mirrored file at http://SaturnianCosmology.Org/
For complete access to all the files of this collection
see http://SaturnianCosmology.org/search.php
==========================================================
The Talk.Origins Archive: Exploring the Creation/Evolution Controversy
Radiometric Dating and the Geological Time Scale
Circular Reasoning or Reliable Tools?
by Andrew MacRae
Copyright © 1997-2004
[Text last updated: October 2, 1998]
[Links updated: September 12, 2004]
*Other Links:*
A Radiometric Dating Resource List
Tim Thompson has collected a large set of links to web pages that
discuss radiometric dating techniques and the age of the earth
controversy.
*Overview*
* Introduction <#intro>
* Background <#back>
o Stratigraphic principles & relative time <#princ>
o Biostratigraphy <#biostrat>
o Radiometric dating: Calibrating the time scale <#radiomet>
* A theoretical example <#theoret>
* Circularity? <#Circularity>
* Specific examples: When radiometric dating "just works" (or not)
<#specific>
* Conclusions <#conclusions>
* References <#refs>
* Other sources <#other>
* Acknowledgements <#acknowl>
*Introduction*
This document discusses the way radiometric dating and stratigraphic
principles are used to establish the conventional geological time scale.
It is not about the theory behind radiometric dating methods, it is
about their /application/, and it therefore assumes the reader has some
familiarity with the technique already (refer to "Other Sources"
<#other> for more information). As an example of how they are used,
radiometric dates from geologically simple, fossiliferous Cretaceous
rocks in western North America are compared to the geological time
scale. To get to that point, there is also a historical discussion and
description of non-radiometric dating methods.
The example used here contrasts sharply with the way conventional
scientific dating methods are characterized by some critics (for
example, refer to discussion in "Common Creationist Criticisms of
Mainstream Dating Methods " in the
Age of the Earth FAQ and Isochron Dating
FAQ ). A common form of criticism is to
cite geologically complicated situations where the application of
radiometric dating is very challenging. These are often characterised
as the norm, rather than the exception. I thought it would be useful to
present an example where the geology is simple, and unsurprisingly, the
method does work well, to show the quality of data that would have to be
invalidated before a major revision of the geologic time scale could be
accepted by conventional scientists. Geochronologists do not claim that
radiometric dating is foolproof (no scientific method is), but it does
work reliably for most samples. It is these highly consistent and
reliable samples, rather than the tricky ones, that have to be falsified
for "young Earth" theories to have any scientific plausibility, not to
mention the need to falsify huge amounts of evidence from other techniques.
This document is partly based on a prior posting composed in reply to
Ted Holden . My thanks to both him and
other critics for motivating me.
*Background*
*Stratigraphic Principles and Relative Time*
Much of the Earth's geology consists of successional layers of different
rock types, piled one on top of another. The most common rocks observed
in this form are sedimentary rocks (derived from what were formerly
sediments), and extrusive igneous rocks (e.g., lavas, volcanic ash, and
other formerly molten rocks extruded onto the Earth's surface). The
layers of rock are known as "strata", and the study of their succession
is known as "stratigraphy". Fundamental to stratigraphy are a set of
simple principles, based on elementary geometry, empirical observation
of the way these rocks are deposited today, and gravity. Most of these
principles were formally proposed by Nicolaus Steno (Niels Steensen,
Danish), in 1669, although some have an even older heritage that extends
as far back as the authors of the Bible. A few principles were
recognized and specified later. An early summary of them is found in
Charles Lyell's Principles of Geology, published in 1830-32, and does
not differ greatly from a modern formulation:
1. The principle of superposition - in a vertical sequence of
sedimentary or volcanic rocks, a higher rock unit is younger than
a lower one. "Down" is older, "up" is younger.
2. The principle of original horizontality - rock layers were
originally deposited close to horizontal.
3. The principle of original lateral extension - A rock unit
continues laterally unless there is a structure or change to
prevent its extension.
4. The principle of cross-cutting relationships - a structure that
cuts another is younger than the structure that is cut.
5. The principle of inclusion - a structure that is included in
another is older than the including structure.
6. The principle of "uniformitarianism" - processes operating in the
past were constrained by the same "laws of physics" as operate today.
Note that these are /principles/. In no way are they meant to imply
there are no exceptions. For example, the principle of superposition is
based, fundamentally, on gravity. In order for a layer of material to be
deposited, something has to be beneath it to support it. It can't float
in mid-air, particularly if the material involved is sand, mud, or
molten rock. The principle of superposition therefore has a clear
implication for the /relative/ age of a vertical succession of strata.
There are situations where it potentially fails -- for example, in cave
deposits. In this situation, the cave contents are younger than both the
bedrock below the cave and the suspended roof above. However, note that
because of the "principle of cross-cutting relationships" <#princ4>,
careful examination of the contact between the cave infill and the
surrounding rock will reveal the true relative age relationships, as
will the "principle of inclusion" <#princ5> if fragments of the
surrounding rock are found within the infill. Cave deposits also often
have distinctive structures of their own (e.g., spelothems like
stalactites and stalagmites), so it is not likely that someone could
mistake them for a successional sequence of rock units.
These geological principles are not /assumptions/ either. Each of them
is a testable hypothesis about the relationships between rock units and
their characteristics. They are applied by geologists in the same sense
that a "null hypothesis" is in statistics -- not necessarily correct,
just testable. In the last 200 or more years of their application, they
are /often/ valid, but geologists do not assume they are. They are the
"initial working hypotheses" to be tested further by data.
Using these principles, it is possible to construct an interpretation of
the sequence of events for any geological situation, even on other
planets (e.g., a crater impact can cut into an older, pre-existing
surface, or craters may overlap, revealing their relative ages). The
simplest situation for a geologist is a "layer cake" succession of
sedimentary or extrusive igneous rock units arranged in nearly
horizontal layers. In such a situation, the "principle of superposition"
<#princ1> is easily applied, and the strata towards the bottom are
older, those towards the top are younger.
*Figure 1.* Sedimentary beds in outcrop, a graphical plot of a
stratigraphic section, and a "way up" indicator example: wave ripples.
Wave ripple in strata
This orientation is not an assumption, because in virtually all
situations, it is also possible to determine the original "way up" in
the stratigraphic succession from "way up indicators". For example, wave
ripples have their pointed crests on the "up" side, and more rounded
troughs on the "down" side. Many other indicators are commonly present,
including ones that can even tell you the angle of the depositional
surface at the time ("geopetal structures"), "assuming" that gravity was
"down" at the time, which isn't much of an assumption :-).
In more complicated situations, like in a mountain belt, there are often
faults, folds, and other structural complications that have deformed and
"chopped up" the original stratigraphy. Despite this, the "principle of
cross cutting relationships" <#princ4> can be used to determine the
sequence of deposition, folds, and faults based on their intersections
-- if folds and faults deform or cut across the sedimentary layers and
surfaces, then they obviously came after deposition of the sediments.
You can't deform a structure (e.g., bedding) that is not there yet!
Even in complex situations of multiple deposition, deformation, erosion,
deposition, and repeated events, it is possible to reconstruct the
sequence of events. Even if the folding is so intense that some of the
strata is now upside down, this fact can be recognized with "way up"
indicators.
No matter what the geologic situation, these basic principles reliably
yield a reconstructed history of the sequence of events, both
depositional, erosional, deformational, and others, for the geology of a
region. This reconstruction is tested and refined as new field
information is collected, and can be (and often is) done completely
independently of anything to do with other methods (e.g., fossils and
radiometric dating). The reconstructed history of events forms a
"relative time scale", because it is possible to tell that event A
occurred prior to event B, which occurred prior to event C, regardless
of the actual duration of time between them. Sometimes this study is
referred to as "event stratigraphy", a term that applies regardless of
the type of event that occurs (biologic, sedimentologic, environmental,
volcanic, magnetic, diagenetic, tectonic, etc.).
These simple techniques have widely and successfully applied since at
least the early 1700s, and by the early 1800s, geologists had recognized
that many obvious similarities existed in terms of the
independently-reconstructed sequence of geologic events observed in
different parts of the world. One of the earliest (1759) relative time
scales based upon this observation was the subdivision of the Earth's
stratigraphy (and therefore its history), into the "Primary",
"Secondary", "Tertiary", and later (1854) "Quaternary" strata based
mainly on characteristic rock types in Europe. The latter two
subdivisions, in an emended form, are still used today by geologists.
The earliest, "Primary" is somewhat similar to the modern Paleozoic and
Precambrian, and the "Secondary" is similar to the modern Mesozoic.
Another observation was the similarity of the fossils observed within
the succession of strata, which leads to the next topic.
*Biostratigraphy*
As geologists continued to reconstruct the Earth's geologic history in
the 1700s and early 1800s, they quickly recognized that the distribution
of fossils within this history was not random -- fossils occurred in a
consistent order. This was true at a regional, and even a global scale.
Furthermore, fossil organisms were more unique than rock types, and much
more varied, offering the potential for a much more precise subdivision
of the stratigraphy and events within it.
The recognition of the utility of fossils for more precise "relative
dating" is often attributed to William Smith, a canal engineer who
observed the fossil succession while digging through the rocks of
southern England. But scientists like Albert Oppel hit upon the same
principles at about about the same time or earlier. In Smith's case, by
using empirical observations of the fossil succession, he was able to
propose a fine subdivision of the rocks and map out the formations of
southern England in one of the earliest geological maps (1815). Other
workers in the rest of Europe, and eventually the rest of the world,
were able to compare directly to the same fossil succession in their
areas, even when the rock types themselves varied at finer scale. For
example, everywhere in the world, trilobites were found lower in the
stratigraphy than marine reptiles. Dinosaurs were found after the first
occurrence of land plants, insects, and amphibians. Spore-bearing land
plants like ferns were always found before the occurrence of flowering
plants. And so on.
The observation that fossils occur in a consistent succession is known
as the "principle of faunal (and floral) succession". The study of the
succession of fossils and its application to relative dating is known as
"biostratigraphy". Each increment of time in the stratigraphy could be
characterized by a particular assemblage of fossil organisms, formally
termed a biostratigraphic "zone" by the German paleontologists Friedrich
Quenstedt and Albert Oppel. These zones could then be traced over large
regions, and eventually globally. Groups of zones were used to establish
larger intervals of stratigraphy, known as geologic "stages" and
geologic "systems". The time corresponding to most of these intervals of
rock became known as geologic "ages" and "periods", respectively. By
the end of the 1830s, most of the presently-used geologic periods had
been established based on their fossil content and their observed
relative position in the stratigraphy (e.g., Cambrian (1835), Ordovician
(1879), Silurian (1835), Devonian (1839), Carboniferous (1822), Permian
(1841), Triassic (1834), Jurassic (1829), Cretaceous (1823), Tertiary
(1759), and Pleistocene (1839)). These terms were preceded by decades by
other terms for various geologic subdivisions, and although there was
subsequent debate over their exact boundaries (e.g., between the
Cambrian and Silurian Periods, which was resolved by proposal of the
Ordovician Period between them), the historical descriptions and fossil
succession would be easily recognizable today.
By the 1830s, fossil succession had been studied to an increasing
degree, such that the broad history of life on Earth was well
understood, regardless of the debate over the names applied to portions
of it, and where exactly to make the divisions. All paleontologists
recognized unmistakable trends in morphology through time in the
succession of fossil organisms. This observation led to attempts to
explain the fossil succession by various mechanisms. Perhaps the best
known example is Darwin's theory of evolution by natural selection. Note
that chronologically, fossil succession was well and independently
established long before Darwin's evolutionary theory was proposed in
1859. Fossil succession and the geologic time scale are constrained by
the observed order of the stratigraphy -- basically geometry -- /not/ by
evolutionary theory.
*Radiometric Dating: Calibrating the Relative Time Scale*
For almost the next 100 years, geologists operated using relative dating
methods, both using the basic principles of geology and fossil
succession (biostratigraphy). Various attempts were made as far back as
the 1700s to scientifically estimate the age of the Earth, and, later,
to use this to calibrate the relative time scale to numeric values
(refer to "Changing views of the history of the Earth"
by Richard Harter and Chris Stassen). Most of the
early attempts were based on rates of deposition, erosion, and other
geological processes, which yielded uncertain time estimates, but which
clearly indicated Earth history was at least 100 million or more years
old. A challenge to this interpretation came in the form of Lord
Kelvin's (William Thomson's) calculations of the heat flow from the
Earth, and the implication this had for the age -- rather than hundreds
of millions of years, the Earth could be as young as tens of million of
years old. This evaluation was subsequently invalidated by the discovery
of radioactivity in the last years of the 19th century, which was an
unaccounted for source of heat in Kelvin's original calculations. With
it factored in, the Earth could be vastly older. Estimates of the age of
the Earth again returned to the prior methods.
The discovery of radioactivity also had another side effect, although it
was several more decades before its additional significance to geology
became apparent and the techniques became refined. Because of the
chemistry of rocks, it was possible to calculate how much radioactive
decay had occurred since an appropriate mineral had formed, and how much
time had therefore expired, by looking at the ratio between the original
radioactive isotope and its product, if the decay rate was known. Many
geological complications and measurement difficulties existed, but
initial attempts at the method clearly demonstrated that the Earth was
very old. In fact, the numbers that became available were significantly
older than even some geologists were expecting -- rather than hundreds
of millions of years, which was the minimum age expected, the Earth's
history was clearly at least billions of years long.
Radiometric dating provides numerical values for the age of an
appropriate rock, usually expressed in millions of years. Therefore, by
dating a series of rocks in a vertical succession of strata previously
recognized with basic geologic principles (see Stratigraphic principles
and relative time <#princ>), it can provide a numerical calibration for
what would otherwise be only an ordering of events -- i.e. relative
dating obtained from biostratigraphy (fossils), superpositional
relationships, or other techniques. The integration of relative dating
and radiometric dating has resulted in a series of increasingly precise
"absolute" (i.e. numeric) geologic time scales, starting from about the
1910s to 1930s (simple radioisotope estimates) and becoming more precise
as the modern radiometric dating methods were employed (starting in
about the 1950s).^1 <#note1>
*A Theoretical Example*
To show how relative dating and numeric/absolute dating methods are
integrated, it is useful to examine a theoretical example first. Given
the background above, the information used for a geologic time scale can
be related like this:
*Figure 2.* How relative dating of events and radiometric (numeric)
dates are combined to produce a calibrated geological time scale. In
this example, the data demonstrates that "fossil B time" was somewhere
between 151 and 140 million years ago, and that "fossil A time" is older
than 151 million years ago. Note that because of the position of the
dated beds, there is room for improvement in the time constraints on
these fossil-bearing intervals (e.g., you could look for a datable
volcanic ash at 40-45m to better constrain the time of first appearance
of fossil B). 1) Raw data 2) Recognition of a unique succession of
events 3) radiometric dating 4) calibrated geologic time
A continuous vertical stratigraphic section will provide the order of
occurrence of events (column 1 of Figure 2 <#fig2>). These are
summarized in terms of a "relative time scale" (column 2 of Figure 2
<#fig2>). Geologists can refer to intervals of time as being "pre-first
appearance of species A" or "during the existence of species A", or
"after volcanic eruption #1" (at least six subdivisions are possible in
the example in Figure 2 <#fig2>). For this type of "relative dating" to
work it must be known that the succession of events is unique (or at
least that duplicate events are recognized -- e.g., the "first ash bed"
and "second ash bed") and roughly synchronous over the area of interest.
Unique events can be biological (e.g., the first appearance of a
particular species of organisms) or non-biological (e.g., the deposition
of a volcanic ash with a unique chemistry and mineralogy over a wide
area), and they will have varying degrees of lateral extent. Ideally,
geologists are looking for events that are unmistakably unique, in a
consistent order, and of global extent in order to construct a
geological time scale with /global/ significance. Some of these events
do exist. For example, the boundary between the Cretaceous and Tertiary
periods is recognized on the basis of the extinction of a large number
of organisms globally (including ammonites, dinosaurs, and others), the
first appearance of new types of organisms, the presence of geochemical
anomalies (notably iridium), and unusual types of minerals related to
meteorite impact processes (impact spherules and shocked quartz). These
types of distinctive events provide confirmation that the Earth's
stratigraphy is genuinely successional on a global scale. Even without
that knowledge, it is still possible to construct local geologic time
scales.
Although the idea that unique physical and biotic events are synchronous
might sound like an "assumption", it is not. It can, and has been,
tested in innumerable ways since the 19th century, in some cases by
physically tracing distinct units laterally for hundreds or thousands of
kilometres and looking very carefully to see if the order of events
changes. Geologists do sometimes find events that are "diachronous"
(i.e. not the same age everywhere), but despite this deserved caution,
after extensive testing, it is obvious that many events really are
synchronous to the limits of resolution offered by the geological record.
Because any newly-studied locality will have independent fossil,
superpositional, or radiometric data that have not yet been incorporated
into the global geological time scale, all data types serve as both an
independent test of each other (on a local scale), and of the global
geological time scale itself. The test is more than just a "right" or
"wrong" assessment, because there is a certain level of uncertainty in
all age determinations. For example, an inconsistency may indicate that
a particular geological boundary occurred 76 million years ago, rather
than 75 million years ago, which might be cause for revising the age
estimate, but does not make the original estimate flagrantly "wrong". It
depends upon the exact situation, and how much data are present to test
hypotheses (e.g., could the range of a fossil be a bit different from
what was thought previously, or could the boundary between two time
periods be a slightly different numerical age?). Whatever the situation,
the current global geological time scale makes /predictions/ about
relationships between relative and absolute age-dating at a local scale,
and the input of new data means the global geologic time scale is
continually refined and is known with increasing precision. This trend
can be seen by looking at the history of proposed geologic time scales
(described in the first chapter of [Harland et al, 1982, p.4-5] <#ref6>,
and see below).
*Circularity?*
The unfortunate part of the natural process of refinement of time scales
is the /appearance/ of circularity if people do not look at the source
of the data carefully enough. Most commonly, this is characterised by
oversimplified statements like:
"The fossils date the rock, and the rock dates the fossils."
Even some geologists have stated this misconception (in slightly
different words) in seemingly authoritative works (e.g., Rastall, 1956
<#Rastall56>), so it is persistent, even if it is categorically wrong
(refer to Harper (1980) <#Harper80>, p.246-247 for a thorough debunking,
although it is a rather technical explanation).
When a geologist collects a rock sample for radiometric age dating, or
collects a fossil, there are independent constraints on the relative and
numerical age of the resulting data. Stratigraphic position is an
obvious one, but there are many others. There is no way for a geologist
to choose what numerical value a radiometric date will yield, or what
position a fossil will be found at in a stratigraphic section. /Every/
piece of data collected like this is an independent check of what has
been previously studied. The data are determined by the /rocks/, not by
preconceived notions about what will be found. Every time a rock is
picked up it is a test of the predictions made by the current
understanding of the geological time scale. The time scale /is/ refined
to reflect the relatively few and progressively smaller inconsistencies
that are found. This is /not/ circularity, it is the normal scientific
process of refining one's understanding with new data. It happens in all
sciences.
If an inconsistent data point is found, geologists ask the question: "Is
this date wrong, or is it saying the current geological time scale is
wrong?" In general, the former is more likely, because there is such a
vast amount of data behind the current understanding of the time scale,
and because every rock is not expected to preserve an isotopic system
for millions of years. However, this statistical likelihood is /not/
assumed, it is /tested/, usually by using other methods (e.g., other
radiometric dating methods or other types of fossils), by re-examining
the inconsistent data in more detail, recollecting better quality
samples, or running them in the lab again. Geologists search for an
explanation of the inconsistency, and will not arbitrarily decide that,
"because it conflicts, the data must be wrong."
If it is a small but significant inconsistency, it could indicate that
the geological time scale requires a small revision. This happens
regularly. The continued revision of the time scale as a result of new
data demonstrates that geologists /are/ willing to question it and
change it. The geological time scale is far from dogma.
If the new data have a large inconsistency (by "large" I mean orders of
magnitude), it is far more likely to be a problem with the new data, but
geologists are not satisfied until a specific geological explanation is
found and tested. An inconsistency often means something geologically
interesting is happening, and there is always a tiny possibility that it
could be the tip of a revolution in understanding about geological
history. Admittedly, this latter possibility is /VERY/ unlikely. There
is almost zero chance that the broad understanding of geological history
(e.g., that the Earth is billions of years old) will change. The amount
of data supporting that interpretation is immense, is derived from many
fields and methods (not only radiometric dating), and a discovery would
have to be found that invalidated practically all previous data in order
for the interpretation to change greatly. So far, I know of no valid
theory that explains how this could occur, let alone evidence in support
of such a theory, although there have been highly fallacious attempts
(e.g., the classic "moon dust" , "decay of the
Earth's magnetic field" and "salt
in the oceans" claims).
*Specific Examples: When Radiometric Dating "Just Works" (or not)*
*A poor example*
There are many situations where radiometric dating is not possible, or
where a dating attempt will be fraught with difficulty. This is the
inevitable nature of rocks that have experienced millions of years of
history: not all of them will preserve their age of origin intact, not
every rock will have appropriate chemistry and mineralogy, no sample is
perfect, and there is no dating method that can effectively date rocks
of /any/ age or rock type. For example, methods with very slow decay
rates will be poor for extremely young rocks, and rocks that are low in
potassium (K) will be inappropriate for K/Ar dating. The real question
is what happens when conditions are ideal, versus when they are
marginal, because ideal samples should give the most reliable dates. If
there are good reasons to expect problems with a sample, it is hardly
surprising if there are!
For example, in the "Dating Game" appendix of his "Bones of Contention"
book (1992) <#Lubenow92>, Marvin Lubenow provided an example of what
happens when a geologically complicated sample is dated -- it can be
very difficult to analyze. He discussed the "KBS tuff" near Lake Turkana
in Africa, which is a redeposited volcanic ash. It contains a /mixture/
of minerals from a volcanic eruption and detrital mineral grains eroded
from other, older rocks. It is also a comparatively "young" sample,
approaching the practical limit of the radiometric methods employed
(conventional K/Ar dating), particularly at the time of the initial
dating attempts in 1969. If the age of this unit were not so crucial to
important associated hominid fossils, it probably would not have been
dated at all because of the potential problems. After some initial and
prolonged troubles over many years, the bed was eventually dated
successfully by careful sample preparation that eliminated the detrital
minerals. Lubenow's work is fairly unique in characterising the normal
scientific process of refining a difficult date as an arbitrary and
inappropriate "game", and documenting the history of the process in some
detail, as if such problems were typical. Another example is "John
Woodmorappe's" paper on radiometric dating (1979) <#Wood78>, which
adopts a "compilation" approach, and gives only superficial treatment to
the individual dates. Among other problems documented in an FAQ by
Steven Schimmrich , many of
Woodmorappe's examples neglect the geological complexities that are
expected to cause problems for some radiometrically-dated samples.
*A good example*
By contrast, the example presented here is a geologically simple
situation -- it consists of several primary (i.e. /not/ redeposited)
volcanic ash deposits with a diverse dateable mineral assemblage
(multiple minerals and methods are possible), found in fossil-bearing
sedimentary rocks in western North America. It demonstrates how
consistent radiometric data can be when the rocks are more suitable for
dating. For most geological samples like this, radiometric dating "just
works". Consider this stratigraphic section from the Bearpaw Formation
of Saskatchewan, Canada (Baadsgaard et al., 1993) <#ref1>:
*Figure 3.* Lithostratigraphy (i.e. the sedimentary rocks),
biostratigraphy (fossils) and radiometric dates from the Bearpaw
Formation, southern Saskatchewan, Canada. Modified from Baadsgaard et
al., 1993 <#ref1>. The section is measured in metres, starting with 0m
at the bottom (oldest). Stratigraphic section, biostratography, and
radiometric dates
This section is important because it places a limit on the youngest age
for a specific ammonite shell -- /Baculites reesidei/ -- which is used
as a zonal fossil in western North America. It consistently occurs below
the first occurrence of /Bacultes jenseni/ and above the occurrence of
/Baculites cuneatus/ within the upper part of the Campanian, the second
to last "stage" of the Cretaceous Period in the global geological time
scale. The biostratigraphic situation can be summarized as a
vertically-stacked sequence of "zones" defined by the first appearance
of each ammonite species:
*Figure 4.* /Baculites/ ammonite zones. Three baculite zones
About 40 of these ammonite zones are used to subdivide the upper part of
the Cretaceous Period in this area. Dinosaurs and many other types of
fossils are also found in this interval, and in broad context it occurs
shortly before the extinction of the dinosaurs, and the extinction of
all ammonites. The Bearpaw Formation is a marine unit that occurs over
much of Alberta and Saskatchewan, and it continues into Montana and
North Dakota in the United States, although it adopts a different name
in the U.S. (the Pierre Shale), mainly for historical and political
reasons, rather than any great geological difference.
The uppermost ash bed, dated by three independent methods (K/Ar, U/Pb,
and Rb/Sr), and from as many as three different minerals (felspar,
biotite, and zircon), yields a date of about 72.5 ± 0.4 million years
ago (Ma) (weighted mean of several analyses. The numbers above are just
summary values). The results for the lower ash bed, although not as
complete as for the upper ash bed (only the Rb/Sr isochron method -- the
U/Pb isochron was discordant, indicating the minerals did not preserve
the date), give the expected result from superpositional relationships
-- it is older by about a million years (73.65 ± 0.59 Ma), taking the
mean values.
Other examples yield similar results - i.e. compatible with the
expectations from the stratigraphy. For example, Baadsgaard and Lerbekmo
(1988) <#ref2> dated the age of the Cretaceous-Tertiary (K/T) boundary
using three methods (K/Ar, Rb/Sr, and U/Pb, again using multiple
minerals) at three localities in the U.S. and Canada. Theoretically, the
K/T boundary should be younger than the /Baculites reesidei/ zone
mentioned above, because the K/T boundary occurs stratigraphically above
this level in the same area and globally. The result? 64.3±1.2 million
years ago is the weighted average from the three localities, and almost
all the results are within 1 million years of each other. The results
are therefore highly consistent given the analytical uncertainties in
any measurement.
Eberth and Braman (1990) <#ref3> described the vertebrate paleontology
and sedimentology of the Judith River Formation, a dinosaur-bearing unit
that occurs stratigraphically below the /Baculites reesidei/ zone (the
Judith River Formation is below the Bearpaw Formation). It should
therefore be older than the results from Baadsgaard et al. (1993)
<#ref1>. An ash bed near the top of the Judith River Fm. yields a date
of 76.11±0.22 million years ago, while one almost 100m lower yields a
date of 78.2±0.2 million years ago (Eberth and Braman, 1990, figure 5)
<#ref3>. Again, this is compatible with the age determined for the
/Baculites reesidei/ zone and its relative stratigraphic position, and
even with the relative position of the two samples within the same
formation.
How do these dates compare to the (then current) geological time scale?
Harland et al. <#ref6> proposed a time scale in 1982 on the basis of
data then available, and prior to the specific studies cited above. Here
are the numbers they applied to the geological boundaries in this
interval, compared to the numbers in the newer studies:
*Figure 5.* Comparison of newer data with the Harland et al., 1982
<#ref6> time scale. [1] is Baadsgaard et al. (1993) <#ref1>; [2] is
Baadsgaard & Lerbekmo (1988) <#ref2>; [3] is Eberth and Braman (1990)
<#ref3>. comparing new radiometric dates to the scale
As you can see, the numbers in the rightmost column are basically
compatible. Skeptics of radiometric dating procedures sometimes claim
these techniques should not work reliably, or only infrequently, but
clearly the results are similar: for intervals that should be about
70-80 million years old, radiometric dates do not yield (for example)
100 or 30 million years, let alone 1000 years, 100 000 years or 1
billion. Most of the time, the technique works exceedingly well to a
first approximation.
However, there are some smaller differences. The Cretaceous/Tertiary
boundary dates differ slightly, but are within the measurement
uncertainties of the new date. The date for the /Baculites reesidei/
zone is at least 0.1 million years off (taking the outside limit of the
data uncertainty), and is below the Campanian/Maastrichtian boundary, so
the inconsistency could be even larger. What to do? Well, standard
scientific procedure is to collect more data to test the possible
explanations -- is it the time scale or the data that are incorrect?
Obradovich (1993) <#ref7> has measured a large number of high-quality
radiometric dates from the Cretaceous Period, and has revised the
geological time scale for this interval. Specifically, he proposes an
age of 71.3 million years for the Campanian/Maastrichtian boundary above
the /Baculites jenseni/ ammonite zone, based on /independent/ dates from
other locations. This is completely compatible with the data in
Baadsgaard et al. (1993) <#ref1>, making it likely the revised, younger
date for the Campanian/Maastrichtian boundary is the correct one versus
Harland et al. (1982) <#ref6>. The other dates are completely consistent
with a lower boundary for the Campanian of 83±1 million years ago, as
suggested by Harland et al. (1982) <#ref6> (which Obradovich revises to
83.5±0.5 Ma). In summary, it looks like the Campanian/Maastrichtian
boundary of Harland et al. (1982) <#ref6> was a little off, but
everything else is basically consistent to within the uncertainties of
measurement.
*Conclusions*
Skeptics of conventional geology might think scientists would expect, or
at least prefer, every date to be perfectly consistent with the current
geological time scale, but realistically, this is not how science works.
The age of a particular sample, and a particular geological time scale,
only represents the /current/ understanding, and science is a process of
refinement of that understanding. In support of this pattern, there is
an unmistakable trend of smaller and smaller revisions of the time scale
as the dataset gets larger and more precise (Harland et al. 1982, p.4-5
<#ref6>). If something were seriously wrong with the current geologic
time scale, one would expect inconsistencies to grow in number and
severity, but they do not.
For example, estimates of the age of boundaries in the Tertiary
regularly varied by 20-30% in the 1930s to 1970s. Since that time, they
have varied by much smaller amounts, rarely approaching 5% (again refer
to Harland et al., 1982, p.4-5 <#ref6>). The same trend can be observed
for other time periods. Palmer (1983) <#Palmer83> and Harland et al.
(1990) <#ref8> present a more recent proposal for the geological time
scale, demonstrating that change is still occurring. The latter includes
an excellent diagram summarizing comparisons between earlier time scales
(Harland et al., 1990, p.8) <#ref8>. Since 1990, there have been still
more revisions by other authors, such as Obradovich (1993) <#ref7> for
the Cretaceous Period, and Gradstein et al. (1995) <#Gradetal95> for the
entire Mesozoic.
*Figure 6.* A recent geological time scale, based on Harland et al.
(1990) <#ref8> Geologic time scale
As another example, Rogers et al. (1993) <#ref4> and Goodwin and Deino
(1989) <#ref5> present radiometric dates that bracket the ages of Late
Cretaceous fossil occurrences (i.e. dates above and below the fossils)
and yield more results that are consistent with predictions from the
current time scale. This is not uncommon. Besides the papers mentioned
here, there are hundreds, if not thousands, of similar papers providing
bracketing ranges for fossil occurrences. The synthesis of work like
this by thousands of international researchers over many decades is what
defines geological time scales in the first place (refer to Harland et
al., 1982 <#ref6>, 1990 <#ref8> for some of the methods). Although
geologists can and do legitimately quibble over the exact age of a
particular fossil or formation (e.g., is it 100 million years old or 110
million?), and genuinely problematic samples do exist, claims that
radiometric dating is so unreliable that the calibration of the
geological time scale could be modified by several orders of magnitude
(10000x, 1000x, or even 10x) are ridiculous from a scientific
standpoint. The data do not support such an interpretation. The methods
work too well most of the time.
In addition, evidence from other aspects of geology (e.g., estimates of
depositional rate and rates of other geological processes) support the
great age of the Earth. Prior to the availability of radiometric dating,
and even prior to evolutionary theory, the Earth was estimated to be at
least hundreds of millions of years old (see above) <#radiomet>.
Radiometric dating has simply made the estimates more precise, and
extended it into rocks barren of fossils and other stratigraphic tools.
The geological time scale and the techniques used to define it are not
circular. They rely on the same scientific principles as are used to
refine any scientific concept: testing hypotheses with data. There are
innumerable independent tests that can identify and resolve
inconsistencies in the data. This makes the geological time scale no
different from other aspects of scientific study.
For potential critics: Refuting the conventional geological time scale
is not an exercise in collecting examples of the worst samples
possible. A critique of conventional geologic time scale should address
the best and most consistent data available, and explain it with an
alternative interpretation, because that is the data that actually
matters to the current understanding of geologic time.
*References* (also refer to "Other sources <#other>")
Baadsgaard, H.; Lerbekmo, J.F.; Wijbrans, J.R., 1993. Multimethod
radiometric age for a bentonite near the top of the /Baculites reesidei/
Zone of southwestern Saskatchewan (Campanian-Maastrichtian stage
boundary?). Canadian Journal of Earth Sciences, v.30, p.769-775.
Baadsgaard, H. and Lerbekmo, J.F., 1988. A radiometric age for the
Cretaceous-Tertiary boundary based on K-Ar, Rb-Sr, and U-Pb ages of
bentonites from Alberta, Saskatchewan, and Montana. Canadian Journal of
Earth Sciences, v.25, p.1088-1097.
Eberth, D.A. and Braman, D., 1990. Stratigraphy, sedimentology, and
vertebrate paleontology of the Judith River Formation (Campanian) near
Muddy Lake, west-central Saskatchewan. Bulletin of Canadian Petroleum
Geology, v.38, no.4, p.387-406.
Goodwin, M.B. and Deino, A.L., 1989. The first radiometric ages from the
Judith River Formation (Upper Cretaceous), Hill County, Montana.
Canadian Journal of Earth Sciences, v.26, p.1384-1391.
Gradstein, F. M.; Agterberg, F.P.; Ogg, J.G.; Hardenbol, J.; van Veen,
P.; Thierry, J. and Zehui Huang., 1995. A Triassic, Jurassic and
Cretaceous time scale. IN: Bergren, W. A. ; Kent, D.V.; Aubry, M-P. and
Hardenbol, J. (eds.), Geochronology, Time Scales, and Global
Stratigraphic Correlation. Society of Economic Paleontologists and
Mineralogists, Special Publication No. 54, p.95-126.
Harland, W.B., Cox, A.V.; Llewellyn, P.G.; Pickton, C.A.G.; Smith, A.G.;
and Walters, R., 1982. A Geologic Time Scale: 1982 edition. Cambridge
University Press: Cambridge, 131p.
Harland, W.B.; Armstrong, R.L.; Cox, A.V.; Craig, L.E.; Smith, A.G.;
Smith, D.G., 1990. A Geologic Time Scale, 1989 edition. Cambridge
University Press: Cambridge, p.1-263. ISBN 0-521-38765-5
Harper, C.W., Jr., 1980. Relative age inference in paleontology.
Lethaia, v.13, p.239-248.
Lubenow, M.L., 1992. Bones of Contention: A Creationist Assessment of
Human Fossils. Baker Book House: Grand Rapids.
Obradovich, J.D., 1993. A Cretaceous time scale. IN: Caldwell, W.G.E.
and Kauffman, E.G. (eds.). Evolution of the Western Interior Basin.
Geological Association of Canada, Special Paper 39, p.379-396.
Palmer, Allison R. (compiler), 1983. The Decade of North American
Geology 1983 Geologic Time Scale. Geology, v.11, p.503-504. [Also
available on-line from the Geological Society of America web site
at
http://www.geosociety.org/pubs/public/geotime1.htm {Now broken link. See
archived copy
instead. -- September 12, 2004 } ]
Rastall, R.H., 1956. Geology. Encyclopaedia Britannica 10, p.168.
Encyclopaedia Britannica, Inc.: Chicago. [As cited in Harper (1980)
<#Harper80>.]
Rogers, R.R.; Swisher, C.C. III, Horner, J.R., 1993. 40Ar/39Ar age and
correlation of the nonmarine Two Medicine Formation (Upper Cretaceous),
northwestern Montana, U.S.A. Canadian Journal of Earth Sciences, v.30,
1066-1075.
Woodmorappe, J. (pseudonym), 1979. Radiometric Geochronology
Reappraised. Creation Research Society Quarterly, v.16, p.102-129.
[Also available in the book "Studies in Flood Geology"
, published by the Institute for
Creation Research .]
*Other Sources*
This document discusses the way radiometric dating is used in geology
rather than the details of how radiometric techniques work. It therefore
assumes the reader has some familiarity with radiometric dating. For a
technical introduction to the methods, I highly recommend these two books:
Dalrymple, G. Brent, 1991. The Age of the Earth. Stanford University
Press: Stanford, 474 pp. ISBN 0-8047-1569-6
Faure, G., 1986. Principles of Isotope Geology, 2nd. edition. John Wiley
and Sons: New York, p.1-589. ISBN 0-471-86412-9
An excellent introduction to radiometric dating can also be found in the
talk.origins FAQ archive:
Age of the Earth FAQ
Isochron dating FAQ
Both are by Chris Stassen .
An excellent source about the integration of radiometric dating,
biostratigraphy (the study of fossil succession) and general
stratigraphic principles is:
Blatt, H.; Berry, W.B.N.; and Brande, S., 1991. Principles of
Stratigraphic Analysis. Blackwell Scientific Publications: Boston, 512p.
ISBN 0-86542-069-6.
The history of the geologic time scale is ably described in:
Berry, W.B.N., 1987. Growth of a Prehistoric Time Scale. Blackwell
Scientific Publications: Boston, 202p.
And a good summary is in "Changing views of the history of the Earth"
by Richard Harter and Chris Stassen.
*Notes*
^1 Technically, these geologic time scales are known as
"geochronologic scales", and there is a conceptually tricky duality to
the scale between the rock, the time represented by the rock, and the
calibration of the relative time to an absolute scale. A profusion of
terms is applied to the different concepts, and, confusingly to the
uninitiated, to the names applied to subdivisions of them (e.g.,
"Cretaceous"). Geologic "Periods" (time) and geologic "Systems" (rock)
are different concepts, even though the same label (e.g., "Cretaceous")
may be applied to them. The semantic difference exists to distinguish
between the different (but relatable) types of observations and
interpretation that go into them. For simplicity sake I am sticking to
the concepts of "relative" and "absolute" (numerical) time, because
these are in common use, and I am glossing over the dual nature of the
subdivisions. These issues are explained in much more detail in the
citations mentioned in "Other Sources" <#other> particularly Blatt (et
al., 1991) <#Blattetal91>.
*Acknowledgements*
This is my third revision of a FAQ on the application of dating methods.
It benefits from the comments of several informal reviewers.
Unfortunately, some were so long ago that I no longer have all their
names :-( But my thanks goes to all of them anyway, and to four recent
ones I do remember: Stanley Friesen, Chris Stassen, Mark Isaak, and
Martyne Brotherton. My thanks also to Brett Vickers for maintaining the
talk.origins archive.
Home Page > | Browse | Search
| Feedback | Links
The FAQ | Must-Read Files
| Index |
Creationism | Evolution
| Age of the Earth
| Flood Geology
| Catastrophism
| Debates