Magnetism and Paleomagnetism
Chapter outline
•Magnetic field and the dipole
•Magnetic measurement (washing)
•Magnetic remenance
•Magneto-stratigraphy
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 ?
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.
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.
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.
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.
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.
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
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).
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.
Compare field charges: mass, electric, magnetic?
Gravity (mass) charge
Electric (Coulomb) charge
Magnetic charge
Only one sign: positive!
Always attractive!
Two signs: plus or minus.
Same sign repulsive force;
opposite sign attractive force.
NO SUCH THING!!
Field is spherical symmetric
and varies as: 1/r*2
Field is spherical symmetric
and varies as: 1/r*2
All magnetism is
relativistic effect of
moving (accelerating)
charge.
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.
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
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).
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.
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
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
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
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
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
Earth’s Magnetic Field
• The magnetic inclination and magnetic latitude are
related by
tan  I   2 tan   
Where I is the inclination
and  is the magnetic latitude
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
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
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
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
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
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
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
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.
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
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°).
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
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
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!
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
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.
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
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
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
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
Earth’s Magnetic Field
• The Earth’s core is above the Curie temperature
• Estimates range from 2300-7300 °K
• No remnant magnetization
• Cannot be source of the Earth’s magnetic field
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
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
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
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
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
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!
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
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
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
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
Magnetostratigraphy
• Ocean floor spreading at mid-ocean ridges
• Form at steady rate
• Oldest are 160 Ma
• Preserve the magnetic reversals for past 160 Ma
Magnetostratigraphy
C is a chron
-significant
interval of
one
polarity
M is for
Mesozoic
Note the
long N
interval
during the
Cretaceous
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
Mi    H
Where M i is the induced magnetization
 is the magnetic susceptibility
and
H is the applied magnetic field
• Susceptibility can vary in direction (anisotropy)
• Called Magnetic fabric
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
Extra Images
Extra Images
Extra Images