Lecture 5: MagneticsLecture 5: Magnetics Much of this information is courtesy of NASAMuch of this information is courtesy of NASA GSFC= Goddard Space Flight CenterGSFC= Goddard Space Flight Center They do a lot on magneticsThey do a lot on magnetics More info at: http://www-spof.gsfc.nasa.gov/Education/Intro.htmMore info at: http://www-spof.gsfc.nasa.gov/Education/Intro.htm

Magnetics: ContentsMagnetics: Contents

Historical Background Historical Background Basic Magnetic theoryBasic Magnetic theory

magnetismmagnetism magnetic metalsmagnetic metals electical magnetismelectical magnetism

The Earth’s magnetic fieldThe Earth’s magnetic field characteristicscharacteristics historyhistory

Why we careWhy we care mapping/navigationmapping/navigation geology/plate tectonicsgeology/plate tectonics

Basic MagnetismBasic Magnetism

A magnetized bar has two A magnetized bar has two poles:poles: the north (N) end points the north (N) end points

northwards and its south (S) northwards and its south (S) S end southwards. S end southwards.

Also, N will repel N of another Also, N will repel N of another magnet, S will repel S, but N magnet, S will repel S, but N and S attract each other. and S attract each other.

The region where this is The region where this is observed is loosely called a observed is loosely called a magnetic field.magnetic field.

Okay, so this is pretty Okay, so this is pretty basic; we need to start basic; we need to start somewhere.somewhere.

What What isis Magnetism? Magnetism?

In nature, magnetic fields are In nature, magnetic fields are produced in:produced in: the rarefied gas of spacethe rarefied gas of space the glowing heat of the glowing heat of

sunspots sunspots the molten core of the Earth. the molten core of the Earth. magnetic mineralsmagnetic minerals

In all cases, magnetism In all cases, magnetism mustmust be produced by be produced by electric currents, but finding electric currents, but finding how those currents are how those currents are produced remains a major produced remains a major challenge.challenge.

Some BackgroundSome Background

The ancient Greeks, originally The ancient Greeks, originally those near the city of those near the city of Magnesia, and also the early Magnesia, and also the early Chinese knew about strange Chinese knew about strange and rare stones (possibly and rare stones (possibly chunks of iron ore struck by chunks of iron ore struck by lightning) with the power to lightning) with the power to attract iron.attract iron.

A steel needle stroked with A steel needle stroked with such a "lodestone" became such a "lodestone" became “magnetic" as well, “magnetic" as well,

Around 1000, the Chinese Around 1000, the Chinese found that such a needle, found that such a needle, when freely suspended, when freely suspended, pointed north-south.pointed north-south.

More BackgroundMore Background

The magnetic compass soon spread to Europe.The magnetic compass soon spread to Europe.

Columbus used it when he crossed the Atlantic Columbus used it when he crossed the Atlantic oceanocean He noted that the needle deviated slightly from

exact north (as indicated by the stars).

And, the deviation changed during the voyage.

Still more background...Still more background...

Around 1600 William Gilbert, physician Around 1600 William Gilbert, physician to Queen Elizabeth I of England, to Queen Elizabeth I of England, proposed an explanation: proposed an explanation: the Earth itself was a giant magnet, the Earth itself was a giant magnet, its magnetic poles are some its magnetic poles are some

distance away from its geographic distance away from its geographic ones ones

As a model, he used a spherical As a model, he used a spherical magnet, which he called , the "little magnet, which he called , the "little Earth" or Terella.Earth" or Terella. He moved a small compass over the He moved a small compass over the

surface of the terrellasurface of the terrella This demonstrated that it always This demonstrated that it always

pointed towards its magnetic poles.pointed towards its magnetic poles.

What is Magnetism?What is Magnetism? Until 1821, only one kind of Until 1821, only one kind of

magnetism was known, the one magnetism was known, the one produced by iron magnets. produced by iron magnets.

Then a Danish scientist, Hans Then a Danish scientist, Hans Christian Oersted discovered Christian Oersted discovered electromagnetism:electromagnetism: He noticed that the flow of He noticed that the flow of

electrical current in a wire electrical current in a wire caused a nearby compass caused a nearby compass needle to move. needle to move.

The new phenomenon was The new phenomenon was studied in France by Andre-studied in France by Andre-Marie Ampere, Marie Ampere,

He concluded that the nature of He concluded that the nature of magnetism was quite different magnetism was quite different from what everyone had from what everyone had believed. believed.

All magnetism is related to electricityAll magnetism is related to electricity

There thus exists There thus exists two kinds of forcestwo kinds of forces associated with electricity:associated with electricity:

electricelectric

magnetic. magnetic.

In 1864 In 1864 James Clerk MaxwellJames Clerk Maxwell demonstrated demonstrated a subtle connection between the two types of a subtle connection between the two types of forceforce

The connection involves the velocity of light. The connection involves the velocity of light. From this connection sprang the idea that light From this connection sprang the idea that light

was an electric phenomenon, was an electric phenomenon, This led to the discovery of radio waves, the This led to the discovery of radio waves, the

theory of relativity and a great deal of present-theory of relativity and a great deal of present-day physics.day physics.

ElectromagnetismElectromagnetism

The fundamental nature of The fundamental nature of magnetism is not associated magnetism is not associated with magnetic poles or iron with magnetic poles or iron magnets, magnets,

It is all bout electric It is all bout electric currents. currents.

The magnetic force is The magnetic force is basically a force between basically a force between electric currentselectric currents

A coil of wire with current A coil of wire with current flowing acts like a strong flowing acts like a strong magnet with magnetic poles magnet with magnetic poles at each end (an at each end (an "electromagnet")."electromagnet").

But What But What isis Magnetism? Magnetism?

Where does this “force” Where does this “force” come from?come from?

We already said that We already said that magnetism and electricity magnetism and electricity always coexistalways coexist when electricty flows, a when electricty flows, a

magnetic field is generatedmagnetic field is generated these co-varythese co-vary

It is all related to It is all related to electricity, regardless of electricity, regardless of where you observe itwhere you observe it..

True regardless of scaleTrue regardless of scale

Magnetism at the Atomic levelMagnetism at the Atomic level

Matter consists of electrically Matter consists of electrically charged particles:charged particles: each atom consists each atom consists

electrons (-) swarming electrons (-) swarming around a nucleus (+).around a nucleus (+).

Imbalances between + /- Imbalances between + /- results in a “static” chargeresults in a “static” charge

Electrons spinning around Electrons spinning around a nucleus represent an a nucleus represent an electric currentelectric current currents produce currents produce

magnetismmagnetism each atom has an inherent each atom has an inherent

magnetic fieldmagnetic field

Atomic MagnetismAtomic Magnetism

Some atoms have net Some atoms have net magnetic moments and magnetic moments and some don’tsome don’t can be explained by quantum can be explained by quantum

physicsphysics well beyond the scope of this well beyond the scope of this

classclass In some substances, these atoms In some substances, these atoms

and their moments naturally line and their moments naturally line up and reinforce each otherup and reinforce each other iron can be magnetized iron can be magnetized natural mineralsnatural minerals

high temperatures overcomes this high temperatures overcomes this thermal motion disrupts alignmentthermal motion disrupts alignment

The Earth’s Magnetic FieldThe Earth’s Magnetic Field

The earth, like many The earth, like many other planets, has a other planets, has a magnetic field. magnetic field.

Why? We honestly Why? We honestly don’t knowdon’t know

best guess is the “dynamo best guess is the “dynamo theory”theory”

The Earth’s Magnetic FieldThe Earth’s Magnetic Field

The earth’s field is best described by “field lines”The earth’s field is best described by “field lines” Do NOT describe lines of equal forceDo NOT describe lines of equal force define direction of force at each pointdefine direction of force at each point Note that they are:Note that they are:

parallel to the earth’s surface at the magnetic equatorparallel to the earth’s surface at the magnetic equator vertical at the magnetic polevertical at the magnetic pole

The Earth’s Magnetic FieldThe Earth’s Magnetic Field

Where Field lines converge, the magnetic force is strong, and spread out where it is weak.

Field lines spread out from one pole and converge towards the other

The magnetic force is strongest near the poles where they come together.

Dynamos (currents from outer space)Dynamos (currents from outer space) How can electric currents be generated in space--or, for that

matter, on the Sun and in the Earth's core?

It appears that some electrically conducting fluid appears to be moving through a magnetic field

plasma in space and on the Sun

molten iron (probably) in the Earth's core.

Dynamos (currents from outer space)Dynamos (currents from outer space)

It can then be shown from the principles of physics that if a closed electric circuit exists

Parts of the current are moving through a magnetic field while other parts are not,

Therefore, an electric current will arise (additional conditions must also be satisfied).

The electric energy needed to drive the current is taken from the motion, which is slowed down.

Magnetics on EarthMagnetics on Earth The earth is a dipole magnet, The earth is a dipole magnet,

with north and south poles with north and south poles that do not exactly coincide that do not exactly coincide with the geographic poles.with the geographic poles. Geographic Pole: where axis of Geographic Pole: where axis of

rotation intersects surfacerotation intersects surface Magnetic Pole: Where Compasses Magnetic Pole: Where Compasses

pointpoint Geomagnetic Pole: Where magnetic Geomagnetic Pole: Where magnetic

lines are verticallines are vertical The field is described by:The field is described by:

Horizontal forceHorizontal force magnitude (gammas)magnitude (gammas) inclinationinclination declinationdeclination polarity (normal or reversed)polarity (normal or reversed)

MagneticsMagnetics

Principal magnetic mineral is magnetite Principal magnetic mineral is magnetite It is contained in basalts (Igneous)It is contained in basalts (Igneous) therefore in most sedimentary rocks as well.therefore in most sedimentary rocks as well.

Almost all rocks can have some magnetism Almost all rocks can have some magnetism but it will differ depending on the rock type but it will differ depending on the rock type and historyand history

Let’s start by considering rock characteristicsLet’s start by considering rock characteristics

The Rock CycleThe Rock Cycle The origin of a rock determines its properties and compositionThe origin of a rock determines its properties and composition

These 3 These 3 are most are most important important in the in the oceanocean

The Rock CycleThe Rock Cycle Sediment becomes sedimentary rock at the bottom of the oceanSediment becomes sedimentary rock at the bottom of the ocean

This This process process takes takes place in place in the oceanthe ocean

The Rock CycleThe Rock Cycle

As they form, both igneous and sedimentary rocks record As they form, both igneous and sedimentary rocks record whatever the Earth’s magnetic field is at the timewhatever the Earth’s magnetic field is at the time

These all These all record the record the Earth’s Earth’s magnetic magnetic fieldfield

Magnetics on EarthMagnetics on Earth

If you heat a rock above it's Curie point If you heat a rock above it's Curie point (roughly 575(roughly 575ooC), it will take the ambient C), it will take the ambient magnetic field as it cools.magnetic field as it cools.

TRM= Thermal Remnant MagnetismTRM= Thermal Remnant Magnetism remnant means that which is retained or remainsremnant means that which is retained or remains igneous rocks onlyigneous rocks only remains when igneous rocks coolremains when igneous rocks cool records earth’s magnetic field at the timerecords earth’s magnetic field at the time inclination, declination, strengthinclination, declination, strength

Magnetics on EarthMagnetics on Earth

Also, magnetic mineral particles settling onto Also, magnetic mineral particles settling onto the sea floor will (on average) align the sea floor will (on average) align themselves with the ambient field.themselves with the ambient field.

DRM = Detrital Remnant MagnetismDRM = Detrital Remnant Magnetism recorded by sediment particles as they settlerecorded by sediment particles as they settle weaker than TRMweaker than TRM may be destroyed by post-depositional actionsmay be destroyed by post-depositional actions

Therefore, both igneous rocks and Therefore, both igneous rocks and sedimentary rocks can sedimentary rocks can record the magnetic record the magnetic fieldfield at the time of their deposition. at the time of their deposition. Igneous rock records are much strongerIgneous rock records are much stronger

MagneticsMagnetics These records of the earth’s field are These records of the earth’s field are

important because of two things:important because of two things: 1) The rock could have been moved since it's formation and 1) The rock could have been moved since it's formation and

the magnetics can tell the original location,the magnetics can tell the original location, 2) The polarity of the earth changes randomly and rocks 2) The polarity of the earth changes randomly and rocks

record this. record this.

The Earth’s The Earth’s Magnetic FieldMagnetic Field

Through its history, the Through its history, the Earth’s field has Earth’s field has reversed many timesreversed many times

north and south north and south magnetic poles switch magnetic poles switch

We have no idea whyWe have no idea why or how long it takesor how long it takes or when it will happen or when it will happen

againagain

Understanding the earth’s magnetic propertiesUnderstanding the earth’s magnetic properties

these reversals are recorded in rocks as they formthese reversals are recorded in rocks as they form rocks recorded during times when the earth’s magnetism was “normal” have rocks recorded during times when the earth’s magnetism was “normal” have

“normal magnetism”“normal magnetism”

Understanding the earth’s magnetic propertiesUnderstanding the earth’s magnetic properties Normal = in the same direction as modern fieldNormal = in the same direction as modern field

adds to earth’s fieldadds to earth’s field results in a positive (higher than normal) anomalyresults in a positive (higher than normal) anomaly

Understanding the earth’s magnetic propertiesUnderstanding the earth’s magnetic properties notice that “normal” field direction varies from place to placenotice that “normal” field direction varies from place to place rocks record the direction as well as the intensityrocks record the direction as well as the intensity direction is more vertical near magnetic polesdirection is more vertical near magnetic poles

The Earth’s The Earth’s Magnetic FieldMagnetic Field

In this way, the history of In this way, the history of the Earth’s magnetic field the Earth’s magnetic field has been recordedhas been recorded both igneous and both igneous and

sedimentary rockssedimentary rocks same pattern all over the same pattern all over the

EarthEarth This diagram is NOT a coreThis diagram is NOT a core

it’s a time chart showing it’s a time chart showing the changesthe changes

it was compiled from many, it was compiled from many, many samples of terrestrial many samples of terrestrial volcanoesvolcanoes

The Earth’s Magnetic FieldThe Earth’s Magnetic Field

This chart shows the This chart shows the individual samplesindividual samples

each one represents lots of each one represents lots of workwork determine agedetermine age determine magnetic determine magnetic

signaturesignature Note that orientation of core Note that orientation of core

is not that importantis not that important normal/reversed can be normal/reversed can be

determined by vertical determined by vertical componentcomponent

works better nearer works better nearer magnetic polesmagnetic poles

The Earth’s Magnetic RecordThe Earth’s Magnetic Record

Now look at the seafloorNow look at the seafloor first where seafloor comes fromfirst where seafloor comes from then how what the magnetic signature looks likethen how what the magnetic signature looks like

At spreading centers, new seafloor is being createdAt spreading centers, new seafloor is being created generated at each sidegenerated at each side symmetricalsymmetrical much more latermuch more later

rocksrocks processesprocesses etc.etc.

Determining magnetic anomaliesDetermining magnetic anomalies

Like gravity, magnetism is described by anomaliesLike gravity, magnetism is described by anomalies An “anomaly” is a place where the magnetism isn’t “normal”An “anomaly” is a place where the magnetism isn’t “normal”

can be either higher or lower than ambient (average) fieldcan be either higher or lower than ambient (average) field essentially all seafloor has either higher or lower magnetism than “average”essentially all seafloor has either higher or lower magnetism than “average”

the discovery of these anomalies was critical to understanding sea floor the discovery of these anomalies was critical to understanding sea floor spreadingspreading

The Earth’s Magnetic The Earth’s Magnetic FieldField

In the 40’s maps of the In the 40’s maps of the seafloor’s magnetism were seafloor’s magnetism were generatedgenerated

needed to find “anamoliesneeded to find “anamolies = submarines= submarines Here’s what they found:Here’s what they found:

-series of parallel, linear -series of parallel, linear anomaliesanomalies

- strange alignments and - strange alignments and offsetsoffsets

initially had wild initially had wild explanations:explanations: sediment pondssediment ponds fault blocksfault blocks

Seafloor magnetic anomaliesSeafloor magnetic anomalies

Later it was found that these Later it was found that these anomalies appear in all anomalies appear in all ocean basinsocean basins

they parallel the ridgesthey parallel the ridges the pattern is the same in the pattern is the same in

each caseeach case the pattern is the same on the pattern is the same on

either side of the ridgeeither side of the ridge this was KEY information in this was KEY information in

solving the plate tectonic solving the plate tectonic puzzlepuzzle

This is Iceland, but the This is Iceland, but the pattern is the same pattern is the same everywhereeverywhere

Sea-FloorSea-FloorMagnetic Magnetic anomalies anomalies explainedexplained

At the spreading centers At the spreading centers (mid ocean ridges):(mid ocean ridges): new sea floor is new sea floor is

generatedgenerated igneous rockigneous rock linear featureslinear features

this new rock records this new rock records the earth’s field as it the earth’s field as it coolscools

Sea-FloorSea-FloorMagnetismMagnetism

This produces This produces “Linear Magnetic “Linear Magnetic Anomalies” (LMAs)Anomalies” (LMAs)

In the 1960s In the 1960s scientists figured scientists figured out that these out that these magnetic anomaly magnetic anomaly stripes were stripes were evidence of sea evidence of sea floor spreadingfloor spreading

Sea-FloorSea-FloorMagnetismMagnetism

Magnetic anomalies can be used to Magnetic anomalies can be used to calculate the rate of sea-floor spreading.calculate the rate of sea-floor spreading. Determine the distance between parallel Determine the distance between parallel

stripesstripes Use magnetic time scale based on terrestrial Use magnetic time scale based on terrestrial

igneous samplesigneous samples The data indicate rates of 1 to 10 cm per yearThe data indicate rates of 1 to 10 cm per year about as fast as your fingernail growsabout as fast as your fingernail grows

Using magnetic anomalies to calculate Using magnetic anomalies to calculate the rate of sea floor spreading.the rate of sea floor spreading.

Sea-FloorSea-FloorMagnetismMagnetism

The anomalies on either side of a spreading center The anomalies on either side of a spreading center are remarkably symmetricalare remarkably symmetrical this shows the same transect reversed and overlaidthis shows the same transect reversed and overlaid note the incredible similaritiesnote the incredible similarities

This is a global signatureThis is a global signature same in every oceansame in every ocean absolute chronologyabsolute chronology

Magnetism SummaryMagnetism Summary

To summarize what we have learned about the Earth’s To summarize what we have learned about the Earth’s magnetic field:magnetic field: described by lines which are horizontal at magnetic equator and described by lines which are horizontal at magnetic equator and

vertical at magnetic polesvertical at magnetic poles generated by rotation of the earth and the liquid, conductive coregenerated by rotation of the earth and the liquid, conductive core reverses polarity irregularlyreverses polarity irregularly polarity recorded by both igneous and sedimentary rockspolarity recorded by both igneous and sedimentary rocks linear magnetic anomalies created at spreading centerslinear magnetic anomalies created at spreading centers symmetrical signal on either sidesymmetrical signal on either side global, absolute signalglobal, absolute signal