Radiometric dating


One thing that’s bothered me about how scientists know how old the Earth is was radiometric dating.  Radiometric dating uses the radioactive decay of one substance into another to determine how old something is.  The more it decayed, the older it is- the decay happens at an essentially constant rate.

I saw a problem with this, though.  If you don’t know how much you had in the first place, how can you backtrack from what you have now?  I figured scientists had some way around this, but I had no idea what it was and this troubled me.

Come to a month or so ago when I started up “Introduction to Astronomy” on Coursera.  Part of astronomy depends on how old stuff is- any substantial revision in the current estimate of an Earth about 4.5 billion years old, and a solar system a few hundred million older, calls a great deal of other knowledge into question.  It’s not really a foundational bit of knowledge, but other stuff won’t make sense if this number is very far off.  So it really bothered me not understanding how scientists get around not knowing how much the rocks they dated started with.

Turns out, it’s simple.  I had expected it to be incredibly complex.  Actually applying it depends on precise measurements and a fair bit of math, but the principle behind it is actually really simple.

Ok, so before I get to the way they dodge needing to know the initial quantity, there are a couple things that need to be known.  First, as planets coalesced, they were molten.  Liquid rock, like lava basically.  Yes, the Earth was at one point a giant ball of lava.  There are some substances, like lead, that escape magma.  If any lead is trapped in a rock, it therefore *must* have formed after the magma cooled into a rock.  If it formed before cooling, it would have promptly escaped.

The other major thing is radioactive decay, and how it works.  There are various types of radioactive decay.  Uranium, for instance, will spontaneously undergo fission- splitting one atom into two smaller atoms.  This occurs at a constant rate- so if you can work out how much you started with, you can determine the age of a sample.  One of the products is lead.  Lead, as mentioned above, doesn’t stick around in magma.  

So, you find a rock.  You know that 100% of the lead in that rock was once uranium, and was formed after the rock cooled from magma.  So you can look at the ratio of uranium to lead in the rock, do a bit of math, and BAM- that’s how long ago the original magma cooled.  So if you date a rock reliably sourced to Earth to 4.5 billion years ago, you can be sure the Earth is at least that old. 

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