Magnetism

1. N lost Magnetism W E S

2. Magnetic Dipole • Experiment: bar magnet, paper, iron filings. • Iron filings will array themselves around the magnetic, along magnetic field lines. • Magnet = dipole (two-pole magnet)

3. The Earth as a Dipole • Earth’s magnetic field is well-approximated as a dipole. • Magnetic field lines radiate from Earth’s N and S magnetic poles • Charged particles trapped on magnetic field lines (like iron filings) = magneto-sphere

4. Earth’s Dipole Magnetic Field • Earth’s magnetic field lines are not symmetrical. • Solar wind (stream of ionized gases from the Sun, 400 km/s) deforms magnetosphere --> compressed on one side, stretched out on the other. • On Sun side: 80 to 60,000 km • Away from Sun: extends >30,000 km

5. Magnetic N isn’t Geographic N • Magnetic field axis is tipped with respect to rotational axis • Difference = 11 degrees • Geographic north = magnetic south

6. Rock Magnetization: Igneous • Volcanic rock erupts and cools. • As rock cools, iron minerals lock in ambient magnetic field. • Acquisition of magnetization occurs at specific temperature called Curie temperature (e.g., 580oC for magnetite and 680oC for hematite). • Blocking temperature is few tens of degrees lower than Curie temperature; is where magnetized grains cannot be reoriented.

7. What causes the magnetic field? • Curie temperature>Temp of core (~ Sun surface temp) = 5000oC. • So analogy of bar magnet is not great -- core of Earth is too hot to allow coherent magnetic signal from magnetism frozen in rock. • Earth’s magnetic field would die away in 20,000 years if not constantly being regenerated (has been in existence for at least 3.5 Ga).

8. Where does the Earth’s magnetic field come from? • Don’t really know. • Somehow linked to rotation of Earth (Venus has similar iron composition in core, but slower rotation period (243 days) and no magnetic field). • Probably has to do with rotation of hot metallic material in Earth’s outer core. Magnetohydrodynamics.

9. Where does the Earth’s magnetic field come from? • “Dynamo effect” or “geodynamo” (like an electric generator). R. Nave

10. Where does the Earth’s magnetic field come from? • Solid inner core = size of moon, temperature of Sun. Overlain by liquid outer core. • Thermal and chemical (compositional) buoyancy drive convection in outer core. • Thermal buoyancy = Earth is cooling slowly • Chemical buoyancy = Iron-rich alloy comprising outer core solidifies, giving off latent heat of fusion.

11. Where does the Earth’s magnetic field come from? • Buoyancy --> fluid rises. • Coriolis force (rotation of planet) shears fluid flow. • Electric current (flow of charged particles) results from conductor (iron) moving through a magnetic field. • Electromagnet: Moving charged particles (like convecting iron) cause magnetic field.

12. Earth’s Magnetic Field • Earth’s total magnetic field = dipole field + non-dipole field • Magnitude of non-dipole field < magnitude of dipole field • International Geomagnetic Reference Field (IGRF) published regularly (every few years). Like go in gravity. Describe local variations in magnetic field with respect to IGRF.

13. Magnetic Reversals • Earth’s magnetic field aperiodically reverses (magnetic north becomes magnetic south). • Reversals have occurred for at least 3.5 Ga. • Reversals take about 10,000 years. • Mean time between reversals ~200,000 years. • Lengths of reversals varies greatly (100s of kyrs to millions of years).

14. Magnetic Reversals • Unclear exactly how reversal occurs. Two possibilities: (1) Existing magnetic field diffuses away and rematerializes in opposite orientation. (2) Poles stay intact and just flip. • Existing data cannot differentiate between these possibilities.

15. Harvard Experiment • Laboratory models have not been successful in reproducing the geodynamo. • Harvard scientists in the mid-90s set up a computer model to simulate the geodynamo: • Programmed in magnetohydrodramic equations • Set up a 3D, spherical Earth • Let it run • Have simulated 300,000 Earth years. • Produced a magnetic field with shape and intensity similar to the Earth’s! • 36,000 years into the simulation, the field spontaneously reversed. Reversal took about 1,000 years.

16. Harvard Experiment PSC Scientific Visualization

17. Harvard Experiment PSC Scientific Visualization

18. Harvard Experiment PSC Scientific Visualization

19. A curious observation • Oceanographic expeditions in the early- to mid-1900s carried magnetometers to measure the magnetic field at sea. (Magnetometers were used to detect submarines). • Subtracted out IGRF to calculate marine magnetic anomalies. • Early published maps of marine magnetic anomalies contained curious striped patterns (e.g., off western N. America, 1961).

20. A curious observation • As time passed, more such maps were collected. • “Stripes” were first understood by Vine and Matthews, as well as Morley, in 1963. • Oceanic crust serves as a “tape recorder” of the orientation of the Earth’s magnetic field. Each time the poles reverse, a “stripe” is created in the magnetic anomaly pattern.

21. A curious observation

22. A curious observation S N “normal” N N

23. A curious observation N S “reverse” N R R N

24. A curious observation S N N R N N R N

25. A curious observation USGS

26. A curious observation • Width of a magnetic “stripe” is determined by (1) how fast the ridge is spreading, and (2) the length of time between magnetic reversals. • How do you calibrate the “stripes” (e.g., assign ages to them)?

27. Rock Magnetization: Sedimentary • Much lower magnetization than sedimentary rocks • Depositional or detrital remanent magnetization (DRM): As sediments are deposited in water, magnetized grains will orient themselves with the Earth’s magnetic field. • Chemical remanent magnetization (CRM): After deposition, chemical growth of iron oxides occurs in situ. After magnetic grains reach a critical size, magnetization is locked in.

28. Calibrating the stripes • Two approaches: terrestrial and marine. • Terrestrial: • Go to the field and measure the magnetic orientation of a rock sample in situ. • Then measure its age in the lab. • Completed in 1966; went back 4 Ma. • Marine: • Collect magnetic anomaly profiles perpendicular to the axis of the Mid-Atlantic Ridge in the South Atlantic. • Assume a constant spreading rate for the last 80 Ma. • Calibrate using seafloor samples collected by DSDP (precursor to ODP), dated using fossils. • Completed in 1968.

29. Calibrating the stripes • With additional marine data, the geomagnetic reversal time scale has undergone continual refinement.

30. Geomagnetic Time Scale • Specific reversal anomalies have been numbered for reference. • 0 to 80 Ma: Anomaly 0 to Anomaly 33 • 125 to 157 Ma: M0 to M27 (M = Mesozoic) • Magnetic Quiet Zone: Cretaceous, 83-124 Ma

31. Using the stripes • Go out on a cruise • Collect marine gravity data. • Reduce to magnetic anomalies. • Make a magnetic anomaly profile. • Wiggle-match.

32. Crustal Age Map • Compile magnetic anomaly picks from many cruises. • Plot locations of specific reversals on map. • Contour ages. • Result = Crustal anomaly age map. IMPORTANT NOTE: Oldest seafloor is ~180 Ma. Everything else has been subducted.

33. Crustal Age Map

34. Plate Reconstructions How do we know where continents were 100 Ma? For a given block of crust,paleomagnetic data can be used to determine the latitude and orientation of the ridge that created it. Use isochron map --> remove intervening blocks of crust.

35. Plate Reconstructions • Limitation: Marine-based reconstructions can only go back 180 Ma. • Earlier than that, must rely on • continental magnetic data • faunal data • etc. • Limitation: Full reconstructions are possible only for ocean basins that contain only ridges. Data are lost at subduction zones (Pacific reconstructions are difficult).