Required Readings - University of British Columbia 1) Magnetic reversals and seafloor spreading 2) Apparent

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  • Electromagnetism and theElectromagnetism and the EarthEarth’’s Magnetic Field:s Magnetic Field:

    • What are its characteristics? • Shape, properties • Does it change?

    • How is it generated? • What does that tell us about the Earth?

    • Applications? Effects?

    Required Readings: 1st edition: Chapter 4 [also: Ch.2 37- 40] 9th US edition: Chapter 17 [also: Ch.19 471-473]

  • Electromagnetic Field Basics

    • An EM field describes the behaviour of forces that are applied to a moving, charged particle

    • Electromagnetic fields are generated at the atomic level • all atoms generate magnetic fields (moving electrons)

    • Large-scale magnetic fields are produced by the alignment of smaller-scale fields

  • Geomagnetism - Characteristics of the EarthGeomagnetism - Characteristics of the Earth’’s Fields Field • the Earth’s magnetic field is complex

    • dominant component (90 %) is a dipole field

    • shape and strength of the field provide clues as to the field’s origin

    Static components: a) Intensity b) Declination (azimuth) c) Inclination (dip)

  • Geomagnetism - Characteristics of the EarthGeomagnetism - Characteristics of the Earth’’s Fields Field • the Earth’s magnetic field is complex

    • dominant component (90 %) is a dipole field • the field changes constantly!

    • variations provide clues as to the field’s origin

    Time scale of variations a) Short b) Historical c) Geological

  • Geomagnetic Field CharacteristicsGeomagnetic Field Characteristics 1) Static:

    • 90% explained by a dipole slightly tilted from the rotational axis

    2) Dynamic: • Secular variation:

    • dipole field axis moving • dipole axis averages out to the rotational axis • intensity always varying

    • Reversals: • two stable orientations of the Earth’s magnetic field

    Clues to what is generating the field

  • Generation of the EarthGeneration of the Earth’’s Magnetic Fields Magnetic Field - Dynamo Theory-- Dynamo Theory-

    What is generating the geomagnetic field?

    Paleomagnetic studies provide a number of clues:

    • the magnetic field cannot be generated by magnetized minerals

    1) Interior is too hot (above Curie temperature)

    2) Outer, cool, crustal layer doesn’t produce a strong enough field

    3) A static source cannot account for field reversals or secular variation. Must be dynamic!

  • What is generating the geomagnetic field?

    Paleomag data indicates the field source must be dynamic

    Requirements: The field source must:

    1) create a nearly dipole field by flowing current  basis of electromagnetism

    2) be a dynamic source - must change with time  reversals (2 “stable” states)  wobble (secular variation)

    3) be self-sustaining over geological time  Earth’s oldest rocks indicate a magnetic field was present

    Source? Likely a “dynamo” operating in the fluid outer core

  • Concept: the electromagnetic dynamo • Complete the circuit so that current can flow

    mechanical energy

    electrical energy

    (a “dynamo”)

    • Current flowing creates a magnetic field

    • With the correct path, flowing current can reinforce • or strengthen the initial field

    • A self-exciting dynamo!

  • Generation of the Geomagnetic Field - Dynamo? 1) Need a conductor

    • Core composition:

    • iron is a conductor in a rotating Earth  electrical current produces a magnetic field

    mostly iron

    • not enough…. Need additional motion

    • Heat convection in outer core • outer core: ~6000oC and 106 atmospheres pressure • fluid (as fluid as water!)

    Convection in fluid outer core: current magnetic field

  • Reversing Geomagnetic Field • fluid iron in outer core (free electrons provide moving charge) • motion: a) earth rotation

    b) convection within outer core • well organized patterns: high intensity dipole field • poorly organized patterns: low intensity dipole field • chaotic patterns: non-dipole field a chance for a reversal!

  • Generation of the Geomagnetic Field - Dynamo? Convection in the core:

    • What is the source of the heat?: - conduction of heat:

    - heated from below (conduction from the inner core) - cooled from above (conduction to base of the mantle)

    and the mantle convects heat away to the surface

    - solidification of the core releases heat - inner core is growing…

    - some radioactive decay

  • Dynamo modelling • understanding improving, but computers not yet powerful enough

    Modelled reversal: -500 years, 0, +500 years (Glatzmaier & Roberts, 1995, 1996)

    3D field snapshot: - primarily dipole

    Blue: outer core Red: inner core Yellow: region in outer core with fastest fluid flow

  • Review: Field Characteristics and the Dynamo 1) Stable state:

    • geocentric dipole

    2) Dynamic properties: Secular variation:

    • dipole field axis wobbles around the rotational axis (but averages out to be aligned with it)

    • field intensity varies Reversals:

    • dipole field can reverse or “flip” (2 stable orientations)

    Geomagnetic Field Source - “Dynamo” • fluid iron in outer core (free electrons provide moving charge)

    • motion: a) earth rotation b) convection within outer core

  • Review: The first half of The Core is better than the last. But that’s like saying the end of a spear someone shoved into your head didn’t hurt as much as the tip.

    - John Larsen

    What basic concepts did the screenwriters have

    wrong?

  • What basic concepts did theWhat basic concepts did the screenwriters have wrong?screenwriters have wrong?

    Scenario 1) Experiment stops core from rotating

    2) Need to start core rotating again or: • No protection from cosmic rays

    (extinction to all life) • Massive seismic activity

    Problems 1) Convection (core rotates with Earth)

    2) No mass extinctions correlate with field reversals (atmosphere protects) • May have genetic mutations…? • Massive seismic activity

  • Paleomagnetism Paleomagnetism –– The Study of The Study of ““FossilFossil”” Magnetic Fields Magnetic Fields • can measure the remanent magnetic field “trapped” in rocks

    • field orientation • field direction • field strength (or intensity)

    • can measure the age of a rock • radioisotope dating or other methods

    • can develop a “magnetic time scale” • description of how the magnetic field has changed through time

    How can we use the geomagnetic field to learn about geologic history

    or human history?

  • Key discoveries using Key discoveries using PaleomagnetismPaleomagnetism

    1) Magnetic reversals and seafloor spreading

    2) Apparent Polar Wander • the plates are moving, not the poles

    • Observed symmetric and parallel magnetic reversals across oceanic ridgecrests

    • the key evidence “proving” plate tectonics (Vine & Matthews, Morley, 1963)

    Magnetic time scale • provides another method of dating ancient events • yields spreading rates in the past

  • Paleomagnetism Measuring the Earth’s field in the past 1) Continents: Lava flows (TRM)

    • episodic lava flows form a layered sequence

    0

    0.5

    1.5

    2

    1

    Ti m

    e (M

    y)

  • Paleomagnetism Measuring the Earth’s field in the past 1) Continents: Lava flows (TRM)

    • episodic lava flows form a layered sequence

    3) Marine sediments (DRM) • near-continous sediment deposition • high sedimentation rates = detailed magnetic record

    2) Oceanic crust (TRM) • near-continous creation of new oceanic crust at mid-ocean ridges

    Paleomagnetic age dates match

    radioisotope age dates

  • Secular Variation vs. Apparent Polar WanderSecular Variation vs. Apparent Polar Wander

    Secular Variation: • real, historically measurable motion of the magnetic poles • the “wobble” of the dipole field about the rotational axis

    • averages out to the rotational axis • caused by changes in the convection patterns in the outer core

    Apparent Polar Wander: • paleomagnetic data could be interpreted to indicate that the magnetic field is not an axial dipole but... • it is actually the plates that are moving!

  • Discoveries Related to Paleomagnetism A. Plate tectonics 2) Apparent Polar Wander

    • Paleopole: Crustal remanent magnetism indicates location of magnetic pole at the time the rocks cooled • as age increases, paleopoles = rotational pole • and even more bizarre, different plates have different paleopoles!

    Explanations? 1) in the past, the magnetic field was not an axial dipole??? 2) each plate has its own independent field???

    or

    1) we have an axial dipole 2) the plates move independently

  • Apparent Polar Wander • Recall: rocks trap the magnetic field at the time they cooled past their Curie temperature • If the rocks (or th