Using new samples of feldspar and pumice they ‘reliably dated’ the tuff at 2.61 million years, which agreed nicely.
Later, this date was confirmed by two other dating methods (paleomagnetism and fission tracks), and was widely accepted.
His Ph D thesis was on isotope ratios in meteorites, including surface exposure dating.
He was employed at Caltech's Division of Geological & Planetary Sciences at the time of writing the first edition.
Further evidence comes from the complete agreement between radiometric dates and other dating methods such as counting tree rings or glacier ice core layers.
Although we now recognize lots of problems with that calculation, the age of 25 my was accepted by most physicists, but considered too short by most geologists. Recognition that radioactive decay of atoms occurs in the Earth was important in two respects: Principles of Radiometric Dating Radioactive decay is described in terms of the probability that a constituent particle of the nucleus of an atom will escape through the potential (Energy) barrier which bonds them to the nucleus.
There are over forty such techniques, each using a different radioactive element or a different way of measuring them.
It has become increasingly clear that these radiometric dating techniques agree with each other and as a whole, present a coherent picture in which the Earth was created a very long time ago.
The only problem is that we only know the number of daughter atoms now present, and some of those may have been present prior to the start of our clock. The reason for this is that Rb has become distributed unequally through the Earth over time.
We can see how do deal with this if we take a particular case. For example the amount of Rb in mantle rocks is generally low, i.e. The mantle thus has a low If these two independent dates are the same, we say they are concordant.