Radioactive potassium dating
The key is to measure an isotope that has had time to decay a measurable amount, but not so much as to only leave a trace remaining.
Given isotopes are useful for dating over a range from a fraction of their half life to about four or five times their half life.
Argon gas, brought up from deep inside the earth within the molten rock, was already present in the lavas when they cooled.
We know the true ages of the rocks because they were observed to form less than 50 years ago.
In using this method, it is assumed that there was no daughter radiogenic argon (Ar*) in rocks when they formed.13 For volcanic rocks which cool from molten lavas, this would seem to be a reasonable assumption. Inset: Andesite of the June 30, 1954 flow, Mt Ngauruhoe, seen at 60 times magnification under a geological microscope. The darker recent lavas were clearly visible and each one easily identified (with the aid of maps) on the northwestern slopes against the lighter-coloured older portions of the cone (Figures 4 and 7).
If the method fails on rocks when we have an independent eye-witness account, then why should we trust it on other rocks where there are no independent historical cross-checks?
However, we do know Someone who was present when all the earth’s rocks formed—the Creator Himself.
Standing roughly in the centre of New Zealand’s North Island, Mt Ngauruhoe is New Zealand’s newest volcano and one of the most active (Figures 1 and 2).
It is not as well publicized as its larger close neighbour MT Ruapehu, which has erupted briefly several times in the last five years.