Paleomagnetism Flashcards
What are the causes of rock magnetism?
- Thermoremnant magnetism
- Chemical remanent magnetism
- Depositional (or detrital remanent magnetism
Thermoremanent magnetism , TRM
- Rock takes on magnetic direction prevailing at time of cooling (or formation)
Curie Temperature
Temperature above which rock loses all magnetism
- 580C for magnetite, 700C for hematite
Chemical Remanent Magentism, CRM
- Chemical reaction, e.g. oxidation, forms ferromagnetic minerals
- Red Beds (sediments) have magnetism formed by CRM (hematite = red colour)
Depositional/Detrital Remanent Magnetism, DRM
- Magnetic domains aligned as sediment is deposited and lithified
- Fine-grained pelagic limestones often have weak but stable DRM
Earth’s magnetic field
- Approximates a dipole
- Generated by dynamic process of convective circulation of electrical charge in the fluid outer core
Dipole field
Axis of dipole defines the geomagnetic pole
- symmetric about dipole axis
- Weakest field at magnetic equator
- Strongest at magnetic pole
What does inclination give?
Latitude the rock was formed at
- Must correct for subsequent tilt of rock
Magnetic declination, D
- Angle between true north and magnetic north for a given location
Where does the needle of a compass point?
Magnetic North
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
What is the inclination at North magnetic pole?
90 degrees
What is inclination at equator?
0 degrees
What is inclination at south magnetic pole?
-90 degrees
Magnetic dipole?
80 - 90 % of Earth’s field,
tan of I = 2 tan lambda (latitude)
What is the assumption of using inclination to determine where a rock formed?
The rock has not been tilted
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
