Magnetism and Paleomagnetism

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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
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?
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

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

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 °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 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

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