Radioactive decay dating rocks
In other words, the hourglass only works when we know its initial condition.
Unlike the hourglass, we do not know how much of each isotope was in the rock in the beginning.
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.
Apart from the fatal problem of not knowing the initial conditions, there is another problem that is just as deadly.
We don’t know what happened to the rock during its ‘lifetime’.
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.
Everything is composed of elements and there are about 90 naturally occurring ones, such as hydrogen, carbon, oxygen and iron.
Radioactive dating is often illustrated with an hour glass.
The sand grains at the top of the sealed glass are like the atoms of the parent isotope in the rock, and those at the bottom like the atoms of the daughter.
‘It’s only had two new heads and three new handles.’ The question is: how old was his axe? When a geologist hammers off a sample of rock he needs to know its history.
Different minerals would have crystallized at different times depending on the way the molten magma cooled.