How dating methods work
by Tas Walker
Images from stock.xchng
Addressing the students, I used a measuring cylinder to illustrate how scientific
dating works. My picture showed a water tap dripping into the cylinder. It was clearly
marked so my audience could see that it held exactly 300 ml of water. The diagram
also showed that the water was dripping at a rate of 50 ml per hour.
I asked, ‘How long has the water been dripping into the cylinder?’
Immediately someone called out, “Six hours.”
“Good. How did you work that out?”
“By dividing the amount of water in the cylinder (300 ml) by the rate (50
ml per hour).”
“Excellent,” I said. “See how easy it is to calculate the age
of something scientifically? Every dating method that scientists use works exactly
the same way. It involves measuring something that is changing with time.”
People began to relax once they understood that the science of dating is not so
difficult. Then I surprised them, “The problem is that six hours is the wrong
answer.”
They look puzzled and disbelieving.
Image from stockxpert
“I set this experiment up and I can tell you that the water has only been
dripping for one hour. Can you tell me what happened?”
After they had composed themselves, someone called out, “The tap was dripping
faster in the past?”
“Perhaps,” I said.
“The cylinder was nearly full when you started?”
“Maybe. But can you see what you are doing?” I asked. “In order
to calculate an age you made assumptions about the past. You assumed the rate had
always been 50 ml per hour and that the cylinder was empty when it started. Based
on those assumptions you calculated the time of 6 hours.”
They nodded.
“You were perfectly happy with that answer. Not one of you challenged it.”
They agreed.
“Then, when I told you the correct answer, do you realize what you did? You
quickly changed your assumptions about the past in order to agree with the age I
told you.”
Scientific dating is not a way of measuring but a way of thinking.
Every scientist must first make assumptions about the past before he can calculate
an age. If the result seems okay then he will happily accept it. But if it does
not agree with other information then he will change his assumptions so that his
answer does agree.
It does not matter if the calculated age is too old or too young. There are always
many assumptions a scientist can make to get a consistent answer.
Suddenly the lights went on. My audience saw, in a nutshell, the way dating methods
work.1 Scientific dating
is not a way of measuring but a way of thinking.
How it works in practice
Replica of skull KNM-ER 1470
A layer of volcanic ash in East Africa, called the KBS tuff, became famous through
the human fossils found nearby.1
Using the potassium-argon method, Fitch and Miller were the first to measure the
age of the tuff. Their result of 212–230 million years did not agree with
the age of the fossils (elephant, pig, ape and tools) so they rejected the date.
They said the sample was contaminated with excess argon.2
Using new samples of feldspar and pumice they ‘reliably dated’ the tuff
at 2.61 million years, which agreed nicely.
Later, this date was confirmed by two other dating methods (paleomagnetism and fission
tracks), and was widely accepted.
Then Richard Leakey found a skull (called KNM-ER 1470) below the KBS tuff,
a skull that looked far too modern to be 3 million years old.
So Curtis and others redated the KBS tuff using selected pumice and feldspar samples,
and obtained an age of 1.82 million years. This new date agreed with the appearance
of the new skull.3
Tests by other scientists using paleomagnetism and fission tracks confirmed the
lower date.
So by 1980 there was a new, remarkably concordant date for the KBS tuff, and this
became the one that was widely accepted.
Which illustrates that, contrary to popular belief, the dating methods are not the
primary way that ages are decided. The dating methods do not lead but follow. Their
results are always ‘interpreted’ to agree with other factors, such as
the evolutionary interpretation of geology and fossils.
References and notes
- For more information see Lubenow, M.L., The pigs took
it all, Creation 17(3):36–38, 1995; <creation.com/pigstook>.
- Fitch, F.J. and Miller, J.A., Radioisotopic age determinations of Lake Rudolf artifact
site, Nature 226(5242):226–228, 1970.
- Curtis, G.H., et al., Age of KBS Tuff in Koobi Fora Formation, East Rudolf,
Kenya, Nature 258:395–398, 4 December 1975.
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Reference
- For further information see: Sarfati, J.,
Diamonds: a creationist’s best friend, Creation 28(4):26–27,
2006 and Walker, T., The way it really is: little-known
facts about radiometric dating, Creation 24(4):20–23,
2002. Return to text.
(Also available in Dutch and Portuguese)
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