Radiometric dating timeline
In hindsight, both theories were deeply misguided, for similar reasons.They assumed that current rates—of sediment deposition and of salt transport by rivers—were the same as historical rates, despite the evidence they had that our own age is one of atypically high geologic activity. The rock cycle, as we now know, is driven by plate tectonics, with sedimentary material vanishing into subduction zones.By 1788 Hutton had formulated a theory of cyclic deposition and uplift, with the earth indefinitely old, showing “no vestige of a beginning—no prospect of an end.” Hutton considered the present to be the key to the past, with geologic processes driven by the same forces as those we can see at work today.This position came to be known as uniformitarianism, but within it we must distinguish between uniformity of natural law (which nearly all of us would accept) and the increasingly questionable assumptions of uniformity of process, uniformity of rate and uniformity of outcome.
It was not until 1926, when (under the influence of Arthur Holmes, whose name recurs throughout this story) the National Academy of Sciences adopted the radiometric timescale, that we can regard the controversy as finally resolved.The second depended on highly dubious theories of formation of the earth and moon and plays relatively little role in this compilation.The third, which by the end was the most acute, presented a problem that outlasted the controversy itself.The Talmudic rabbis, Martin Luther and others used the biblical account to extrapolate back from known history and came up with rather similar estimates for when the earth came into being.The most famous came in 1654, when Archbishop James Ussher of Ireland offered the date of 4004 B. Within decades observation began overtaking such thinking.