Axis of dipole defines the geomagnetic pole - symmetric about dipole axis - Weakest field at magnetic equator - Strongest at magnetic pole

Paleomagnetism Flashcards by Cassie Bosma

What are the causes of rock magnetism?

Thermoremnant magnetism
Chemical remanent magnetism
Depositional (or detrital remanent magnetism

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Thermoremanent magnetism , TRM

Rock takes on magnetic direction prevailing at time of cooling (or formation)

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Curie Temperature

Temperature above which rock loses all magnetism

- 580C for magnetite, 700C for hematite

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Chemical Remanent Magentism, CRM

Chemical reaction, e.g. oxidation, forms ferromagnetic minerals
Red Beds (sediments) have magnetism formed by CRM (hematite = red colour)

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Depositional/Detrital Remanent Magnetism, DRM

Magnetic domains aligned as sediment is deposited and lithified
Fine-grained pelagic limestones often have weak but stable DRM

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Earth’s magnetic field

Approximates a dipole

- Generated by dynamic process of convective circulation of electrical charge in the fluid outer core

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Dipole field

Axis of dipole defines the geomagnetic pole

symmetric about dipole axis
Weakest field at magnetic equator
Strongest at magnetic pole

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What does inclination give?

Latitude the rock was formed at

- Must correct for subsequent tilt of rock

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Magnetic declination, D

Angle between true north and magnetic north for a given location

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Where does the needle of a compass point?

Magnetic North

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Magnetic inclination, I

Angle of magnetic field to Earth’s surface i.e. below horizontal
Inclination is larger than latitude, magnetic field lines come in steeply near poles

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What is the inclination at North magnetic pole?

90 degrees

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What is inclination at equator?

0 degrees

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What is inclination at south magnetic pole?

-90 degrees

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Magnetic dipole?

80 - 90 % of Earth’s field,

tan of I = 2 tan lambda (latitude)

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What is the assumption of using inclination to determine where a rock formed?

The rock has not been tilted

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What does magnetic declination in a rock provide?

Direction to magnetic pole

- Magnetic latitude, can plot geographic position of magnetic pole

Non-dipole component

Total field minus dipole field
Looks like a Clown’s face
Field lines enter eyes (N. Am., Russia/China) and mouth (South), exit nose (African equator)
Drifting West at 0.2%/yr over periods of 1800yrs

Origin of non-dipole component?

small current sources near core-mantle boundary

Time variations in Earth’s magnetic field, Dipole component

Geomagnetic pole, random walk about North Pole
Averaged over past 1800 yrs,
Geomagnetic pole = rotation axis
Intensity decreased 10% over past 200yrs, variations plus/minus 50% over past 10,000yrs
Long time scales (50ka - 1Ma) of polarity reversal

Paleomagnetism Assumes

Formation time for rocks is much longer than drift period of 1800 yrs
or
Ages of different samples span an age much greater than 1800 yrs

If you measure magnetic declination and inclination in a rock, what pole will it give you?

Rotation pole

- Magnetic pole moves both randomly (dipole) and rotates about N. pole (non-dipole), average position = (true) N. pole

Geocentric Axial Dipole Hypothesis

Average magnetic pole is aligned along axis of rotation

Continental Drift

If continent drifts away from pole then latitude changes
Change in mag. inclination preserved in rock record over time
Apparent shift in pole position

If a continent drifts only rel. to longitude (change will be in declination, not inclination), can magnetic measurements detect drift? Why?

- No | - With same latitude but different longitude, the pole position is the same

Apparent polar wander?

- As continent moves away from mag. pole, inclination and declination preserved in rocks changes through time - Relative to the continent the pole appears to wander

Apparent polar wander path?

A plot of the magnetic pole position through time, viewed relative to a given continent, according to rocks of different age on the continent

If the continents are different but have rocks of the same age and different apparent polar wander path what does this indicate?

- Continents have moved relative to each other

What can be concluded by rocks of the same age having the same magnetic polarity, normal or reversed?

- Earth's magnetic field reverses

Reversal mechanism?

- Complex | - Poorly understood

How are magnetic reversal observations made possible?

- Development of K-Ar dating for rocks < 5Ma, plus/minus 2% accuracy - 5Ma max b/c error is plus/minus 0.1Ma which is larger than many reversals

What is the intensity of the field at magnetic epoch boundaries?

- Near zero

Oceanic record

- Stripes generally parallel to ocean ridge, symmetric about ridge - Anomaly strength in layer 2 (seds, pillow basalts, sheeted dykes)

Magnetometer observations

- Magnetometer observations, stripes are positive ('normal' direction) and negative (reversely magnetized) anomalies relative to the present-day background magnetic field

Marie Tharp

- Put together seafloor maps (particularly Atlantic) - Noticed V-shaped notch at mount. ridge crest - Idea was dismissed as girl talk but she was right and boss stole the credit

Magnetic Tape Recorder

- Model, quantitative estimates of motion, e.g. 92km from ridge to 4Ma stripe - Use to get spreading rate

Half-spreading rate, what is it relative to?

- Relative to Ridge

Full-spreading rate?

- How fast a rock on one side of the ridge moves w/ regard to rock on the other side = Dist/time ex. 92km/4Ma = 23km/Ma = 23mm/yr - Divide by 2 = 1/2 spreading rate

Why are the stripes in the N. Pacific wider than the Atlantic?

- Pacific travelled 2x as far at the same age as Atlantic | - Pacific traveled at a faster rate

Deep Sea Drilling Program (DSDP) Leg 3

- 1968 - Critical test of theory - Sampled seds just above volcanic layer, dated by paleontology (microfossil) - Found age increases uniformly with distance from ridge - Velocity same as determined from magnetic stripes (predicted age from magnetic stripes = measured age from fossils)

Why was paleontology instead of K-Ar?

- fossils more precise