Although these particular isotopes are not used to date rocks, they illustrate that radioisotope decay (radiodecay) is not always constant.
Second, rocks observed to form on a particular date often show radioisotope age estimates far exceeding their actual ages. Helens was only ten years old, it showed a radioisotope age estimate of 340,000 years!
It is similar to assuming that the constriction in an hourglass has always been the same diameter, and the same number of sand grains passes every minute.
Radioisotope decay rates are renowned for constancy under normal conditions, so this assumption appears reasonable.
But two observations and two clues omitted from physics textbook discussions of radiodating show that these radioisotope “clocks” are broken.
First, scientists have observed that radioactive isotope (radioisotope) decay rates do fluctuate, including Th-228, Rn-22, and Si-32.
Geologists do not directly measure the age of a rock.
They choose rocks containing radioactive “parent” isotopes that emit particles and radiation to become a different “daughter” element and measure ratios of elements to their isotopes.
Attempts to transform these ratios into dates are where this becomes problematic.
Assigning a date requires that the rate at which the parent decays into the daughter element has been the same throughout the rock’s history.
One clue was abundant helium trapped in tiny zircon crystals inside granite.