Why is potassium 40 useful in dating igneous rocks
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Other radiometric techniques, like carbon14, are useful for much more recent dates.
The technique can be used for dates ranging from earth's beginning, 4550 mya (4.55bn in US terminology) to about 100,000 years ago.The half-life of potassium40 is roughly 1.25 billion years, so four half-lives are about five billion years, or older than the earth.Another factor that can skew measurements is reheating of the rock, which can partially reset the clock by releasing some of the stored argon into the atmosphere.This is not a significant problem for igneous rocks, but it affects the usefulness of the techniques for other forms of rock that have more complex heating histories.
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Because argon is an inert gas, if it is not physically trapped in a rock, it will diffuse into the atmosphere.
In molten rock almost all of the argon will be released into the atmosphere; so in volcanic material, when the rock cools and hardens, the argon begins to accumulate in the crystals, effectively starting the clock.
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