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That's because it doesn't influence decay rates. The age of stones is measured by effects of decaying nuclei or other forms of radiation on them, from within (Uranium-Lead or other forms of isotope ratio dating) and without ((thermo)luminescence). Once the rock is formed, it stops releasing (some) products of decay or replenishing less stable isotopes, which is not unlike removing a living organism from C14 cycle after it dies. EDIT: Do keep in mind that the above explanation is somewhat idealised. Also, wikipedia has a nice overview of chronological dating methods if you are interested.I'll guess that something about hot lava prevents some kind of radioactive decay from occurring, but I can't imagine how.