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EOS410PltTec > Paleomagnetism > Flashcards

Paleomagnetism Flashcards

1
Q

What are the causes of rock magnetism?

A
  • Thermoremnant magnetism
  • Chemical remanent magnetism
  • Depositional (or detrital remanent magnetism
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2
Q

Thermoremanent magnetism , TRM

A
  • Rock takes on magnetic direction prevailing at time of cooling (or formation)
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3
Q

Curie Temperature

A

Temperature above which rock loses all magnetism

- 580C for magnetite, 700C for hematite

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4
Q

Chemical Remanent Magentism, CRM

A
  • Chemical reaction, e.g. oxidation, forms ferromagnetic minerals
  • Red Beds (sediments) have magnetism formed by CRM (hematite = red colour)
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5
Q

Depositional/Detrital Remanent Magnetism, DRM

A
  • Magnetic domains aligned as sediment is deposited and lithified
  • Fine-grained pelagic limestones often have weak but stable DRM
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6
Q

Earth’s magnetic field

A
  • Approximates a dipole

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

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7
Q

Dipole field

A

Axis of dipole defines the geomagnetic pole

  • symmetric about dipole axis
  • Weakest field at magnetic equator
  • Strongest at magnetic pole
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8
Q

What does inclination give?

A

Latitude the rock was formed at

- Must correct for subsequent tilt of rock

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9
Q

Magnetic declination, D

A
  • Angle between true north and magnetic north for a given location
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10
Q

Where does the needle of a compass point?

A

Magnetic North

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11
Q

Magnetic inclination, I

A
  • 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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12
Q

What is the inclination at North magnetic pole?

A

90 degrees

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13
Q

What is inclination at equator?

A

0 degrees

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14
Q

What is inclination at south magnetic pole?

A

-90 degrees

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15
Q

Magnetic dipole?

A

80 - 90 % of Earth’s field,

tan of I = 2 tan lambda (latitude)

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16
Q

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

A

The rock has not been tilted

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17
Q

What does magnetic declination in a rock provide?

A
  • Direction to magnetic pole

- Magnetic latitude, can plot geographic position of magnetic pole

18
Q

Non-dipole component

A
  • 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
19
Q

Origin of non-dipole component?

A

small current sources near core-mantle boundary

20
Q

Time variations in Earth’s magnetic field, Dipole component

A
  • 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
21
Q

Paleomagnetism Assumes

A
  • 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
22
Q

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

A
  • Rotation pole

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

23
Q

Geocentric Axial Dipole Hypothesis

A

Average magnetic pole is aligned along axis of rotation

24
Q

Continental Drift

A
  • If continent drifts away from pole then latitude changes
  • Change in mag. inclination preserved in rock record over time
  • Apparent shift in pole position
25
Q

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

A
  • No

- With same latitude but different longitude, the pole position is the same

26
Q

Apparent polar wander?

A
  • 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
27
Q

Apparent polar wander path?

A

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

28
Q

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

A
  • Continents have moved relative to each other
29
Q

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

A
  • Earth’s magnetic field reverses
30
Q

Reversal mechanism?

A
  • Complex

- Poorly understood

31
Q

How are magnetic reversal observations made possible?

A
  • 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
32
Q

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

A
  • Near zero
33
Q

Oceanic record

A
  • Stripes generally parallel to ocean ridge, symmetric about ridge
  • Anomaly strength in layer 2 (seds, pillow basalts, sheeted dykes)
34
Q

Magnetometer observations

A
  • Magnetometer observations, stripes are positive (‘normal’ direction) and negative (reversely magnetized) anomalies relative to the present-day background magnetic field
35
Q

Marie Tharp

A
  • 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
36
Q

Magnetic Tape Recorder

A
  • Model, quantitative estimates of motion, e.g. 92km from ridge to 4Ma stripe
  • Use to get spreading rate
37
Q

Half-spreading rate, what is it relative to?

A
  • Relative to Ridge
38
Q

Full-spreading rate?

A
  • 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
39
Q

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

A
  • Pacific travelled 2x as far at the same age as Atlantic

- Pacific traveled at a faster rate

40
Q

Deep Sea Drilling Program (DSDP) Leg 3

A
  • 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)
41
Q

Why was paleontology instead of K-Ar?

A
  • fossils more precise

EOS410PltTec (12 decks)

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