Last Time: Magnetic Methods (Intro) Governed by Laplace’ eqn :  2 u = f (sources) Differs from gravity in that the source is a dipole rather than a monopole Magnetic field strength relates to force exerted by one “monopole” on the other: Intensity of induced magnetization by an external field is (where k is magnetic susceptibility ) Crustal minerals can be diamagnetic ( k ~ –10 -5 ); paramagnetic ( k ~ 0.02–0.2); antiferromagnetic ( k ~ 0.05); or ferrimagnetic ( k ~ 0.5–10) Core minerals are ferromagnetic … Geology 5660/6660 Applied Geophysics 26 Mar 2014 © A.R. Lowry 2014 For Fri 26 Mar: Burger (§7.4–7.6)

Last Time Cont’d: Magnetic Methods (Intro) Magnetic anomaly is the measured perturbation of the external (core) magnetic field’s vector magnitude by crustal magnetization (i.e., vector component of the total change in the field in the direction of measurement!) Must know strength & direction of the Earth’s main field! Magnetic field strength decays as 1/r 3 Total intensity of magnetization: where remanent magnetization I R is in the direction of H E at the time of magnetization… Geology 5660/6660 Applied Geophysics 26 Mar 2014 © A.R. Lowry 2014 For Fri 26 Mar: Burger (§7.4–7.6)

Field Exercises Tomorrow: West Cache Fault near Mendon, UT (Thanks for the legwork Michael!)

special_studies/SS-98.pdf Two events on a 7 m (!) scarp: One 4.4–4.8 ka; the other 15–25 ka. Long- term slip rate ~ 0.13 mm/year… Would imply a ~27 ka recurrence for a 3.5 m event (but with caveats!)

The Earth’s Main Field (Core Field): Earth’s main field is generated by convection of Earth’s fluid Ni-Fe outer core. As the solid inner core cools & grows, released heat drives thermo-chemical convection. Motion of the electrically-conductive molten iron produces electric currents which in turn generate a magnetic field. Rotation of the Earth  Coriolis forces which cause a Magnetohydrodynamic Dynamo Effect, in which magnetic fields organize in a way that amplifies the current flow. Positive feedbacks are self-stabilizing & produce a very large, predominantly dipolar magnetic field (with smaller higher-order terms).

Core-generated magnetic field is a vector quantity so has magnitude and direction. Most often described by: intensity H E (i.e., magnitude) inclination i (  from horiz) declination d (  from true N) These vary depending on location on Earth’s surface, and also change nonlinearly with time! Intensity H E varies from ~25k nT at equator to ~65k nT at poles Here blue is negative ( i is positive down) HEHE i d World magnetic model 2010

Westward drift of “non-dipole” field (& precession of magnetic about rotation pole)  declination changes relatively rapidly

IGRF Inclination 1995 Can express vector where are geographical East, North, Up directions. Then intensity inclination i is the angle of field direction from horizontal (positive downward): declination d is angle (positive clockwise) from true north to magnetic north:

Intensity of Earth’s total dipole field also changes through time… Paleointensity measurements are very noisy but they and models of core dynamo suggest field is strong immediately after a reversal & weakens (~ exponential decay) for some period to near zero, then jumps to high value… Reversal may or not accompany the jump. This “sawtooth” pattern is basis for suggestion by some that Earth will experience a reversal in next ~2000 years. But it’s not nearly that predictable!

500 yrs before reversal mid-reversal500 yrs after reversal Glatzmeier modeling revealed: Solid inner core magnetized opposite main field; forced to rotate by applied torque  precession (~0.2°/yr for real Earth) Inner core stabilizes field dipole; long time required to diffuse outer core field to inner core controls reversal timescale