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:

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? True regardless of scale
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. The Rock Cycle These 3 are most important in the ocean
The origin of a rock determines its properties and composition
These 3 are most important in the ocean

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

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

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-Floor Magnetic 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-Floor Magnetism 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-Floor Magnetism
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-Floor Magnetism
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