The way it really is: little-known facts about radiometric dating
Long-age geologists will not accept a radiometric date unless it matches their pre-existing
expectations.
by Tas Walker
Many people think that radiometric dating has proved the Earth is millions of years
old. That’s understandable, given the image that surrounds the method. Even
the way dates are reported (e.g. 200.4 ± 3.2 million years) gives the impression
that the method is precise and reliable (box below).
However, although we can measure many things about a rock, we cannot directly measure
its age. For example, we can measure its mass, its volume, its colour, the minerals
in it, their size and the way they are arranged. We can crush the rock and measure
its chemical composition and the radioactive elements it contains. But we do not
have an instrument that directly measures age.
Before we can calculate the age of a rock from its measured chemical composition,
we must assume what radioactive elements were in the rock when it formed.1 And then, depending on the assumptions we make, we can
obtain any date we like.
It may be surprising to learn that evolutionary geologists themselves will not accept
a radiometric date unless they think it is correct—i.e. it matches what they
already believe on other grounds. It is one thing to calculate a date. It is another
thing to understand what it means.
So, how do geologists know how to interpret their radiometric dates and what the
‘correct’ date should be?
Field relationships
A geologist works out the relative age of a rock by carefully studying where the
rock is found in the field. The field relationships, as they are called, are of
primary importance and all radiometric dates are evaluated against them.
For example, a geologist may examine a cutting where the rocks appear as shown in
Figure 1. Here he can see that some curved sedimentary rocks have been cut vertically
by a sheet of volcanic rock called a dyke. It is clear that the sedimentary rock
was deposited and folded before the dyke was squeezed into place.
Figure 1
Figure 2 Cross-section
By looking at other outcrops in the area, our geologist is able to draw a geological
map which records how the rocks are related to each other in the field. From the
mapped field relationships, it is a simple matter to work out a geological cross-section
and the relative timing of the geologic events. His geological cross-section may
look something like Figure 2.
Clearly, Sedimentary Rocks A were deposited and deformed before the Volcanic Dyke
intruded them. These were then eroded and Sedimentary Rocks B were deposited.
The geologist may have found some fossils in Sedimentary Rocks A and discovered
that they are similar to fossils found in some other rocks in the region. He assumes
therefore that Sedimentary Rocks A are the same age as the other rocks in the region,
which have already been dated by other geologists. In the same way, by identifying
fossils, he may have related Sedimentary Rocks B with some other rocks.
Creationists would generally agree with the above methods and use them in their
geological work.
From his research, our evolutionary geologist may have discovered that other geologists
believe that Sedimentary Rocks A are 200 million years old and Sedimentary Rocks
B are 30 million years old. Thus, he already ‘knows’ that the igneous
dyke must be younger than 200 million years and older than 30 million years. (Creationists
do not agree with these ages of millions of years because of the assumptions they
are based on.2)
Because of his interest in the volcanic dyke, he collects a sample, being careful
to select rock that looks fresh and unaltered. On his return, he sends his sample
to the laboratory for dating, and after a few weeks receives the lab report.
Let us imagine that the date reported by the lab was 150.7 ± 2.8 million
years. Our geologist would be very happy with this result. He would say that the
date represents the time when the volcanic lava solidified. Such an interpretation
fits nicely into the range of what he already believes the age to be. In fact, he
would have been equally happy with any date a bit less than 200 million years or
a bit more than 30 million years. They would all have fitted nicely into the field
relationships that he had observed and his interpretation of them. The field relationships
are generally broad, and a wide range of ‘dates’ can be interpreted
as the time when the lava solidified.
What would our geologist have thought if the date from the lab had been greater
than 200 million years, say 350.5 ± 4.3 million years? Would he have concluded
that the fossil date for the sediments was wrong? Not likely. Would he have thought
that the radiometric dating method was flawed? No. Instead of questioning the method,
he would say that the radiometric date was not recording the time that the rock
solidified. He may suggest that the rock contained crystals (called xenocrysts)
that formed long before the rock solidified and that these crystals gave an older
date.3 He may suggest that some other
very old material had contaminated the lava as it passed through the earth. Or he
may suggest that the result was due to a characteristic of the lava—that the
dyke had inherited an old ‘age’.
The error is not the real error
The convention for reporting dates (e.g. 200.4 ± 3.2 million years) implies
that the calculated date of 200.4 million years is accurate to plus or minus 3.2
million years. In other words, the age should lie between 197.2 million years and
203.6 million years. However, this error is not the real error on the date. It relates
only to the accuracy of the measuring equipment in the laboratory. Even different
samples of rock collected from the same outcrop would give a larger scatter of results.
And, of course, the reported error ignores the huge uncertainties in the assumptions
behind the ‘age’ calculation. These include the assumption that decay
rates have never changed. In fact, decay rates have been increased in the laboratory
by factors of billions of times.1
Creationist physicists point to several lines of evidence that decay rates have
been faster in the past, and propose a pulse of accelerated decay during Creation
Week, and possibly a smaller pulse during the Flood year.2
References
-
Woodmorappe, J., Billion-fold acceleration of radioactivity
demonstrated in laboratory, TJ15(2):4–6,
2001. Return to text.
- Vardiman, L.,
Snelling, A.A. and Chaffin, E.F., Radioisotopes
and the age of the Earth, Institute for Creation Research, El Cajon, California
and Creation Research Society, St. Joseph, Missouri, USA, 2000. Return
to text.
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What would our geologist think if the date from the lab were less than 30 million
years, say 10.1 ± 1.8 million years? No problem. Would he query the dating
method, the chronometer? No. He would again say that the calculated age did not
represent the time when the rock solidified. He may suggest that some of the chemicals
in the rock had been disturbed by groundwater or weathering.4 Or he may decide that the rock had been affected by a
localized heating event—one strong enough to disturb the chemicals, but not
strong enough to be visible in the field.
No matter what the radiometric date turned out to be, our geologist would always
be able to ‘interpret’ it. He would simply change his assumptions about
the history of the rock to explain the result in a plausible way. G. Wasserburg,
who received the 1986 Crafoord Prize in Geosciences, said, ‘There are no bad
chronometers, only bad interpretations of them!’5
In fact, there is a whole range of standard explanations that geologists use to
‘interpret’ radiometric dating results.
Why use it?
Someone may ask, ‘Why do geologists still use radiometric dating? Wouldn’t
they have abandoned the method long ago if it was so unreliable?’ Just because
the calculated results are not the true ages does not mean that the method is completely
useless. The dates calculated are based on the isotopic composition of the rock.
And the composition is a characteristic of the molten lava from which the rock solidified.
Therefore, rocks in the same area which give similar ‘dates’ are likely
to have formed from the same lava at about the same time during the Flood. So, although
the assumptions behind the calculation are wrong and the dates are incorrect, there
may be a pattern in the results that can help geologists understand the relationships
between igneous rocks in a region.
Contrary to the impression that we are given, radiometric dating does not prove
that the Earth is millions of years old. The vast age has simply been assumed.2 The calculated radiometric ‘ages’ depend
on the assumptions that are made. The results are only accepted if they agree with
what is already believed. The only foolproof method for determining the age of something
is based on eyewitness reports and a written record. We have both in the Bible.
And that is why creationists use the historical evidence in the Bible to constrain
their interpretations of the geological evidence.
What if the rock ages are not ‘known’ in advance—does radio-dating
give coherent results?
Recently, I conducted a geological field trip in the Townsville area, North Queensland.
A geological guidebook,1 prepared
by two geologists, was available from a government department.
The guidebook’s appendix explains ‘geological time and the ages of rocks.’
It describes how geologists use field relationships to determine the relative ages
of rocks. It also says that the ‘actual’ ages are measured by radiometric
dating—an expensive technique performed in modern laboratories. The guide
describes a number of radiometric methods and states that for ‘suitable specimens
the errors involved in radiometric dating usually amount to several percent of the
age result. Thus … a result of two hundred million years is expected to be
quite close (within, say, 4 million) to the true age.’
This gives the impression that radiometric dating is very precise and very reliable—the
impression generally held by the public. However, the appendix concludes with this
qualification: ‘Also, the relative ages [of the radiometric dating results]
must always be consistent with the geological evidence. … if a contradiction
occurs, then the cause of the error needs to be established or the radiometric results
are unacceptable’.
This is exactly what our main article explains. Radiometric dates are only accepted
if they agree with what geologists already believe the age should be.
Townsville geology is dominated by a number of prominent granitic mountains and
hills. However, these are isolated from each other, and the area lacks significant
sedimentary strata. We therefore cannot determine the field relationships and thus
cannot be sure which hills are older and which are younger. In fact, the constraints
on the ages are such that there is a very large range possible.
We would expect that radiometric dating, being allegedly so ‘accurate,’
would rescue the situation and provide exact ages for each of these hills. Apparently,
this is not so.
Concerning the basement volcanic rocks in the area, the guidebook says, ‘Their
exact age remains uncertain.’ About Frederick Peak, a rhyolite ring dyke in
the area, it says, ‘Their age of emplacement is not certain.’ And for
Castle Hill, a prominent feature in the city of Townsville, the guidebook says,
‘The age of the granite is unconfirmed.’
No doubt, radiometric dating has been carried out and precise ‘dates’
have been obtained. It seems they have not been accepted because they were not meaningful.
Reference
- Trezise, D.L. and Stephenson, P.J., Rocks and landscapes
of the Townsville district, Department of Resource Industries, Queensland,
1990. Return to text.
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References and notes
- In addition to other unprovable assumptions, e.g. that the decay
rate has never changed. Return to text. Return to text.
- Evolutionary geologists believe that the rocks are millions of
years old because they assume they were formed very slowly. They have worked
out their geologic timescale based on this assumption. This timescale deliberately
ignores the catastrophic effects of the Biblical Flood, which deposited the rocks
very quickly. Return to text.
- This argument was used against creationist work that exposed problems
with radiometric dating. Laboratory tests on rock formed from the 1980 eruption
of Mt St Helens gave ‘ages’ of millions of years. Critics claimed
that ‘old’ crystals contained in the rock contaminated the result.
However, careful measurements by Dr Steve Austin
showed this criticism to be wrong. See Swenson, K.,
Radio-dating in rubble, Creation23(3):23–25,
2001. Return to text.
- This argument was used against creationist work done on a piece
of wood found in sandstone near Sydney, Australia, that was supposed to be 230 million
years old. Critics claimed that the carbon-14 results were ‘too young’
because the wood had been contaminated by weathering. However, careful measurements
of the carbon-13 isotope refuted this criticism. See
Snelling, A.A., Dating dilemma: fossil wood in ‘ancient’
sandstone, Creation21(3):39–41, 1999.
Return to text.
- Wasserburg, G.J., Isotopic abundances: inferences on solar system
and planetary evolution, Earth and Planetary Sciences Letters86:129–173,
150, 1987. Return to text.
(Article available in Spanish)
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