The point is that not all methods are applicable to all rocks of all ages.One of the primary functions of the dating specialist (sometimes called a geochronologist) is to select the applicable method for the particular problem to be solved, and to design the experiment in such a way that there will be checks on the reliability of the results.They observed that every rock formation, no matter how ancient, appeared to be formed from still older rocks.

The main point is that the ages of rock formations are rarely based on a single, isolated age measurement.

On the contrary, radiometric ages are verified whenever possible and practical, and are evaluated by considering other relevant data.

Lord Kelvin and Clarence King calculated the length of time required for the Earth to cool from a white-hot liquid state; they eventually settled on 24 million years.

James Joly calculated that the Earth’s age was 89 million years on the basis of the time required for salt to accumulate in the oceans.

The discovery of radioactivity in 1896 by Henri Becquerel, the isolation of radium by Marie Curie shortly thereafter, the discovery of the radioactive decay laws in 1902 by Ernest Rutherford and Frederick Soddy, the discovery of isotopes in 1910 by Soddy, and the development of the quantitative mass spectrograph in 1914 by J. Thomson all formed the foundation of modern isotopic dating methods.

But it was not until the late 1950s that all the pieces were in place; by then the phenomenon of radioactivity was understood, most of the naturally occurring isotopes had been identified and their abundance determined, instrumentation of the necessary sensitivity had been developed, isotopic tracers were available in the required quantities and purity, and the half-lives of the long-lived radioactive isotopes were reasonably well known.

The K-Ar clock works primarily on igneous rocks, i.e., those that form from a rock liquid (such as lava and granite) and have simple post-formation histories.

It does not work well on sedimentary rocks because these rocks are composed of debris from older rocks.

Radiometric dating is based on the decay of long-lived radioactive isotopes that occur naturally in rocks and minerals.

These parent isotopes decay to stable daughter isotopes at rates that can be measured experimentally and are effectively constant over time regardless of physical or chemical conditions.

By the early 1960s, most of the major radiometric dating techniques now in use had been tested and their general limitations were known.