8 Physicists have carefully measured the radioactive decay rates of parent radioisotopes in laboratories over the last 100 or so years and have found them to be essentially constant (within the measurement error margins).
Furthermore, they have not been able to significantly change these decay rates by heat, pressure, or electrical and magnetic fields.
So geologists have assumed these radioactive decay rates have been constant for billions of years.
When we look at sand in an hourglass, we can estimate how much time has passed based on the amount of sand that has fallen to the bottom.
Radioactive rocks offer a similar “clock.” Radioactive atoms, such as uranium (the parent isotopes), decay into stable atoms, such as lead (the daughter isotopes), at a measurable rate.
These basalts yield ages of up to 1 million years based on the amounts of potassium and argon isotopes in the rocks.
But when we date the rocks using the rubidium and strontium isotopes, we get an age of 1.143 billion years.
Yet the same uranium decay also produced abundant helium, but only 6,000 years worth of that helium was found to have leaked out of the tiny crystals. Not Billions (Master Books, Green Forest, Arkansas, 2005), pages 65–78.