Potassium is chemically incorporated into common minerals, notably hornblende, biotite and potassium feldspar, which are component minerals of igneous rocks.
Argon, on the other hand, is an inert gas; it cannot combine chemically with anything.
For every 100 K-40 atoms that decay, 11 become Ar-40.
The method relies on satisfying some important assumptions: Given careful work in the field and in the lab, these assumptions can be met.
The rock sample to be dated must be chosen very carefully.
If the mineral composition of the two sample is different, so that the sample for measuring the potassium is richer or poorer in potassium than the sample used for measuring the argon, then this will be a source of error.
Potassium-Argon Dating Potassium-Argon dating is the only viable technique for dating very old archaeological materials.
Developed in the 1950s, it was important in developing the theory of plate tectonics and in calibrating the geologic time scale.
Potassium occurs in two stable isotopes (Ar atoms trapped inside minerals.
Any alteration or fracturing means that the potassium or the argon or both have been disturbed.
The site also must be geologically meaningful, clearly related to fossil-bearing rocks or other features that need a good date to join the big story.
By comparing the proportion of K-40 to Ar-40 in a sample of volcanic rock, and knowing the decay rate of K-40, the date that the rock formed can be determined.