Lecture 5: Magnetics

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

2. Magnetics: Contents • Historical Background • Basic Magnetic theory • magnetism • magnetic metals • electical magnetism • The Earth’s magnetic field • characteristics • history • Why we care • mapping/navigation • geology/plate tectonics

3. Basic Magnetism • A magnetized bar has two poles: • the north (N) end points northwards and its south (S) S end southwards. • Also, N will repel N of another magnet, S will repel S, but N and S attract each other. • The region where this is observed is loosely called a magnetic field. • Okay, so this is pretty basic; we need to start somewhere.

4. What is Magnetism? • In nature, magnetic fields are produced in: • the rarefied gas of space • the glowing heat of sunspots • the molten core of the Earth. • magnetic minerals • In all cases, magnetism must be produced by electric currents, but finding how those currents are produced remains a major challenge.

5. Some Background • The ancient Greeks, originally those near the city of Magnesia, and also the early Chinese knew about strange and rare stones (possibly chunks of iron ore struck by lightning) with the power to attract iron. • A steel needle stroked with such a "lodestone" became “magnetic" as well, • Around 1000, the Chinese found that such a needle, when freely suspended, pointed north-south.

6. More Background • The magnetic compass soon spread to Europe. • Columbus used it when he crossed the Atlantic ocean • He noted that the needle deviated slightly from exact north (as indicated by the stars). • And, the deviation changed during the voyage.

7. Still more background... • Around 1600 William Gilbert, physician to Queen Elizabeth I of England, proposed an explanation: • the Earth itself was a giant magnet, • its magnetic poles are some distance away from its geographic ones • As a model, he used a spherical magnet, which he called , the "little Earth" or Terella. • He moved a small compass over the surface of the terrella • This demonstrated that it always pointed towards its magnetic poles.

8. What is Magnetism? • Until 1821, only one kind of magnetism was known, the one produced by iron magnets. • Then a Danish scientist, Hans Christian Oersted discovered electromagnetism: • He noticed that the flow of electrical current in a wire caused a nearby compass needle to move. • The new phenomenon was studied in France by Andre-Marie Ampere, • He concluded that the nature of magnetism was quite different from what everyone had believed.

9. All magnetism is related to electricity • There thus exists two kinds of forces associated with electricity: • electric • magnetic. • In 1864 James Clerk Maxwell demonstrated a subtle connection between the two types of force • The connection involves the velocity of light. • From this connection sprang the idea that light was an electric phenomenon, • This led to the discovery of radio waves, the theory of relativity and a great deal of present-day physics.

10. Electromagnetism • The fundamental nature of magnetism is not associated with magnetic poles or iron magnets, • It is all bout electric currents. • The magnetic force is basically a force between electric currents • A coil of wire with current flowing acts like a strong magnet with magnetic poles at each end (an "electromagnet").

11. But What is Magnetism? • Where does this “force” come from? • We already said that magnetism and electricity always coexist • when electricty flows, a magnetic field is generated • these co-vary • It is all related to electricity, regardless of where you observe it. • True regardless of scale

12. Magnetism at the Atomic level • Matter consists of electrically charged particles: • each atom consists electrons (-) swarming around a nucleus (+). • Imbalances between + /- results in a “static” charge • Electrons spinning around a nucleus represent an electric current • currents produce magnetism • each atom has an inherent magnetic field

13. Atomic Magnetism • Some atoms have net magnetic moments and some don’t • can be explained by quantum physics • well beyond the scope of this class • In some substances, these atoms and their moments naturally line up and reinforce each other • iron can be magnetized • natural minerals • high temperatures overcomes this • thermal motion disrupts alignment

14. The Earth’s Magnetic Field • The earth, like many other planets, has a magnetic field. • Why? We honestly don’t know • best guess is the “dynamo theory”

15. The Earth’s Magnetic Field • The earth’s field is best described by “field lines” • Do NOT describe lines of equal force • define direction of force at each point • Note that they are: • parallel to the earth’s surface at the magnetic equator • vertical at the magnetic pole

16. The 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.

17. 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.

18. 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.

19. Magnetics on Earth • The earth is a dipole magnet, with north and south poles that do not exactly coincide with the geographic poles. • Geographic Pole: where axis of rotation intersects surface • Magnetic Pole: Where Compasses point • Geomagnetic Pole: Where magnetic lines are vertical • The field is described by: • Horizontal force • magnitude (gammas) • inclination • declination • polarity (normal or reversed)

20. Magnetics • Principal magnetic mineral is magnetite • It is contained in basalts (Igneous) • therefore in most sedimentary rocks as well. • Almost all rocks can have some magnetism but it will differ depending on the rock type and history • Let’s start by considering rock characteristics

21. These 3 are most important in the ocean The Rock Cycle • The origin of a rock determines its properties and composition

22. The Rock Cycle • Sediment becomes sedimentary rock at the bottom of the ocean This process takes place in the ocean

23. These all record the Earth’s magnetic field The Rock Cycle • As they form, both igneous and sedimentary rocks record whatever the Earth’s magnetic field is at the time

24. Magnetics on Earth • If you heat a rock above it's Curie point (roughly 575oC), it will take the ambient magnetic field as it cools. • TRM= Thermal Remnant Magnetism • remnant means that which is retained or remains • igneous rocks only • remains when igneous rocks cool • records earth’s magnetic field at the time • inclination, declination, strength

25. Magnetics on Earth • Also, magnetic mineral particles settling onto the sea floor will (on average) align themselves with the ambient field. • DRM = Detrital Remnant Magnetism • recorded by sediment particles as they settle • weaker than TRM • may be destroyed by post-depositional actions • Therefore, both igneous rocks and sedimentary rocks can record the magnetic field at the time of their deposition. • Igneous rock records are much stronger

26. Magnetics • These records of the earth’s field are important because of two things: • 1) The rock could have been moved since it's formation and the magnetics can tell the original location, • 2) The polarity of the earth changes randomly and rocks record this.

27. The Earth’s Magnetic Field • Through its history, the Earth’s field has reversed many times • north and south magnetic poles switch • We have no idea why • or how long it takes • or when it will happen again

28. Understanding the earth’s magnetic properties • these reversals are recorded in rocks as they form • rocks recorded during times when the earth’s magnetism was “normal” have “normal magnetism”

29. Understanding the earth’s magnetic properties • Normal = in the same direction as modern field • adds to earth’s field • results in a positive (higher than normal) anomaly

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

31. The Earth’s Magnetic Field • In this way, the history of the Earth’s magnetic field has been recorded • both igneous and sedimentary rocks • same pattern all over the Earth • This diagram is NOT a core • it’s a time chart showing the changes • it was compiled from many, many samples of terrestrial volcanoes

32. The Earth’s Magnetic Field • This chart shows the individual samples • each one represents lots of work • determine age • determine magnetic signature • Note that orientation of core is not that important • normal/reversed can be determined by vertical component • works better nearer magnetic poles

33. The Earth’s Magnetic Record • Now look at the seafloor • first where seafloor comes from • then how what the magnetic signature looks like • At spreading centers, new seafloor is being created • generated at each side • symmetrical • much more later • rocks • processes • etc.

34. Determining magnetic anomalies • Like gravity, magnetism is described by anomalies • An “anomaly” is a place where the magnetism isn’t “normal” • can be either higher or lower than ambient (average) field • essentially all seafloor has either higher or lower magnetism than “average” • the discovery of these anomalies was critical to understanding sea floor spreading

35. The Earth’s Magnetic Field • In the 40’s maps of the seafloor’s magnetism were generated • needed to find “anamolies • = submarines • Here’s what they found: • -series of parallel, linear anomalies • - strange alignments and offsets • initially had wild explanations: • sediment ponds • fault blocks

36. Seafloor magnetic anomalies • Later it was found that these anomalies appear in all ocean basins • they parallel the ridges • the pattern is the same in each case • the pattern is the same on either side of the ridge • this was KEY information in solving the plate tectonic puzzle • This is Iceland, but the pattern is the same everywhere

37. Sea-FloorMagnetic anomalies explained • At the spreading centers (mid ocean ridges): • new sea floor is generated • igneous rock • linear features • this new rock records the earth’s field as it cools

38. Sea-FloorMagnetism • This produces “Linear Magnetic Anomalies” (LMAs) • In the 1960s scientists figured out that these magnetic anomaly stripes were evidence of sea floor spreading

39. Sea-FloorMagnetism • Magnetic anomalies can be used to calculate the rate of sea-floor spreading. • Determine the distance between parallel stripes • Use magnetic time scale based on terrestrial igneous samples • The data indicate rates of 1 to 10 cm per year • about as fast as your fingernail grows

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

41. Sea-FloorMagnetism • The anomalies on either side of a spreading center are remarkably symmetrical • this shows the same transect reversed and overlaid • note the incredible similarities • This is a global signature • same in every ocean • absolute chronology

42. Magnetism Summary • To summarize what we have learned about the Earth’s magnetic field: • described by lines which are horizontal at magnetic equator and vertical at magnetic poles • generated by rotation of the earth and the liquid, conductive core • reverses polarity irregularly • polarity recorded by both igneous and sedimentary rocks • linear magnetic anomalies created at spreading centers • symmetrical signal on either side • global, absolute signal