Re: [asa] geological dating

Moorad asked:

Is radioactive decay done separately on both the fossil and the
> corresponding strata and thus corroborated?
>

There are many different decay series that are used in radioactive dating.
Each can be used only on a particular range of minerals under specific
conditions, and each has a particular half life. Except for very recent
fossils (several tens of thousands of years or less) that can potentially be
dated by C14, fossils cannot be directly dated using radiometric methods.
Also, only certain types of rocks can be dated -- only those with the
required minerals and that meet other necessary conditions. The rock types
most easily dated are igneous rocks (that date the time of cooling) and
volcanic ash deposits (dating the time of eruption), and some metamorphic
minerals (which date the time of metamorphosis). Sedimentary rocks are not
easily dated, and dated mineral grains within sedimentary rocks date the age
of the igneous or metamorphic rocks from which those grains were eroded.
The ages of sedimentary layers are usually dated by correlation with the
radiometrically dated rocks. There are some diagenetic minerals and mineral
cements in sedimentary rocks that can be dated.

There are also a range of non-radiometric dating methods that provide other
independent means of determining the ages of fossil-bearing rocks.

Typically a number of independent dating techniques are used to determine
the age of particular rocks or strata. It is the agreement between such
independent methods that give considerable confidence to the calculated
dates.

Davis Young published a series of articles in the ASA journal "Perspectives"
a couple of years ago. He discusses all of the major radiometric and
non-radiometric methods in use. An excellent popular book on radioactive
dating is "Ancient Earth, Ancient Skies" by C. Brent Dalrymple. Also the
new book by Davis Young and Ralphy Stearley "The Bible, Rocks, and Time"
devotes significant space to radiometric dating and to the development of
the geologic time scale.

Keith

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Received on Mon Oct 12 14:30:25 2009