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Geomagnetics Geomagnetics { Including paleomagnetism 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 Ridge 1.0 Ma boundary between paleomagnetic “stripes” dated by radiometric analysis. Measurement of map yields a distance of positive negative 50 km and a date of 1.0 Ma. 50 km Find: Spreading rate at ocean ridge in cm/year. rate = 50km/1.0Ma = 5x106cm/1x106year = 5cm/year 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 ‐φX )= cos(λ) * sin (D) cos (λ P) if sin λ ≥ sin(λ P) * sin(λ X) Sin (180 + φ P ‐φX )= 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 ‐φX )= cos(16.1) * sin (80) cos (18.45) φ P ‐φX = 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): 47.00degrees Sample Longitude (jX): 20.00degrees Inclination (I): 30.00degrees Declination (D): 80.00degrees Paleo‐Latitude: l= 16.10211375degrees Sin(Mag. Pole Latitude): Sin(lP)= 0.316622744unitless Magnetic Pole Latitude: lP= 18.45880521degrees Sin(l)= 0.277350098sin(lP)*sin(lX)= 0.231563 Sin(fP‐fX)= 0.99749211(jP‐jX)= 85.9413397fP= 105.94 Sin(180+fP‐fX)= 0.99749211(180+fP‐fX)= 85.9413397fP= ‐74.06 Magnetic Pole Longitude (fP): 105.94degrees Changes in the Paleomagnetic “Stripe” Trend Changes in the trend of paleomagnetic stripes may indicate subduction of pre‐existing triple points.
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