Creationism, Science and the Age of the Earth: Part II
Radioactive Minerals: The Ultimate Natural Clock
1. Radiometric Dating
Most atoms in the universe are stable, and will retain the same nuclear mass over time. Other atoms, however, are unstable to varying degrees, and lose either electrons or neutrons and protons. This process is called decay. Either through a one-step simple decay, or a multi-step decay series, these radioactive atoms decay "into" another, lighter type of atom, at which point they become stable, and no longer decay. The rate at which this process occurs is denoted by the term half-life, or decay constant. Half-life represents the amount of time, in years, that it takes for half of a given sample of a given radioactive nuclide to decay into its corresponding decay product, or daughter nuclide. The interesting thing about this process of radioactive decay, the thing that makes radiometric dating possible, is that the rate at which this process occurs is extremely uniform throughout time. The decay processes utilized by radiometric dating, as we will see below, are almost completely unaffected by even extreme external conditions of pressure, magnetism, heat, and chemistry. There is no known physical process which could produce decay rate changes of the magnitude required by the YEC timescale.
Two different types of decay are relevant to radiometric dating. The most common is called beta decay, in which a proton decays into neutron, releasing an electron and an antineutrino in the process. Another type is called alpha decay, in which the nucleus of a radioactive atom ejects two nuetrons and two protons, essentially a Helium nuclei. Other types of decay, such as electron capture decay, do not involve a transmutation of the element into a daughter product, and hence have no bearing whatsoever on radiometric ages.
In principle, the method of radiometric dating is very simple. When certain minerals are formed (quartz, biotite, feldspars, glauconite, etc.), they assimilate a small amount of radioactive atoms into their crystal structure. Over time, these radioactive atoms decay into their stable daughter elements. Since the rate at which this decay occurs can be measured directly, the ratio of parent to daughter atoms in the rock are indications of how much decay, and hence time, has occured since the mineral cystallized, or, in the case of radiocarbon dating, the amount of time since an organism has died.
Actually, the situation is a little more complex than this, since most rocks will start with some amount of the daughter atom to begin with. Luckily for geochronologists, however, several methods exist which allow for the determination of the initial amounts of daughter nuclide. As we consider the different methods individually, we will see how each solves this problem.
a) C-14 Dating
C-14 dating, commonly referred to as Carbon-dating, is not used to date minerals, but organic material. Thus, for example, carbon dating can be used to date wood and plant material, such as samples found in tombs and ancient structures all over the world. Dating of such material has shown a conclusive agreement between C-14 dates and historically established real-time dates.
Since the half-life of C-14 is only about 5.7 thousand years, amounts of C-14 become nearly undetectable after about 60 thousand years. C-14, along with Beryllium 10 and Chlorine 36, is constantly being produced in the upper atmosphere by "cosmic rays." C-14 is produced from Nitrogen 14 specifically. When C-14 is produced, it almost immediately binds with free oxygen to form CO2, which settles into the lower atmosphere and oceans, and enters the biosphere via photosynthesis. When an animal or plant is alive, it is constantly rebuilding itself from the carbon in its environment, and hence a certain amount of the C-14 atoms will be incorporated into the plant or animal. When the plant or animal dies, it ceases to recycle its Carbon atoms with those in the atmosphere, and it will retain C-14 and C-12 in the same ratio present in its environment.
Therefore, as the sample ages, and as C14 atoms gradually decay into C12 atoms, the ratio of C-12 to C14 increases. Since this ratio increases in proportion to the amount of time that has passed since the sample organism's death, the ratio serves as an indication of the sample's age.
Let's say, for example, that the atmospheric ratio of C-12/C-14 has a constant throughout time of 10/1 (actually, the ratio is closer to about 99.99 to .01). Let's also suppose that our sample has a ratio of C12/C14 atoms of 100/5. Since this is about half of the C-14 which originally would have been present in the sample, assuming an invariable amount of atmospheric C14, we can conclude that about one half-life has passed, and that the sample is about 5.75 thousand years old. Likewise, if the sample has a ratio of of C12-C14 at 100/2.5. This would indicate that two half-lives, or about 11.5 thousand years, have passed since the organism's death.
On the other hand, contrary to our first assumption, we now know that the ratio of C-12 to C-14 has NOT remained constant over time. Rates of C-14 production have changed over the past several thousand years, from about 10% greater than current values to about 10% lower than current values. These fluctuations are largely the result of both solar activity and the strength of the Earth's magnetic field, which affects the "cosmic ray" production of cosmogenic isotopes, and hence the production of C-14. All is not lost, though, since the ratio of C12-C14, for any point in the past 10 years, can be directly calibrated by using tree rings (dendrocalibration). Using varve laminae and ice core layers, moreover, the C12-C14 ratios can be calibrated almost as far back as C-14's useful range of 40 thousand years.
The results of these calibrations show that, even though the atmospheric C-12/C-14 ratio has fluctuated over time, the affect of these fluctuations on actual dates has been negligible. For the most part, these fluctuations seem to have resulted in ages which are slightly too young. The comparison of C-14 dates with tree ring dates shows that the amount of fluctuation of C-14 rates has not been greater than about 10%. Likewise, Minze Stuiver of the University of Washington found that magnetic ages of the lake sediments, over a period of 22,000 years, remained within 500 years of the radiocarbon ages throughout the entire period. He reported that the concentration of C-14 in the atmosphere during that long interval did not vary by more than 10 percent (Stuiver, 1976, p. 835). "Thus," writes Strahler, "the available evidence is sufficient to validate the radiocarbon method of age determination within an error of about 10% for twice as long as the [young-earth] creation scenario calls for" (p. 157). Finally, and most recently, varves from Lake Suigetsu in Japan have allowed the radiocarbon time scale to be calibrated as far back as 43,000 B.C. Again the use of independent cross-checks on radiocarbon method have shown it to be accurate within a small margin of error.
b) Potassium 40-Argon 40 Dating
Whereas C-14 has a half-life of about 5.75 thousand years, and can not be extended past about 50 thousand years, Potassium 40 has a half-life of about 1.25 billion years, giving it a much wider chronologic range of application. This method is applied primarily to igneous rocks, which gives the method certain advantages. For one, it allows for the accurate estimation of daughter nuclide initially present in the sample. Igneous rocks form when volcanic magma cools and crystallizes. As the new igneous rocks cools, it takes on a very tight crystalline structure, leaving no room for Argon, which will not form chemical bonds with the rock. As a result, Argon is pushed out of the rock as it cystallizes. Brent Dalrymple notes:
"The K-Ar dating method is the only decay scheme that can be used with little or no concern for the initial precense of the daughter isotope. This is because Ar40 is an inert gas that does not combine chemically with any other element and so escapes easily when rocks when they are heated. Thus, while a rock is molten the Ar40 formed by the decay of K40 escapes from the liquid" (p 91).
Aside from this, there is still a way to check for initial levels of Ar40. All you need to do is check for the presence of Ar36 in the sample. Since Ar36 is not radiogenic but chemically identical to Ar40, it can be used to determine the amount of initial Ar40, or alternatively to measure for contamination after the rock's formation. If Ar36 is found in the sample, then Ar40 was initally present also. Since the ratio of Ar36 to Ar40 is known, one can determine the amount of initial Ar40 by comparing it with the current amount of Ar36. Dalrymple again:
"In the process of analysis, a correction must be made for atmosheric Ar present in most minerals and in the experimental apparatus. This correction is easily made by measuring the amount of Ar36 in the sample to be dated and, using the known composition of atmospheric Argon, subtracting the appropriate amount of Ar40 due to atmospheric contamination . . . This correction can be made very accurately and has no appreciable effect on the calculated age . . ." (p 93).
NOTE: Argon40-Argon39 dating methods and related analytical instruments have now become sensitive enough to date historical events. For instance, scientists at the Berkeley Geochronology Center recently dated feldspars from Vesuvius at 1,925 years ago +/-94 yrs. We know from Pliny the Younger that this eruption occured in 79AD. See "40Ar/39Ar Dating into the Historical Realm: Calibration Against Pliny the Younger." P. R. Renne, W. D. Sharp, A. L. Deino, G. Orsi, L. Civetta. Science. Volume 277, Number 5330, Issue of 29 Aug 1997, pp. 1279-1280.
c) U/Pb and Th/Pb Dating
U235, U238, and Th232 all decay into lead isotopes, with half-lives of over a billion years. Since these isotopes commonly occur in the same mineral, they can often be used to cross-check each a sample's date. Just as we saw with K-Ar and C-14 dating, this dating method can only be applied to a certain class of sample, samples for which the amount of initial daughter nuclide, in this case Pb, can be estimated. Dalrymple again:
"There are a few common minerals . . . that contain large amounts of U and Th. These minerals, of which zircon is the most common, do not occur in large volumes but do occur in small amounts in many rocks. Zircon is also exceeding low in intitial Pb, so the U-Pb and Th-Pb methods can be applied to this mineral, and a few others, with little concern for initial Pb" (101).
Uranium>Thorium>Lead dating has a greater margin of error than many other radiometric methods. For one, daughter contamination is more problematic, since Lead is a volatile element, and leaches easily into and out of a sample if it is heated some time after formation. For this reason, U>Th>Pb dating is rarely used as a simple accumulation clock. Most often, they are employed in conjuction with more complicated methods of analyses, known as isochron and discordia analyses. As we will see below, these techniques solve the daughter problem, since this method in allows one to determine a) the original amount of daughter nuclide, and b) whether or not the sample has been contaminated since its formation.
2. Rb-Sr Dating and the Isochron Method
Since Rb-Sr dating methods are usually used in conjunction with the isochron method, we will use these isotopes to illustrate how the method works. In a simple accumulation clock, age is determined by comparing the amount of remaining parent to its daughter nuclide, which has accumulated in the sample over time. If we restrict the method to rocks whose initial composition can be estimated, such as igneous rocks formed at normal pressure, then the age of the rock can be determined. For many rocks, however, the determination of initial levels of daughter nuclide is problematic. This is where the isochron method comes in. Dalrymple notes:
"The isochron method has two significant advantages over simple accumulation clocks. First, it circumvents the problem of the amount of the initial daughter. That information need not be known -- it is one of the answers provided by the method. Second, the method is self-checking, providing the user with information about the degree to which the sample has behaved as a closed system" (103).
The method works by taking a third measurement into account, which is the amount of another isotope of the same element as the daughter nuclide. In the case of Rb-Sr dating, this third nuclide is Sr86. Since Sr86 is not radiogenic, the amount of it in the rock will remain constant over time. Since Sr87 IS radiogenic, it will increase in abundance over time. Therefore, the ratio of Sr87/Sr86 will INCREASE over time. If the rock remains a closed system over time, this ratio will increase uniformly, over time, in all parts of the rock. Consider the following hypothetical isochron, which includes 4 data points, each of which represents an isotope ratio for four different minerals taken from the same rock:
Figure 1: A hypothetical isochron of four samples. At the time of crystallization (T=0), the rock will yield a flat isochron. Since greater slope equals greater age, a newly formed rock will yield zero slope. This is based on the fact that, even though all samples contain different amounts of Rb87, Sr86, and Sr87, they initially contain them in the same ratios. Since the ratio Rb87/Sr86 will decrease over time, the data points will move down on the x-axis (to the left). Since the ratio Sr87/Sr86 will increase over time, the data points will also move up on the y-axis. Of the four samples plotted on our isochron, the one on the right starts with the most Rb87, so it will move furthest on both axes. Likewise, since the one of the far left starts with the smallest ratio of Rb87/Sr86, it will move less far on both axes. The upper three isochrons represent measurements taken at three later times, T=1, T=2, and T=3. If all the samples remain closed systems, their plots will move uniformly left on the x-axis, and uniformly up on the x-axis.
The important point is this: if the system remains undisturbed over time, if no parent or daughter nuclides enter or exits the rock, then at any time (t') after the rock's crystallization, the slope of the isochron will change in proportion to the age of the rock. If the rock remains a closed system, all of the samples of the rock will plot on a straight isochron. When this is the case, the amount of initial daughter is indicated by the point at which the isochron intersects the y-axis, in this case at about 2.2. If either parent or daughter nuclide enters the system, the points will not fall on a straight isochron.
If the system is disturbed after formation, the data points will drift differentially away from the isochron, depending upon the ability of the minerals in question to absorb or lose either parent or daughter nuclide. In the following diagram, all four samples have been disturbed in different ways:
Figure 2: samples from a disturbed isotopic system fail to plot upon an isochron. Sample one has gained Sr87, and so moves up the y-axis. Sample 2 has lost Sr87, so it will move down the y-axis. Sample 3 has gained Rb87, and so moves right on the x-axis. Sample four has lost Rb87, and so moves left on the x-axis. Since these data points will not plot colinearly, no slope can be determined. Hence, no age for the rock can be determined.
From this discussion, we can see how the isochron method solves the potential problems of simple accumulation dating. First, if contamination occurs in the sample, it will be reflected in the isochron graph itself by the failure of the samples to plot on a line. The rock samples will yield different ratios of Sr87/Sr86 and Rb87/Sr86, indicating that the originally homogenous mixture has been altered at some point after its crystallization. With a simple accumulation clock, the four samples would simply yield different ages. With the isochron, the four samples yield no age at all. Second, when the data DO fall on a straight line, and the rock has remained a closed system, the amount of initial daughter will be indicated by the point where the isochron intersects the y-axis.
Note also the very long half-life of Rb87, at about 4.47 billion years. Unless the Earth was older than about 40-50 million years, it should be impossible to date ANY rock with the Rb-Sr method. In this case, the isochrons should all fall into two categories: -- those that fail to plot on a line at all due to contamination, and those that plot on a horizontal line, indicating less than 40 million years or so had had passed since the time of the rock's crystallization. The fact that thousands of sloped isochrons exist at all is sufficient evidence for an Earth much older than 50 million years.
3. Criticisms of Radiometric Dating
a) Decay rates may have varied in the past.
The constancy of radioactive decay rates has been repeatedly confirmed by experiment. Despite attempts to change decay rates with extreme amounts of heat, pressure and the like, no significant changes in decay rates have ever been produced. Brent Dalrymple, summarizing the experimental evidence, says:
"Both theory and experiment show that changes in alpha, beta, and e.c. [electron capture] decay rates are not only rare, but exceedingly small. Even the largest observed change of 0.18% in Be7 would have a negligible effect on a calculated radiometric age. Also of importance is the fact that no changes have ever been detected for any of the isotopes used for dating and none of significance are theoretically expected. Of the physical and chemical processes that affect meteorites and rocks from the Earth and Moon, including pressure, temperature, gravity, and magnetic and electric fields, none should affect radiometric dating to any significant degree. Thus we can be confident that for all practical purposes the radiometric clocks used for geologic dating 'tick' at unchanging rates" (1991, p 89).
The constancy of decay rates is a consequence of the fact that the atomic nucleus is rarely if at all affected by chemical or electromagnetic forces. Chris Stassen, in his Talk.Origins "Age of the Earth" debate, says similarly:
"Significant changes to rates of radiometric decay of isotopes relevant to geological dating have never been observed under any conditions. Emery (1972) is a comprehensive survey of experimental results and theoretical limits on variation of decay rates. Note that the largest changes reported by Emery are both irrelevant (they do not involve isotopes or modes of decay used for [radiometric dating]), and miniscule (decay rate changed by of order 1%) compared to the change needed to compress the apparent age of the Earth into the young-Earthers' timescale"
Stassen's last point is especially important. In order to accomodate the vast ages of earth history revealed by isotope geology into the 7000 or so years of the young-earth time scale, decay rates must have been many thousands of times faster in the past then they are today. And then, to make the creationist case even more absurd, the decay-rates have to become fixed at their modern values after the flood. But as we've seen, creationists can't point to any mechanism whereby decay rates can be changed over time. Even under extreme chemical, electrical, and magnetic conditions not likely to have ever obtained on earth, decay rates of the nuclides used in radiometric dating show no variation at all.
b) Radiometric dating techniques have yielded false ages when applied to rocks of known ages. For example, K-Ar dating of 1801 pillow lava yielded greatly inflated ages.
Not all methods can be applied to all types of rocks or to all geologic contexts. For each of the isotopic dating systems used by geologists, tests have been conducted to determine which minerals and geologic settings are suitable, and which are not. In the case of K-Ar dating, it was suggested that the system should only work when applied to volcanics that are allowed to cool slowly. In magma that is cooled quickly, for example extruded under water, Argon cannot escape, and hence the K-Ar clock is not "reset." Since initial Argon remains in the rock, the age derived from subaqeously extruded pillow lava is not likely to reflect the actual age of crystallization.
To test this idea, geologists selected an area of basalt that was known to have formed during an eruption in 1801, and used the K-Ar method to date the outer surface. The oldest date obtained was 22 million years, thus demonstrating that such rocks indeed retained much initial Ar, and thus were not suitable for K-Ar dating. As Brent Dalrymple reported:
"Two studies independently discovered that the glassy margins of submarine pillow basalts . . . trap Ar-40 dissolved in the melt before it can escape. This effect is most serious in the rims of the pillows and increases in severity with water depth. The excess Ar-40 content approaches zero toward the pillow interiors, which cool more slowly and allow the Ar-40 to escape . . . The purpose of these two studies was to determine, in a controlled experiment with samples of known age, the suitability of submarine pillow basalts for dating, because it was suspected that such samples might be unreliable . . . The results clearly indicated that these rocks were unsuitable for dating, and so they are not generally used for this purpose" (qtd. in Strahler, p. 206)
The creationists have made an obviously invalid leap from the fact that the rims of pillow lava retain initial Ar-40 to the sweeping conclusion that all K-Ar dates are totally unreliable. This is a nonsequiter. To say that Funkhauser et al's results somehow invalidate the K-Ar dating method is about as sensible as saying that because one "clock" gave a wrong time yesterday, all clocks through all time and space are inaccurate. The important point to remember here is that different systems have different realms of applicability, and these realms of applicability can be determined experimentally.
c) Rock samples may have become contaminated over time. In U-Pb dating, for example, Lead could migrate into the sample, giving the sample an inflated appearance of age. Alternatively, Lead could migrate out of the sample, yielding an attenuated age. If either parent or daughter nuclides enter or leave the sample over time, the radiometric date is likely to inaccurate.
Indeed, rocks can become contaminated over time, but methods exist for determing whether or not this has occured. For example, many rocks have several types of mineral inclusions. The permeability of these different minerals to P and D varies. Therefore, if a rock sample is contaminated, the amount of contamination will vary according to the minerals in the sample. This will result in different dates for different types of minerals, and will in turn will result in the failure of those mineral ages to plot on a straight isochron. Since this is the case, the isochron method allows contaminated samples to be recognized.
And this leads me to my next point, which centers around concordant ages. While creationists busy themselves pointing out examples of flawed radiometric data, they offer no explanation for cases in which the evidence is unambiguous and consistent! Take the Amistoq Gneiss in Greenland, for example, one of the oldest geological formations in the world (dates listed in Dalrymple, 141-42).
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Here we have five different decay schemes all agreeing, within their margins of error, on the same age. These different dating schemes utilize different nuclides (Rb, U, Th, Lu), which decay by different mechanisms (alpha and beta decay), and at widely varying rates. If all these methods are not actually measuring the gneiss's age, how is this remarkable agreement to be explained?
What are the chances of this occuring if radiometric ages are nothing but random numbers? Let's suppose that all of the dates above contain a mere three samples each, for a total of 15 samples agreeing on the same age, within 100m years. Assuming a possible date anywhere between 0 and 5 billion years, there are 50 possible "units" of time. The probability of any one sample falling on one particular 100m year unit is 1/50. The probability that 2 samples will fall on this same block is 1/50 x 1/50, or 1/2500, and so forth with each additional sample. Now, the probability that 15 different samples would all fall on the same 100m year block is equal to 1 chance in (50^15), an astronomically small chance. Such apparent "miracles" occur quite frequently with earth rocks, moon rocks, and meteorites. To the geophysicist, this agreement not only makes sense, but is predicted by the physical theories underlying radiometric dating. To the young-earther, however, concordant ages can only be the most remarkable of coincidences.
In the case of the moon, the evidence is even more clear. Broadly speaking, the surface of the moon can be divided into two classes, the dark lunar "seas," or maria, and lighter-colored, highly-cratered, lunar highlands. Since the lunar marias are actually large craters that were later "filled-in" with lava, common sense dictates that the cratered areas which rise above the the maria should be older than the maria itself. Radiometric dating of lunar rocks confirms this view. Of the dozens of samples of mare basalt collected by the Russian Luna and American Appollo missions, nearly all date to between 3-3.9 billion years. Since lunar rocks are obviously scarce and precious samples, and have much to tell us about the age of the solar system, they have been subjected to a great deal of experimental scrutiny. Dalrymple summarizes the results of extensive radiometric analyses of one Appollo 11 sample:
"Most impressive is the concordance of ages measured by different methods and from different phases seperated from the same sample. An example is 10072, a sample of high-K mare basalt collected by the Appollo 11 astronauts. A seven-point Rb-Sr isochron and a six-point Sm-Nd isochron both give ages of 3.57Ga. Ar40/Ar39 age spectrum dating of this sample has been done on the whole rock, plagioclase feldspar, ilmenite, and pyroxene. The two analyses of the plagioclase, which yielded the most convincing plataeus . . . agree exactly with the Rb-Sr and Sm-Nd ages. The age spectrum plateaus for the whole rock, ilmenite, and pyroxene are discontinuous through the highest temperature increments . . . Nevertheless, the ages found from these imperfect plateaus agree with the other results within the analytic uncertainties . . . Such consistency among methods indicates clearly that the Ar-Ar, Rb-Sr, and Sm-Nd isotopic clocks in the lunar rocks are accurate and being properly read" (228-9).
Rocks from the highland areas, as we said above, were expected to yield ages greater than those for the maria. This is confirmed by radiometric ages of highland rocks, which nearly all fall between about 3.9 billion and 4.5 billion years old. Of the highland rock samples collected by the Appollo missions, most date to about 4.0 billion, and about 12 date older 4.2 billion. Again, we see dozens of rocks, dated with different methods and by different laboratories, converging on the same dates.
Meteoric rocks, unlike earth rocks and even moon rocks, are thought to have been essentially undisturbed since the time of the formation of the solar system. Thus, meteoric rocks, as a group, should yield ages greater than both the earth and moon. Once again, this is confirmed by radiometric dating methods. Whereas the oldest rocks on earth are just under 4 billion years old, and only a small portion of moon rocks are older than this, nearly all meteoric rocks have the same age, about 4.5 billion years.
4. Fission Track Dating
Here we have yet another method, very precise, of proving the great age of the earth. Fission track dating is similar to normal radiometric dating, but even more reliable, since it completely sidesteps the problems of original nuclide content and contamination. The basis of this method is the fact that fission products of Uranium 238 have high energy. When U238 trapped inside a mineral (usually volcanic glass, zircon, sphene, apatite, muscovite biotite or tektites) releases these decay products, it etches a "track" inside the mineral, leaving conclusive indications as to the original amount of U238 inside the mineral. By comparing the reconstructed original amount of U238 with the current amount of U238, and extrapolating backwards using the decay constant, minerals can be dated precisely. That is, we can determine the time of crystallization (usually, again, of minerals formed volcanically). Using this dating technique, ages can be obtained with very little error for samples between 20M and 1B years.
These dates could only be altered in one direction of time. If the mineral was partially remelted or annealed, by metamorphicism or heating, for example, and some of the tracks dissappeared, this would yield a younger age for the sample, not an older one. Likewise, there can be no concern about sample contamination, since FT dates directly measure the amount of U238 which has decayed inside a mineral, NOT the ratio of parent atoms to daughter atom. Of course, as I said above, isochron dating methods already allowed scientists to self-check for contamination, since contamination would result in the failure of data point to plot an isochron.
Works Cited
Dalrymple, Brent G. (1991). The Age of the Earth. Stanford University Press.
Strahler, Arthur. (1988). Science and Earth History.
Stuiver, Minze. (1976). First Miami conference on isotope climatology and paleoclimatology. EOS, vol.57, no.1, pp.830-836