Potassium 40 dating
Young rocks have low levels of Ar, so as much as several kilograms may be needed.
Rock samples are recorded, marked, sealed and kept free of contamination and excessive heat on the way to the lab.
Terrified residents are fleeing Tokyo today as the Fukushima nuclear plant destroyed by last week's tsunami threatened to send a cloud of radioactive dust across Japan.
Any alteration or fracturing means that the potassium or the argon or both have been disturbed.
A precise amount of argon-38 is added to the gas as a "spike" to help calibrate the measurement, and the gas sample is collected onto activated charcoal cooled by liquid nitrogen.
Then the gas sample is cleaned of all unwanted gasses such as H, nitrogen and so on until all that remains are the inert gasses, argon among them.
That is, a fresh mineral grain has its K-Ar "clock" set at zero.
The method relies on satisfying some important assumptions: Given careful work in the field and in the lab, these assumptions can be met.