations indicate F is pointed downward, negative inclinations indicate F is pointed upward. Inclination varies from -90 to 90 degrees
  • 13. Total mag intensity • F 14
  • 14. Magnetic Inclination 15
  • 15. Magnetic declination 16
  • 16. 10th Generation IGRF Results Field Model Results Location Latitude Longitude Altitude Karachi Date 25 degs N 67 degs E 0.00 km 2008 Component Field Value Secular Variation Declination 0.528 degrees 2.6 arcmin/year Inclination 38.376 degrees 6.3 arcmin/year Horizontal Intensity 35170 nT 2.2 nT/year North Component 35169 nT 2.0 nT/year East Component 324 nT 26.1 nT/year Vertical Intensity 27852 nT 107.4 nT/year Total Intensity 44863 nT 68.4 nT/year
  • 17. paths of magnetic field Earth’s Magnetic Field [iron filings on a sheet over a magnet] bar magnet coil where rotation axis intersects the surface 18
  • 18. Ampere’s law & mag 19
  • 19. Mag Units • Parameter • • • • • • • • • Magnetic moment (m) Magnetization (M) Magnetic Field (H) Magnetic Induction (B) Permeability of free space (μ0) Susceptibility (χ) total (m/H) by volume (M/H) by mass (m/m ·1/H) • 1 H = kg m2A−2s−2, SI unit Am2 Am −1 Am −1 T Hm−1 m3 m3kg−1 1 emu = 1 G cm3, cgs unit Conversion emu emu cm−3 Oersted (oe) Gauss (G) 1 1 A m2 = 103 emu 1 Am−1 = 10−3 emu cm−3 1 Am−1 = 4π x 10−3 oe 1 T = 104 G 4π x 10−7 Hm−1 = 1 emu oe−1 1 m3 = 106 4π emu oe−1 emu cm−3 oe−1 1 S.I. = 1/4π emu cm−3 oe−1 3 −1 emu g−1 oe−1 1 m kg = 103 4π emu g−1 oe−1 B = μo(H +M), 1 T = kg A−1 s−2 20
  • 20. lines of magnetic field intersect the Earth’s surface at an angle magnetic latitude (inclination) & direction to north (declination) are easily found, but magnetic LONGITUDE can not be deduced due to the symmetry of the magnetic field about its axis magnetic declination: [at a certain location] the difference (angle) between geographic/true north and magnetic north [azimuth of horizontal component of magnetic field] (degrees east or west of true north) magnetic inclination: [at the Earth’s surface] the angle between the magnetic field 21 and the horizontal (degrees, +90º to 0º to –90º)
  • 21. Fundamentals of Magnetism magnetic dipole created by rotation of an electron around atom’s nucleus [dipole is presented by an arrow pointing NS] non-magnetic material: atoms tilt all different ways, so dipoles cancel each other out [zero net magnetization] permanent magnet: dipole lock into alignment, so they add to each other and produce a strong cumulative magnetization non-magnetic material permanent magnet 22
  • 22. Magnetism of Rocks remanent magnetisation: the ability to retain magnetisation in the absence of a field or in the presence of a different magnetic field alignment and hence magnetisation disappears as soon as the field is removed directions of the magnetisations of the magnetic atoms spontaneous align [iron materials & its compounds, e.g. magnetite] • Most rocks contain some ferromagnetic minerals [compounds of iron] • the atomic magnets of tiny ferromagnetic crystals or grains are aligned along one of the crystallographic directions (called easy axes) and the grains have strong magnetisation for their size • if a magnetic field is applied the individual grain magnetisations will each tend to rotate into an easy axis closer to that of the field and in this way obtain a remanence 23
  • 23. Mag minerals classification 24
  • 24.  one way to magnetise a rock is by applying a magnetic field  another way is by heating [usually to demagnetise a rock sample] Curie & blocking temperatures progressive thermal demagnetisation Blocking temperature: a range of temperatures characteristic for individual minerals to be thermally demagnetised Curie temperature or Curie point for a specific material: temperature above which the material becomes paramagnetic [the individual atomic magnets cease to align with one another, and the spontaneous magnetisation 25 necessary for ferromagnetism disappears]
  • 25. Intensity of Magnetization 26
  • 26. Mag susceptibility 27
  • 27. rock magnetism acquired at a [πάλαιο, greek] =“older age” = age of rock formation Paleomagnetism & Mineral Magnetism 28
  • 28. Dynamo theory 29
  • 29. Earth’s magnetic field • 90% is coming from internal dipolar source. • The remaining 10% of the magnetic field cannot be explained in terms of simple dipolar sources. It is attributed to external solar activity, • Complex models of the Earth's magnetic field have been developed and are available. • If the Earth's field were simply dipolar with the axis of the dipole oriented along the Earth's rotational axis, all declinations would be 0 degrees (the field would always point toward the north). As can be seen, the observed declinations are quite complex
  • 30. 31
  • 31. Units Associated with Magnetic Poles • N / (Amp - m). A N / (Amp - m) is referred to as a tesla (T) F= G m1 m2 r2
  • 32. 33
  • 33. Mag variations 34
  • 34. Mag Storm 35
  • 35. 10th Generation IGRF Results Field Model Results Location Latitude Longitude Altitude Karachi Date 25 degs N 67 degs E 0.00 km 2008 Component Field Value Secular Variation Declination 0.528 degrees 2.6 arcmin/year Inclination 38.376 degrees 6.3 arcmin/year Horizontal Intensity 35170 nT 2.2 nT/year North Component 35169 nT 2.0 nT/year East Component 324 nT 26.1 nT/year Vertical Intensity 27852 nT 107.4 nT/year Total Intensity 44863 nT 68.4 nT/year
  • 36. Rocks magnetization 37
  • 37. Secular Variation: the slow, somewhat irregular, change in the direction of the magnetic field every few years updated maps are produced [International Geomagnetic Reference Field] to give both the updated declination and its rate of change 38
  • 38. Magnetism of Rocks remanent magnetisation: the ability to retain magnetisation in the absence of a field or in the presence of a different magnetic field alignment and hence magnetisation disappears as soon as the field is removed directions of the magnetisations of the magnetic atoms spontaneous align [iron materials & its compounds, e.g. magnetite] • Most rocks contain some ferromagnetic minerals [compounds of iron] • the atomic magnets of tiny ferromagnetic crystals or grains are aligned along one of the crystallographic directions (called easy axes) and the grains have strong magnetisation for their size • if a magnetic field is applied the individual grain magnetisations will each tend to rotate into an easy axis closer to that of the field and in this way obtain a remanence 39
  • 39. Mineral magnetism Magnetic susceptibility (χ): the ability of a rock to become temporarily magnetised while a magnetic field is applied to it paramagnetic materials  become magnetised only when the field is present ferromagnetic materials  increase their magnetisation while a field is applied } this temporary magnetisation is called induced magnetisation strength of the magnetic field 40
  • 40. Different types of remanent magnetisation Thermal Remanent Magnetisation (TRM) as magma cools it passes through the Curie temperature as the atomic magnets of magnetic material grains align spontaneously to form one or more magnetic domains. As the rock cools through its range of blocking temperatures, a net magnetisation is “frozen” in  strong magnetisation Chemical Remanent Magnetisation (CRM) chemical alteration of a nonmagnetic iron mineral into a magnetic one, e.g. weathering, or precipitating iron oxides usually haematite from water percolating through the rock, example: cement in sandstones forming “red bed”)  because the process leads to haematite formation which is magnetically weak, CRM leads to weak, though measurable, magnetisation Detrital or Depositional Remanent Magnetisation (DRM) as existing magnetised grains are deposited (rock erosion products, e.g. basaltic lava) together with other material to form a waterlain sediment, they tend to align their magnetisations with the field, like tiny compass needles, as they settle through the water  weak; cases where the direction of DRM may not align closely with the inclination of the Earth’s field due to turbulence in the depositional flow and more importantly because flattened grains tend to land flat on the floor as pieces/flakes of paper settle through the water Viscous Remanent Magnetisation (VRM) if it happens that thermal fluctuations (ambient temperature) taking place over long periods of time are not too far from any rock blocking temperatures, the rock is remagnetised in the direction of field at the time  slow, partial magnetisation (like a compass needle in very thick oil), is very common in rock samples and is removed by reheating to 100-220ºC 41
  • 41. Thermal Remanent Magnetisation (TRM)  one way to magnetise a rock is by applying a magnetic field  another way is by heating [usually to demagnetise a rock sample] when a magma cools (solidifies including the formation of grains of magnetic minerals) it passes through the Curie temperature as the atomic magnets of magnetic material grains align spontaneously to form one or more magnetic domains. As the rock cools through its range of blocking temperatures, a net magnetisation is “frozen” in  resulting in thermal remanent magnetisation (TRM) Measuring reheating temperatures • an igneous rock had originally a primary remanence • reheated by an intrusion: if above its highest blocking temperature  all its primary remanence will be demagnetised and the rock remagnetises in the Earth’s field at that time giving wrong remanence for the initial formation age of the rock (none awareness of the intrusion thermal effect) • reheated by an intrusion: not sufficient temperature to destroy all primary remanence  primary remanence is retained and a second remanence is added as the rock cools • Natural Remanent Magnetisation (NRM) : progressive demagnetisation by reheating remanence of a rock sample regardless of how it is magnetised will be a mix (vector  strength & direction) of the two remanences 42
  • 42. Paleomagnetism: the magnetism of a rock acquired long time ago, often when they are formed [provides inclination, declination of the location where the rock was formed] measurements have shown that when the secular variation is averaged over ten thousand years or more, it coincides with the direction of the rotation axis, and so with the true north; this simplifies paleomagnetic interpretations  present field: normal or N-polarity  opposite field: reversed or R-polarity [there have been times during the Earth history when the magnetic poles have been interchanged] 43
  • 43. Paleomagnetism: measuring a paleomagnetic direction rock-sample at a location at present magnetic equator oriented samples (azimuth & dip recorded) are needed [6-8 samples taken from the same rock formation at some distance to reduce errors] (2) does not parallel the Earth’s present field [exhibits a declination angle] laboratory measurements: (paleo-inclination & paleo-declination)  spinner magnetometer (1) its inclination is not 0º (as expected for its magnetic equator location) 44
  • 44. Magnetostratigraphy: changes of magnetic field direction (normal/reverse) leave their records in the rocks and are used to establish a stratigraphic order or even to date the rocks 45
  • 45. Paleomagnetic reversals recorded by basalt at mid-ocean ridges 46
  • 46. need for continuous sections  basaltic lava succession in the ocean floor 47
  • 47. need for continuous sections  basaltic lava succession in the ocean floor 48
  • 48. Magnetic Polarity Timescale (1) 49
  • 49. Magnetic Polarity Timescale (2) 50
  • 50. lines of magnetic field intersect the Earth’s surface at an angle magnetic latitude (inclination) & direction to north (declination) are easily found, but magnetic LONGITUDE can not be deduced due to the symmetry of the magnetic field about its axis magnetic declination: [at a certain location] the difference (angle) between geographic/true north and magnetic north [azimuth of horizontal component of magnetic field] (degrees east or west of true north) magnetic inclination: [at the Earth’s surface] the angle between the magnetic field 51 and the horizontal (degrees, +90º to 0º to –90º)
  • 51. Apparent Polar Wander (APW) paths (1) 52
  • 52. Apparent Polar Wander (APW) paths (3) 2 alternative explanations “true polar-wander” model: continent is fixed, so to explain polar-wander paths, the magnetic pole must move substantially the magnetic pole does move a little, but it never strays very far from the geographic pole continental-drift model: magnetic pole is fixed near the geographic pole, and the continent drifts relative to the pole 53
  • 53. Apparent Polar Wander (APW) paths (2) 54
  • 54. Apparent Polar Wander (APW) paths & relative continental movements Europe & North America 280-180 Ma: APW paths coincide as Europe and North America moved together as a unit when both were part of Pangea. When Pangea broke up, they began to develop separate paths Europe & Siberia APW paths  can show that a single mass is formed from smaller parts: Europe & Siberia similar APW paths back to Triassic, but differ 55 for older ages as the two collided in the Triassic
  • 55. Paleomagnetic Directions cluster of directions: replace by an average, or mean, direction, plus an error α95 confidence limit [statistic]: a cone with this half-angle has a 95% probability of containing the true direction of magnetization α95 circle of confidence [on stereonet] 56
  • 56. Paleopoles: paleolatitudes & rotations rock-sample present location: 10ºN paleomagnetic lab-measurements: declination=20º inclination=+49º  tan I = 2 tan λ  paleolatitude=30ºN  apparent-North pole was at 60º (90º-λ) away from rock location apparent North pole: relative to our rock sample Paleopole is found: by traveling 60º around the Earth along a great circle, starting from the present rock location, in the direction of declination, 20º different from present pole  rock has moved different declination  rock has rotated about a vertical axis different inclination  moved N-S or tilted but cannot say is the rock has changed its paleo-longitude [due to axial symmetry of the dipole field] 57
  • 57. Apparent Polar Wander (APW) paths & relative continental movements APW paths can show if there has been relative movement between land masses [provided the APW paths cover the same time span] 58