Paleomagnetism

Question 1

What are the causes of rock magnetism?

Answer 1

• Thermoremnant magnetism
• Chemical remanent magnetism
• Depositional (or detrital remanent magnetism

Question 2

Thermoremanent magnetism , TRM

Answer 2

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

Question 3

Curie Temperature

Answer 3

Temperature above which rock loses all magnetism

- 580C for magnetite, 700C for hematite

Question 4

Chemical Remanent Magentism, CRM

Answer 4

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

Question 5

Depositional/Detrital Remanent Magnetism, DRM

Answer 5

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

Question 6

Earth’s magnetic field

Answer 6

• Approximates a dipole

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

Question 7

Dipole field

Answer 7

Axis of dipole defines the geomagnetic pole

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

Question 8

What does inclination give?

Answer 8

Latitude the rock was formed at

- Must correct for subsequent tilt of rock

Question 9

Magnetic declination, D

Answer 9

• Angle between true north and magnetic north for a given location

Question 10

Where does the needle of a compass point?

Answer 10

Magnetic North

Question 11

Magnetic inclination, I

Answer 11

• 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

Question 12

What is the inclination at North magnetic pole?

Answer 12

90 degrees

Question 13

What is inclination at equator?

Answer 13

0 degrees

Question 14

What is inclination at south magnetic pole?

Answer 14

-90 degrees

Question 15

Magnetic dipole?

Answer 15

80 - 90 % of Earth’s field,

tan of I = 2 tan lambda (latitude)

Question 16

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

Answer 16

The rock has not been tilted

Question 17

What does magnetic declination in a rock provide?

Answer 17

• Direction to magnetic pole

- Magnetic latitude, can plot geographic position of magnetic pole

Question 18

Non-dipole component

Answer 18

• 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

Question 19

Origin of non-dipole component?

Answer 19

small current sources near core-mantle boundary

Question 20

Time variations in Earth’s magnetic field, Dipole component

Answer 20

• 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

Question 21

Paleomagnetism Assumes

Answer 21

• 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

Question 22

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

Answer 22

• Rotation pole

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

Question 23

Geocentric Axial Dipole Hypothesis

Answer 23

Average magnetic pole is aligned along axis of rotation

Question 24

Continental Drift

Answer 24

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

Question 25

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

Answer 25

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

Question 26

Apparent polar wander?

Answer 26

- 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

Question 27

Apparent polar wander path?

Answer 27

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

Question 28

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

Answer 28

- Continents have moved relative to each other

Question 29

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

Answer 29

- Earth's magnetic field reverses

Question 30

Reversal mechanism?

Answer 30

- Complex | - Poorly understood

Question 31

How are magnetic reversal observations made possible?

Answer 31

- 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

Question 32

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

Answer 32

- Near zero

Question 33

Oceanic record

Answer 33

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

Question 34

Magnetometer observations

Answer 34

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

Question 35

Marie Tharp

Answer 35

- 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

Question 36

Magnetic Tape Recorder

Answer 36

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

Question 37

Half-spreading rate, what is it relative to?

Answer 37

- Relative to Ridge

Question 38

Full-spreading rate?

Answer 38

- 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

Question 39

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

Answer 39

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

Question 40

Deep Sea Drilling Program (DSDP) Leg 3

Answer 40

- 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)

Question 41

Why was paleontology instead of K-Ar?

Answer 41

- fossils more precise