Freethought & Rationalism Archive

Camaban · 06-14-2002, 07:02 PM

Radiometric dating on recently (relatively, at least) molten rock.

I'd heard that it's unreliable there, true or false?

Bill · 06-14-2002, 07:58 PM

It depends upon what you mean by "radiometric dating." There are different methods, and each method is suitible for some particular situation.

Of course, if we are speaking of "very" recent, then the whole business gets tied up in the fact that the radioactive decay doesn't hardly begin until the rock cools to some degree, The whole idea is to measure how long since the rock cooled, but there is a huge uncertainty zone when you are very near the time of cooling.

Strictly as an illustration (I don't have the exact numbers off the top of my head), if the usual margin of error was the greater of plus or minus 5% or 5 years, trying to measure it within a couple of years of cooling could give you very unpredictable results.

== Bill

Camaban · 06-14-2002, 08:43 PM

Bill: This is what provoked me to ask the question (if you don't like linking to other forums from here, my apologies)

<a href="http://www.theologyonline.com/vbulletin/showthread.php?s=&postid=72001#post72001" target="_blank">http://www.theologyonline.com/vbulletin/showthread.php?s=&postid=72001#post72001</a>

Quote:

risback
i agree with you on the radio metic dating techniques
They are faulty to the max
Robert Gentry(I think it was ) did a test on the 10 year old lava dome at M.t Saint Helens
Needless to say it was way off The test said something like 3.5ma "old "+/- a bit
Also they tested some rocks from a mountain here in N.Z which erupted sometime in the 1940-50s and the same thing occured roughly 3.5 ma was the "age"given.
Also In Aussie a fossilised tree was was found that had been buried and at some stage in its life lava flowed around its trunk ,
a few mtrs from its roots (you can see the burn mark)
The tree was carbon 14 dated to around 35000 yrs old but the lava was 3.5ma+/- a bit. Which is right?
RM is based on three Assumptions
1) that the decay rate is constant .(yea right!)
2) that the parent /daughter isotopes have not been altered by leaching in or out of the subject being tested .
3)and that sample A had this amount of isotope x at the beginning . (like we were there 50 billion ma ago to measure it .)
If any one of these quesses are wrong the hole thing is thrown out of whack!!
BTW answersingenesis has a article on this showing that in the lab they have showin that radiometic decay can be sed up by as much as a billion fold .
More cogitative material for ya

Just wanting the opinion of someone who knows what he/she is talking about/.

thanks

Dr.GH · 06-14-2002, 09:36 PM

A good deal of research has been done to establish what kinds of situations are not good candidates for radiometric dating.

Most methods used on volcanics date the last time a sample was completely melted. (note that there are some techniques that can compensate for partial reworking but with a loss of accuracy.)

Neither the Mt. St. Helens pyroclastic flow material nor the building lava dome would be good candidates for analysis.

A good book to read is on different methods plus a general history of dating:

Dalrymple, G. Brent,
1991 The Age of the Earth Stanford: Stanford University Press

Here are some online sources, and there are some very well informed people who are regulars on this BB.

<a href="http://home.austarnet.com.au/stear/hiding_the_numbers_woody_henke.htm" target="_blank">Hiding the Numbers to Defame Radiometric Dating</a>

<a href="http://asa.calvin.edu/ASA/resources/Wiens.html" target="_blank">Radiometric Dating A Christian Perspective by Roger C. Wiens</a>

S2Focus · 06-14-2002, 10:13 PM

Quote:

Originally posted by Camaban:
<strong>Radiometric dating on recently (relatively, at least) molten rock.

I'd heard that it's unreliable there, true or false?</strong>

If you are referring to the use of K-Ar dating on lava that's just a few years old, here's something to ponder:

If you were going to mail a box of cookies to your Dad for Father's day, would you use a truck-scale to weigh it in order to determine how much postage to pay? Why or why not?

Camaban · 06-15-2002, 12:28 AM

Cheers

beausoleil · 06-15-2002, 02:22 AM

Radioactive decay is going on all the time. What happens as a magma cools and forms a rock is that minerals form and retain the decay products where they form. It's the time of 'closure' that a chronometer dates.

What the people are talking about here is the excess argon 'problem' in K-Ar dating. The Earth's mantle has an excess of 40Ar, so if the material of a rock didn't lose all it's gas as it erupted, simply dividing 40Ar by potassium will give a false old age. Typically this happens when a basalt contains crystals that were solid befoir it erupted (e.g. xenocrysts), or when a rock wasn't completely degassed. There are 2 flaws with their analysis:

1. People generally use the argon-technique and look for a correlation between potassium and argon rather than just calculating a ratio (different minerals in a rock have different potassium contents, so they have different argon contents from potassium decay since they formed). The age is calculated from the correlation, and the existence of a correlation tests the hypothesis that the argon was generated in the rocj since the rock formed.

2. The excess 40Ar in the mantle is not primordial - it was generated by decay of potassium over the 4.5 Ga of the Earth's existence. Using it to defend a young earth is paradoxical.

Coragyps · 06-15-2002, 05:57 PM

Dalrymple, of Age of the Earth fame, has a table in another publication somewhere ( c'mon, feed me the link!) which has been seized on by AiG and the like. He has a few, five or so, instances of "modern" or historically dated lavas which give "incorrect" dates - he presumable offers reasons for these. AiG uses this part. The part of the table they don't use are the 20+ lavas that give a correct, "zero" age. And this with the "truck-scale to weigh cookies" that S2F mentioned.

06-14-2002, 07:02 PM	#1
Camaban Regular Member Join Date: Jun 2002 Location: Australia Posts: 473	Radiometric dating on recently (relatively, at least) molten rock Radiometric dating on recently (relatively, at least) molten rock. I'd heard that it's unreliable there, true or false?

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06-14-2002, 07:58 PM	#2
Bill Veteran Member Join Date: Dec 2002 Location: Gatorville, Florida Posts: 4,334	It depends upon what you mean by "radiometric dating." There are different methods, and each method is suitible for some particular situation. Of course, if we are speaking of "very" recent, then the whole business gets tied up in the fact that the radioactive decay doesn't hardly begin until the rock cools to some degree, The whole idea is to measure how long since the rock cooled, but there is a huge uncertainty zone when you are very near the time of cooling. Strictly as an illustration (I don't have the exact numbers off the top of my head), if the usual margin of error was the greater of plus or minus 5% or 5 years, trying to measure it within a couple of years of cooling could give you very unpredictable results. == Bill

06-14-2002, 09:36 PM	#4
Dr.GH Veteran Member Join Date: Jul 2001 Location: Dana Point, Ca, USA Posts: 2,115	A good deal of research has been done to establish what kinds of situations are not good candidates for radiometric dating. Most methods used on volcanics date the last time a sample was completely melted. (note that there are some techniques that can compensate for partial reworking but with a loss of accuracy.) Neither the Mt. St. Helens pyroclastic flow material nor the building lava dome would be good candidates for analysis. A good book to read is on different methods plus a general history of dating: Dalrymple, G. Brent, 1991 The Age of the Earth Stanford: Stanford University Press Here are some online sources, and there are some very well informed people who are regulars on this BB. <a href="http://home.austarnet.com.au/stear/hiding_the_numbers_woody_henke.htm" target="_blank">Hiding the Numbers to Defame Radiometric Dating</a> <a href="http://asa.calvin.edu/ASA/resources/Wiens.html" target="_blank">Radiometric Dating A Christian Perspective by Roger C. Wiens</a>

06-15-2002, 12:28 AM	#6
Camaban Regular Member Join Date: Jun 2002 Location: Australia Posts: 473	Cheers

06-15-2002, 02:22 AM	#7
beausoleil Senior Member Join Date: May 2002 Location: US and UK Posts: 846	Radioactive decay is going on all the time. What happens as a magma cools and forms a rock is that minerals form and retain the decay products where they form. It's the time of 'closure' that a chronometer dates. What the people are talking about here is the excess argon 'problem' in K-Ar dating. The Earth's mantle has an excess of 40Ar, so if the material of a rock didn't lose all it's gas as it erupted, simply dividing 40Ar by potassium will give a false old age. Typically this happens when a basalt contains crystals that were solid befoir it erupted (e.g. xenocrysts), or when a rock wasn't completely degassed. There are 2 flaws with their analysis: 1. People generally use the argon-technique and look for a correlation between potassium and argon rather than just calculating a ratio (different minerals in a rock have different potassium contents, so they have different argon contents from potassium decay since they formed). The age is calculated from the correlation, and the existence of a correlation tests the hypothesis that the argon was generated in the rocj since the rock formed. 2. The excess 40Ar in the mantle is not primordial - it was generated by decay of potassium over the 4.5 Ga of the Earth's existence. Using it to defend a young earth is paradoxical.

06-15-2002, 05:57 PM	#8
Coragyps Veteran Join Date: Aug 2001 Location: Snyder,Texas,USA Posts: 4,411	Dalrymple, of Age of the Earth fame, has a table in another publication somewhere ( c'mon, feed me the link!) which has been seized on by AiG and the like. He has a few, five or so, instances of "modern" or historically dated lavas which give "incorrect" dates - he presumable offers reasons for these. AiG uses this part. The part of the table they don't use are the 20+ lavas that give a correct, "zero" age. And this with the "truck-scale to weigh cookies" that S2F mentioned.

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