Presentation on theme: "Magnetism and Paleomagnetism"— Presentation transcript:
1Magnetism and Paleomagnetism Chapter outlineMagnetic field and the dipoleMagnetic measurement (washing)Magnetic remenanceMagneto-stratigraphy
2Earth’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 ?
3Relation between spin axis POLE that define true north and magnetic POLE that approximately defines the primary magnetic field dipole orientationThe 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.
4Magnetic 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.
5All 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.
6So 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.
7Earth’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.
8Geodynamo’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
9History 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).
10What 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.
11Compare field charges: mass, electric, magnetic? Electric (Coulomb) chargeTwo signs: plus or minus.Same sign repulsive force; opposite sign attractive force.Field is spherical symmetric and varies as: 1/r*2Magnetic chargeNO SUCH THING!!All magnetism is relativistic effect of moving (accelerating) charge.Gravity (mass) chargeOnly one sign: positive!Always attractive!Field is spherical symmetric and varies as: 1/r*2
12Earth’s Magnetic Field The Earth’s PRIMARY magnetic field interacts with rocks to provide a REMANENT magnetic field record.Provides a fossil compass recordused to ascertain conditions of the formation of the rocksCan be used to track the movements of the rocksCan also be used to investigate the subsurface for mineral explorationUnderstanding its origin due to flow of conductive iron liquid in outer core is fundamental to understanding evolution of earth’s atmosphere.
13Paleomagnetism & Rock Magnetism Paleomagnetism utilizes the fossil magnetism preserved in rocksCan be used to measure the movements of the rocksCan be due to plate movementsCan result from tectonic tiltingRequires an understanding of how rocks acquire a remanent magnetizationRequires access to the rocks
14Magnetic Field A magnet (dipole) produces a magnetic field The field lines map out thedirection and magnitudeof the force (torque) that a compass (a bar magnet which is a magnetic dipole).
15Dipole Magnetic FieldWhere the field lines are dense (close), the magnetic field is strongMKS Units of a magnetic field isTesla (T)On the surface of the Earth the magnetic field ranges from 60,000 nT at the pole to 30,000 nT at the equatorCurrent 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.
16Magnetic Field A Magnetic field can be produced by a magnet or a current in a coilThe Earth’s magnetic fieldis more complicatedIt is produced by electricalcurrents in the liquid outercore
17Earth’s Magnetic Field GeodynamoElectrical currents produced byconvective currents of convectivefluids in the liquid outer coreNot fully understoodWe will call it a magneticdipoleMeans that the sourcevolume is far from wherewe measure the field
18Earth’s Magnetic Field The Earth’s magnetic field does not align with theEarth’s rotational axisPresently tilted 11.5°Magnetic North differsfrom geographic (true) NTermed declination
19Earth’s Magnetic Field The Earth’s magnetic field lines intersect the surface of the Earth at an angleAt the poles, it is nearlyverticalAt the equator, it is nearlyhorizontalTermed inclinationCan be measured with a compassPositive when points downNegative when points up
20Earth’s Magnetic Field Where the axis of the Earth’s magnetic field intersects the surface of theEarth is called the north andsouth magnetic polesMagnetic equator andmagnetic latitude aresimilarly definedThe Earth’s magnetic field issymmetric about the magneticaxis
21Earth’s Magnetic Field The magnetic inclination and magnetic latitude are related by
22Earth’s Past Magnetic Field From observatory records going back a few hundred years, we know that the magnetic axis continually changes directionSlow and somewhat irregularCalled secular variation
23Earth’s Past Magnetic Field From paleomagnetic (fossilized magnetic remanence) records in rocks, we find that the Earth’s magnetic axis wobbles about the rotational axisCompletes a cycle in around a couple of thousand yearsAveraged over several thousand years, the Earth’s magnetic field is a geocentric, axial dipoleUsing average inclination to calculate magnetic latitude, we find the true paleolatitude
24Earth’s Past Magnetic Field At times in the Earth’s history, the magnetic poles have been interchangedPolarity reversalsOccur at irregular intervals, on order of MyrTime for reversal to take place is order of KyrGeologically shortRare to find rocks from the transitionsCurrent state of the field is normal (N)Reversed state is termed R polarityExcursions of the magnetic poles also occurs
25PaleomagneticismRocks retain magnetism acquired long ago, often when they formedCalled paleomagnetismProcess will be addressed laterConsider a pile of Tertiary lavasEach eruption cools in a few yearsRecords instantaneous field directionDeposited over thousands of yearsThe lava pile will average out secular variation
26Measuring Paleomagnetic Directions Sample of the rocks are requiredGenerally a short corePenetrates through weatheringVery important to have three dimensional orientation of the sampleMay have to use a sun compass to measure azimuthIf the rock has been tilted, this must be measuredUsually 6-8 samples separated by few meters
27Measuring Paleomagnetic Directions In the laboratory, short cylinders are cut out and measured with a magnetometerCylinder is spun, causing its magnetism to produce a current in a nearby coil, which can be used to measure the magnetic fieldThis is repeated for the other two orthogonal directionsConvert the data into declination and inclination using the sample orientation
28Measuring Paleomagnetic Directions Performed for several cylinders from each corePlotted on a stereonet to give a stereoplot of the directionsPositive inclination (downward) is plotted with open circlesIf the samplescluster, we canassume that themagnetization hasnot changed overtime
29Measuring Paleomagnetic Directions Magnetization is usually reported as a mean direction and an errorWe assume that the samples are scattered randomlyStatistics of small number of samples is diceyMore samples are always better!Error is reported as α95A cone with thishalf angle has a95% probabilityof containing thetrue direction.
30Apparent PoleRocks 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 apartWe calculate the position of the magnetic north pole at the time of magnetizationActually where the pole was relative to the rock sampleCalled the apparent poleExample: rock formed at the equator (I = 0°)Later moved to the south poleI=0° => infer it was magnetized at equator
31Apparent PoleExample: 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 circlein declination direction (20°).
32Apparent PoleIf :the apparent paleopole isn’t at the present magnetic poleThe rock must have moved (assume sec. var. ave. out)the declination is not due northThe rock must have been rotatedthe inclination does not correspond to its current latitudeThe rock must have been moved N or S, or tiltedTilting can often be recognized and corrected forBecause of symmetry of the Earth’s magnetic fieldCannot determine longitudinal movementStill useful for determining climatic effects
33Apparent Polar Wander If a continent has moved N or S over time Paleopoles of rocks of successive ages will changeTrace out path called Apparent Polar WanderWe 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
34Apparent Polar WanderThe two paths for the period Ordovician to Jurassic are not the sameThey do have same general shapeIf we ‘close the Atlantic’, the paths are same until the Triassic when they divergeBoth landmasses weretogetherLongitudinalinformation!
35Apparent Polar WanderCan also be used to determine if a continent is made up of smaller (once separate) partsAPW paths for Siberia and Europe are the same going back to the TriassicPrior to that they wereseparateIf two continents move apartwhile at same latitude, theirpole remains the sameCannot detect movement
36Magnetism of RocksMagnetization 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 magnetismParamagnetism: temporary field that goes away when applied field is removed.Ferromagnetism: permanent field that remains when applied field is removed.
37Magnetism 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 structureOn average, they are random, hence the internal field is 0When 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 fieldRequires energy to rotateWhen field is removed, theyremain alignedRemnant magnetization
38Magnetism of RocksMagnetic materials above a certain size (0.001 to 1mm) form magnetic domainsDomains have high alignmentBounded by domain wallsTend to align with crystal imperfectionsDifficult to move => remnant magnetizationEasier to change magnetization of multi-domain materialsLess remnantmagnetization
39Blocking Temperatures If the temperature of material is slowly raised, thermal oscillations will cause the domain walls to move or rotateIn the absence of a magnetic field, randomizes the domainsDifferent domain walls require different temperatures to move themDifferent Blocking temperaturesLeads to progressive thermaldemagnetization
40Curie TemperatureIf the temperature of material is high enough, the individual atomic magnets cease to alignSpontaneous magnetization disappearsCharacteristic temperature of the materialCurie temperature, TcAlways higher than blocking temperature
41Earth’s Magnetic Field The Earth’s core is above the Curie temperatureEstimates range from °KNo remnant magnetizationCannot be source of the Earth’s magnetic field
42Thermal Remnant Magnetization If the temperature of material is slowly lowered in the presence of an external magnetic fieldSome of the domains will align as it goes below the domains blocking temperatureDifferent domain have different blocking temperaturesAs the temperature is lowered, more domains will align until a net magnetization is ‘frozen inThermal Remnant MagnetizationStronger than if applied to acool rockCan persist through Geologictime
43Partial ReheatingIf a rock is reheated partway through its blocking temperatures, it can be partially remagnetized to align with the new external magnetic field.Secondary remanenceRocks must be examined for reheating!Primary and secondary remanence add together to formNatural Remanent MagnetizationPrimary remanence can be retrieved in the laboratory by heating in theabsence of anymagnetic field
443D Magnetic Vectors Magnetic vectors are inherently 3D Component diagrams (or 3D axis) are inconvenientProject the vectors onto two planes and plotStereoplots only showdirection, no magnitude
45Reheating Temperatures Intrusions (dykes) can cause reheatingMagnetization Directions of D & L antiparallelA lava sample close to the contact is reheatedChange is small until T=515°CRapidly moves toward L=> Lava was heated to 515 °CDykeLava
46Magnetic Minerals Magnetite is the mineral with the greatest remanence Maghaemite has a fairly high magnetizationImportant in soilsResponsible for magnetization of archaeological sitesCompound as well as concentration of iron determinesGrain size is also importantFine grains may be single domain, highly remanent
47Magnetization at Ambient Temp Sediments do not have thermal remanenceMagnetization takes place at ambient temperatureChemical remanent magnetization (CRM)Chemical alteration of non-mag minerals into magnetic minerals (weathering, precipitating FeO2)Depositional remanent magnetization (DRM)Influenced by flowsViscous remanent magnetization (VRM)Blocking temperature slightly above ambient TOver long time, temperature fluctuations causes slow, partial magnetizationAll Natural Remanent Magnetization not when it formed!
48Cleaning Unwanted Magnetization Thermal demagnetization can remove secondary remanenceSlow, and may change the nature of the mineralsAlternating field demagnetizationUses alternating magnetic fieldProgressively stronger fieldSample is tumbled in space to randomize the induced magnetization from the applied fieldBoth depend upon the secondary remanence being easier to removeNot true for chemical remanenceCleaning or washing of unwanted dirty magnetism
49Field Tests Fold Test Can determine if the magnetization was acquired before or after the foldingCan also be applied to tiltingConglomeration testCompare magnetization of the clastsBaked contact testDyke lava example
50Magnetostratigraphy Reversals of the Earth’s magnetic field Global Occur abruptlyEasy to recognizeAllows us to establish stratigraphic orderAllows us to date rocks
51MagnetostratigraphySuppose we have an isolated lava that we measure both the age and remanent magnetizationThere are reversals at 0.7, 1.6, 1.9 & 2.4 MaSuppose that we have a continuous succession of 50 lavas extruded at short intervalsOldest is 10 MaInterval is 200,000 yrNo such lavas exist
52Magnetostratigraphy Ocean floor spreading at mid-ocean ridges Form at steady rateOldest are 160 MaPreserve the magnetic reversals for past 160 Ma
53Magnetostratigraphy C is a chron -significant interval of one polarity M is for MesozoicNote the long N interval during the Cretaceous
54Mineral MagnetismInherent magnetic properties can also be used to measure geological processesWhen an external field is applied, materials become more magnetizedMagnetic susceptibility (χ) is a measure of the induced magnetization due to an external magnetic fieldSusceptibility can vary in direction (anisotropy)Called Magnetic fabric
55Mineral MagnetismThese properties can be used to measure geologic eventsDifferent materials have different susceptibilitiesCores in glacial varves can be used to count the varvesDue to seasonal differences in deposition particlesFlow can causealignmentMagnetic fabricin dyke swarmsVertical flow nearcenter, horizontalflow away fromcenter