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

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

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

Magnetism and Paleomagnetism

Chapter outline

•Magnetic field and the dipole

•Magnetic measurement (washing)

•Magnetic remenance

•Magneto-stratigraphy

Page 2: 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 ?

Page 3: Magnetism and Paleomagnetism Chapter outline Magnetic field and the dipole Magnetic measurement (washing) Magnetic remenance Magneto-stratigraphy

Relation between spin axis POLE that define true north and magnetic POLE that approximately defines the primary magnetic field dipole orientation

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.

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 ?

Page 4: Magnetism and Paleomagnetism Chapter outline Magnetic field and the dipole Magnetic measurement (washing) Magnetic remenance Magneto-stratigraphy

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.

Page 5: Magnetism and Paleomagnetism Chapter outline Magnetic field and the dipole Magnetic measurement (washing) Magnetic remenance Magneto-stratigraphy

All magnetic fields derive from moving electric charges (current)

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.

B field around a wire with current flow I.

Page 6: Magnetism and Paleomagnetism Chapter outline Magnetic field and the dipole Magnetic measurement (washing) Magnetic remenance Magneto-stratigraphy

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.

Page 7: Magnetism and Paleomagnetism Chapter outline Magnetic field and the dipole Magnetic measurement (washing) Magnetic remenance Magneto-stratigraphy

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.

Page 8: Magnetism and Paleomagnetism Chapter outline Magnetic field and the dipole Magnetic measurement (washing) Magnetic remenance Magneto-stratigraphy

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

Page 9: Magnetism and Paleomagnetism Chapter outline Magnetic field and the dipole Magnetic measurement (washing) Magnetic remenance Magneto-stratigraphy

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).

Page 10: Magnetism and Paleomagnetism Chapter outline Magnetic field and the dipole Magnetic measurement (washing) Magnetic remenance Magneto-stratigraphy

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.

Page 11: Magnetism and Paleomagnetism Chapter outline Magnetic field and the dipole Magnetic measurement (washing) Magnetic remenance Magneto-stratigraphy

Compare field charges: mass, electric, magnetic?

Gravity (mass) charge

Only one sign: positive!Always attractive!

Field is spherical symmetric and varies as: 1/r*2

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.

Page 12: Magnetism and Paleomagnetism Chapter outline Magnetic field and the dipole Magnetic measurement (washing) Magnetic remenance Magneto-stratigraphy

• 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.

Earth’s Magnetic Field

Page 13: Magnetism and Paleomagnetism Chapter outline Magnetic field and the dipole Magnetic measurement (washing) Magnetic remenance Magneto-stratigraphy

• 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

Paleomagnetism & Rock Magnetism

Page 14: Magnetism and Paleomagnetism Chapter outline Magnetic field and the dipole Magnetic measurement (washing) Magnetic remenance Magneto-stratigraphy

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).

Page 15: Magnetism and Paleomagnetism Chapter outline Magnetic field and the dipole Magnetic measurement (washing) Magnetic remenance Magneto-stratigraphy

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.

Page 16: Magnetism and Paleomagnetism Chapter outline Magnetic field and the dipole Magnetic measurement (washing) Magnetic remenance Magneto-stratigraphy

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

Page 17: Magnetism and Paleomagnetism Chapter outline Magnetic field and the dipole Magnetic measurement (washing) Magnetic remenance Magneto-stratigraphy

Earth’s Magnetic FieldGeodynamo

• 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

Page 18: Magnetism and Paleomagnetism Chapter outline Magnetic field and the dipole Magnetic measurement (washing) Magnetic remenance Magneto-stratigraphy

• 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

Page 19: Magnetism and Paleomagnetism Chapter outline Magnetic field and the dipole Magnetic measurement (washing) Magnetic remenance Magneto-stratigraphy

• 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

Page 20: Magnetism and Paleomagnetism Chapter outline Magnetic field and the dipole Magnetic measurement (washing) Magnetic remenance Magneto-stratigraphy

• 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

Page 21: Magnetism and Paleomagnetism Chapter outline Magnetic field and the dipole Magnetic measurement (washing) Magnetic remenance Magneto-stratigraphy

• The magnetic inclination and magnetic latitude are related by

Earth’s Magnetic Field

tan 2 tan

Where is the inclination

and is the magnetic latitude

I

I

Page 22: Magnetism and Paleomagnetism Chapter outline Magnetic field and the dipole Magnetic measurement (washing) Magnetic remenance Magneto-stratigraphy

• 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

Page 23: Magnetism and Paleomagnetism Chapter outline Magnetic field and the dipole Magnetic measurement (washing) Magnetic remenance Magneto-stratigraphy

• 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

Page 24: Magnetism and Paleomagnetism Chapter outline Magnetic field and the dipole Magnetic measurement (washing) Magnetic remenance Magneto-stratigraphy

• 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

Earth’s Past Magnetic Field

Page 25: Magnetism and Paleomagnetism Chapter outline Magnetic field and the dipole Magnetic measurement (washing) Magnetic remenance Magneto-stratigraphy

• 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

Paleomagneticism

Page 26: Magnetism and Paleomagnetism Chapter outline Magnetic field and the dipole Magnetic measurement (washing) Magnetic remenance Magneto-stratigraphy

• 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

Page 27: Magnetism and Paleomagnetism Chapter outline Magnetic field and the dipole Magnetic measurement (washing) Magnetic remenance Magneto-stratigraphy

• 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

Page 28: Magnetism and Paleomagnetism Chapter outline Magnetic field and the dipole Magnetic measurement (washing) Magnetic remenance Magneto-stratigraphy

• 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

Page 29: Magnetism and Paleomagnetism Chapter outline Magnetic field and the dipole Magnetic measurement (washing) Magnetic remenance Magneto-stratigraphy

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.

Measuring Paleomagnetic Directions

Page 30: Magnetism and Paleomagnetism Chapter outline Magnetic field and the dipole Magnetic measurement (washing) Magnetic remenance Magneto-stratigraphy

• 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

Page 31: Magnetism and Paleomagnetism Chapter outline Magnetic field and the dipole Magnetic measurement (washing) Magnetic remenance Magneto-stratigraphy

• 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

Page 32: Magnetism and Paleomagnetism Chapter outline Magnetic field and the dipole Magnetic measurement (washing) Magnetic remenance Magneto-stratigraphy

• 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 Pole

Page 33: Magnetism and Paleomagnetism Chapter outline Magnetic field and the dipole Magnetic measurement (washing) Magnetic remenance Magneto-stratigraphy

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

Page 34: Magnetism and Paleomagnetism Chapter outline Magnetic field and the dipole Magnetic measurement (washing) Magnetic remenance Magneto-stratigraphy

• 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

Page 35: Magnetism and Paleomagnetism Chapter outline Magnetic field and the dipole Magnetic measurement (washing) Magnetic remenance Magneto-stratigraphy

• 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

Apparent Polar Wander

Page 36: Magnetism and Paleomagnetism Chapter outline Magnetic field and the dipole Magnetic measurement (washing) Magnetic remenance Magneto-stratigraphy

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

Page 37: Magnetism and Paleomagnetism Chapter outline Magnetic field and the dipole Magnetic measurement (washing) Magnetic remenance Magneto-stratigraphy

• 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

Page 38: Magnetism and Paleomagnetism Chapter outline Magnetic field and the dipole Magnetic measurement (washing) Magnetic remenance Magneto-stratigraphy

• 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

Magnetism of Rocks

Page 39: Magnetism and Paleomagnetism Chapter outline Magnetic field and the dipole Magnetic measurement (washing) Magnetic remenance Magneto-stratigraphy

• 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

Blocking Temperatures

Page 40: Magnetism and Paleomagnetism Chapter outline Magnetic field and the dipole Magnetic measurement (washing) Magnetic remenance Magneto-stratigraphy

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

Curie Temperature

Page 41: Magnetism and Paleomagnetism Chapter outline Magnetic field and the dipole Magnetic measurement (washing) Magnetic remenance Magneto-stratigraphy

• 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

Earth’s Magnetic Field

Page 42: Magnetism and Paleomagnetism Chapter outline Magnetic field and the dipole Magnetic measurement (washing) Magnetic remenance Magneto-stratigraphy

• 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

Thermal Remnant Magnetization

Page 43: Magnetism and Paleomagnetism Chapter outline Magnetic field and the dipole Magnetic measurement (washing) Magnetic remenance Magneto-stratigraphy

• 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

Partial Reheating

Page 44: Magnetism and Paleomagnetism Chapter outline Magnetic field and the dipole Magnetic measurement (washing) Magnetic remenance Magneto-stratigraphy

• 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

3D Magnetic Vectors

Page 45: Magnetism and Paleomagnetism Chapter outline Magnetic field and the dipole Magnetic measurement (washing) Magnetic remenance Magneto-stratigraphy

• 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

Reheating Temperatures

Dyke

Lava

Page 46: Magnetism and Paleomagnetism Chapter outline Magnetic field and the dipole Magnetic measurement (washing) Magnetic remenance Magneto-stratigraphy

• 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

Magnetic Minerals

Page 47: Magnetism and Paleomagnetism Chapter outline Magnetic field and the dipole Magnetic measurement (washing) Magnetic remenance Magneto-stratigraphy

• 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!

Magnetization at Ambient Temp

Page 48: Magnetism and Paleomagnetism Chapter outline Magnetic field and the dipole Magnetic measurement (washing) Magnetic remenance Magneto-stratigraphy

• 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

Cleaning Unwanted Magnetization

Page 49: Magnetism and Paleomagnetism Chapter outline Magnetic field and the dipole Magnetic measurement (washing) Magnetic remenance Magneto-stratigraphy

• 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

Field Tests

Page 50: Magnetism and Paleomagnetism Chapter outline Magnetic field and the dipole Magnetic measurement (washing) Magnetic remenance Magneto-stratigraphy

• 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

Page 51: Magnetism and Paleomagnetism Chapter outline Magnetic field and the dipole Magnetic measurement (washing) Magnetic remenance Magneto-stratigraphy

• 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

Page 52: Magnetism and Paleomagnetism Chapter outline Magnetic field and the dipole Magnetic measurement (washing) Magnetic remenance Magneto-stratigraphy

• Ocean floor spreading at mid-ocean ridges

• Form at steady rate

• Oldest are 160 Ma

• Preserve the magnetic reversals for past 160 Ma

Magnetostratigraphy

Page 53: Magnetism and Paleomagnetism Chapter outline Magnetic field and the dipole Magnetic measurement (washing) Magnetic remenance Magneto-stratigraphy

C is a chron

-significant interval of one polarity

M is for Mesozoic

Note the long N interval during the Cretaceous

Magnetostratigraphy

Page 54: Magnetism and Paleomagnetism Chapter outline Magnetic field and the dipole Magnetic measurement (washing) Magnetic remenance Magneto-stratigraphy

• 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

Mineral Magnetism

Where is the induced magnetization

is the magnetic susceptibility

and is the applied magnetic field

i

i

M H

M

H

Page 55: Magnetism and Paleomagnetism Chapter outline Magnetic field and the dipole Magnetic measurement (washing) Magnetic remenance Magneto-stratigraphy

• 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

Mineral Magnetism

Page 56: Magnetism and Paleomagnetism Chapter outline Magnetic field and the dipole Magnetic measurement (washing) Magnetic remenance Magneto-stratigraphy

Extra Images

Page 57: Magnetism and Paleomagnetism Chapter outline Magnetic field and the dipole Magnetic measurement (washing) Magnetic remenance Magneto-stratigraphy

Extra Images

Page 58: Magnetism and Paleomagnetism Chapter outline Magnetic field and the dipole Magnetic measurement (washing) Magnetic remenance Magneto-stratigraphy

Extra Images