Electromagnetism and theElectromagnetism and theEarthEarth’’s Magnetic Field:s Magnetic Field:

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

• How is it generated?• What does that tell us aboutthe 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 forcesthat are applied to a moving, charged particle

• Electromagnetic fields are generated at theatomic level• all atoms generate magnetic fields (movingelectrons)

• Large-scale magnetic fields are produced bythe 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 cluesas to the field’s origin

Static components:a) Intensityb) 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 variationsa) Shortb) Historicalc) Geological

Geomagnetic Field CharacteristicsGeomagnetic Field Characteristics1) Static:

• 90% explained by a dipole slightly tilted from therotational 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 magnetizedminerals

1) Interior is too hot (above Curie temperature)

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

3) A static source cannot account for field reversals orsecular 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 coreRed: inner coreYellow: region in outer core with fastest fluid flow

Review: Field Characteristics and the Dynamo1) 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 variesReversals:

• 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 rotationb) convection within outer core

Review: The first half of The Coreis better than the last. But that’slike saying the end of a spearsomeone shoved into your headdidn’t hurt as much as the tip.

- John Larsen

What basic concepts didthe screenwriters have

wrong?

What basic concepts did theWhat basic concepts did thescreenwriters have wrong?screenwriters have wrong?

Scenario1) 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

Problems1) Convection (core rotates with Earth)

2) No mass extinctions correlate withfield 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 changedthrough 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 reversalsacross 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

PaleomagnetismMeasuring the Earth’s field in the past1) Continents: Lava flows (TRM)

• episodic lava flows form a layered sequence

0

0.5

1.5

2

1

Tim

e (M

y)

PaleomagnetismMeasuring the Earth’s field in the past1) Continents: Lava flows (TRM)

• episodic lava flows form a layered sequence

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

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

Paleomagnetic age datesmatch

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 outercore

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

Discoveries Related to PaleomagnetismA. Plate tectonics 2) Apparent Polar Wander

• Paleopole: Crustal remanent magnetism indicates location of magneticpole 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 dipole2) 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 the plates they comprise) move,

then it appears that the pole has moved!

Apparent PolarWander Path

Apparent Polar Wander• the plates wander, not the magnetic or rotational poles

Apparent polar wander curves for the North American and Eurasian plates

N.A.Eurasian

a) different plates, different polar wander curvesb) if you reconstruct the plate positions

• apparent polar wander curves match!

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 outercore

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

Summary: Key discoveries using Paleomagnetism

1) Magnetic reversals and seafloor spreading

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

• Observed symmetric and parallel magnetic reversalsacross 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

Effects of the geomagnetic field?Effects of the geomagnetic field?

• Extinctions?• No, but possible increased geneticmutation rate…

• Navigation• human and animal

• Interactions with charged particles from thesun generate strong currents in the atmosphere• aurora borealis and aurora australis• major failures of power grids, satellites