Download Magnetic Fields

Electromagnetism and the
Earth’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 Earth’s Field
• the Earth’s magnetic field is complex
• dominant component (90 %) is a dipole field
Static components:
a) Intensity
b) Declination (azimuth)
c) Inclination (dip)
• shape and strength of the field provide clues
as to the field’s origin
Geomagnetism - Characteristics of the Earth’s Field
• the Earth’s magnetic field is complex
• dominant component (90 %) is a dipole field
• the field changes constantly!
Time scale of variations
a) Short
b) Historical
c) Geological
• variations provide clues as to the field’s origin
Geomagnetic 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 Earth’s Magnetic Field
- Dynamo TheoryWhat 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: mostly iron
• iron is a conductor in a rotating Earth
 electrical current
produces a magnetic field
• not enough…. Need additional motion
• Heat
convection in outer core
o
• outer core: ~6000 C 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
3D field snapshot:
- primarily dipole
Blue: outer core
Red: inner core
Yellow: region in
outer core with
fastest fluid flow
Modelled reversal: -500 years, 0, +500 years (Glatzmaier & Roberts, 1995, 1996)
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 the
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 – The Study of “Fossil” 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 Paleomagnetism
1) Magnetic reversals and seafloor spreading
• Observed symmetric and parallel magnetic reversals
across oceanic ridgecrests
• the key evidence “proving” plate tectonics
(Vine & Matthews, Morley, 1963)
2) Apparent Polar Wander
• the plates are moving, not the poles
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
Time (My)
0.5
1
1.5
2
Paleomagnetism
Measuring the Earth’s field in the past
1) Continents: Lava flows (TRM)
• episodic lava flows form a layered sequence
2) Oceanic crust (TRM)
• near-continous creation of
new oceanic crust at
mid-ocean ridges
3) Marine sediments (DRM)
• near-continous sediment
deposition
• high sedimentation rates =
detailed magnetic record
Paleomagnetic age dates
match
radioisotope age dates
Secular 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 the plates they comprise) move,
then it appears that the pole has moved!
Apparent Polar
Wander Path
Apparent Polar Wander
• the plates wander, not the magnetic or rotational poles
N.A.
Eurasian
Apparent polar wander curves for the North American and Eurasian plates
a) different plates, different polar wander curves
b) if you reconstruct the plate positions
• apparent polar wander curves match!
Secular 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!
Summary: Key discoveries using Paleomagnetism
1) Magnetic reversals and seafloor spreading
• Observed symmetric and parallel magnetic reversals
across oceanic ridgecrests
• the key evidence “proving” plate tectonics
(Vine & Matthews, Morley, 1963)
2) Apparent Polar Wander
• the plates are moving, not the poles
Magnetic time scale
• provides another method of dating ancient events
• yields spreading rates in the past
Effects of the geomagnetic field?
• Extinctions?
• No, but possible increased genetic
mutation rate…
• Navigation
• human and animal
• Interactions with charged particles from the
sun generate strong currents in the atmosphere
• aurora borealis and aurora australis
• major failures of power grids, satellites