An hour glass is only useful if it is not disturbed.
But after rocks crystallize from molten magma, they can be heated and cooled; they can be affected by metamorphic events and groundwater.
Some minerals may have crystallized even later still when ground waters in the area percolated through the pores of the rock.
He explores the geology of the area so he can understand the geological history, and where his particular sample fits into the sequence of geological events.
He checks out the ages other geologists have assigned to the different rocks in the region.
This means that, on its own, a radioactive ‘date’ is meaningless. You may be surprised to learn that a geologist would never collect a rock at random and send it off for radioactive dating on its own. Every radioactive date has to be interpreted before anyone can say what it means.
What happens is that the geologist will carefully record exactly where he collected the rock.
There is a radioactive form of thorium that also decays into lead.
There is an isotope of samarium that decays into neodymium, and one of rubidium that decays into strontium.
There are many different radioactive isotopes that are used for radiometric dating.
For example, there is a radioactive form of potassium (potassium-40) that decays into argon (argon-40).
Radioactive decay is where the parent atoms change as a result of radioactive decay into daughter atoms, like the individual grains of sand falling from the top to the bottom of the glass.