Radiometric Dating
from The Evolution Evidence Page

Scientists have settled on the age of the earth of about 4.6 billion years as a result of research started almost 50 years ago. This conclusion was based upon carefully designed and conducted experiments that compared the ratios in rock samples of parent elements to daughter elements ( some of which would have been from radioactive decay of the parent, some of which may have been present in the sample at the time of formation). Since radioactive decay is known to occur at a constant rate, the age of a rock can be determined from the ratio of the parent element to the daughter element. The concerns about these dating methods were exactly the same that creationists continue to raise - presence of the daughter element at the time the rock was formed and possible loss / gain of either the parent or daughter element at some point in the history of the rock. For this reason, the tests were designed to account for those possibilities.

Initial daughter element can often be accounted for by either measuring the amount of an isotope of the daughter element (the ratio of isotopes are almost always constant). Another possibility is (as in the case of the potassium - argon - K-Ar method) that because the daughter element is gaseous, it would escape from the rock when the rock was molten. Once the rock cooled, the gaseous daughter would be trapped in the rocks crystal structure and could no longer escape. By experimentation, scientists have determined which rocks are suitable for various dating techniques. For K-Ar, for example, igneous rocks are good candidates for testing because they formed directly from molten magma and have a simple history. Metamorphic rocks do not work well because heating events in their history have allowed the escape of Argon (daughter element) and thus will indicate an age too young for the sample. Sedimentary rocks do not work because they are made up of a mixture of deposits of many other types of rocks, each of which would point to a different age. At any rate, scientists have devoted a great deal of effort to determining exactly which dating methods are appropriate for which types of rocks.

The other problem to avoid when dating rocks is the possibility that changes to the rock have caused loss or gain of either the parent or daughter element - this would lead to a false date (too old if parent element were lost, too young if daughter element were lost). I know of two methods that have been designed that can account for this possibility - isochron dating and the uranium-thorium-lead discordia / concordia method (actually three independent age calculations for one sample). Both of these methods have internal checks for the possible loss / gain of elements to the rock.

Creationists want the world to think that geologists just grab a rock and throw any old radiometric test at it and poof - there's the age of the rock. Reality is far more complex. If you examine the extensive research in the field of geochronology, you will see that one of the most important criteria in dating a sample lies in choosing an appropriate dating method for the sample. From G. Brent Dalrymple (see below):

One of the principal tasks of the geochronologist is to select the type of the material used for a dating analysis. A great deal of effort goes into the sample selection, and the choices are made before the analysis, not on the basis of the results. Mistakes are sometimes made but are usually caught by the various checks employed in the well-designed experiment.

The most compelling argument for an age of the earth of 4.5 billion years are the large number of independent tests that have been used to confirm this date. These tests have been performed on what are thought to be the earth's oldest surviving rocks, meteorites, and moon rocks. These tests have consistently given the same ages for each of these objects. Examples include:

The Earths Oldest Rock's

Description Technique Age (in billions of years)
Amitsoq gneisses (western Greenland) Rb-Sr isochron 3.70 +- 0.12
Amitsoq gneisses (western Greenland) 207Pb-206Pb isochron 3.80 +- 0.12
Amitsoq gneisses (western Greenland) (zircons) U-Pb discordia 3.65 +- 0.05
Amitsoq gneisses (western Greenland) (zircons)  Th-Pb discordia 3.65 +- 0.08
Amitsoq gneisses (western Greenland) (zircons) Lu-Hf isochron 3.55 +- 0.22
Sand River gneisses (South Africa) Rb-Sr isochron 3.79 +- 0.06

These are the oldest of the rocks dated on the earth so far (as of 1997). These are metamorphic rocks and thus have had some of their "history" lost - metamorphosis fully or partially resets the radiometric ages of rocks pointing to younger ages than the true age of the original rock. Older rocks may have been lost due to erosion or have not yet been discovered.

For many more examples of the consistancy of dating the same rocks with multiple methods, see Consistent Radiometric Dates by Joe Meert, a Geologist at the University of Florida. Dr. Meert's examples not only show that multiple radiometric methods come up with consistent dates for samples from the same locations, but that these results are also consistant with the paleomagnetic signature of the rocks, the position where the rocks would be expected to be (due to continental drift) at the time they were formed, and the cooling curves for the rocks. (Cooling curves deal with the fact that the different radiometric isotopes become "frozen" in the rocks at different temperatures. The higher the closure temperature for an isotope, the older the rock will be as dated by that isotope.) All of this consistancy rules out all of the arguments creationists attempt to make against radiometric dating techniques.
 

Oldest Lunar Rocks

Mission Technique Age (in billions of years)
Apollo 17 Rb-Sr isochron 4.55 +- 0.1
Apollo 17 Rb-Sr isochron 4.60 +- 0.1
Apollo 17 Rb-Sr isochron 4.49
Apollo 17 Rb-Sr isochron 4.43 +- 0.05
Apollo 17 Sm-Nd isochron 4.23 +- 0.05
Apollo 17 Sm-Nd isochron 4.34 +- 0.05
Apollo 16 40Ar/39Ar 4.47
Apollo 16 40Ar/39Ar 4.42

The following, although not among the oldest lunar rocks, show the consistency of ages found for the same sample (note the sample numbers) as determined by different dating techniques.
 
Description  Sample # Technique Age (in billions of years)
Apollo 14 - highlands A1 basalt 14053 Rb-Sr isochron 3.96
Apollo 14 - highlands A1 basalt 14053 40Ar/39Ar 3.95
Apollo 17 - highlands High-T1 basalt 75055 Rb-Sr isochron 3.83
Apollo 17 - highlands High-T1 basalt 75055 40Ar/39Ar 3.76
Luna 16   - highlands B-1 40Ar/39Ar 3.42
Luna 16   - highlands B-1 Rb-Sr isochron 3.45
Apollo 15 - highlands olivine basalt 15555 Rb-Sr isochron 3.32
Apollo 15 - highlands olivine basalt 15555 40Ar/39Ar 3.31
Apollo 12 - mare quartz basalt 12051 40Ar/39Ar 3.27
Apollo 12 - mare quartz basalt 12051 Rb-Sr isochron 3.26
Apollo 12 - mare quartz basalt 12051 40Ar/39Ar 3.24
Apollo 12 - mare quartz basalt 12051 Rb-Sr isochron 3.16

Finally, note this stunning example from a single moon rock sample (from Age of the Earth, Dalrymple, 1991 - a "must read" for anyone who wants the true story on radiometric dating):
 
Description Sample # Technique Age (in billions of years)
Apollo 11 - High-K basalt 10072 40Ar/39Ar whole rock 3.49 +- 0.05
Apollo 11 - High-K basalt 10072 40Ar/39Ar whole rock 3.52 +- 0.04
Apollo 11 - High-K basalt  10072 40Ar/39Ar plagioclase  3.57 +- 0.05
Apollo 11 - High-K basalt 10072 40Ar/39Ar plagioclase 3.56 +- 0.06
Apollo 11 - High-K basalt 10072 40Ar/39Ar ilmenite 3.58 +- 0.05
Apollo 11 - High-K basalt 10072 40Ar/39Ar pyroxene 3.55 +- 0.05
Apollo 11 - High-K basalt 10072 Rb-Sr isochron 3.57 +- 0.05
Apollo 11 - High-K basalt 10072 Sm-Nd isochron 3.57 +- 0.03

Don't forget, these ages were all obtained for the same sample!
 
 

The Oldest Meteorites

Description Technique Age (in billions of years)
Juvinas (achondrite) Mineral isochron  4.60 +- 0.07
Allende (carbonaceous chondrite) Mixed isochron 4.5 - 4.7
Colomera (silicon inclusion, iron met.) Mineral isochron 4.61 +- 0.04
Enstatite chondrites Whole-rock isochron 4.54 +- 0.13
Enstatite chondrites Mineral isochron 4.56 +- 0.15
Carbonaceous chondrites Whole-rock isochron 4.69 +- 0.14
Amphoterite chondrites Whole-rock isochron 4.56 +- 0.15
Bronzite chondrites Whole-rock isochron 4.69 +- 0.14
Hypersthene chondrites  Whole-rock isochron 4.48 +- 0.14
Krahenberg (amphoterite) Mineral isochron 4.70 +- 0.1
Norton County (achondrite)  Mineral isochron 4.7 +- .1

See Radiometric Dating A Christian Perspective by Roger C. Wiens for more examples of age of the earth and meteorite dates. The more I learn about radiometric dating, the more I appreciate the quality of this article. It really offers a complete, concise, and understandable explanation of radiometric dating for the non-scientist.
 

All of the above data are from G. Brent Dalrymple, Radiometric Dating, Geologic Time, and The Age of the Earth: A Reply to "Scientific" Creationism published by the U.S. Department of the Interior (Open-File Report 86-110), 1986. This is an excellent report by a real scientist rebutting the distortions that creation apologists have been putting out for 35 years. It documents several examples of Henry Morris (the "Father of Creation Science") and H.S. Slusher reaching invalid conclusions and quoting researchers out of context. I strongly recommend that anyone interested in the scientific vs. Genesis (fundamentalist interpretation) age of the earth controversy read this paper. It should be available in most university libraries in the U.S.
 

Creation "Science" Arguments Against Radiometric Dating

Now, what are the creationist arguments against radiometric dating? (See my previous post on radiometric dating for details on how these techniques work.) First, creationists have no data that determines the age of the earth to be 6 - 10 thousand years old. Instead of trying to develop tests to actually determine the earth's age (whatever it may be), creationists are only interested in attempting to tear down the well founded results of scientists (after 50 years of geochronological research alone) that the earth is on the order of 4.5 billion years old. What have creationists gotten for all of these years of work?

Well, the first popular technique of creationists is to try to find examples of bad radiometric dates and offer these up as proof that radiometric dating is totally unreliable. Henry Morris, in his 1974 book "Scientific Creationism" made the claim that K/Ar dates for lava from the 1801 Hulalalei volcano lava flow ranged from 160 million years to 3 billion years old. With results like these, radiometric dating must be totally useless - how could scientists possibly be so stupid as to believe that radiometric dating could be of any merit when a simple lava flow gives such a wide range of dates - right in their own research? As usual with such claims, we'll have to dig a little further to get at the truth. Morris cited a study by Funkhouser and Norton - "Radiogenic Helium and Argon in Ultramafic Inclusions from Hawaii" published in the Journal of Geophysics Research, vol. 73, pgs. 4601 - 4607. What Morris didn't bother to inform his readers was that the study concerned xenoliths - fragments of foreign rock broken of by the magmas intrusion through the crust. Since they are not completely melted by the magma, they are in fact much older than the lava flow (with K/Ar dating, what you are really dating is how long it has been since the rock solidified from the molten state - at this point the daughter Argon can no longer escape from the rock). That particular study confirmed (as geologists already suspected) that xenoliths could not be properly dated with the K/Ar method. The Funkhouser and Norton study clearly stated that the "ages" determined by K/Ar dating had no geological meaning and also determined that the xenoliths contained excess argon trapped in "air" bubbles inside the rock. Of course there was no mention of these details by Morris or any of the internet postings I have seen that still list this "problem" - despite the fact that Dalrymple answered it over 10 years ago. In the same paper, Dalrymple goes on to describe in detail several other quotes taken out of context by creationists. See Reference to a case where the given method did not work by Chris Stassen and Woodmorappe's Collection of Bad Dates by for more examples of this popular creationist technique.

The second creationist technique is to provide supposed evidences of a young age for the earth. These usually involve measuring the rate of change of some (any) environmental phenomenon, assuming that the rate has never changed, and calculating an age (always called an upper limit) for the earth. These ages vary from 100 years (accumulation of aluminum in the ocean) to 260 million years (accumulation of sodium in the ocean). Examples include changes in the earth's magnetic field, accumulation of helium in the atmosphere, and accumulation of metals in the ocean. The problem with all of these methods is that they are too complex - no one knows all of the sources or sinks (method of removal) for all of these currently - and certainly in the past. Most of these techniques have been examined in the scientific literature and found to be useless for determining an age of the earth.

Creationists make the claim that the earth's magnetic field has been steadily decreasing over time. They claim that this trend indicates the earth could not have supported life more than 20,000 years ago because of the huge strength of the the earth's magnetic field. Besides making the huge leap of faith that these trends (for which they have no explanation) continued at the same rate into the distant past, they ignore the fact that the decrease in the earth's dipole has been offset by an increase in the non dipole component of the earth's magnetic field - leading to a constant strength for the earth's magnetic field as measured over the past 50 years. Paleomagnetic measurements have also shown that the earth's dipole has fluctuated over the past 8000 years - not constantly decaying as creationists claim. See the Dalrymple paper mentioned in my previous post for more details. Also, see
Decay of the Earth's magnetic field.
Creation "Science" and Magnetic Fields
CREATIONISTS AND "MAGNETIC FIELD DECAY"
and Is the Earth's Magnetic Field Decaying Exponentially?
for more problems with this disproven "theory".

The next big "proof" for a young earth involves the accumulations of various metal ions (including salt - the sodium of sodium chloride is a metal ion when dissolved in water) dissolved in sea water. This claim ignores the evidence that these ions are not accumulating in sea water - they are at or near equilibrium in sea water. Although creationists want to talk about sources of these ions, they don't want to talk about ways these ions leave the ocean - precipitation, continental uplift. etc. See the following:
Problems with "Dating" the Earth from Metal Influx Rates in the Ocean.
and How Long Did it Take to Salt the Oceans?.

Another favorite claim of creationists is that the amount of Helium in the earth's atmosphere indicates a young age for the earth. Once again, they are quick to look at sources of Helium (radioactive decay) and ignore or underestimate ways in which Helium escapes the earth's atmosphere. See:

Accumulation of Helium in the atmosphere

Finally, creationists like to claim that there has been no success in dating historical volcanic flows. Well, here are a few examples for you - the following eruptions were given zero ages from K/Ar dating. Zero is exactly what you would expect from a radioisotope with a billion year half life. I would suggest that you use an isotope with a shorter half life - but, radioisotopes with a half life of 80 million years (or less) are extremely rare in rocks (unless they are products of a longer lived parent) - could this be because they all decayed to their daughter elements in the earth's 4.5 billion history? See Dave Matson's article on this topic for details.
 

Historical Volcanic Lavas Dated at Zero from K40/Ar40
 
Location Year of Eruption
Mt. Milhara, Japan 1951
Sakurajima, Japan 1946
Kilauea, Hawaii  1750
Kilauea, Hawaii 1955
Mauna Loa, Hawaii 1907
Mt. Etna, Sicily 252
Mt. Etna, Sicily 1329
Mt. Etna, Sicily 1444
Mt. Etna, Sicily 1536
Mt. Etna, Sicily 1669
Mt. Etna, Sicily 1886
Mt. Vesuvius, Italy 1944
Askja, Iceland ~1500 BCE
Lakagigar, Iceland 1783
Ngauruhoe, New Zealand 1954
Paracutin, Mexico 1944
Augustine, Alaska 1935
Cinder Cone, California 1851

Here are the 3 historic volcanic sites with excess Argon (as of 1969).
 
Location  Year of Eruption Apparent Age (millions of years)
Hualalei, Hawaii 1801 1.1
Sunset Crater, Arizona 1065 0.22
Mt. Etna, Sicely 1792 0.15

Note that all 3 of the above lavas contain xenolyths - foreign stones that aren't part of the lava. These xenolyths contribute excess Argon to the analysis leading to an older than expected date. Also note that while these dating errors are significant for dating very recent rocks, an error of 1 million years is insignificant in rocks older than say 50 to 100 million years.

All of the above was from Age of the Earth by G. Brent Dalrymple, pg. 132 - 134.

The simple fact is, Creationists have NO evidence that points to a 6 - 10 thousand year age for the earth! This belief is an article of faith for creationists and they have never found any significant scientific data to back it up. The best they can do is to try to find flaws in decades of mainstream research (by thousands of researchers) that indicate the effectiveness of radiometric dating and to try to come up with doubtful scenarios that put an "upper limit" on the age of the earth. As was shown above by multiple sources, these schemes do not work. To see just how bad the ceationist model is , see Creationist Geologic Time Scale: an attack strategy for the sciences by Don Wise.

Finally, I would like to raise the issue of motive in these questions of the age of the earth. Scientists have no reason to claim an age of the earth of 4.5 billion years except for the results of decades of research and thousands of tests by independent researchers indicating the effectiveness of radiometric dating and that 4.5 billion years is the correct age of the earth. They have no stake in this date - other than that the facts support it. As the geochronologist Dalrymple said, "I have no reason whatsoever to want the age of the earth to be any more or less than it happens to be. I would take great delight in proving that the earth is only 10,000 years old if it were possible to do so."

Creationists, on the other hand , have an extreme interest in proving the age of the earth to be 6 - 10 thousand years - it is required by their fundamentalist (literal) Biblical theology. To them, the earth must have been created in six literal days six thousand years ago. Any other possibility is a violation of their deeply held religious beliefs. In fact, all of the creation "science" organizations require members to adhere to clearly religious "creeds" that require them to accept the authority of the Bible in all matters. While this may be good religion, it is very, very bad science! Can we really trust such people to make a fair and unbiased attempt to determine the true age of the earth? We certainly haven't seen any evidence of it so far.

For more information, see Tim Thompson's exhaustive A Radiometric Dating Resource List .