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As a bonus, the intercept ($$b$$) of the line tells us the value of $$\text_$$ because we know the line was flat when the age of the rock was zero. Note that the values of the axes are actually normalized by Sr86 because the mass spectrometers used to take these measurements are much more accurate at relative values than they are at absolutes.

The animation in Figure 2 shows the flat line and how it increases with time. It works because Sr86 is stable and not radiogenic and therefore stays constant with time.

The data have been replotted from the tables of [2].On the other hand, since only lead is involved (instead of two chemical species), the lead-lead clock is resilient against situations where the samples were recently weathered or otherwise "opened".We will skip the isochron derivation, but you can find it elsewhere[1].You may have heard that the Earth is 4.54 billion years old. If the half-life of a material is 100 years and you have 1 kg of it, 100 years from now you will only have 0.5 kg of it.This was calculated by taking precise measurements of things in the dirt and in meteorites and using the principles of radioactive decay to determine an age. The rest will have decayed into a different nuclide (called a nuclide).