Up to now we have focused on the development of local and global geologic histories without numbers. We know methods for working out the sequence of events and making global correlations (biostratigraphy), but have not discussed how we can estimate the duration and absolute age of events.
How can we put numbers on our geologic histories?
Some interesting steps on the way to about 4.5 billion years:
-> Bishop Usher, Biblical chronology, Oct. 26th, 4004 B.C.. Even a rudimentary study of the earth shows a history much to long and complicated to be stuffed into some 6000 years. Many naturalists/theologists decided the earth must be older than a biblical model would indicate.
-> Count Buffon, 1749, extrapolated from cooling rates for cannonballs to the cooling rate of the earth; a leap to 110,000 years or longer in duration.
-> James Hutton, late 1700s, "no vestige of a beginning , no prospect of an end"; the earth of infinite age.
-> Charles Darwin, 1859, Origin of Species, 300 million years since the end of the Mesozoic, based on rates of erosion of a large dome structure.
-> Other estimates made in the 1800s were based on how long it would take for the sea to become as salty as it is, how long would it take to deposit a sequence of rocks given reasonable deposition rates. 100s of millions of years a common answer.
-> Annual deposits: Seasonal changes can cause changes in sedimentation or growth that allow a quantitative estimate for the formation of a deposit or feature. Varves, speleothems, tree rings are some examples.
Photo of recent varved deposits, with each pair representing one year. From USGS site http://pubs.usgs.gov/of/2004/1216/tz/tz.html
-> Lord Kelvin, 1862, a cooling earth revisited:
Radioactive decay and geochronology.
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