Science in Christian Perspective

The Impact of Geological Dating Upon the Interpretation of Biblical Chronology
HERBERT FEELY
Columbia University

From: JASA 7 (September 1955): 46-49.

What impact has geologic dating upon the interpretation of Biblical Chronology? In the last few days many of you gentlemen, who should know much better than I, have indicated that it is slight; that aside from the lengthening of a few gaps in the chronologies, no alternations have been necessary. In so far as this is true it is indeed excellent. It is a sign of the accuracy of Evangelical theology. However a highly vocal, even if not large group, labelling themselves all things from theologists to geologists, have arisen to denounce geologic dating and geology itself as tools of the Devil which are seeking to destroy our faith in the Bible as the word of God. I haven't heard such a view expressed here so I won't waste your time trying to refute it.

Instead I would like to discuss briefly the theories and techniques of geologic dating. I do this not because the techniques are in themselves of especial importance to Christians, but because the results gotten from them seem to bother some. If this were not so I'm sure that this subject would not have been put on the program.

Let's consider first the reasons why the study of geology convinced men of the great antiquity of the Earth long before good quantitative methods of calculating its age were available. Then lets consider the nuclear physical data and geologic relationships upon which geochronology is based, and finally the degree of respect which theologians should give to geologic data when setting forth their interpretations of Biblical data.

Let's consider one example of this activity, a proc involving great magnitudes of space, mass, and ti Vast quantities of material are being eroded by t Mississippi, its tributaries, and parallel streams. T material is ultimately carried down to these rivers and deposited as sediment Coast. Measures of the rate of sedimentation off t shores of Texas, Louisiana, Mississippi, and the of the Gulf Coast states indicate average rates of much less than a few inches per year. Yet explora tion by oil companies has shown that the deposits a as much as 28,000 feet thick. The structures and tex tures within these rocks are the same in the deeply buried beds as in those at the surface indicating that all formed even as they are forming today. Great periods of time must have been required for their accumulation. The nature of sediments indicate the the floor of the basin has slowly and sunk while the sediments have been piled Yet the Gulf of Mexico is but one of many such basins which have existed at one time or another during the history of the Earth collecting millions of cubic moles of sediment.

Such rocks as these now seen forming along the Gulf Coast and in other sedimentary basins are found all around the earth. Some are still soft and uncompacted but most are consolidated into hard rock though still maintaining the same structures and textures as the soft sediments. Some are partially or largely recrystallized with loss of many of their original textures and some are completely recrystallized so that all, or almost all, primary structures are obliterated and their original sedimentary character is more inferred than observed.

Detailed studies of such masses of sedimentary rocks have shown parallel sequences in neighboring localities both in lithology, that is in the chemical composition and phyisical nature of the rock, and in their fossil content. These have permitted age correlations of beds in sections measured at different localities. Lithologic correlations are good only over short distances since they depend strongly on uniformity of environmental conditions during their deposition. However they do serve to substantiate the validity of parallel correlations made by use of certain fossils, or better yet, certain groups of fossils. Certainly not all age correlations made on these bases have proven to be correct, but experience shows stratigraphers which criteria are valid and which are not. Those which don't work are eliminated from stratigraphic methods. And let it not be thought that the validity of stratigraphic correlation by means of fossils in any way substantiates the theory of evolution any more than that of progressive creation or perhaps even modification of first creation. As with all scientific methods stratigraphic correlations and sequences cannot be applied without good judgment and adequate testing by the researchers.

Interspersed among the sedimentary rocks are igneous rocks which have formed by cooling of lava or magma, that is, fluid rock melts. Some have formed at the surface of the earth by extrusion from volcanoes or fissures, while others such as the feeders to volcanoes cooled and solidified below the surface.

The interrelations among the sediments and igneous rock units cutting across them or interbedded with them have been of great usefulness in working out the history of the rock sequences at many localities.

Deeper within the crust of the earth where higher temperature and pressure prevail, recrystallization of both sediments and igneous rocks has taken place. These processes are only relatively recently beginning to be understood in any detail.

Careful study of the rocks at many localities around the earth, using all methods of investigation available, has permitted the construction of a geologic history of the earth in general and of more detailed histories for multitudes of particular regions.

Our understanding of geologic history, to be sure, contains many gaps and uncertainties. Then, too future discoveries will doubtless necessitate many alterations and amendments. However the tremendous weight of factual evidence already available, and now serving as the foundation upon which the history has been built assures us that by and large the present history must be correct.

Studies of earth history show that tremendously long time periods must have been required for all that has taken place. An example is found in the petrified forests of Yellowstone Park where successive fossil forests are found, one above the other, where each has grown, been wiped out by volcanic ash, where the ash has weathered into soil at its upper surface, where another forest has grown only to be wiped out by another ash fall, and so on through several cycles. Yet these beds involved represent but a tiny fraction of the geologic column.

Thus a study of geology gives us the order of magnitude of earth history but doesn't give us good quantitative measurements. For such measurements geologists have turned to determinations of the radioactivity of rocks and minerals.

The phenomenon of radioactivity has been known and studied f or almost 60 years but it has only been efficiently applied to geologic dating rather recently.

Radioactive decay results from an attempt by energetically unstable nuclei to reach a condition of greater stability. This is usually achieved by ejection from the nucleus of particles of matter or quanta of energy.

Much careful study by physicists has proven that the total activity of a radioactive isotope is directly proportional to the amount present. This decay can be expressed mathematically by a relatively simple differential equation containing one constant. The value of this constant, which may be expressed as the "half life" is different for each radioactive isotope and depends upon the particular energetics involved in its decay. The half life is the time required for half of any original amount of the species to decay. Thus Carbon-14 has a half life of 5580 years. That is, if I have one pound of C-14 today, in 5580 years only 1/2 pound will be left. The other half pound will have decayed to Nitrogen-14. Half lives range from those which are extremely long, such as the 62 billion years of Rubidium-87, to those which are extremely short, as the three ten millionths of a second of Polonium-212.

Typical decay schemes are shown by two radioactive isotopes important in geologic dating: Uranium238 and Rubidium-87. Uranium-238, the most abundant isotope of uranium, has a half life of 4.5 billion years. It decays by the emission of an alpha particle to Thorium-234. This is itself unstable and has a half life of 24.5 days, decaying to Protactinium-234. The protactinium decays to Uranium-234, this is to Thorium-230, and so on until finally lead-206 is formed. This isotope is stable and will increase in amount as the Uranium-238 and the 16 intermediate products forming from it decay.

To measure the age of a rock or mineral one generally needs to know only the amount of a radioactive species in it, the amount of the daughter product which has been formed by its decay, and the rate of this decay.

The amount of the isotopes present may sometimes he measured by standard methods of chemical analysis but frequently mass spectrometers are used for determinations of isotope ratios of an element, and Geiger counters, scintillation counters, and other instruments for measuring radioactivity are used to determine the amount of a radioactive species present.

Ideally only minerals which contain none of the (laughter product as a primary constituent should be dated. Daughter product initially present will make the mineral seem older than it really is. Thus Rubidiuin-87 ages are measured only for lepidolite and biotite micas which have negligible content of the strontium daughter. Potassitim-40-Argon-40 ages and Uranium-Heliurn ages can be run on any mineral or rock since the argon and helium are gasses and are not included as primary constituents in rocks in measurable quantities. Similarly the best Uranium-Lead ages are gotten from minerals low in initial lead content.

Sometimes however mass spectrometer analysis permit-, corrections for initial daughter content and permits ages to be determined front minerals such as uranium ore containing primary lead. The isotopic composition of the primary lead may be gotten front associated lead minerals and this may be subtracted from the lead content of the uranium mineral. Ages from such minerals, however, seldom give as good precision as ages from lead-free minerals.

Another problem for the geologist measuring ages is the possibility of loss of daughter product during the history of the rock, giving ages which are too low. Thus argon formed from potassium decay and helium formed from uranium decay may leak out of a rock. Similarly either parent or daughter species may be leached out of the rock late in its history giving a spurious parent-daughter ratio, and hence an incorrect age. This is an especially important problem in leaduranium age work.

These and similar problems must be overcome by the geochronologists. Their net result is to reduce the number of possible age methods which can be relied upon. Thus the Uranium-Helium method is practically defunct. The most recent trend in geochronology has been the measurement of single rock bodies by several methods to gain intercalibration. Results have been excellent, showing which methods are dependable and which have flaws, and what the nature of the flaws are.

Another age method, probably of. greater pop interest than those thus far discussed, uses carbon-14 This isotope has a relatively short half-life compared to the age of the earth, and is, therefore, not useful in measurements on rocks, or, in fact, on any mater ial more than about 30,000 years old. However it is im portant in the dating of many archeological material and very recent sediments. Because of its short half life no carbon-14 formed at the creation of the uni verse, which apparently took place about 5 billion yea ago, could now be detected. However cosmic radiation. bombardment of nitrogen in upper atmosphere produces carbon-14 as one if its products. This radio active carbon is chemically the same as and so is taken into the carbon cycle of the atmosphere, hydrosphere, and biosphere. All carbon in this cycle, such as carbon dioxide in the air, carbonate in surface sea water, and carbon in the organic pounds of plants and animals, has, within a per cent or so, the same content of this isotope.

When something is removed from the carbon cycle such as when a plant or animal dies and is buried or when carbonate precipitates from sea water to form sediments, it ceases to receive carbon-14 into itself and that which it contains slowly disintegrates at the half-life of 5580 years. Thus the carbon-14 content of the carbon of an ancient tree-trunk or sea shell is a direct function of its age, and by measuring this content we may determine its age.

As with the other dating methods, so this one con tains many sources of possible error. Contamination is a constant worry and a blunder in interpretation by a geologist or archaeologist can't be overcome by even the most expensive geiger counter. But these sources of error are known and constant care taken to eliminate them.

As we all know, geologic age determinations and even geology itself have been attacked by many of the hyperorthodox as invalid because they fail to uphold one or another interpretation of scripture. Numerous arguments have been brought forward to implement this attack. It is worthwhile, therefore, to discuss some of the possible errors in geochronology. First, how

However, as with all scientific measurements, those of geochronology are subject to statistical errors and to errors of interpretation. The half-lives of carbon14, uranium-233, and all other radioactive isotopes used in age determination must be measured. These are determined by nuclear physicists using the best techniques available, but the job is a difficult one and accuracies better than a per cent or two are hard to achieve, Similarly measurements of the radioactivity of a sample by standard counting techniques or of isotopic ratios by mass spectrometry may be accurate only to within a per cent or so of the true value. Similar statistical errors are unavoidable in other steps of the determinations. However these could only change a calculated age by a few per cent at most, and an age of 500 million years is scarcely different than one of 515 million.

Another stimulus for the control or elimination of errors is the spirit of competition among the various laboratories engaged in geological age determinations. Each laboratory is only too eager to discover and pounce upon mistakes made by another lab. When something is agreed upon by all you can be quite sure that the evidence is very strong.

It may be safely stated then that the methods of geochronology have been well tested and have been found to work. Certainly they aren't perfect or inerrant but then no one claims that they are. They are a useful tool in the deciphering the history of the earth, and that's all that they are meant to be.

A few sages have chosen to argue against geochronology on the philosophical rather than the physical level, opining that extrapolation of present day physical laws back into the past is invalid, or that God created the earth with a history built into it. Certainly the first view leads logically to the second because of the orderliness of the geologic history discovered when backward extrapolation of today's physical laws is made.

Since this is primarily a philosophical question I won't attempt to discuss it here, but I do think that we must decide whether we believe that God, in letting us study His Creation, is trying to mislead us into unbelief or is showing us the grandeur of His Works, I think, that as Christians, we may only believe that He is showing us His Greatness and Majesty to increase our reverence and love for Him, "Speak to the earth, and it shall teach thee. Who knoweth not in all these, that the hand in whose hand is the the breath of all mankind."

How much then should the theologian be influenced by geochronological data relevant to his interpretations of scripture? I believe he should give it weight equal to that given to data from his own field of investigation. The exact amount of weight given, of course, depending on the precision of the measurements made and on the validity of the interpretations made from the measurements. Such judgments are not always easy to make of course, but careful study of reports almost always gives adequate information.