Science and the Bible – Lesson 6
By Michael McCann
In our last lesson we looked at some examples from geology that showed detailed, complex, coherent, and discoverable evidence that the earth is far older than a few thousand years. I could have multiplied similar examples all day long if necessary. Should you want to examine the geologic record in more detail I suggest this book. If the Grand Canyon piques your interest there is a new book out that details evidence for the age of the canyon and how it could not have been formed in one flood event.
One thing I did not do in the last lesson was specify how old the earth might be or how we could know that. It’s such an important topic that I did not want to shoehorn it into the last lesson. Of course I’m talking about radiometric dating. The discussion today will be taken from Radiometric Dating – A Christian Perspective by Dr. Roger C. Wiens (). It was written for Christian laymen by a Christian.
Radiometric dating (often called radioactive dating) is a technique used to date materials such as rocks, usually based on a comparison between the observed abundance of a naturally occurring radioactive isotope and its decay products, using known decay rates.
All ordinary matter is made up of combinations of chemical elements, each with its own atomic number, indicating the number of protons in the atomic nucleus. Additionally, elements may exist in different isotopes, with each isotope of an element differing in the number of neutrons in the nucleus. A particular isotope of a particular element is called a nuclide. Some nuclides are inherently unstable. That is, at some point in time, an atom of such a nuclide will spontaneously transform into a different nuclide. This transformation may be accomplished in a number of different ways, including radioactive decay, either by emission of particles (usually electrons (beta decay), positrons or alpha particles) or by spontaneous fission, and electron capture.
Atoms of a radioactive nuclide decays exponentially at a rate described by a parameter known as the half-life, usually given in units of years when discussing dating techniques. After one half-life has elapsed, one half of the atoms of the nuclide in question will have decayed into a “daughter” nuclide or decay product.
The mathematical expression that relates radioactive decay to geologic time, is:
D = D0 + N(t) (eλt − 1)
t is age of the sample,
D is number of atoms of the daughter isotope in the sample,
D0 is number of atoms of the daughter isotope in the original composition,
N is number of atoms of the parent isotope in the sample at time t (the present), given by N(t) = N0e-λt, and
λ is the decay constant of the parent isotope, equal to the inverse of the radioactive half-life of the parent isotope times the natural logarithm of 2.
Now don’t worry if you don’t follow the math above. My reason for giving it is (1) to be precise and complete and (2) to show you that the math is in fact simple algebra. As long as a lab can analyze the amount of parent isotope and daughter isotope, the amount of original daughter isotope can be calculated and the age of the rock from when it cooled from the last time it was molten can be calculated. It really is just simple physics and math.
Let’s give an example from Wiens. Let’s say we are going to date a rock using the Rubidium-Strontium method. Rubidium-87 decays to strontium-87 with a half-life of 48.8 million years; so it is a good method to date older rocks.
From Figure 4 of Weins. A rubidium-strontium three-isotope plot. When a rock cools, all its minerals have the same ratio of strontium-87 to strontium-86, though they have varying amounts of rubidium. As the rock ages, the rubidium decreases by changing to strontium-87, as shown by the dotted arrows. Minerals with more rubidium gain more strontium-87, while those with less rubidium do not change as much.
Note that in this example at least 5 different minerals that compose the one rock are checked. The geologist doesn’t just “date” the rock. Notice that at any given time, the minerals all line up–a check to ensure that the system has not been disturbed. This is called an isochron. If the minerals don’t line up then something is wrong and the particular rock is NOT used to assign a date.
From Figure 5 of Weins. The original amount of the daughter strontium-87 can be precisely determined from the present-day composition by extending the line through the data points back to rubidium-87 = 0. This works because if there were no rubidium-87 in the sample, the strontium composition would not change. The slope of the line is used to determine the age of the sample. As Weins puts it:
As the rock starts to age, rubidium gets converted to strontium. The amount of strontium added to each mineral is proportional to the amount of rubidium present. This change is shown by the dashed arrows, the lengths of which are proportional to the rubidium/strontium ratio. The dashed arrows are slanted because the rubidium/strontium ratio is decreasing in proportion to the increase in strontium-87/strontium-86. The solid line drawn through the samples will thus progressively rotate from the horizontal to steeper and steeper slopes.
All lines drawn through the data points at any later time will intersect the horizontal line (constant strontium-87/strontium-86 ratio) at the same point in the lower left-hand corner. This point, where rubidium-87/strontium-86 = 0 tells the original strontium-87/strontium-86 ratio. From that we can determine the original daughter strontium-87 in each mineral, which is just what we need to know to determine the correct age.
There are now well over forty different radiometric dating techniques, each based on a different radioactive isotope. Most dating techniques involve multiple tests using different methods and on different minerals within a rock (isochrons).
For example some of the oldest rocks on earth are found in Western Greenland. Because of their great age, they have been especially well studied. The table below gives the ages, in billions of years, from twelve different studies using five different techniques on one particular rock formation in Western Greenland, the Amitsoq gneisses.
Note that scientists give their results with a stated uncertainty. They take into account all the possible errors and give a range within which they are 95% sure that the actual value lies. The top number, 3.60±0.05, refers to the range 3.60+0.05 to 3.60-0.05. The size of this range is every bit as important as the actual number. A number with a small uncertainty range is more accurate than a number with a larger range. For the numbers given above, one can see that all of the ranges overlap and agree between 3.55 and 3.74 billion years as the age of the rock. Several studies also showed that, because of the great ages of these rocks, they have been through several mild metamorphic heating events that disturbed the ages given by potassium-bearing minerals (not listed here). As pointed out earlier, different radiometric dating methods agree with each other most of the time, over many thousands of measurements.
All of the different dating methods agree–they agree a great majority of the time over millions of years of time. Some Christians make it sound like there is a lot of disagreement, but this is not the case. The disagreement in values needed to support the position of young-Earth proponents would require differences in age measured by orders of magnitude (e.g., factors of 10,000, 100,000, a million, or more). The differences actually found in the scientific literature are usually close to the margin of error, usually a few percent, not orders of magnitude! 3.55 to 3.74 billion is a 5% difference, but 3.5 to 0.000006 billion (6,000 years) is a 58,333,333% difference.
Vast amounts of data overwhelmingly favor an old Earth. Several hundred laboratories around the world are active in radiometric dating. Their results consistently agree with an old Earth. Over a thousand papers on radiometric dating were published in scientifically recognized journals in the last year, and hundreds of thousands of dates have been published in the last 50 years. Essentially all of these strongly favor an old Earth.
Radioactive decay rates have been measured for over sixty years now for many of the decay clocks without any observed changes. And it has been close to a hundred years since the uranium-238 decay rate was first determined. Both long-range and short-range dating methods have been successfully verified by dating lavas of historically known ages over a range of several thousand years.
And finally radiometric dating of certain Biblical archaeological sites confirm that the biblical history is true and accurate. For example:
Carbon-14 Dating of Copper Smelting in Edom (Jordan) Confirm Biblical Date of King Solomon’s Kingdom
The 14C dates associated with smelting debris layers from Khirbat en-Nahas demonstrate intensive 10th-9th century B.C. industrial metallurgical activities conducted by complex societies. High-precision radiocarbon dating at Khirbat en-Nahas establishes a date earlier than that suggested by previous studies utilizing pottery finds. The accuracy of 14C dating calls into question previous studies based solely upon pottery evidence. The current dating of the site to the 10th-9th century B.C. agrees with biblical dates for Solomon’s rule of the area.
1.Levy, T. E., T. Higham, C. B. Ramsey, N. G. Smith, E. Ben-Yosef, M. Robinson, S. Münger, K. Knabb, J. P. Schulze, M. Najjar, and L. Tauxe. 2008. High-precision radiocarbon dating and historical biblical archaeology in southern Jordan. Proc. Natl. Acad. Sci. USA 105:16460ï¿½16465.
2.Copper ruins in Jordan bolster biblical record of King Solomon, Los Angeles Times, October 28, 2008.
You can’t have it both ways, dear evangelical reader. The carbon-14 dating works when it confirms the Bible but doesn’t work when it says things are older than 6,000 years?
The earth appears to be old? The earth is old…
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Photo by KoolCats Photography at Flickr. Creative Commons License