Do the Earth's magnetic poles ever change places?
Is it possible for the poles on Earth to switch places? Would we feel any effects from this happening? Why does this happen?
The poles on the Earth have changed places - many times! We can tell this has happened because the magnetic moment of the rocks that make up the ocean floor have an alternating direction. Which direction they exhibit depends on which way the poles were oriented when the rocks were being formed at the mid-ocean ridge.
During a reversal, which can take thousands of years, the magnetic poles start to wander away from the region around the spin poles, and eventually end up switched around. Sometimes this wandering is slow and steady, and other times it occurs in several jumps. One of the things that does consistently happen during a reversal is that the strength of the magnetic field decreases to almost zero. This is the part that has a lot people worried, as the magnetic field blocks a lot of incoming solar radiation that may be harmful to life.
Based on current research, the effects on humans and the Earth would actually be pretty negligible ("The Core" is hilarious. but a total scientific nightmare). Most of the harmful radiation the magnetic field blocks would be absorbed by the atmosphere, and wouldn't reach the surface (this is why it will be difficult to colonize Mars - no magnetic field OR atmosphere!). A few poorly built satellites might stop working, but overall, not much would happen to humans.
The cause of the reversals isn't well understood. The magnetic field is created by the Earth's "dynamo," or the extremely complicated set of currents of liquid iron in the outer core. Some models have shown that a reversal is the result of the reorganization of the currents, but we probably won't know for sure until it happens.
Here are some links on reversals: http://en.wikipedia.org/wiki/Magnetic_polarity_reversal http://en.wikipedia.org/wiki/Paleomagnetism
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