Reversals of the Geomagnetic Field Secular variations- historic to modern changes in the field Archaeomagnetism: changes during the Holocene Reversals

  • View
    215

  • Download
    2

Embed Size (px)

Text of Reversals of the Geomagnetic Field Secular variations- historic to modern changes in the field...

  • Slide 1
  • Reversals of the Geomagnetic Field Secular variations- historic to modern changes in the field Archaeomagnetism: changes during the Holocene Reversals of the dipole polarity Reversal chronology for past 5 million years: the terrestrial record reversal chronology for past 200 million years: the seafloor spreading tape recorder
  • Slide 2
  • Locations of the north pole of the dipole component of the geomagnetic field from 1945- 2000.
  • Slide 3
  • -30 to 800 BP 800 to 1940 BP 1940 to 3690 BP Calibrated radiocarbon years before present, (B.P, AD1950=0) Average pole position for all data (94 poles): 88.4 N 23.8 W 1.6 degrees from geographic North Pole The north magnetic pole during the past 3700 years. -30 to 3690 BP
  • Slide 4
  • units: nT/yr contour interval: 5 nT/yr Main field: 30,000 to 60,000 nT
  • Slide 5
  • units: minutes/yr contour interval: 2 min/yr
  • Slide 6
  • units: minutes/yr contour interval: 1 min/yr
  • Slide 7
  • Years before present (BP) Dipole moment determined from the strength of magnetization of archaeological material (archaeomagnetic results from TRM in ancient hearths and pottery)
  • Slide 8
  • dipole component non-dipole component Schematic plot of magnetic field variations in time
  • Slide 9
  • A snapshot of the 3D magnetic field structure simulated with the Glatzmaier-Roberts geodynamo model. Magnetic field lines are blue where the field is directed inward and yellow where directed outward. The rotation axis of the model Earth is vertical and through the center. A transition occurs at the core-mantle boundary from the intense, complicated field structure in the fluid core, where the field is generated, to the smooth, potential field structure outside the core. The field lines are drawn out to two Earth radii. Magnetic field is wrapped around the "tangent cylinder" due to the shear of the zonal fluid flow.
  • Slide 10
  • About 36,000 years into the simulation the magnetic field underwent a reversal of its dipole moment (Figure 3), over a period of a little more than a thousand years. The intensity of the magnetic dipole moment decreased by about a factor of ten during the reversal and recovered immediately after, similar to what is seen in the Earth's paleomagnetic reversal record. Our solution shows how convection in the fluid outer core is continually trying to reverse the field but that the solid inner core inhibits magnetic reversals because the field in the inner core can only change on the much longer time scale of diffusion [2]. Only once in many attempts is a reversal successful, which is probably the reason why the times between reversals of the Earth's field are long and randomly distributed. 500yrs before middle of reversal 500yrs after
  • Slide 11
  • The key to determining the chronology of the geomagnetic field reversals is to be able to date the time during which robust magnetizations were attained in a given rock sample. The classical work was done in the latter half of the last century on basaltic rocks, which cool rapidly and acquire a strong thermo-remanent magnetization (TRM). These rocks can be dated effectively with the Potassium-Argon method, which uses the decay of K-40 into the chemically inert Ar-40. Ar-40 is trapped and accumulates in the rock only since the last time the rock was melted the time when the basalt was extruded and solidified. While liquid, the prior Ar-40, a gas, leaves the magma. Since the basalt is extruded on the surface, cooling is rapid and the acquisition of TRM occurs soon after the trap is set for accumulation of Ar-40.
  • Slide 12
  • Reversal captured in Columbia River basalt flows ( Steens Mtn., Oregon: Miocene, 15.5 Ma)
  • Slide 13
  • Steens Mtn: Kiger Gorge from the Steens Mountain Loop Road
  • Slide 14
  • Steens Mtn: View northwest from the short trail/road to the summit.
  • Slide 15
  • High resolution record of geomagnetic field reversal Mankinen, et al., 1985, J. Geophys. Res., v. 90, p, 10400 3500 yrs 3600 yrs 5000 yrs Reversal captured in Columbia River basalt flows ( Steens Mtn., Oregon: Miocene, 15.5 Ma) Extrusions at rate of about 43 m/1000 yrs
  • Slide 16
  • Steens Mtn results: VGPs in time
  • Slide 17
  • Magnetizations (DRM) recovered from deep ocean sediments
  • Slide 18
  • Note minimum intensities during reversals Magnetizations (DRM) recovered from deep ocean sediments
  • Slide 19
  • Geomagnetic field reversal chronology for past 5 million years based mainly on K-Ar dating of terrestrial volcanic rocks Why only to 5 Ma?
  • Slide 20
  • Geomagnetic field reversal chronology for past 5 million years based mainly on K-Ar dating of terrestrial volcanic rocks Why only to 5 Ma? Errors in K-Ar dates become too large compared to reversal periods
  • Slide 21
  • The chronology of geomagnetic field reversals earlier than 5 Ma is well preserved in the magnetization of basalts extruded on the ocean floor in the process of sea-floor spreading.
  • Slide 22
  • Slide 23
  • Age, Ma moho Seafloor spreading model lithosphere crust upper mantle Schematic representation of upper crustal magnetized layer 1200 deg C convecting mantle
  • Slide 24
  • Age, Ma moho Seafloor spreading is a tape recorder of the geomagnetic field! crust upper mantle The recording head of the tape recorder The tape drive The reversal chronology recorded on land
  • Slide 25
  • Marine magnetic anomalies Ships tow magnetometers which measure the total intensity of the geomagnetic field, the magnitude of the geomagnetic field vector, often symbolized by F, or F obs, to denote that it is the observed total intensity. These measurements lead to a plot of F obs versus distance along the track.
  • Slide 26
  • magnetic field intensity,F obs 0 Smoothly varying global field plus small, short wavelength effects due to crustal magnetizations distance along ship track
  • Slide 27
  • Marine Magnetic anomalies Ships tow magnetometers which measure the total intensity of the geomagnetic field, the magnitude of the geomagnetic field vector, often symbolized by F, or F obs, to denote that it is the observed total intensity. These measurements lead to a plot of F obs versus distance along the track. The main internal geomagnetic field (produced in the outer core), F g, is determined for the earth as a function of time as the International Geomagnetic Reference Field (IGRF). The IGRF field can then be subtracted from the observed value to produce a total intensity anomaly, F = F obs - F g F results only from effects of rocks magnetized near the surface, and can thus be compared with models of the magnetization of the ocean bottom rocks.
  • Slide 28
  • magnetic field intensity,F obs distance along ship track intensiy anomaly, F 0 0 Smoothly varying global field plus small, short wavelength effects of crustal magnitizations distance along ship track Total intensity anomaly, F
  • Slide 29
  • Marine Magnetic anomalies The rocks with the strongest magnetizations by far are the basalts extruded and rapidly cooled, acquiring thermo-remanent magnetization (TRM) via the process of seafloor spreading.
  • Slide 30
  • + ++++++++++++++ ---------------- J Magnetic field lines for vertically downwards magnetization in cross-sectional view
  • Slide 31
  • + ++++++++++++++ ---------------- J Magnetic field lines for vertically upwards magnetization
  • Slide 32
  • + ++++++++++++++ ---------------- J Magnetic field due to magnetized prism taken along the surface above the prism (directions only) Earths field, He Vertically downwards magnetization parallel to vertical earths field ocean surface
  • Slide 33
  • + ++++++++++++++ ---------------- J Magnetic field due to magnetized prism taken along the surface above the prism (directions only) Earths field, He Magnetized prism field adds to Earths field, F positive
  • Slide 34
  • + ++++++++++++++ ---------------- J Magnetic field due to magnetized prism taken along the surface above the prism (directions only) Earths field, He Magnetized prism field perpendicular to He, F = 0
  • Slide 35
  • + ++++++++++++++ ---------------- J Magnetic field due to magnetized prism taken along the surface above the prism (directions only) Earths field, He Magnetized prism field subtracts from He, F negative
  • Slide 36
  • sea surface ocean bottom Basalt magnetized upon solidification along axis of spreading ridge axis of seafloor spreading Direction of modern geomagnetic field distance along track Intensity anomaly, F reversal + -
  • Slide 37
  • sea surface ocean bottom Basalt magnetized upon solidification along axis of spreading ridge axis of seafloor spreading Direction of modern geomagnetic field distance along track Intensity anomaly, F reversal Magnetization increases main field Magnetization decreases main field
  • Slide 38
  • Global bathymetry, showing ocean ridge system
  • Slide 39
  • East Pacific Rise Mid-Atlantic Ridge Global bathymetry, showing ocean