Geomagnetics
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Including paleomagnetism
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Magnetic Poles
Magnetic Declination
Polar Wandering
Paleomagnetism
Spreading Rates calculated from paleomagnetic stripes
Topics to be covered…
Earth’s magnetic field varies widely
• Earth’s geographic and magnetic poles do not coincide
• The angular azimuth variation is termed declination
• The position of the magnetic poles relative to geographic poles varies over time • Note that the positive “North” end of a compass magnet seeks the negative (south) pole of the Earth
• A magnet aligning itself with Earth’s magnetic field has a steeper inclination at higher latitude
Global Declination Values
Solar Wind and the Magnetosphere
• Variations in the Solar Wind may affect the strength and orientation of the Geomagnetic field
• The overall shape of the Geomagnetic field is controlled by the Solar Wind
Origin of the Magnetic Field
• Produced by convection “rolls” in the liquid metallic outer core
Magnetic Polar Wandering Path
• Although the magnetic pole wanders it does not move far from geographic pole
• Variations are due to pertubations in flow regime in the outer core
• Paleomagnetic poles that plot at low latitudes are the result of plate tectonic rotations
Paleomagnetic Polar Wandering
• Paleo‐Polar Wandering over wide geographic areas is only apparent‐ the true pole position never strays far from the geographic pole
• The actual reason for Paleo‐Polar Wandering is plate tectonic motions
• Latitude migration changes the apparent latitude of the paleo‐pole
• Longitude migration around a rotation axis non‐parallel to the magnetic pole axis will shift the apparent longitude
• Plate rotation will change the apparent position of the paleo‐pole South African Apparent Polar Wandering Paleozoic through Mesozoic
• Left Diagram: raw data uncorrected
• Right Diagram: corrected for deformation, etc.
Apparent Wandering Paths and Past Tectonic Motion
• Because Plates are constantly changing their relative positions each has a unique path
• If continents are fitted to original Pangean configuration the paths coincide
Paleomagnetism and Seafloor Spreading
Magnetic Reversals
• Over time the Earth’s magnetic field polarity can reverse
• Reversals have occurred many times over the past several million years
• Models predict that the reversal may occur as rapidly as 24‐48 hours
Causes of Magnetic Field Reversals
• Reversals may be inherently chaotic as predicted by certain computer models of a liquid outer core
• Reversals may be triggered by impact events disrupting the flow regime in the outer core
• Subduction of oceanic slabs may disrupt flow in the outer core
• Extreme sun spot activity may disrupt the ionosphere
Effects of Geomagnetic Reversals
• Several scientists have hypothesize that prominent reversals correlate with extinction events
• Disappearance of the magnetic field would allow more ionizing radiation to penetrate the atmosphere
• The lack of a Van Allen belt would allow the solar wind to gradually erode the atmosphere
• The periodicity of reversals appears random over time
• Besides the increase in radiation there is no known negative effect on biological activity associated with a lack of magnetic field
Calculation of Spreading Rates from Paleomagnetic Reversals
Given: A map of the seafloor with the boundary between paleomagnetic “stripes” dated by radiometric analysis. Measurement of map yields a distance of 50 km and a date of 1.0 Ma.
1.0 Ma
Ridge
positive
50 km
Find: Spreading rate at ocean ridge in cm/year.
rate = 50km/1.0Ma = 5x106cm/1x106year = 5cm/year
negative
Calculation of Paleomagnetic Latitude
• P is the position of a magnetite‐bearing basalt, B is the total field at P, I is the angle of inclination, Hθ and Zr are the horizontal and vertical components of the total field
• O is the center of the earth
Tan I = 2 tan λ
Where λ is the paleolatitude of the basalt flow
Calculation of Paleo‐Pole Latitude & Longitude
• D is the measured remnant declination
• λ P is the latitude of the paleo‐pole
• λ X is the latitude of the present sample location
• λ is the paleolatitude of the sample
Sin λ P = sin(λ X ) * sin (λ) + cos(λ X) * cos(λ) * cos (D)
Sin (φ P ‐ φ X )= cos(λ) * sin (D)
cos (λ P) if sin λ ≥ sin(λ P) * sin(λ X)
Sin (180 + φ P ‐ φ X )= cos(λ) * sin (D)
cos (λ P) if sin λ < sin(λ P) * sin(λ X)
Example Calculation for Paleolatitude
Magnetic measurements on a basalt flow presently at (47N, 20E) yielded an angle of inclination of 30˚ on the remnant magnetization. Tan I = 2 tan λ
λ = tan ‐1 (tan 30/2)
λ = 16.1
Therefore, when the basalt was erupted it was at latitude 16.1N. Example Calculation for Paleo‐Pole Position
Using previous example basalt location of (47N, 20E) with measured declination D= 80˚, and calculated λ = 16.1˚
Sin λ P = sin(47) * sin(16) + cos(47) * cos(16.1) * cos(80)
λ P = 18.45°N
Sin (16.1) >= Sin(18) * Sin(47)
0.277 >= 0.231 Sin (φ P ‐ φ X )= cos(16.1) * sin (80)
cos (18.45)
φ P ‐ φ X = 85.94˚ therefore φ P = 105.9°E
Example Spreadsheet Layout for Paleo‐
Latitude & Paleo‐ Magnetic Pole Position Calculation
Paleolatitude and PaleoPole Calculations
Sample Latitude (lX):
Sample Longitude (jX):
Inclination (I):
Declination (D):
47.00 degrees
20.00 degrees
30.00 degrees
80.00 degrees
Paleo‐Latitude:
l=
16.10211375 degrees
Sin(Mag. Pole Latitude):
Sin(lP)=
0.316622744 unitless
Magnetic Pole Latitude:
lP =
18.45880521 degrees
Sin(l)=
0.277350098 sin(lP)*sin(lX)=
Sin(fP‐fX)=
Sin(180+fP‐fX)=
Magnetic Pole Longitude (fP):
0.99749211 (jP‐jX)=
0.99749211 (180+fP‐fX)=
105.94 degrees
0.231563
85.9413397 fP=
85.9413397 fP=
105.94
‐74.06
Changes in the Paleomagnetic “Stripe” Trend
Changes in the trend of paleomagnetic stripes may indicate subduction of pre‐existing triple points