Lecture%20#13%20Geol%203950%20notes%202010%20CR%20Stern

Lecture%20#13%20Geol%203950%20notes%202010%20CR%20Stern -...

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Lecture #13 notes, Geology 3950 Spring 2010: CR Stern Magnetic reversals The earth has a magnetic field generated by circulation of charged particles (electrons) in the liquid iron outer core. Due to the earth’s rotation, which constrains how these charged particles move, the earth’s magnetic field has many of the characteristics, most significantly the geometry of the magnetic field lines, as if a simple dipole bar magnet was inside the earth’s core (figure 1 below), but it is not since the temperature of the core of the earth is too hot to support the presence of a stable permanent magnet. Earth’s dipole magnetic field is oriented approximately north-south, and the south magnetic pole currently is aligned with the geographic north rotational pole, deviating by 11.5 degrees The earth’s magnetic field is an important shield against solar winds and radiation and diverts much of this radiation around the magnetic field, or magnetosphere, into space (figure 2), except in rare solar flares when this solar radiation enters the earth’s atmosphere along the magnetic field lines near the poles and creates the aurora borealis.
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The magnetic field also captures charged particles in the Van Allen belts (figure 3 below), which encircle the earth in the upper atmosphere around the equator, and these particles further shield the earth for nasty cosmic and solar radiation. The sun also has a magnetic field that encircles the solar system (figure 4) and shields the solar system from cosmic rays. Neither the Moon nor Mars have magnetic fields because they do not have liquid iron cores. The earth’s magnetic field is not a stable feature, but varies in its orientation relative to the earth’s geographic rotational orientation (called polar wandering – see figure 5 below), direction of the north-south dipole (in either a normal or reversed direction) and strength (called the secular variation of the field strength). The changing orientation of the earth’s magnetic field with respect to the geographic rotational axis has been measured very carefully over the last few hundred years or so by navigators (figure 5) and shows smaller to greater deviations from the rotational north.
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Information of the behavior of the magnetic field in the pre-instrumental, pre-historic and geologic past is obtained from what is called “remnant magnetism” contained in rocks. When a hot magma cools from >1000°C to form a solid rocks, tiny magnetic minerals -- iron oxides -- in the rock line up like little bar magnets along the direction of the earth’s magnetic field and preserve information about the orientation of the magnetic field lines and strength of the field at the time the rock cooled. Similar minerals in sedimentary
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This note was uploaded on 03/02/2010 for the course GEOL 3950 taught by Professor Charles during the Spring '08 term at Colorado.

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Lecture%20#13%20Geol%203950%20notes%202010%20CR%20Stern -...

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