Argon-Argon dating is similar to Potassium-Argon dating, but it requires less material.
There's a more basic explanation here: books.google.com/…
A problem is that it takes two separate measurements to get the potassium concentration and the argon isotope ratios, increasing the uncertainty.
Argon-argon dating gets around many of the issues by measuring only multiple isotopes of argon.
While the moment in time at which a particular nucleus decays is random, a collection of atoms of a radioactive nuclide decays exponentially at a rate described by a parameter known as the half-life, usually given in units of years when discussing dating techniques.
After one half-life has elapsed, one half of the atoms of the substance in question will have decayed.
The Washington Post article Scientists discover hundreds of footprints left at the dawn of modern humanity describes the geological dating of stratified layers of mud by analyzing and dating minerals within each layer.