That is, a fresh mineral grain has its K-Ar "clock" set at zero.Given careful work in the field and in the lab, these assumptions can be met.

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.

The rock samples are crushed, in clean equipment, to a size that preserves whole grains of the mineral to be dated, then sieved to help concentrate these grains of the target mineral.

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.

Lava flows that lie above and below rock beds with ancient human fossils are a good—and true—example.

The mineral sanidine, the high-temperature form of potassium feldspar, is the most desirable.

But micas, plagioclase, hornblende, clays and other minerals can yield good data, as can whole-rock analyses.

By Andrew Alden The potassium-argon (K-Ar) isotopic dating method is especially useful for determining the age of lavas.

Developed in the 1950s, it was important in develping plate tectonics and in calibrating the geologic time scale.

Potassium occurs in two stable isotopes (Ar atoms trapped inside minerals.

What simplifies things is that potassium is a reactive metal and argon is an inert gas: Potassium is always tightly locked up in minerals whereas argon is not part of any minerals. So assuming that no air gets into a mineral grain when it first forms, it has zero argon content.