Pole reversal?

Question

How do Scientists now there have been 400 pole reversals? What evidence are they going by? What actually happens when these reversals take place?

Answer 1

When magnetic crystals settle out of water or crystallize out of a lava or magma, they will orient according to the earth's magnetic field at the time. Examining rocks with ages known from other methods of dating allow stratigraphers to determine the strength and polarity of the magnetic field at a given time

Answer 2

The pole reversals don't happen abruptly.  The poles are moving all the time.  A complete 180 degree reversal take s thousands of years.  The magnetic striping along the mid-Atlantic Ridge is evidence of the reversals.  When the rift opens and magma wells up, the liquid lava aligns itself with the magnetic poles due to its magnetite content.  Then it hardens in this configuration.  The stripes have alternating magnetic direction.

The north magnetic pole was first discovered in 1831 and when it was revisited in 1904, explorers found that the pole had moved 31 miles.

Studies have shown that the strength of the Earth's magnetic shield has decreased 10 percent over the past 150 years. During the same period, the north magnetic pole wandered about 685 miles out into the Arctic, according to a new analysis by Stoner.

The rate of the magnetic pole's movement has increased in the last century compared to fairly steady movement in the previous four centuries, the Oregon researchers said.

At the present rate, the north magnetic pole could swing out of northern Canada into Siberia. If that happens, Alaska could lose its Northern Lights, which occur when charged particles streaming away from the sun interact with different gases in Earth's atmosphere.

The magnetic poles are part of the magnetic field generated by liquid iron in Earth's core and are different from the geographic poles, the surface points marking the axis of the planet's rotation.

Scientists have long known that magnetic poles migrate and in rare cases, swap places. Exactly why this happens is a mystery.

"This may be part of a normal oscillation and it will eventually migrate back toward Canada,'' Joseph Stoner, a paleomagnetist at Oregon State University, said Thursday at an American Geophysical Union meeting.

Answer 3

The polarity of the magnetic field can clearly be seen in maps of the magnetic polarity on lines perpendicular to the mid-Atlantic ridge.  When the outflowing magma cools, the orientation of the magnetic crystals is fixed at the orientation of the magnetic field when the crystals form.  The maps of the floor of the Atlantic show stripes of reversing magnetic fields over the time of the formation of the oceanic crust.

Answer 4

By the alignment of rocks at either side of the mid-ocean ridge.

The seafloor is characterized by mid-ocean ridges. Hess hypothesized that the topography of the seafloor could be explained if the seafloor moves sideways, away from the oceanic ridges. Hess postulated that magma rose from the interior of the Earth and formed new oceanic crust along the mid-ocean ridges. This hypothesis was tested using paleomagnetism. When lava is extruded at any mid-ocean ridge, the rock it forms becomes magnetized and acquires the magnetic polarity that exists at the time the lava cools. As the crust moves away from mid-ocean ridges, it contains a continuous record of the Earth's changing magnetic polarity.

This record shows that the  poles have reversed several times in Earth's history.