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Paleomagnetism Ch. 10: Page 1
Chapter 10: Paleomagnetism Homework Solutions
- e Give at least two possible explanations for why it is not ro
Th declination of a lava is 8°. ze a)
The block has been rotated b) The magnetic pole whenthe block was magnetized does not coincide with the rotational
c)
pole The block has been tilted d) There has been secondary remanent magnetism
- What are true north, geographic north, magnetic north?
rue north and geographic north are the same and are aligned with the Earth’s spin axis. Earth’s surface.
T
Magnetic north is where the magnetic axis of the Earth intersects with the
They would be the same if the layers (interfaces) are flat and continuous.
- Explain what these test for and how they differ: (a) Fold test, (b) Conglomerate test, (c ) Baked contact test?
a) Fold test checks whether the magnetic remanence was caused before or after the folding. b) Conglomerate test checks whether the magnetic remanence was caused before or after the deposition of the clasts. ) Baked contact test checks magnetic remanence was caused before or after the intrusion reheated the rock.
c
- You wish to travel due north; you have only a magnetic compass to provide a bearing, but you know that for your location the magnetic inclination is +5° and the declination is 8°. Then with respect to the direction of your compass needle, which of the following should you choose?
Declination is by definition the direction to true north. Thus, travel 8° W of magnetic North.
- When a particular sample was thermally demagnetized, its intensity increased up to 250°C, after which it decreased. Why was this?
The rock had secondary remanence that was opposite to the primary remanence which resulted in diminishing the overall remanent magnetic field. As thermal demagnetization occurred, the secondary remanence was removed (washed), increasing the total field in the process. As the temperature continues to increase, the primary demagnetization is removed progressively through the range of blocking temperatures.
Paleomagnetism Ch. 10: Page 2
- e g
Th Earth’s magnetic field is similar to the field that would be produced by a huge bar be a magnet. above the Currie Point (~600°C)
ma net at its center. Give two reasons why it cannot a) Below depths of 15‐20 km, the Earth’s temperature is b) A simple bar magnet cannot produce field reversals.
- Which of the following relations between blocking temperature, Tb , and Curie Temperature, Tc , is corr c e t?
i i) Tc is always less than Tb.
- If you wished to identify the magnetic mineral in a rock, would it be more useful to measure it Curie or blocking temperature?
Blocking temperatures are a range for all the different domain walls, impurities, etc. of a particular mineral. However, Curie temperature is characteristic of a mineral. Thus, mea uring the Curie temperature would allow determination of the magnetic minerals present.
s
- Two rocks have the same iron content, yet their remanent magnetizations differ in strength. Give at least three reasons why the strength of the remanence of a rock do e s not depend just on its iron content?
a) Magnetic field variation due to latitude. b) Grain size affects domain stability c) Different secondary remanence history d) ifferent chemical formula, i.e., magnetite, hematite, maghaemite all have different aturation remanence.
D
s
- The declination and inclination of rocks from the western end of an island a few kilometers long are 20° and +30 °, whereas the declination and inclination of rocks from the eastern end are 35° and +30°. What explanation can you offer?
Since they have the same inclination, most likely the two rocks were magnetized at the same latitude. However, since their inclinations are different, they have different poles, ost likely because of relative rotation between the two. Another explanation is that the sland is made up of two distinct accreted blocks.
m i
- An American geologist, finding on arrival in New Zealand that his compass does not rotate freely, blames the airline for damage in transit. But there could be another explanation. Explain.
Because the Earth’s magnetic field is a vector and since the field lines almost always intersect the Earth’s surface at an inclination that is different than zero, compasses are
Paleomagnetism Ch. 10: Page 4
- A small area of continent has rocks of a range of ages. Their paleomagnetic directions were measured, giving the data below (dates are accurate to about ±4 Ma, paleomagnetic directions to ±2°). Plot the paleolatitudes against time. What can you deduce about the continent?
Age (Ma) Inclination (deg) Latitude (deg)
300 ‐ 54 ‐34.
250 ‐ 56 ‐36.
190 ‐ 55 ‐35.
160 ‐ 50 ‐30.
140 ‐ 38 ‐21.
80 0 0.
60 10 5.
0 43 25.
The continent moved from the southern hemisphere northward at a relatively constant rate beginning around 170 Ma.
‐40.
‐30.
‐20.
‐10.
350 300 250 200 150 100 50 0 Latitude
(deg)
Age (Ma)
Latitude vs Age for Continent
Paleomagnetism Ch. 10: Page 5
- A small area of continent has rocks of a range of ages. Their paleomagnetic directions were measured, giving the data below (dates are accurate to about ±4 Ma, aleomagnetic directions to ±2°). Plot the paleolatitudes against time. What can you educe about the continent?
p d
Age (Ma) Declination
Inclination (deg) Latitude (deg) 0 0 36 19. 25 6 26 13.71 moving N, slight rotation to 6°
40 10 19 9.
moving N, slight rotation to 10°
65 10 ‐ 6 ‐3.
moving N, slight rotation to ‐ 3°
100 10 ‐ 38 ‐21.
moving N, slight rotation to ‐ 21° 140 190 61 42.05 polar reversal 160 10 ‐ 69 ‐52.
140 190 ‐ 61 ‐42.05 corrected for polar reversal
‐60.
‐40.
‐20.
200 150 100 50 0 Latitude
(deg)
Age (Ma)
Latitude vs Age for Continent
Correction for reversal
b) Based on same argument of (a), minimum duration is the minimum length of an R period, only one in Campanian, ~4 Ma. The maximum is any time during the Campanian, ~9 Ma and would have had to overlap the time 79 Ma.
Alternative: Since the rock doesn’t (explicitly) contain a ‘sandwich (NRN)’, the minimum nterval would be small to zero. If the succession is not uniform in time, it could span the ntire Campanian.
i e
With only a few at the base, the age is probably less than 80 Ma.
Paleomagnetism Ch. 10: Page 7
