Author: Chris Stassen Subject: FAQ: Age of the Earth Updated: 02/08/
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Author: Chris Stassen
Subject: FAQ: Age of the Earth
Updated: 02/08/94
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Overview:
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(I) How old is the earth, and how do we know?
(II) Common creationist "dating methods"
(III) Common creationist criticisms of mainstream dating methods
(IV) Suggested further reading
(V) References
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(I) How old is the earth, and how do we know?
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The generally accepted age for the earth (and the rest of the solar
sytem) is about 4.5 billion years. This value is derived from several
different lines of evidence.
Unfortunately, the age cannot be computed directly from material that
is solely from the earth. There is evidence that energy from the
earth's accumulation caused the surface to be molten. Further, the
processes of erosion and crustal recycling have apparently destroyed
all of the earliest surface.
The oldest rocks which have been found so far date to about
3.8 to 3.9 billion years ago (by several radiometric dating methods).
Some of these rocks are sedimentary, and include minerals which are
themselves as old as 4.1 to 4.2 billion years. Rocks of this age are
relatively rare, however rocks at least 3.5 billion years in age have
been found on all continents.
While these values do not compute an age for the earth, they
do establish a lower limit (the earth must be at least as old as any
formation on it). This lower limit is at least concordant with the
independently derived figure of 4.54 billion years for the earth's
actual age.
The most direct means for calculating the earth's age is a Pb/Pb
isochron age, derived from samples of the earth and meteorites.
This involves measurement of three isotopes of lead (Pb-206, Pb-207,
and either Pb-208 or Pb-204). A plot is constructed of Pb-206/Pb-204
versus Pb-208/Pb-204.
If the solar system formed form a common pool of matter,
which was uniformly distributed in terms of Pb isotope ratios, then
the initial plots for all objects from that pool of matter would
fall on a single point. However, amounts of Pb-206 and Pb-207 will
change in some samples, as these isotopes are decay end-products of
U (U-238 decays to Pb-206, and U-235 decays to Pb-207).
If the source of the solar system was also uniformly distributed
with respect to U isotope ratios, then this change will cause the
data points to move away from each other, but they will always fall on
a single line. And from the slope of the line we can derive the
amount of time which has passed since the pool of matter became separated
into individual objects. (See the "Isochron Dating FAQ" for more
detail.)
A creationist would object to all of the "assumptions" listed
above. However, the test for these assumptions is the plot of the data
itself. The actual underlying assumption is that, if those requirements
have not been met, there is no reason for the data to plot on a line.
The resulting plot for five meteorites that contained uranium, a single
data point for all meteorites that do not, and one for modern ocean
sediments. It looks like this:
Y-axis: ratio of Pb[207]/Pb[204]
X-axis: ratio of Pb[206]/Pb[204].
+---------------------------------------------------------------------+
| 7 |
| |
30 + |
| |
| 6 |
| |
| |
20 + |
| |
| 4 5 |
| 3 |
| 2 |
10 + 1 |
| |
| |
+------+------+------+------+------+------+------+------+------+------+
10 20 30 40 50
Data points: (1) Iron Meteorites; (2) Beardsley; (3) Modern sediments
and young galenas; (4) Saratov; (5) Elenovka; (6) Richardton; (7) Nuevo
Laredo. I can't really do it justice in ASCII, I recommend interested
parties to get the original. (Dalrymple 1986, Figure 12)
The slope of the line in the above chart gives an age of 4.55 +/- 0.07
billion years.
Most of the other measurements for the age of the earth rest upon
calculating an age for the solar system by dating objects which are
less geologically active (such as meteorites). Below is a table of
radiometric ages derived from groups of meteorites:
======================= ====== ====== ===============
Number
Type Dated Method Age (x10^9 yr)
======================= ====== ====== ===============
Chondrites 13 Sm-Nd 4.21 +/- 0.76
Carbonaceous chondrites 4 Rb-Sr 4.37 +/- 0.34
Chondrites (undist. H) 38 Rb-Sr 4.50 +/- 0.02
Chondrites (all) 50 Rb-Sr 4.43 +/- 0.04
H Chondrites (undist.) 17 Rb-Sr 4.52 +/- 0.04
H Chondrites 15 Rb-Sr 4.59 +/- 0.06
L Chondrites (rel. und.) 6 Rb-Sr 4.44 +/- 0.12
L Chondrites 5 Rb-Sr 4.38 +/- 0.12
LL Chondrites (undist.) 13 Rb-Sr 4.49 +/- 0.02
LL Chondrites 10 Rb-Sr 4.46 +/- 0.06
E Chondrites (undist.) 8 Rb-Sr 4.51 +/- 0.04
E Chondrites 8 Rb-Sr 4.44 +/- 0.13
Eucrites (polymict) 23 Rb-Sr 4.53 +/- 0.19
Eucrites 11 Rb-Sr 4.44 +/- 0.30
Eucrites 13 Lu-Hf 4.57 +/- 0.19
Diogenites 5 Rb-Sr 4.45 +/- 0.18
Iron (+ St. Severin) 8 Re-Os 4.57 +/- 0.21
======================= ====== ====== ===============
(After Dalrymple 1991, p. 291; duplicate studies on identical meteorite
types omitted.)
As shown in the table, there is excellent agreement on about 4.5 billion
years, between hundreds of different meteorites and by several different
dating methods.
Further, studies on individual meteorites generally give concordant ages
by multiple radiometric means. For example:
======================= ====== ====== ===============
Meteorite Dated Method Age (x10^9 yr)
======================= ====== ====== ===============
Guarena w-rock Ar-Ar 4.44 +/- 0.06
13 sam Rb-Sr 4.46 +/- 0.08
----------------------- ------ ------ ---------------
Olivenza 18 sam Rb-Sr 4.53 +/- 0.16
w-rock Ar-Ar 4.49 +/- 0.06
----------------------- ------ ------ ---------------
Saint Severin 4 sam Sm-Nd 4.55 +/- 0.33
10 sam Rb-Sr 4.51 +/- 0.15
w-rock Ar-Ar 4.43 +/- 0.04
----------------------- ------ ------ ---------------
Juvinas 5 sam Sm-Nd 4.56 +/- 0.08
5 sam Rb-Sr 4.50 +/- 0.07
----------------------- ------ ------ ---------------
Y-75011 9 sam Rb-Sr 4.50 +/- 0.05
7 sam Sm-Nd 4.52 +/- 0.16
======================= ====== ====== ===============
(After Dalrymple 1991, p. 286; meteorites dated by only a single means
omitted, duplicated methods omitted.)
Also note that the meteorite ages (both when dated mainly by Rb-Sr
dating in groups, and by multiple means individually) are in exact
agreement with the solar system "model lead age" produced earlier.
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(II) Common creationist "dating methods":
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Creationists have several methods which they claim to give "upper
limits" to the age of the earth, much lower than the age calculated
above (usually in the thousands of years). Those which appear the
most often in talk.origins are reproduced below:
1. Accumulation of Helium in the atmosphere
2. Decay of the Earth's magnetic field
3. Accumulation of meteoritic dust on the moon
4. Accumulation of metals into the oceans
Note that these aren't necessarily the "best" or most difficult to
refute of creationist young-earth arguments. However, they are quite
popular in modern creationist literature (even though they should not
be!) and they are the ones which we have to answer in talk.origins the
most often.
--------------------------------
1. Accumulation of Helium in the atmosphere
The creationst argument goes something like this: Helium-4 is created
by radioactive decay (alpha particles are helium nuclei) and is
constantly added to the atmosphere. Helium is not light enough to
escape the earth's gravity (unlike hydrogen), and it will therefore
accumulate over time. The current level of helium in the atmosphere
would accumulate in less than two hundred thousand years, therefore
the earth is young. (I believe this argument was originally put forth
by creationist Melvin Cook, in a letter to the editor which was
published in _Nature_.)
But helium can and does escape from the atmosphere, at rates
calculated to be nearly identical to rates of production. In
order to "get" a young age from their calculations, creationists
"handwave away" mechanisms by which Helium can escape. For
example, Morris says:
"There is no evidence at all that Helium 4 either does,
or can, escape from the exosphere in significant amounts."
(Morris 1974, p. 151)
But Morris is wrong. Surely one cannot "invent" a good dating
mechanism by simply ignoring processes which work in the opposite
direction of the process which the date is based upon. Dalrymple
says:
"Banks and Holzer (12) have shown that the polar wind can
account for an escape of 2 to 4 x 10^6 ions/cm^2.sec of
[4]He, which is nearly identical to the estimated production
flux of (2.5 +- 1.5) x 10^6 atoms/cm^2.sec. Calculations
for [3]He lead to similar results, i.e., a rate virtually
identical to the estimated production flux. Another possible
escape mechanism is direct interaction of the solar wind with
the upper atmosphere during the short periods of lower
magnetic-field intensity while the field is reversing.
Sheldon and Kern (112) estimated that 20 geomagnetic-field
reversals over the past 3.5 million years would have assured
a balance between helium production and loss."
(Dalrymple 1984, p. 112)
Dalrymple's references:
(12) Banks, P. M. & T. E. Holzer. 1969. High-latitude plasma
transport: the polar wind. Geophys. Res. J. 74: 6317-6332.
(112) Sheldon, W. R. & J. W. Kern. 1972. Atmospheric helium and
geomagnetic field reversals. Geophys. Res. J. 77: 6194-6201.
This argument also appears in the following creationist literature:
(Baker 1976, pp. 25-26)
(Brown 1989, pp. 16 and 52)
(Jansma 1985, p. 61)
(Whitcomb and Morris 1961, pp. 384-385)
(Wysong 1976, pp. 161-163)
--------------------------------
2. Decay of the Earth's magnetic field
The creationist argument: the dipole component of the magnetic field
has decreased slightly over the time that it has been measured.
Assuming the generally accepted "dynamo theory" for the existence of
the earth's magnetic field is wrong, the mechanism might instead be
free currents of magnetic material, which have been losing speed ever
since the creation event. An exponential fit (assuming a half-life of
1400 years on 130 years' worth of measurements) yields an impossibly
high magnetic field even 8000 years ago, therefore the earth must be
young. The main proponent of this argument was the late Thomas Barnes.
There are several things wrong with this "dating" mechanism. It's
hard to just _list_ them all. The primary three are that the
rejection of the dynamo theory is not solidly based, that there is
overwhelming evidence that the magnetic field has reversed itself
(rendering any unidirectional extrapolation useless), and that it
completely ignores the nondipole component of the field.
That last part is more important than it may sound. The earth's
magnetic field is often split in two components when measured. The
"dipole" component is the part which approximates a theoretically
perfect field around a single magnet, and the "nondipole" component is
the ("messy") remainder. Since about the turn of the century, the
slight decrease in the dipole component has been nearly completely
compensated by an increase in the nondipole component of the field.
(In other words, the measurements show that the field has been
diverging from a theoretical ideal magnet more than it has been
changing in strength.) The extrapolation therefore does not really
rest on the change in strength of the field.
For information, see (Dalrymple 1984, pp. 106-108) or (Strahler,
1987, pp. 150-155).
This argument also appears in the following creationist literature:
(Baker 1976, p. 25)
(Brown 1989, pp. 17 and 53)
(Jackson 1989, pp. 37-38)
(Jansma 1985, pp. 61-62)
(Morris 1974, pp. 157-158)
(Wysong 1976, pp. 160-161)
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3. Accumulation of meteoritic dust on the moon
This argument: A single measurement of the rate of meteoritic dust
influx to the earth gave a value in the millions of tons per year.
While this is negligible compared to the processes of erosion on the
earth (a shoebox-full per acre per year), there are no such processes
on the moon. The moon must receive a similar amount of dust (perhaps
25% as much per unit surface area due to its lesser gravity), and
there should be a very large dust layer (about a hundred feet thick)
if the moon is several billion years old.
Morris says, regarding the dust influx rate:
"The best measurements have been made by Hans Pettersson,
^^^^^^^^^^^^^^^^^
who obtained the figure of 14 million tons per year (1)."
(Morris 1974, p. 152)
Pettersson stood on a mountain top and collected dust there with a
device intended for measuring smog levels. He published calculations
which measured the amount of nickel he collected, assumed that nickel
was only present in meteoritic dust, and assumed that some percentage
of meteoritic dust was nickel, to get his final figures (that first
assumption was wrong and caused his published figures to be a vast
overestimate).
Pettersson's calculation resulted in the a figure of about 15 million
tons per year. He believed that estimate to be an over-estimate, and
indicated in the paper that 5 million tons per year was a much more
likely figure.
Much more accurate measurements were available, from satellite
penetration data (no possibility of earthly contamination), by the
time Morris published _Scientific Creationism_. These more accurate
measurements give the value of about 18,000 to 25,000 tons per year.
These measurements agree with levels of meteoritic dust levels trapped
in sediments on earth. (That is, they are verified by an independent
cross-check.)
Morris chooses to pick obsolete data with known problems, and call it
the "best" measurement available. His calculations are based on a
figure that is nearly three orders of magnitude too high. With the
proper values, the expected depth of meteoritic dust on the moon is
less than one foot.
For further information, see (Dalrymple 1984, pp. 108-111) or
(Strahler 1987, pp. 143-144).
This argument also appears in the following creationist literature:
(Baker 1976, p. 25)
(Brown 1989, pp. 17 and 53)
(Jackson 1989, pp. 40-41)
(Jansma 1985, pp. 62-63)
(Whitcomb and Morris 1961, pp. 379-380)
(Wysong 1976, pp. 166-168)
--------------------------------
4. Accumulation of metals into the oceans
In 1965, _Chemical Oceanography_ published a list of some metals'
"residency times" in the ocean. This calculation was performed by
dividing the amount of various metals in the oceans by the rate at
which rivers bring the metals into the oceans.
Several creationists have reproduced this table of numbers, claiming
that these numbers gave "upper limits" for the age of the oceans
(therefore the earth) because the numbers represented the amount of
time that it would take for the oceans to "fill up" to their present
level of these various metals from zero.
First, let us examine the results of this "dating method." Most
creationist works do not produce all of the numbers, only the ones
whose values are "convenient." The following list is more complete:
Al - 100 Pb - 2k Ba - 84k Ag - 2.1M
Fe - 140 Si - 8k Sn - 100k K - 11M
Ti - 160 Ni - 9k Zn - 180k Sr - 19M
Cr - 350 Co - 18k Rb - 270k Li - 20M
Th - 350 Hg - 42k Sb - 350k Mg - 45M
W - 1000 Bi - 45k Mo - 500k Na - 260M
Mn - 1400 Cu - 50k Au - 560k
(In the above list, "k" = 1,000 years, "M" = 1,000,000 years)
Now, let us critically examine this method as a method of finding an
age for the earth.
1. The method ignores known mechanisms which remove metals from the oceans:
a) Many of the listed metals are in fact *known* to be at or near
equilibrium; that is, the rates for their entering and leaving the
ocean are the same to within uncertainty of measurement. One cannot
derive a date from a process at equilibrium. (It could go on
forever without changing concentration of the ocean.)
b) Even the metals which are not known to be at equilibrium are
known to be very close to it. I have seen a similar calculation
on uranium, failing to note that the uncertainty in the efflux
estimate is larger than its distance from equilibrium. To
calculate a *true* upper limit, we must calculate the *maximum*
upper limit, using all values at the appropriate extreme of their
measurement uncertainty. We must perform the calculations on the
highest possible efflux rate, and the lowest possible influx rate
(within the measurement error). If that gets us to equilibrium,
then no upper limit can be derived.
c) In addition, *even if* we knew exactly the rates at which
metals were removed from the oceans, and *even if* these rates did
not match the influx rates, these numbers are still wrong. It
would probably require solving a differential equation, and any
reasonable approximation MUST "figure in" the efflux rate. Any
creationist who presents these values as an "upper limit" has
missed this factor entirely. These published values are only
"upper limits" when the efflux rate is zero (which is known to be
false for all the metals). Any efflux decreases the rate at which
the metals build up, invalidating the alleged "limit."
2. The method simply does not work. Ignoring the three problems
above, the results are scattered randomly (5 < 1k years, 5 in 1k-9k
years, 5 in 10k- 99k years, 6 in 100k-999k years, 6 > 1M years).
Also, the only two results that agree are 350 years, and Aluminum
gives 100 years. If this is a valid method, then the earth is
less than the lowest value -- 100 years -- in age.
3. These "dating methods" do not actually date anything, which
prevents independent confirmation. (Is a 19M year "limit" [Sr] a
"confirmation" of a 42k year "limit" [Hg]?) Independent
confirmation is very important for dating methods -- scientists
generally do not place much confidence in a date that is only
computed from a single measurement.
4. These methods depend on uniformity of a process which is almost
certainly not uniform. There is no reason to believe that influx
rates have been constant throughout time. There is reason to
expect that, due to a relatively large amount of exposed land,
today's erosion (and therefore influx) rates are higher than typical
past rates.
5. There is no "check" built into these methods. There is no way to
tell if the calculated result is good or not. The best methods
used by geologists to perform dating have a built-in check which
identifies undateable samples. The only way a creatonist can
"tell" which of these methods produce bad values is to throw out
the results that he doesn't like.
One might wonder why creationist authors have found it worthy of
publishing. Yet, it is quite common. This argument also appears in
the following creationist literature:
(Baker 1976, p. 25)
(Brown 1989, p. 16)
(Morris 1974, pp. 153-156)
(Morris & Parker 1987, pp. 284-284 and 290-291)
(Wysong 1976, pp. 162, 163)
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Conclusion:
Obviously, these are a pretty popular set of "dating" mechanisms; they
appear frequently in creationist literature from the 1960s through the
late 1980s. They appear in talk.origins more often than any other
young-earth arguments. And they are all built upon a distortion of
the data.
A curious and unbiased observer could quite reasonably refuse to even
listen to the creationists until they "clean house" and stop pushing
these arguments. If I found "Piltdown Man" in a modern biology text
as evidence for human evolution, I'd throw the book away. (If I
applied the same standards to the large collection of creationist
materials that I own, none would remain.)
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(III) Common creationist criticisms of mainstream dating methods:
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Most creationist criticisms of radiometric dating can be categorized
into a few groups. These include:
--------------------------------
1) Reference to a case where the given method did not work.
This is perhaps the most common objection of all. Creationists point
to instances where a given method produced a result that is clearly
wrong, and then argue that therefore all such dates may be ignored.
Such an argument fails on two counts:
a) First, an instance where a method fails to work does not imply
that it does not ever work. The question is not whether there are
"undateable" objects, but rather whether or not ALL objects cannot
be dated by a given method. The fact that one wristwatch has
failed to keep time properly cannot be used as a justification for
discarding all watches.
How many creationists would see the same time on five
different clocks and then feel free to ignore it? Yet, when five
radiometric dating methods agree on the age of one of the earth's
oldest rock formations (Dalrymple 1986, p. 44), it is dismissed
without a thought.
b) Second, these arguments fail to address the fact that radiometric
dating produces results in line with "evolutionary" expectations
about 95% of the time (Dalrymple 1992, personal correspondence).
The claim that the methods produce bad results essentially at
random does not explain why these "bad results" are so consistently
in line with mainstream science.
--------------------------------
2) Claims that the assumptions of a method may be violated.
Certain assumptions are involved with all radiometric dating methods.
These generally include constancy of decay rate and lack of contamination
(gain or loss of parent or daughter isotope). These two assumptions are
the most frequently attacked:
a) Constancy of radioactive decay rates.
Rates of radiometric decay (the ones relevant to radiometric
dating) are thought to be based mainly fundamental properties of
matter, such as the probability per unit time that a certain
particle can "tunnel" out of the nucleus of the atom. The nucleus
is well-insulated and therefore is relatively immune to larger-scale
effects such as pressure or temperature. Significant changes to
rates of radiometric decay relevant to geological dating have never
been observed under any conditions.
A short digression on mechanisms for radioactive decay,
taken from
E-Mail Fredric L. Rice / The Skeptic Tank