The raw material of Earth Time

One of the problems of living in the 'jungle' of geology for so long is that it becomes proverbially difficult to see 'jungle for the trees'. As in most endeavours, when you work very closely with a subject, aspects that initially are quite alien become taken for granted. To the uninitiated (that is, the majority of people) many geological phenomena such as folded rocks or the opening and closing of past oceans can seem really weird, surprising or plain crazy. For me, thinking and talking about geological Earth Time is now almost second nature. But when I first ask normal, intelligent non-geologists how Earth Time is measured, they look askance and then volunteer 'years perhaps' as an answer. They suspect that either I am simple or it's some sort of devious question, since the answer seems so obvious.

'Yes, but where do we get this measure of years from?' To that replies invariably include 'rocks I suppose' and 'radiocarbon dating' from those who have some familiarity with archaeology as opposed to geology. There is an intuitive acknowledgement that somehow or other, since the material of the Earth is basically rock and mineral matter, that must be the ultimate source of Earth Time. But how?

Most people with any scientific background do know something about radioactivity and have heard of radiocarbon dating, because it is the measure of prehistoric time most frequently mentioned in the media. Geologists use the same radiometric method but have to employ different chemical elements. Radiocarbon measures can only be used for carbon-based materials that have been formed over the last 40,000 years or so. Those 40,000 years or around 1000 human generations might seem a long time by our egocentric human standards, but they barely signify in the immense 4,500,000,000 year span of Earth Time.

However, rocks and minerals also play another important role in the measure of Earth Time. Like diary pages that record the months, weeks and days in a year, rocks record time intervals in the history of the Earth. The problem is that the diary for Earth Time is not easily read. Over the last 200 and more years, many generations of Earth scientists have struggled to decipher the testimony and chronicle of the rocks. Despite the seemingly impossible task of recovering the record of the rocks, there was an expectation that as the pages of the chronicle were pieced together, deciphered and put into correct order, the details of the Genesis account would be verified and amplified from the testimony of the rocks. Curiously, as we shall see, some of the geological evidence did seem, at first, to verify some aspects of the Genesis account, especially that of the Flood.

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