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The Talk.Origins Archive: Exploring the Creation/Evolution Controversy
A Criticism of the
ICR's Grand Canyon Dating Project
by Chris Stassen
Copyright © 1994-2003
[Last Update: February 18, 2003]
Other Links:
Answers in Genesis Feedback
Jonathan Sarfati of Answers in Genesis attempts to address this
FAQ, but it appears that he read only the fragment submitted to
AiG's feedback system. As a result, Sarfati raises no
substantive issues, and many of his criticisms are refuted by
the full text.
Contents:
1. General information on the Grand Canyon
2. ICR's claims
3. Background on ICR's claims and isochrons
4. Criticism of ICR's claims
5. Summary
6. Response to Criticisms
7. References
General information on the Grand Canyon
T he Grand Canyon looks something like this:
CAPTION: Figure 1. Idealized and simplified diagram of the Grand
Canyon
Grand Canyon figure
There are a number of lava flows on the plateau that the canyon is cut
into (yellow in Figure 1, above). These lava flows are Cenozoic in
age, and some of them spill into the canyon. The walls of the canyon
are mostly cut into horizontal rock layers of Paleozoic age (green in
Figure 1, above). There is an angular unconformity at the bottom of
the Paleozoic layers. An angular unconformity is the result of tilting
and eroding of the lower layers before the upper ones are deposited.
These tilted and eroded layers are Precambrian in age (blue in Figure
1, above).
The geological relationships of the various formations are quite
clear. The lava flows which spill into the canyon must be younger than
the canyon. The canyon must be younger than the rock layers that it
cuts into. The sediments above the angular unconformity must be
younger than the sediments below it.
Grand Canyon figure The ordering of events which resulted in Figure 1
must be:
1. The blue layers are deposited.
2. The blue layers are tilted and eroded.
3. The green layers are deposited.
4. The canyon is cut into the green and blue layers.
5. The lava flows occur.
Even young-earth creationists would agree with this relative
sequencing of events. They would argue for a much shorter absolute
timescale than mainstream geologists would accept, but the relative
sequence is agreed upon by all parties.
ICR's claims
Dr. Steven Austin, chairman of the Geology Department at the Institute
for Creation Research, claimed (1992) that he had derived an Rb/Sr
isochron for the plateau flows, which indicates an age of about 1.3
billion years.
One particular Precambrian layer known as the Cardenas Basalt has been
dated by radiometric methods to about 1.1 billion years in age. The
Cenozoic flows sampled by ICR thus are claimed to yield an age which
is about 200 million years older than the Cardenas Basalt. But the
Cardenas Basalt cannot be younger than the plateau flows, due to the
geological relationships discussed in the first section of this
document.
Austin says that his isochron age is the result of a ""research
project"" (1992, p. i) undertaken by the ICR to ""test the ages
assigned by the best radioactive isotope dating methods"" (1992, p.
i). Dr. Austin suggests that the slope of his isochron line
(indicating great age) is ""unexpected"" (1992, p. iii) and that his
result ""challenges the basic assumptions upon which the isochron
dating method is based"" (1992, p. iv).
In other words, Austin claims that he has produced a seemingly
reliable isochron age which must necessarily be wrong, and therefore
the Rb-Sr isochron dating method, which is considered to be among the
more reliable of radiometric dating methods, must be considered
suspect.
Background on ICR's claims and isochrons
* The damaging paper trail
In order to understand what is going on, it is useful to examine
the paper trail. Prior to ICR starting the Grand Canyon Dating
Project, Austin (1988) produced a similar isochron -- this time
1.5 billion years -- for the same lava flows. He used data taken
out of a mainstream scientist's paper (Leeman 1975) to construct
the plot.
Leeman's paper contains quite a bit more data than Austin used,
with sufficient scatter to suggest that the resulting isochron
probably is either an "inherited" reflection of the mantle source
age or has no significance at all. However, Austin narrowed down
the data set to flows which fell into a particular stratigraphic
range -- ""stages III and IV of Hamblin's later classification","
said Austin (1988) -- and those selected data points fell quite
close to a single line.
In his 1988 paper, Austin noted that this sort of "false isochron"
is well known, and explained in the mainstream literature. He
cited a discussion of it in Faure (1986, pp. 145-147), a popular
textbook/handbook on isotope dating methods.
* Isochron dating methods
For general information on isochron dating methods, see
talk.origins' Isochron Dating FAQ. Further information is
available in Dalrymple (1991, pp. 102ff), a semi-technical
description, and Faure (1986, pp. 117ff), a college-level
textbook.
* The requirements of isochron dating
One of the requirements for an isochron to signify the age of an
object, is that the data points be derived from samples of
materials which were isotopically homogeneous (with respect to
each other) when the object formed, and all separated and ceased
chemical exchange at the object's time of formation. Faure (1986,
p. 121) writes, discussing the derivation of the isochron
technique from basic principles:
If the strontium in such a magma was isotopically homogeneous
throughout the cooling period, we may assume that all the diverse
rocks that formed from the magma had the same initial ^87Sr/^86Sr
ratio. Moreover, we may assume that the time required for
crystallization of the magma was relatively short and that all
rocks produced by this process have very nearly the same age. Under
these conditions, Equation 8.3 is the equation of a family of
straight lines in the slope-intercept form: [...]
That is why isochron results are usually considered reliable if
the data points are derived from the individual minerals of a
single igneous rock sample, or on multiple samples of a single
lava flow. The molten state allows isotopic homogenization, the
solidification ceases that process, and therefore the expected
result is the time since the solidification occurred.
It is possible for the data points to fall on an isochron line if
this requirement is violated. The result will still have the same
meaning: the time since all of the samples were isotopically
homogenized with respect to each other. However, that result does
not have to be the time since each sample formed. Often it will be
the isotopic age of the common source of the samples. That result
could also be the age of the samples themselves, but only in the
case where their common source was isotopically homogeneous --
i.e., zero-age -- when the samples formed from it.
For example, as discussed in the talk.origins Age of the Earth
FAQ, young Earth sediments and meteorite whole-rock measurements
all sit on a Pb/Pb isochron that gives the age of the Solar
System:
CAPTION: Figure 2. Pb/Pb isochron of terrestrial and meteorite samples
(Scanned from Dalrymple (1986) with permission)
Lead-Lead Isochron diagram for Terrestrial and Meteorite Samples.
That isochron tells us the time since the samples were
isotopically homogenized with respect to each other -- the time
since the meteorites and Earth formed from the solar nebula. It
does not imply that the young Earth sediments themselves are 4.5
billion years old.
This is a well-known and expected behavior of isochrons. No
competent geologist would be fooled by this sort of "inherited"
isochron age, because it is quite obvious, as the samples are
collected, whether the date must reflect the individual samples'
time of formation. This is discussed in more detail in the
"Violation of cogenetic requirement" section of the Isochron
Dating FAQ.
Criticism of ICR's claims
* This was not a "test" of Rb-Sr dating.
It is misleading for Austin to claim that he set out to "test"
Rb/Sr isochron dating. The paper trail -- the 1988 Impact article
-- documents that Austin knew he'd get a mantle age from
whole-rock measurements of those lava flows, long before the ICR
obtained a single rock sample of their own.
If isotopic dating methods are as unreliable as Austin would like
us to believe, why did he have to rig his test -- by only
selecting rock samples which were known in advance to fail it? If
a mainstream scientist were to fix a test in this manner, their
reputation would be demolished when that fact was uncovered.
* The wrong meaning is assigned to the dates.
Before the Grand Canyon Dating Project began, in his 1988 Impact
article, Austin admitted in print that the selected lava flows
fell into two different stratigraphic stages. That is, the very
information which he used to select the flows, also clearly
indicates that they did not all occur at the same time. In his
subsequent book (1994, p. 125), Austin indicated that his five
data points came from four different lava flows plus an extracted
"phenocryst" (large mineral which likely formed in the magma
chamber and was not molten in the lava flow). We had known from
the Impact articles that Austin's samples were not all cogenetic;
years later we found out by his own admission that no two of them
are so.
In fact, as discussed above, the selection of non-cogenetic
samples is sometimes used intentionally by isotope geologists. It
is known to be a way to have an isochron dating method "look back"
beyond a recent event to an earlier event -- the age of the common
source of the samples. Thus, it is misleading for Austin to
pretend that his resulting isochron plot should be expected to
represent the age of the flows themselves.
A geologist in my acquaintance suggested that this FAQ should be
very short:
It should merely state that Austin has confirmed what mainstream
geologists have known all along: that the lithospheric mantle
underlying the Grand Canyon must be older than the Cardenas Basalt.
The mantle is the source of much of the sampled flows' material,
and Austin's sampling technique matches the technique one would
use to obtain a minimum for the age of the flows' source.
* It's an insufficient case against isotope dating.
Austin (1992) suggests that he has ""tested"" the dating method.
He claims that the false isochron, that he knew would result, is
""unexpected"." He goes as far as implying that all isotopic ages
can be ignored when he suggests that nobody has ever
""successfully dated a Grand Canyon rock"." The first two claims
are falsehoods, as shown above, and the third cannot be justified
by ICR's Grand Canyon Dating Project.
Young-Earth creationists cannot escape the fact that a large
majority of isotope dating results are well-aligned with
mainstream predictions, and equally well-aligned with geological
relationships which even young-earthers would accept. For example,
intrusive formations consistently date as being younger than the
formations that they cut across. A laundry-list of anomalous dates
will not change that fact. That only shows that the methods
sometimes fail, which is not in dispute.
If Austin wishes to make a case that all isotopic results are
unreliable (which he desires to do, in order to prop up the
timescale that he accepts for religious reasons), he is going to
have to do better than he has done here. All the ICR's Grand
Canyon Dating Project shows is that a sample selection geared to
yield the age of the flows' source... apparently does yield the
age of the flows' source.
Summary
The ICR's Grand Canyon Dating Project does not strike a telling blow
against the reliability of isochron dating. The conditions which
caused the "false isochron" in this case are fairly well-understood,
and easy to avoid by proper sample selection. In fact, the resulting
age in this case may well be meaningful and accurate. The problem is
not the age itself but rather Austin's sleight-of-hand in trying to
pass off the result as necessarily the age of the flows rather than a
minimum age of their source.
The attempt to abuse the meaning of a single contrived date -- which
was produced only by a sample selection geared to dating a different
event, and only for samples whose results were known by Austin in
advance -- says a lot more about the level of competence or honesty in
this creation "science" research program, than it says about the
validity of isochron dating methods.
Even if given credit for discovering this case (which he clearly
doesn't deserve, as his use of Leeman's data proves), Austin has only
managed to "call into question" a particular sampling technique.
However, this sampling technique was known by mainstream geologists to
behave in this manner long before Austin published on the topic, and
this behavior is often intentionally used by geologists. Austin was
aware of this, as his 1988 reference to Faure shows.
Response to Criticisms
I recently received a critique of this FAQ. Unfortunately, it was
submitted anonymously and it didn't address the key issues above.
Since I couldn't get permission to reproduce the claims verbatim, I
will summarize the creationist claims, and respond to them here. I
would recommend that future prospective critics attempt to deal
directly and explicitly with the three items in the "criticisms"
section above.
* Austin took care to ensure that the samples were cogenetic by
selecting lava flows of only Hawaiite basalt, in the same area,
which occurred by mainstream reckoning within the last few million
years.
The "type" of rock is not sufficient to establish the samples being
cogenetic. Since the stratigraphic evidence indicates that the flows
did not all occur at the same time, the case could only be made by
other isotopic analysis such as taking internal isochrons of the
individual flows. That data is lacking from Austin's published works.
Besides, this line of argument does not address the fact that the
result is a known and expected behavior of isochrons. As discussed
above, whole-rock samples of multiple flows yields the time since
their common source was isotopically homogeneous. It could also be the
age of the flows, but it does not have to be. If it is not the flows'
age, that is not a "problem" with isochron dating, and it is not
relevant to the large number of Rb/Sr isochrons which were computed
from mineral separations of a single object.
* Austin's claims cannot be misleading because he presented this
data at a GSA (Geological Society of America) conference and they
would not have allowed a dishonest presentation.
At the GSA meeting, Austin discussed the inheritance of a mantle age.
He didn't pretend that the age of the flows was the expected result,
and he didn't make the false claim that his result was sufficient to
call all isochron dating into question. This is a transparent attempt
to place a GSA "seal of approval" on Austin's unsupportable Impact
claims. (In my opinion, the anonymous critic is engaging in a little
sleight-of-hand of his own.)
* An Impact article is so short that only a single point can be
made, therefore Austin should be excused for a misleading or
inaccurate appearance to his statements which might simply be a
result of brevity. Those wishing the full argument should look to
Austin's book instead.
The length of the medium is not a legitimate excuse for a blatant
falsehood (the claim that Austin set out to "test" Rb/Sr dating) or
for the shenanigans involving the sampling technique versus the
expected meaning of the resulting age. Further, there is no material
in Austin's book which legitimizes the false and misleading claims in
his Impact article.
In addition, the Impact articles (which are free and available online)
receive much wider distribution than Austin's book (which costs $20).
At least a dozen creationists arguing against isotope geology have
referred me to the Impact articles, and not a single one of them had
ever looked at the book. The claims in Impact are all that most
creationists ever see. Therefore they must be accurate on their own.
References
* Austin, Steven A., ed., 1994. Grand Canyon: Monument to
Catastrophe. Plus Communications, ISBN 0-932766-33-1.
Back to "wrong meaning"
* Austin, Steven A., 1992. "Excessively Old "Ages" For Grand Canyon
Lava Flows," in Impact #224 (February).
Back to "ICR claims" or "insufficient case"
* Austin, Steven A., 1988. "Grand Canyon lava flows: A survey of
isotope dating methods," in Impact #178 (April).
Back to "paper trail" or "not a test"
* Dalrymple, G. Brent, 1991. The Age of the Earth. California:
Stanford University Press, ISBN 0-8047-1569-6.
Back to "isochron dating methods"
* Faure, Gunter, 1986. Principles of Isotope Geology, Second
Edition. New York: John Wiley and Sons, ISBN 0-471-86412-9.
Back to "fictitious isochrons", "isochron dating", or "isochron
requirements"
* Leeman, W. P., 1974. "Late Cenozoic Alkali-Rich Basalt from the
Western Grand Canyon Area, Utah and Arizona: Isotopic Composition
of Strontium" in Geological Society of America Bulletin 85
(November), pp. 1691-1696.
Back to "paper trail"
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