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How Old Is The Earth, And How Do We Know?
The generally accepted age for the Earth and the rest of the solar sytem is about 4.55 billion years (plus or minus about 1%). 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 (on the Earth) 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 that are at least 3.5 billion years in age have been found on North America, Greenland, Australia, Africa, and Asia.
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 4.55 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-207/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 or Faure 1986, chapter 18 for technical detail.)
A young-earther 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 points to fall 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:
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 expected to have formed with the planets but are not geologically active (and therefore cannot erase evidence of their formation), such as meteorites. Below is a table of radiometric ages derived from groups of meteorites:
------------------------------ PREFORMATTED -------------------------------
======================= ====== ====== =============== 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.)
---------------------------- END PREFORMATTED -----------------------------
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, the oldest age determinations of individual meteorites generally give concordant ages by multiple radiometric means.
Common Young-Earth \"Dating Methods\"
Young-earthers 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:
Accumulation of Helium in the atmosphere Decay of the Earth\'s magnetic field Accumulation of meteoritic dust on the moon Accumulation of metals into the oceans
Note that these aren\'t necessarily the \"best\" or most difficult to refute of young-earth arguments. However, they are quite popular in modern creation-\"science\" 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 young-earth 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 Mormon young-earther 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, young-earthers \"handwave away\" mechanisms by which Helium can escape. For example, Henry 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 young-earth 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 an initially created field which has been losing strength 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 four are:
While there is no complete model to the geodynamo (certain key properties of the core are unknown), there are reasonable starts and there are no good reasons for rejecting such an entity out of hand. If it is possible for energy to be added to the field, then the extrapolation is useless. There is overwhelming evidence that the magnetic field has reversed itself, rendering any unidirectional extrapolation on its strength useless. Even some creationists admit to that these days--e.g., (Humphreys 1988). Much of the energy in the field is probably locked in toroidal fields that are not even visible external to the core. This means that the extrapolation rests on the assumption that fluctuations in the observable portion of the field accurately represent fluctuations in its total energy. The extrapolation completely ignores the nondipole component of the field. Even if we grant that it is permissible to ignore portions of the field that are internal to the core, Barnes\' extrapolation also ignores portions of the field which are visible and instead rests on extrapolation of a theoretical entity.
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\" components are the (\"messy\") remainder. A study in the 1960s showed that the decrease in the dipole component since the turn of the century had been nearly completely compensated by an increase in the strength of the nondipole components 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 actually changing in strength.) Barnes\' 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)
[*]
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 (about a shoebox-full of dust 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) [emphasis added]
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).
There is a recent creationist technical paper on this topic which admits that the depth of dust on the moon is concordant with the mainstream age and history of the solar system (Snelling and Rush 1993). Their abstract concludes with:
\"It thus appears that the amount of meteoritic dust and meteorite debris in the lunar regolith and surface dust layer, even taking into account the postulated early intense bombardment, does not contradict the evolutionists\' multi-billion year timescale (while not proving it). Unfortunately, attempted counter-responses by creationists have so far failed because of spurious arguments or faulty calculations. Thus, until new evidence is forthcoming, creationists should not continue to use the dust on the moon as evidence against an old age for the moon and the solar system.\"
Even though the creationists themselves have refuted this argument, (and refutations from the mainstream community have been around for at least a decade longer than that), the \"moon dust\" argument continues to be propagated in their \"popular\" literature, and continues to appear in talk.origins on a regular basis:
(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:
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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)
---------------------------- END PREFORMATTED -----------------------------
Now, let us critically examine this method as a method of finding an age for the earth.
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 relatively 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.\"
The method simply does not work. Ignoring the three problems above, the results are scattered randomly (5 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.
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.
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.
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 undatable 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)
[*]
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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Common Creationist Criticisms of Mainstream Dating Methods
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:
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 \"undatable\" 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.
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:
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 by Steve Carlip:
For the case of alpha decay, [...] the simple underlying mechanism is quantum mechanical tunneling through a potential barrier. You will find a simple explanation in any elementary quantum mechanics textbook; for example, Ohanion\'s Principles of Quantum Mechanics has a nice example of alpha decay on page 89. The fact that the process is probabilistic, and the exponential dependence on time, are straightforward consequences of quantum mechanics. (The time dependence is a case of \"Fermi\'s golden rule\" --- see, for example, page 292 of Ohanion.)
An exact computation of decay rates is, of course, much more complicated, since it requires a detailed understanding of the shape of the potential barrier. In principle, this is computable from quantum chromodynamics (an extremely well-tested theory), but in practice the computation is much too complex to be done exactly. There are, however, reliable approximations available, and in addition the shape of the potential can be measured experimentally.
For beta decay, the underlying fundamental theory is different; one begins with electroweak theory (for which Glashow, Weinberg and Salam won their Nobel prize) rather than quantum chromodynamics.
As described above, the process of radioactive decay is predicated on rather fundamental properties of matter. In order to explain old isotopic ages on a young Earth by means of accelerated decay, an increase of six to ten orders of magnitude in rates of decay would be needed (depending on whether the acceleration was spread out over the entire pre-Flood period, or accomplished entirely during the Flood).
Such a huge change in fundamental properties would have plenty of noticeable effects on processes other than radioactive decay (taken from by Steve Carlip):
So there has been a lot of creative work on how to look for evidence of such changes.
A nice (technical) summary is given by Sisterna and Vucetich in Physical Review D44 (1991), p. 3096. Among the phenomena they look at are:
searches for changes in the radius of Mercury, the Moon, and Mars (these would change because of changes in the strength of interactions within the materials that they are formed from); searches for long term (\"secular\") changes in the orbits of the moon and the earth --- measured by looking at such diverse phenomena as ancient solar eclipses and coral growth patterns; ranging data for the distance from earth to Mars, using the Viking spacecraft; data on the orbital motion of a binary pulsar PSR 1913+16; observations of long-lived isotopes that decay by beta decay (Re 187, K 40, Rb 87) and comparisons to isotopes that decay by different mechanisms; the Oklo natural nuclear reactor (mentioned in another posting); experimental searches for differences in gravitational attraction between different elements (Eotvos-type experiments); absorption lines of quasars (fine structure and hyperfine splittings); laboratory searches for changes in the mass difference between the K0 meson and its antiparticle.
While it is not obvious, each of these observations is sensitive to changes in the physical constants that control radioactive decay. For example, a change in the strength of weak interactions (which govern beta decay) would have different effects on the binding energy, and therefore the gravitational attraction, of different elements. Similarly, such changes in binding energy would affect orbital motion, while (more directly) changes in interaction strengths would affect the spectra we observe in distant stars.
The observations are a mixture of very sensitive laboratory tests, which do not go very far back in time but are able to detect extremely small changes, and astronomical observations, which are somewhat less precise but which look back in time. (Remember that processes we observe in a star a million light years away are telling us about physics a million years ago.) While any single observation is subject to debate about methodology, the combined results of such a large number of independent tests are hard to argue with.
The overall result is that no one has found any evidence of changes in fundamental constants, to an accuracy of about one part in 10^11 per year.
Contamination may have occurred.
This is addressed in the most detail in the[*]Isochron Dating FAQ, for all of the methods discussed in the \"age of the earth\" part of this FAQ are isochron (or equivalent) methods, which have a check built in that detect most forms of contamination.
It is true that some dating methods (e.g., K-Ar and carbon-14) do not have a built-in check for contamination, and if there has been contamination these methods will produce a meaningless age. For this reason, the results of such dating methods are not treated with as much confidence.
Also, similarly to item (1) above, pleas to contamination do not address the fact that radiometric results are nearly always in agreement with old-earth expectations. If the methods were producing completely \"haywire\" results essentially at random, geologists would not use them.
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