Dr. Dorothy Hanna
Dr. David Hanson
Jim ZanardiFavorite Links:
Post-talk Comments on The Marks of Time
Comments on Dr. David Hanson Skywatch Talk – The Marks of Time, including references to Lord Kelvin contribution and controversy
Kasoff, Secretary of the
David Hanson gave both a historical survey of the pre-scientific and modern scientific age determination for the earth. Even though this listener had a reasonable prior understanding of the subject, the talk content and Q&A was interesting. Two ideas raised at the talk are presented below for your comment and enjoyment.
1. A convective heat transfer model proposed in 1895 by John Perry was more important than the new source of heat generation from radioactive materials, for greatly increasing the predicted age of the earth. Lord Kelvin model is based on thermal gradients in a conductive sphere. The whole sphere rate of conduction was based on measurements taken in mining shafts. These measurements in the crustal rock greatly underestimated the rate of heat transfer in the earth’s interior. The more efficient convective transfer of heat from a molten or plastic interior stretched the age determination from Kelvin’s prediction of 100 million (108) years to 2 billion (2 x 109) years. See the link to John Perry’s neglected critique of Kelvin’s age for the Earth: A missed opportunity in geodynamics:
The following teaching essay describes the conflict in the prediction of the Sun's and Earth's ages during the 19th century. The authors describe how the mathematical physicist, Lord Kelvin dismissed the views of Darwin and the geologists as non-scientific.
Find Lord Kelvin's 1868 historic refutation of the muddle-headed geologic thinking at the following link: http://zapatopi.net/kelvin/papers/on_geological_time.html
It is amazing how certain Lord Kelvin was about a young age for the Sun and Earth. The presentation perplexed Charles Darwin, since a key assumption of his 1859 evolutionary theory was a vast time span for natural selection to create the great diversity of life.
2. Paraphrasing Dorothy Hanna, PhD question - Does accurate radiometric dating rely on the assumption that daughter atoms are absent during initial solidification of sample?
Responses to question based on web search after the talk:
a) Several independent methods yield essentially the same earth/planetary formation age of 4.55 x 109 years. Of course, there are some examples of the poor research methods in the published literature, but good techniques have yielded very consistent dates over the last 55 years of research
b) A specific class of minerals called Zircons used in U-Pb radiometric dating strongly reject lead during crystallization, thereby minimizing the possibility of daughter atoms in the formation of the sample.
c) Mathematically sophisticated radiometric methods can account for the present of daughter atom in the initial sample
See following links for more detailed information that are the basis for above responses:
a) http://en.wikipedia.org/wiki/Uranium-lead_dating. The following important point is made in this article – “Uranium-lead dating is usually performed on the mineral zircon (ZrSiO4), though it can be used on other minerals such as monazite, titanite, and baddeleyite. Zircon incorporates uranium and thorium atoms into its crystalline structure, but strongly rejects lead. Therefore we can assume that the entire lead content of the zircon is radiogenic. Where this is not the case, a correction must be applied.”
c) http://en.wikipedia.org/wiki/Lead-lead_dating. Independent method using Geochron that can account for daughter atom in initial sample – very mathematical
d) http://en.wikipedia.org/wiki/Age_of_the_Earth#cite_note-kp-18. Good overview article on the Age of the Earth
Recommended textbook accessible to both scientists and non-scientists - Dalrymple, G. Brent, 1991. The Age of the Earth. California: Stanford University Press, ISBN 0-8047-1569-6
Lastly, a following link to the Decay Chain for U-238/Pb-206 is given with associated table of radionuclei half-lives. http://en.wikipedia.org/wiki/Decay_chain. The relative short half-lives of the intermediate radionuclei between U-238 and Pb-206 simplify the age calculation, since parent and daughter atoms will represent 99.99% of sample. The simplified mathematics do not reinforce the argument supporting the reliability of radiometric dating, just makes it easier to understand.
Skywatch Schedule 2012-13 |
Winter Observing Tips by Alan Witzgall |
Neptune - The Last Gas Giant |
Post-talk Comments on The Marks of Time |
Info Links & books about Saturn, The Ringed Planet |
Rise and Set Time for Sun, 2012|
Fraction of Moon Illuminated, 2012 | Rise and Set Time for Moon, 2012 | HOME | WRITE US