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Because lead (the stable daughter of uranium) has a very different arrangement of electrons, it does not make its way into the crystal as it is forming.The formation of crystals in the magma marks the moment that the radio-isotopic clock starts ticking.Scientists usually express this as an age range (e.g., one billion years plus or minus half a million years), meaning that they are very confident that the true date falls somewhere within that range.With modern techniques, these ranges have gotten narrower and narrower, and consequently, even very ancient rocks can be dated quite precisely.
Of course, in this case "shortly" is meant in terms of geologic timescales.
Slightly different dating techniques are used with different radioactive elements, but the same basic logic of estimating backwards based on radioactive decay remains the same.
The geology behind radioisotopic dating Though the basic logic behind radioisotopic dating relies on nuclear physics and quantum theory, many geologic processes also factor into our ability to date a particular rock. How do they know that the rock isn't contaminated with elements that would throw off the dating?
And in the next 704 million years, it will decay leaving behind ¼ gram, and in the next 704 million years, it will decay leaving behind ⅛ gram and so on.
At the same time, the amount of the element that it decays into (in this case lead-207), will increase accordingly, as shown below. At what point on the graph would you expect the ratio of uranium to lead to be about 39 to 61?