1. Magnetism and Paleomagnetism
Chapter outline
Magnetic field and the dipole
Magnetic measurement (washing)
Magnetic remenance
Magneto-stratigraphy

2. Earth’s PRIMARY magnetic field with solar wind blowing on it
Earth’s PRIMARY magnetic field with solar wind blowing on it. The solar wind is high kinetic energy charged particles emitted from Sun.
The solar wind deforms the earths primary magnetic field: note close field line spacing on Sun side and wide field lines on non-sun side of earth.
What causes auroa-borealis ?

3. Relation between spin axis POLE that define true north and magnetic POLE that approximately defines the primary magnetic field dipole orientation
The earth’s primary magnetic field can be approximated as a ‘big bar magnet’. But, the ‘big bar magnet’ is only a good metaphor .
This dipole field approximation is very useful for predicted magnetic field near earth’s surface to facilitate the study of paleo-magnetism.
WHY ? What does arrow direction manifest ?
If the magnetic and spin axis poles change, then WHERE is the real north pole ?
Use the stars , whose motion with respect to our planet is too small to be measured, which can provide a reference frame. Note this is how we discover precession of the earth’s spin axis (2000 yrs ago!). Note: we can see black holes moving around the gallactic center.

4. Magnetic dipole (dipoles as a concept in general)
A dipole has two parameters:
Direction of the axis in 3-space (vector) and the polarity of the ‘north/south’ pole.
A scalar magnetic dipole strength in Amps/m*2. The Earth’s dipole is 10*22 Amp/m*2.
Like electric charges, for magnetic fields, the same poles repulse and opposite poles attract.
Note a compass is a magnetic dipole.
Note the compass S-pole is attracted towards the N-pole and the compasses N-pole is attracted towards the S-pole.

5. All magnetic fields derive from moving electric charges (current)
B field around a wire with current flow I.
To make an electro-magnetic, wrap a wire around a magnetic conductor (nail) and hook up a battery to permit electrical current to flow. The direction of current flow give polarity of magnetic dipole.

6. So where is the moving electric charge to make magnetism?
Two places:
When charged particles move in a fluid (gas or liquid): e.g., the earth’s outer core or in gas nebula clouds in intra gallatic space or a current in a wire.
An electron and proton have a magnetic dipole which is an intrinsic property required by quantum mechanics. In certain ferromagnetic substance, such as iron, the unpaired outer electrons in the high F orbitals do an extraordinary thing, they will all line up when the temperature (thermal agitation) is small enough (the curie temperature). Its called exchange interaction.

7. Earth’s Geodynamo that makes primary magnetic field
Liquid iron in outer core can both conduct electricity AND convective flow!
Thus it can create a spiraling flow (tangent yellow cylinder around inner core above) that produce a self-reinforcing dynamo that generates the earth’s primary magnetic field . When the flow reverse, the polarity of magnetic field reverses.

8. Geodynamo’s in other solar system planets?
Mercury: Little magnetic dynamo, 1% earth’s field strength.
Venus: Field at least 100,000 less than earth’s field. Why? The planet almost certainly has a liquid iron core like the earth. But, Venus only rotates once every 220 days.
Mars: No primary field now, but evidence for magnetic remanence. Small planetary radius means the liquid iron core solidified in first Ga.
Jupiter: largest dynamo of planets, 14 times stronger field than earth. Dynamo is core of liquid hydrogen.
Saturn, Uranus, Neptune: all have magnetic dynamos and strong fields.
Jupiter Aureo Borealis

9. History of magnetic force
700 BC Greek’s found loadstone which is a highly magnetized rock (due to magnetite)
400 Chinese discovery that loadstone ‘whittled’ into a needle points about north-south.
1175 Compass make it to Europe (Venice) and spawns the ‘Age of Discovery’.
1269 Peregrinus, a French Crusader, describes a floating compass and concept of poles.
1601 William Gilbert publishes ‘De Magneta’ saying earth is like a huge bar magnet. Start of the scientific method with Francis Bacon’s publications.
1745 ‘Leyden Jar’ is made that can store and discharge electricity.
1770 Ben Franklin does a lot of electrical experiments (e.g., the kite).
1800 Volta makes first battery: greatly increase amount of current available to experimenters.
1820 Oersted, by accident, finds that a changing electric field (current) deflects a compass. This provides the first link between electric and magnetic phenomena.
1882 Maxwell discovers theory of electromagnetism (light is just an EM-wave!!)
1905 Einstein’s special relatively leads to understanding of magnetic field as relativistic effect of moving charge when speed of electromagnetic waves is finite (c).

10. What is a charge and its field?
A charge is a quantity that is the source of a field that extends into space. For gravity, the charge is mass (kg) and for electromagnetism the charge is electric (coulombs).
The field strength is proportional the amount of charge (kg or coulombs). The closer the field line are together; the stronger the field locally is.
The field can perform the miracle of action at distance: i.e., apply a force and do work on another object proportional to the objects charge.
It took physicists until 1890 or so to accept the concept that a force field that can do work without two object touching.

11. Compare field charges: mass, electric, magnetic?
Electric (Coulomb) charge
Two signs: plus or minus.
Same sign repulsive force; opposite sign attractive force.
Field is spherical symmetric and varies as: 1/r*2
Magnetic charge
NO SUCH THING!!
All magnetism is relativistic effect of moving (accelerating) charge.
Gravity (mass) charge
Only one sign: positive!
Always attractive!
Field is spherical symmetric and varies as: 1/r*2

12. Earth’s Magnetic Field
The Earth’s PRIMARY magnetic field interacts with rocks to provide a REMANENT magnetic field record.
Provides a fossil compass record
used to ascertain conditions of the formation of the rocks
Can be used to track the movements of the rocks
Can also be used to investigate the subsurface for mineral exploration
Understanding its origin due to flow of conductive iron liquid in outer core is fundamental to understanding evolution of earth’s atmosphere.

13. Paleomagnetism & Rock Magnetism
Paleomagnetism utilizes the fossil magnetism preserved in rocks
Can be used to measure the movements of the rocks
Can be due to plate movements
Can result from tectonic tilting
Requires an understanding of how rocks acquire a remanent magnetization
Requires access to the rocks

14. Magnetic Field A magnet (dipole) produces a magnetic field
The field lines map out the
direction and magnitude
of the force (torque) that a compass (a bar magnet which is a magnetic dipole).

15. Dipole Magnetic Field
Where the field lines are dense (close), the magnetic field is strong
MKS Units of a magnetic field is
Tesla (T)
On the surface of the Earth the magnetic field ranges from 60,000 nT at the pole to 30,000 nT at the equator
Current flow through loop (b) makes magnetic field dipole.
The bar magnetic is a form of fossil remanent magnetism where the current flow is derived from the electrons.

16. Magnetic Field A Magnetic field can be produced by a magnet or
a current in a coil
The Earth’s magnetic field
is more complicated
It is produced by electrical
currents in the liquid outer
core

17. Earth’s Magnetic Field
Geodynamo
Electrical currents produced by
convective currents of convective
fluids in the liquid outer core
Not fully understood
We will call it a magnetic
dipole
Means that the source
volume is far from where
we measure the field

18. Earth’s Magnetic Field
The Earth’s magnetic field does not align with the
Earth’s rotational axis
Presently tilted 11.5°
Magnetic North differs
from geographic (true) N
Termed declination

19. Earth’s Magnetic Field
The Earth’s magnetic field lines intersect the surface of the Earth at an angle
At the poles, it is nearly
vertical
At the equator, it is nearly
horizontal
Termed inclination
Can be measured with a compass
Positive when points down
Negative when points up

20. Earth’s Magnetic Field
Where the axis of the Earth’s magnetic field intersects the surface of the
Earth is called the north and
south magnetic poles
Magnetic equator and
magnetic latitude are
similarly defined
The Earth’s magnetic field is
symmetric about the magnetic
axis

21. Earth’s Magnetic Field
The magnetic inclination and magnetic latitude are related by

22. Earth’s Past Magnetic Field
From observatory records going back a few hundred years, we know that the magnetic axis continually changes direction
Slow and somewhat irregular
Called secular variation

23. Earth’s Past Magnetic Field
From paleomagnetic (fossilized magnetic remanence) records in rocks, we find that the Earth’s magnetic axis wobbles about the rotational axis
Completes a cycle in around a couple of thousand years
Averaged over several thousand years, the Earth’s magnetic field is a geocentric, axial dipole
Using average inclination to calculate magnetic latitude, we find the true paleolatitude

24. Earth’s Past Magnetic Field
At times in the Earth’s history, the magnetic poles have been interchanged
Polarity reversals
Occur at irregular intervals, on order of Myr
Time for reversal to take place is order of Kyr
Geologically short
Rare to find rocks from the transitions
Current state of the field is normal (N)
Reversed state is termed R polarity
Excursions of the magnetic poles also occurs

25. Paleomagneticism
Rocks retain magnetism acquired long ago, often when they formed
Called paleomagnetism
Process will be addressed later
Consider a pile of Tertiary lavas
Each eruption cools in a few years
Records instantaneous field direction
Deposited over thousands of years
The lava pile will average out secular variation

26. Measuring Paleomagnetic Directions
Sample of the rocks are required
Generally a short core
Penetrates through weathering
Very important to have three dimensional orientation of the sample
May have to use a sun compass to measure azimuth
If the rock has been tilted, this must be measured
Usually 6-8 samples separated by few meters

27. Measuring Paleomagnetic Directions
In the laboratory, short cylinders are cut out and measured with a magnetometer
Cylinder is spun, causing its magnetism to produce a current in a nearby coil, which can be used to measure the magnetic field
This is repeated for the other two orthogonal directions
Convert the data into declination and inclination using the sample orientation

28. Measuring Paleomagnetic Directions
Performed for several cylinders from each core
Plotted on a stereonet to give a stereoplot of the directions
Positive inclination (downward) is plotted with open circles
If the samples
cluster, we can
assume that the
magnetization has
not changed over
time

29. Measuring Paleomagnetic Directions
Magnetization is usually reported as a mean direction and an error
We assume that the samples are scattered randomly
Statistics of small number of samples is dicey
More samples are always better!
Error is reported as α95
A cone with this
half angle has a
95% probability
of containing the
true direction.

30. Apparent Pole
Rocks magnetized at the same time but at different latitudes have different magnetic directions (inclinations)
Makes it difficult to recognize if those rocks (or the continents they are riding on) have moved apart
We calculate the position of the magnetic north pole at the time of magnetization
Actually where the pole was relative to the rock sample
Called the apparent pole
Example: rock formed at the equator (I = 0°)
Later moved to the south pole
I=0° => infer it was magnetized at equator

31. Apparent Pole
Example: drill cores from lavas formed hundreds of Ma ago which are now at 10° N latitude.
The measured declination of the sample is 20° (EofN)
The measured inclination is +49° >> Paleo-latitude = 30°N
=> North pole was 60° from present position of rocks (90°-30°)
Paleopole is 60° along great circle
in declination direction (20°).

32. Apparent Pole
If :
the apparent paleopole isn’t at the present magnetic pole
The rock must have moved (assume sec. var. ave. out)
the declination is not due north
The rock must have been rotated
the inclination does not correspond to its current latitude
The rock must have been moved N or S, or tilted
Tilting can often be recognized and corrected for
Because of symmetry of the Earth’s magnetic field
Cannot determine longitudinal movement
Still useful for determining climatic effects

33. Apparent Polar Wander If a continent has moved N or S over time
Paleopoles of rocks of successive ages will change
Trace out path called Apparent Polar Wander
We assume that secular variations of the earth’s magnetic pole average to zero; therefore, true motion of landmasses can be found.
We can compare the movements of two continents if we look at the APW over the same time span

34. Apparent Polar Wander
The two paths for the period Ordovician to Jurassic are not the same
They do have same general shape
If we ‘close the Atlantic’, the paths are same until the Triassic when they diverge
Both land
masses were
together
Longitudinal
information!

35. Apparent Polar Wander
Can also be used to determine if a continent is made up of smaller (once separate) parts
APW paths for Siberia and Europe are the same going back to the Triassic
Prior to that they were
separate
If two continents move apart
while at same latitude, their
pole remains the same
Cannot detect movement

36. Magnetism of Rocks
Magnetization of rocks takes place at the atomic scale/ The ability to lock in remanent magnetism depends on the ‘exchange interactions’ between the F electron orbitals in transition elements.
Two basic kinds of magnetism
Paramagnetism: temporary field that goes away when applied field is removed.
Ferromagnetism: permanent field that remains when applied field is removed.

37. Magnetism of Rocks Most rocks contain ferromagnetic minerals
If the grains of the ferromagnetic materials are tiny, the atomic magnetization aligns with an ‘easy axis’ which is determined by the crystal structure
On average, they are random, hence the internal field is 0
When an external field is applied, if the field is strong enough, individual grains will rotate to an ‘easy axis’ that is closest to the applied field
Requires energy to rotate
When field is removed, they
remain aligned
Remnant magnetization

38. Magnetism of Rocks
Magnetic materials above a certain size (0.001 to 1mm) form magnetic domains
Domains have high alignment
Bounded by domain walls
Tend to align with crystal imperfections
Difficult to move => remnant magnetization
Easier to change magnetization of multi-domain materials
Less remnant
magnetization

39. Blocking Temperatures
If the temperature of material is slowly raised, thermal oscillations will cause the domain walls to move or rotate
In the absence of a magnetic field, randomizes the domains
Different domain walls require different temperatures to move them
Different Blocking temperatures
Leads to progressive thermal
demagnetization

40. Curie Temperature
If the temperature of material is high enough, the individual atomic magnets cease to align
Spontaneous magnetization disappears
Characteristic temperature of the material
Curie temperature, Tc
Always higher than blocking temperature

41. Earth’s Magnetic Field
The Earth’s core is above the Curie temperature
Estimates range from °K
No remnant magnetization
Cannot be source of the Earth’s magnetic field

42. Thermal Remnant Magnetization
If the temperature of material is slowly lowered in the presence of an external magnetic field
Some of the domains will align as it goes below the domains blocking temperature
Different domain have different blocking temperatures
As the temperature is lowered, more domains will align until a net magnetization is ‘frozen in
Thermal Remnant Magnetization
Stronger than if applied to a
cool rock
Can persist through Geologic
time

43. Partial Reheating
If a rock is reheated partway through its blocking temperatures, it can be partially remagnetized to align with the new external magnetic field.
Secondary remanence
Rocks must be examined for reheating!
Primary and secondary remanence add together to form
Natural Remanent Magnetization
Primary remanence can be retrieved in the laboratory by heating in the
absence of any
magnetic field

44. 3D Magnetic Vectors Magnetic vectors are inherently 3D
Component diagrams (or 3D axis) are inconvenient
Project the vectors onto two planes and plot
Stereoplots only show
direction, no magnitude

45. Reheating Temperatures
Intrusions (dykes) can cause reheating
Magnetization Directions of D & L antiparallel
A lava sample close to the contact is reheated
Change is small until T=515°C
Rapidly moves toward L
=> Lava was heated to 515 °C
Dyke
Lava

46. Magnetic Minerals Magnetite is the mineral with the greatest remanence
Maghaemite has a fairly high magnetization
Important in soils
Responsible for magnetization of archaeological sites
Compound as well as concentration of iron determines
Grain size is also important
Fine grains may be single domain, highly remanent

47. Magnetization at Ambient Temp
Sediments do not have thermal remanence
Magnetization takes place at ambient temperature
Chemical remanent magnetization (CRM)
Chemical alteration of non-mag minerals into magnetic minerals (weathering, precipitating FeO2)
Depositional remanent magnetization (DRM)
Influenced by flows
Viscous remanent magnetization (VRM)
Blocking temperature slightly above ambient T
Over long time, temperature fluctuations causes slow, partial magnetization
All Natural Remanent Magnetization not when it formed!

48. Cleaning Unwanted Magnetization
Thermal demagnetization can remove secondary remanence
Slow, and may change the nature of the minerals
Alternating field demagnetization
Uses alternating magnetic field
Progressively stronger field
Sample is tumbled in space to randomize the induced magnetization from the applied field
Both depend upon the secondary remanence being easier to remove
Not true for chemical remanence
Cleaning or washing of unwanted dirty magnetism

49. Field Tests Fold Test Can determine if the magnetization was
acquired before or after the folding
Can also be applied to tilting
Conglomeration test
Compare magnetization of the clasts
Baked contact test
Dyke lava example

50. Magnetostratigraphy Reversals of the Earth’s magnetic field Global
Occur abruptly
Easy to recognize
Allows us to establish stratigraphic order
Allows us to date rocks

51. Magnetostratigraphy
Suppose we have an isolated lava that we measure both the age and remanent magnetization
There are reversals at 0.7, 1.6, 1.9 & 2.4 Ma
Suppose that we have a continuous succession of 50 lavas extruded at short intervals
Oldest is 10 Ma
Interval is 200,000 yr
No such lavas exist

52. Magnetostratigraphy Ocean floor spreading at mid-ocean ridges
Form at steady rate
Oldest are 160 Ma
Preserve the magnetic reversals for past 160 Ma

53. Magnetostratigraphy C is a chron -significant interval of one polarity
M is for Mesozoic
Note the long N interval during the Cretaceous

54. Mineral Magnetism
Inherent magnetic properties can also be used to measure geological processes
When an external field is applied, materials become more magnetized
Magnetic susceptibility (χ) is a measure of the induced magnetization due to an external magnetic field
Susceptibility can vary in direction (anisotropy)
Called Magnetic fabric

55. Mineral Magnetism
These properties can be used to measure geologic events
Different materials have different susceptibilities
Cores in glacial varves can be used to count the varves
Due to seasonal differences in deposition particles
Flow can cause
alignment
Magnetic fabric
in dyke swarms
Vertical flow near
center, horizontal
flow away from
center

56. Extra Images

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58. Extra Images