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The length of the ruler (b) and diameter of the coin (c) are 1.25 m and 2 cm, respectively.Earth's magnetic field periodically reverses such that the north magnetic pole becomes the south magnetic pole.Argon-argon isotopic dating allowed Sprain and Renne to date the ash layers far more precisely than has been done before.“What’s incredible is that you go from reverse polarity to a field that is normal with essentially nothing in between, which means it had to have happened very quickly, probably in less than 100 years,” said Renne.As the earth rotates, these electric currents produce a magnetic field that extends outward into space.This process, in which the rotation of a planet with an iron core produces a magnetic field, is called a dynamo effect.Magnetism occurs whenever electrically charged particles are in motion.The Earth's molten core has electric currents flowing through it.

The fact that past events have not been associated with mass extinctions seems not to register in these theories.

There has been plenty of speculation that we're heading for another such event quite soon.

Now evidence suggests that the last such event happened much faster than previously thought – suggesting the next one could too.

The Earth's magnetic field experiences reversals such that north becomes south. Researchers have dated volcanic ash that was formed immediately before the last reversal.

This result and chronology of the associated sedimentary rock identifies the age of the reversal as 780,000 years ago.

This volcanic ash contains small crystals called zircons.