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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 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.Second, the rock or mineral must not lose or gain either potassium or argon from the time of its formation to the time of analysis.By many experiments over the past three decades, geologists have learned which types of rocks and minerals meet these requirements and which do not.They observed that every rock formation, no matter how ancient, appeared to be formed from still older rocks.Comparing these rocks with the products of present erosion, sedimentation, and earth movements, these earliest geologists soon concluded that the time required to form and sculpt the present Earth was immeasurably longer than had previously been thought.

The K-Ar method is probably the most widely used radiometric dating technique available to geologists.

There were other estimates but the calculations were hotly disputed because they all were obviously flawed by uncertainties in both the initial assumptions and the data.

Unbeknownst to the scientists engaged in this controversy, however, geology was about to be profoundly affected by the same discoveries that revolutionized physics at the turn of the 20th century.

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.

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For example, a method based on a parent isotope with a very long half-life, such as C method can only be used to determine the ages of certain types of young organic material and is useless on old granites.