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Visit Stack Exchange Earth Science Stack Exchange is a question and answer site for those interested in the geology, meteorology, oceanography, and environmental sciences. Sign up to join this community 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.
The calcium-potassium age method is seldom used, however, because of the great abundance of nonradiogenic calcium in minerals or rocks, which masks the presence of radiogenic calcium.
On the other hand, the abundance of argon in the Earth is relatively small because of its escape to the atmosphere during processes associated with volcanism.
Using the argon-argon dating technique, by which scientists measure the decay of an isotope called Argon-40 into Argon-39 in order to find the age of crystals, they came up with a rough approximation of the footprints' age: 19,000 years at the oldest, 10,000 or 12,000 years at the youngest.
Potassium-40 is radioactive but has such a long half-life that it is primordial - it has been around since the earth was being formed. About 10.7% decays to Argon-40 by emitting a positron.
Because of this, we can assume that the potassium-40:potassium-39 ratio in minerals at any time is a constant. If the argon-40 stays trapped in the crystal and you can measure the ratio of potassium-40 to argon-40, then you know how long it has been since the mineral formed.
This also assumes that there is no other source of argon like trapped air.
As the K-40 in the rock decays into Ar-40, the gas is trapped in the rock.
In this simulation, a unit of molten rock cools and crystallizes. Note that time is expressed in millions of years on this graph, as opposed to thousands of years in the C-14 graph.