Geo,paleomagnetism

MAGNETICS

Introduction

Geo & Paleo

Magnetism

Role of Geo & Paleo inGeophysics & 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.

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.

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.

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.

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.

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

Magnetic Lines of Force Associated with Magnetic Dipoles

• Mag Bar

9

Earth’s magnetism: due to electric currents in liquid outer core

Magnetic field is measured (units): in Teslas [T, too large] nanoTesla (nT, 10-9 T)

11.5º

magnetic dipole[the source producing the magneticfield is far from where were aremeasuring its field; within the core]

Earth’s Mag field

10

11

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

Total mag intensity

• F

14

Magnetic Inclination

15

Magnetic declination

16

10th Generation IGRF ResultsField Model Results

Location Latitude Longitude Altitude Date

Karachi 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

18

paths of magnetic field[iron filings on a sheet over a magnet]

bar magnet

coil

Earth’s Magnetic Field

where rotation axisintersects the surface

Ampere’s law & mag

19

Mag Units• Parameter SI unit cgs unit Conversion

• Magnetic moment (m) Am2 emu 1 A m2 = 103 emu• Magnetization (M) Am−1 emu cm−3 1 Am−1 = 10−3 emu cm−3• Magnetic Field (H) Am−1 Oersted (oe) 1 Am−1 = 4π x 10−3 oe• Magnetic Induction (B) T Gauss (G) 1 T = 104 G• Permeability of free space (μ0) Hm−1 1 4π x 10−7 Hm−1 = 1• Susceptibility (χ)• total (m/H) m3 emu oe−1 1 m3 = 106 4π emu oe−1• by volume (M/H) - emu cm−3 oe−1 1 S.I. = 1/4π emu cm−3 oe−1• by mass (m/m ·1/H) m3kg−1 emu g−1 oe−1 1 m3kg−1 = 103 4π emu g−1 oe−1

• 1 H = kg m2A−2s−2, 1 emu = 1 G cm3, B = μo(H +M), 1 T = kg A−1 s−2

20

21

lines of magnetic field intersectthe Earth’s surface at an angle

magnetic inclination: [at the Earth’s surface] the angle between the magnetic field and the horizontal (degrees, +90º to 0º to –90º)

magnetic declination: [at a certain location] the difference (angle)between geographic/true north and magnetic north [azimuthof horizontal component of magnetic field] (degrees east orwest of true north)

magnetic latitude (inclination) & direction to north (declination)are easily found, but magnetic LONGITUDE can not be dedu-ced due to the symmetry of the magnetic field about its axis

22

Fundamentals of Magnetism

magnetic dipole created by rotation of an electron around atom’s nucleus [dipole is presented by an arrow pointing NS]

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

23

Magnetism of Rocks

remanent magnetisation: the ability to retain magnetisation in theabsence of a field or in the presence of a different magnetic field

alignment and hence magnetisationdisappears as soon as the field is removed

directions of the magnetisations of themagnetic 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

Mag minerals classification

24

25

Curie & blocking temperatures one way to magnetise a rock is by applying a magnetic field another way is by heating [usually to demagnetise a rock sample]

progressive thermal demagnetisation

Blocking temperature:a range of temperatures characteristicfor individual minerals to be thermallydemagnetised

Curie temperature or Curie pointfor a specific material: temperatureabove which the material becomesparamagnetic[the individual atomic magnets cease to align withone another, and the spontaneous magnetisationnecessary for ferromagnetism disappears]

Intensity of Magnetization

26

Mag susceptibility

27

28

PaleoPaleomagnetismmagnetism&&

Mineral MagnetismMineral Magnetism

rock magnetism acquired at a [πάλαιο, greek] =“older age” = age of rock formation

Dynamo theory

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

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

33

Mag variations

34

Mag Storm

35

10th Generation IGRF ResultsField Model Results

Location Latitude Longitude Altitude Date

Karachi 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

Rocks magnetization

37

38

Secular Variation: the slow, somewhat irregular, change in the direction of the magnetic field

every few years updated maps areproduced [International GeomagneticReference Field] to give both theupdated declination and its rate of change

39

Magnetism of Rocks

remanent magnetisation: the ability to retain magnetisation in theabsence of a field or in the presence of a different magnetic field

alignment and hence magnetisationdisappears as soon as the field is removed

directions of the magnetisations of themagnetic 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

40

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 iscalled induced magnetisation

strength of the magnetic field

41

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 moremagnetic 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 usuallyhaematite 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 water-lain 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 rockblocking 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

42

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 grainsof magnetic minerals) it passes through the Curie temperature asthe atomic magnets of magnetic material grains align spontaneouslyto form one or more magnetic domains. As the rock cools throughits 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) : remanence of a rock sample regardless of how it is magnetised will be a mix (vector strength & direction) of the two remanences

progressive demagnetisation by reheating

43

measurements have shown that when the secularvariation is averaged over ten thousand years ormore, it coincides with the direction of the rotationaxis, 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]

Paleomagnetism: the magnetism of a rock acquiredlong time ago, often when they are formed [provides incli-nation, declination of the location where the rock was formed]

44

rock-sample at a location at present magnetic equator rock-sample at a location at present magnetic equator

(1) its inclination is not 0º (as expected for its magnetic equator location)

Paleomagnetism: measuring a paleomagnetic direction

(2) does not parallel the Earth’s present field [exhibits a declination angle]

oriented samples (azimuth & dip recorded)are needed [6-8 samples taken from the samerock formation at some distance to reduce errors]

laboratory measurements: (paleo-inclination &paleo-declination) spinner magnetometer

45

Magnetostratigraphy:changes of magnetic field direction (normal/reverse) leave their records in the rocks and are used to establisha stratigraphic order or even to date the rocks

Paleomagnetic reversals recorded by basalt at mid-ocean ridges

46

47

need for continuous sections basaltic lava succession in the ocean floor

48

need for continuous sections basaltic lava succession in the ocean floor

49

Magnetic Polarity Timescale (1)

50

Magnetic PolarityTimescale (2)

51

lines of magnetic field intersectthe Earth’s surface at an angle

magnetic inclination: [at the Earth’s surface] the angle between the magnetic field and the horizontal (degrees, +90º to 0º to –90º)

magnetic declination: [at a certain location] the difference (angle)between geographic/true north and magnetic north [azimuthof horizontal component of magnetic field] (degrees east orwest of true north)

magnetic latitude (inclination) & direction to north (declination)are easily found, but magnetic LONGITUDE can not be dedu-ced due to the symmetry of the magnetic field about its axis

52

Apparent Polar Wander (APW) paths (1)

53

Apparent Polar Wander (APW) paths (3)

2 alternative explanations

“true polar-wander” model:continent is fixed, so to explain polar-wanderpaths, the magnetic pole must move substantially

the magnetic pole does move a little, but itnever 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

54

Apparent Polar Wander (APW) paths (2)

55

Apparent Polar Wander (APW) paths & relative continental movements

Europe & SiberiaEurope & North America

280-180 Ma: APW paths coincide as Europe and NorthAmerica moved together as a unit when both were partof Pangea. When Pangea broke up, they began todevelop separate paths

APW paths can show that a single mass isformed from smaller parts: Europe & Siberiasimilar APW paths back to Triassic, but differfor older ages as the two collided in the Triassic

56

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 a95% probability of containing thetrue direction of magnetization

α95 circle of confidence [on stereonet]

57

Paleopoles: paleolatitudes & rotations

rock-sample present location: 10ºNpaleomagnetic lab-measurements: declination=20ºinclination=+49º tan I = 2 tan λ paleolatitude=30ºN apparent-North pole was at 60º (90º-λ) away from rock location

Paleopole is found: by traveling 60ºaround the Earth along a great circle,starting from the present rock location,in the direction of declination, 20º

apparent North pole:relative to our rocksample

different from present pole rock has moveddifferent declination rock has rotated about

a vertical axisdifferent inclination moved N-S or tilted

but cannot say is the rock haschanged its paleo-longitude[due to axial symmetry of the dipole field]

58

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]