Changing Views of the History of the Earth
One of these techniques is called the lead-lead technique because it determines the ages from the lead isotopes alone. This was a minimum acceptable age consistent with geology. Radiocarbon dating of these finding indicate very active life in at least semiarid conditions within the past 11, years - a far cry from 25 million years. Yet, some Christians question whether we can believe something so far back in the past. The chemical properties of rubidium and strontium are so dissimilar that minerals which readily accept rubidium into their crystal structure tend to exclude strontium and vice versa.
It seems that the whole rock analysis method and the resulting optimization problem may underestimate the significance of other production pathways, i. It was first used in , about a century ago. This involves uranium isotopes with an atomic mass of Radiometric dating is based on index fossils whose dates were assigned long before radioactivity was discovered. Supporting this view is the presence of thin bands of lignite near the top of the section, with a cm coal layer just underlying the capping basalt. The very fact that these flows cover such great distances indicate that the individual flows traveled at a high rate of speed in order to avoid solidification before they covered such huge areas as they did. In some cases a batch of the pure parent material is weighed and then set aside for a long time and then the resulting daughter material is weighed.
It is clear that mixing of pre-existent materials will yield a linear array of isotopic ratios. We need not assume that the isotopes, assumed to be daughter isotopes, were in fact produced in the rock by radioactive decay. Thus the assumption of immense ages has not been proven. The straight lines, which seem to make radiometric dating meaningful, are easily assumed to be the result of simple mixing.
This preliminary study of the recent evolutionary literature would suggest that there are many published Rb-Sr isochrons with allegedly measured ages of hundreds of millions of years which easily meet the criteria for mixing, and are therefore more cogently indicative of recent origin.
Kramer and others 78 and Arndts and Overn 8 have come to an incorrect conclusion because they have ignored several important facts about the geochemistry of Rb-Sr systems and the systematics of isochrons.
First, the chemical properties of rubidium and strontium are quite different, and thus their behavior in minerals is dissimilar. Both are trace elements and rarely form minerals of their own. It is chemically similar to potassium and tends to substitute for that element in minerals in which potassium is a major constituent, such as potassium feldspar and the micas muscovite and biotite. It commonly substitutes for calcium in calcium minerals, such as the plagioclase feldspars.
The chemical properties of rubidium and strontium are so dissimilar that minerals which readily accept rubidium into their crystal structure tend to exclude strontium and vice versa. Thus, rubidium and strontium in minerals tend to be inversely correlated; minerals high in rubidium are generally low in strontium and vice versa.
This relation, however, is a natural consequence of the chemical behavior of rubidium and strontium in minerals and of the decay of 87 Rb to 87 Sr over time, and has nothing to do with mixing.
Second, mixing is a mechanical process that is physically possible only in those rock systems where two or more components with different chemical and isotopic compositions are available for mixing. Examples include the mingling of waters from two streams, the mixing of sediment from two different source rocks, and the contamination of lava from the mantle by interactions with the crustal rocks through which it travels to the surface.
Mixing in such systems has been found 49 , 70 , but the Rb-Sr method is rarely used on these systems. The Rb-Sr isochron method is most commonly used on igneous rocks, which form by cooling from a liquid. Mineral composition and the sequence of mineral formation are governed by chemical laws and do not involve mixing.
In addition, a rock melt does not contain isotopic end members that can be mechanically mixed in different proportions into the various minerals as they form, nor could such end members be preserved if they were injected into a melt. Fourth, if isochrons were the result of mixing, approximately half of them should have negative slopes. In fact, negative slopes are exceedingly rare and are confined to those types of systems, mentioned above, in which mechanical mixing is possible and evident.
An example is the meteorite Juvinas Figure 3. Thus, even using the criteria developed by Arndts and Overn 8 and Kramer and others 78 , the 4. Therefore, arguments advance by Arndts and Overn 8 and by Kramer and others 78 are based on premises that are geochemically and logically unsound, and their conclusion that isochrons are due to mixing rather than to decay of 87 Rb over geologic time is incorrect.
The radioactivity of carbon is very weak and even with all its dubious assumptions the method is not applicable to samples that supposedly go back 10, to 15, years. In those intervals of time the radioactivity from the carbon would become so weak that it could not be measured with the best of instruments. Claims have been made that dating can be done back to from 40 to 70 thousand years, but it seems highly improbable that instruments could measure activity of the small amounts of C 14 that would be present in a sample more than 15, years old.
This statement was as untrue when it was first written in , ed. Modern counting instruments, available for more than two decades, are capable of counting the 14 C activity in a sample as old as 35, years in an ordinary laboratory, and as old as 50, years in laboratories constructed with special shielding against cosmic radiation.
New techniques using accelerators and highly sensitive mass spectrometers, now in the experimental stage, have pushed these limits back to 70, or 80, years and may extend them beyond , years in the near future. Before discussing some of their claims, it is worth discussing briefly the types of radioactive decay and the evidence that decay is constant over the range of conditions undergone by the rocks available to scientists. Most radioactive decay involves the ejection of one or more sub-atomic particles from the nucleus.
Alpha decay occurs when an alpha particle a helium nucleus , consisting of two protons and two neutrons, is ejected from the nucleus of the parent isotope.
Beta decay involves the ejection of a beta particle an electron from the nucleus. Gamma rays very small bundles of energy are the device by which an atom rids itself of excess energy. Because these types of radioactive decay occur spontaneously in the nucleus of an atom, the decay rates are essentially unaffected by physical or chemical conditions. The reasons for this are that nuclear forces act over distances much smaller than the distances between nuclei, and that the amounts of energy involved in nuclear transformations are much greater than those involved in normal chemical reactions or normal physical conditions.
This combination of the strength of nuclear binding and the insulation of the nucleus is the reason why scientists must use powerful accelerators or atomic reactors to penetrate and induce changes in the nuclei of atoms.
A great many experiments have been done in attempts to change radioactive decay rates, but these experiments have invariably failed to produce any significant changes. Measurements of decay rates under differing gravitational and magnetic fields also have yielded negative results.
Although changes in alpha and beta decay rates are theoretically possible, theory also predicts that such changes would be very small 42 and thus would not affect dating methods. Under certain environmental conditions, the decay characteristics of 14 C, 60 Co, and Ce, all of which decay by beta emission, do deviate slightly from the ideal random distribution predicted by current theory 5 , 6 , but changes in the decay constants have not been detected.
There is a fourth type of decay that can be affected by physical and chemical conditions, though only very slightly. This type of decay is electron capture e. Because this type of decay involves a particle outside the nucleus, the decay rate may be affected by variations in the electron density near the nucleus of the atom. For example, the decay constant of 7 Be in different beryllium chemical compounds varies by as much as 0.
The only isotope of geologic interest that undergoes e. Measurements of the decay rate of 40 K in different substances under various conditions indicate that variations in the chemical and physical environment have no detectable effect on its e. Another type of decay for which small changes in rate have been observed is internal conversion IC. Slusher , p. For example, in the first edition of his monograph on radiometric dating, Slusher claims that the decay rate of 57 Fe has been changed by as much as 3 percent by electric fields; however this is an IC decay, and 57 Fe remains Fe.
Note, however, that even a 3 percent change in the decay constants of our radiometric clocks would still leave us with the inescapable conclusion that the Earth is more than 4 billion years old. These changes are irrelevant to radiometric dating methods. Morris 92 claims that free neutrons might change decay rates, but his arguments show that he does not understand either neutron reactions or radioactive decay. Neutron reactions do not change decay rates but, instead, transmute one nuclide into another.
The result of the reaction depends on the properties of the target isotope and on the energy of the penetrating neutron. There are no neutron reactions that produce the same result as either beta or alpha decay. An n,p neutron in, proton out reaction produces the same change in the nucleus of an atom as e. If enough free neutrons did exist, they would produce other measurable nuclear transformations in common elements that would clearly indicate the occurrence of such a process.
Morris 92 also suggests that neutrinos might change decay rates, citing a column by Jueneman 72 in Industrial Research.
Jueneman, however, does not propose that decay rates would be changed, nor does he state how the clocks would be reset; in addition, there is no evidence to support his speculation. Neutrinos are particles that are emitted during beta decay. They have no charge and very small or possibly no rest mass.
Because they have no charge and little or no mass, neutrinos do not interact much with matter — most pass unimpeded right through the Earth — and they can be detected experimentally only with great difficulty. The chance that neutrinos could have any effect on decay rates or produce nuclear transmutations in sufficient amounts to have any significant effect on our radiometric clocks is exceedingly small. Dudley himself rejects the conclusions drawn from his hypothesis by Slusher and Rybka , noting that the observed changes in decay rates are insufficient to change the age of the Earth by more than a few percent Dudley, personal communication, , quoted in 20 , p.
Thus, even if Slusher and Rybka were correct — which they are not — the measured age of the Earth would still exceed 4 billion years. Slusher , and Rybka also claim that the evidence from pleochroic halos 6 indicates that decay rates have not been constant over time:. In a review of the subject, however, Gentry 52 concludes that the data from pleochroic halo studies are inconclusive on this point — the uncertainties in the measurements and other factors are too great.
Rybka claims that experimental evidence suggests that decay rates have changed over time:. Two cases where it appears that the half life is increasing with time are as follows.
Glasstone has the half life for Protactinium as 3. For the half life of Radium , Glasstone has He has failed to consider all of the data. The various values for the half lives of Ra and Pa reported in the literature since are given in Table 3. It is clear that there is no increase in the values as a function of time. The differences in the reported half lives are a consequence of improved methods and instruments, and the care with which the individual measurements were made.
For example, Kirby and others 74 argue convincingly that the measurements of the half life of Ra reported from to Table 3 suffered from inadequate experimental methods and are not definitive. Kirby and his colleagues carefully measured this half life by two different methods and obtained values of The weighted mean of these two measurements is I should also mention that the two references cited by Rybka are textbooks, not the publications in which the original data were reported; the dates of publication of these texts, therefore, do not reflect the years in which the measurements were made or reported.
Rybka also explores the consequences of a hypothetical change over time of the decay constant, but his results are due solely to his arbitrary changes in the decay formula — changes for which there is neither a theoretical basis nor a shred of physical evidence.
There have been no changes observed in the decay constants of those isotopes used for dating, and the changes induced in the decay rates of other radioactive isotopes are negligible. These observations are consistent with theory, which predicts that such changes should be very small.
Any inaccuracies in radiometric dating due to changes in decay rates can amount to, at most, a few percent. Several creationist authors have criticized the reliability of radiometric dating by claiming that some of the decay constants, particularly those for 40 K, are not well known 28 , 29 , 92 , Since the decay rate is also unsettled, values of these constants are chosen which bring potassium dates into as close correlation with uranium dates as possible.
There seems to be some difficulty in determining the decay constants for the K 40 -Ar 40 system. Geochronologists use the branching ratio as a semi-empirical, adjustable constant which they manipulate instead of using an accurate half-life for K These statements would have been true in the s and early s, when the K-Ar method was first being tested, but they were not true when Morris 92 and Slusher wrote them.
By the mid- to late s the decay constants and branching ratio of 40 K were known to within a few percent from direct laboratory counting experiments 2.
Today, all the constants for the isotopes used in radiometric dating are known to better than 1 percent. Morris 92 and Slusher have selected obsolete information out of old literature and tried to represent it as the current state of knowledge.
In spite of the claims by Cook 28 , 29 , Morris 92 , Slusher , , DeYoung 37 and Rybka , neither decay rates nor abundance constants are a significant source of error in any of the principal radiometric dating methods. The reader can easily satisfy himself on this point by reading the report by Steiger and Jaeger and the references cited therein.
Statements similar to this one by Slusher are also made by Morris These statements spring from an argument developed by Cook 28 that involves the use of incorrect assumptions and nonexistent data. In the late s, Nier published Pb isotopic analyses on 21 samples of uranium ore from 14 localities in Africa, Europe, India, and North America and calculated simple U-Pb ages for these samples. Some of these data were later compiled in the book by Faul 46 that Cook 28 cites as the source of his data.
Table 4 lists the data for one typical sample. Cook notes the apparent absence of thorium and Pb, and the presence of Pb. He reasons that the Pb could not have come from the decay of Th because thorium is absent, and could not be common lead because Pb, which is present in all common lead, is absent. He reasons that the Pb in these samples could only have originated by neutron reactions with Pb and that Pb, therefore, would also be created from Pb by similar reactions:.
Cook 28 then proposes that these effects require corrections to the measured lead isotopic ratios as follows: He presents an equation for making these corrections:. This calculation is repeated by both Morris 92 and Slusher Cook 28 , Morris 92 , and Slusher all note that this ratio is close to the present day production ratio of Pb and Pb from U and U, respectively, and conclude, therefore, that the Katanga ores are very young, not old.
For example, Slusher states:. First, Pb is not absent in the Katanga samples; it simply was not measured! In his report, Nier states:. Actually, in 20 of the 21 samples investigated the amount of common lead is so small that one need not take account of the variations in its composition.
In a number of samples where the abundance of Pb was very low no attempt was made to measure the amount of it as the determination would be of no particular value. Second, the neutron-capture cross sections for Pb and Pb are not equal, as Cook 28 assumes, but differ by a factor of 24 0. The correct radiometric age is, of course, the scientific value of million years.
This fact is readily acknowledged by Cook:. In spite of evidence that the neutron flux is only a millionth as large as it should be to account for appreciable n, effects, there are several well documented examples that seem to demonstrate the reality of this scheme.
See more information about "Strata" Smith and his original geologic map of England. Click on the map to see a larger version. The Principles of Dendochronology. Dendrochronology -- Tree Rings: Tree-Ring dating is based on the principle that the growth rings on certain species of trees reflect variations in seasonal and annual rainfall.
Trees from the same species, growing in the same area or environment will be exposed to the same conditions, and hence their growth rings will match at the point where their lifecycles overlap. Earth's oldest living inhabitant "Methuselah" at 4, years, has lived more than a millennium longer than any other tree. See Oldest Living Organism. The Sheffield Laboratory now has a continuous master sequence for England going back to about BC.
This is made up of numerous regional tree-ring chronologies, particularly in the medieval and post-medieval periods, for which the laboratory now has more than reference chronologies from many areas. The Ultimate Tree-ring Pages: This really must be the ultimate web resource for this topic. You will find information about tree-rings, current research, and examples of practical applications of this science.
There are over radiocarbon dating laboratories around the world producing radiocarbon assays for the scientific community. The Carbon14 technique has been and continues to be applied and used in many, many different fields including hydrology, atmospheric science, oceanography, geology, palaeoclimatology, archaeology and biomedicine.
An excellent series of short movies take students through a course of explanation and demonstration of C14 methods. Oxford Radiocarbon Accelerator Unit. About research in radiocarbon methodology. Includes many protocols for adjusting results to account for fluctuations in atmospheric C For learning more about radiocarbon methods, laboratories and databases.
An excellent article about the process and its limitations, written without scientific jargon. Radiometric Dating -- A Christian Perspective: Wiens, Los Alamos National Laboratory. Also discusses other dating methodologies. This article should be a "must read" for any person interested in factualy accurate information on dating methods. Radiometric Dating Film Clips: By comparing the proportion of K to Ar in a sample of volcanic rock, and knowing the decay rate of K, the date that the rock formed can be determined.
A series of movie clips walks you through the process. Gives the simple principles of how the process works. More on the basics from the United States Geological Service. Discussed six isoptopes commonly used to date very ancient rocks. Reliability of Radiometric Dating. Similar to this webpage, it presents many links to articles about radiometric dating and the age of the earth, some of which I do not list here for want of space.
Isochron methods avoid the problems which can potentially result during radiometric testing. These are very nice pages from www. Age of the Earth: The most compelling argument for an age of the earth of 4. 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 of a number of consistent dates derived from different methods are given. A short but clear explanation about radioactive isotopes commonly used for determining ages of rocks with graphics and putting numbers on the geologic time scale, extending it back before the occurance of abundant index fossils. This is a relatively new method intended to to improve the precision of uranium and thorium istopy methods.
It excludes contamination and weathering of travertines and makes possible more precise dating of thin deposits of secondary carbonates. No web-based resource for this method is available. A team of University of Massachusetts geologists is exploring a new way to determine the ages of ancient rocks, and refining our understanding of the timing and rates of the geologic events that have shaped the planet.
The new method offers greater efficiency, and access to a much more detailed geologic record than current dating methods. Obsidian hydration dating is based on the fact that a fresh surface is created on a piece of obsidian in the tool manufacturing, or flintknapping, process. Obsidian contains about 0. When a piece of obsidian is fractured, atmospheric water is attracted to the surface and begins to diffuse into the glass.
This results in the formation of a water rich hydration rind that increases in depth with time. The hydration process continues until the fresh obsidian surface contains about 3. This is the saturation point. Each element can have a number of different isotopes, that is,. A portion of the chart of the nuclides showing isotopes of argon and potassium, and some of the isotopes of chlorine and calcium. Isotopes shown in dark green are found in rocks.
Isotopes shown in light green have short half-lives, and thus are no longer found in rocks. Short-lived isotopes can be made for nearly every element in the periodic table, but unless replenished by cosmic rays or other radioactive isotopes, they no longer exist in nature.
So each element occupies a single row, while different isotopes of that element lie in different columns. For potassium found in nature, the total neutrons plus protons can add up to 39, 40, or Potassium and are stable, but potassium is unstable, giving us the dating methods discussed above.
Besides the stable potassium isotopes and potassium, it is possible to produce a number of other potassium isotopes, but, as shown by the half-lives of these isotopes off to the side, they decay away.
Now, if we look at which radioisotopes still exist and which do not, we find a very interesting fact. Nearly all isotopes with half-lives shorter than half a billion years are no longer in existence. For example, although most rocks contain significant amounts of Calcium, the isotope Calcium half-life , years does not exist just as potassium, , , etc. Just about the only radioisotopes found naturally are those with very long half-lives of close to a billion years or longer, as illustrated in the time line in Fig.
The only isotopes present with shorter half-lives are those that have a source constantly replenishing them. Chlorine shown in Fig.
In a number of cases there is. Some of these isotopes and their half-lives are given in Table II. This is conclusive evidence that the solar system was created longer ago than the span of these half lives! On the other hand, the existence in nature of parent isotopes with half lives around a billion years and longer is strong evidence that the Earth was created not longer ago than several billion years.
The Earth is old enough that radioactive isotopes with half-lives less than half a billion years decayed away, but not so old that radioactive isotopes with longer half-lives are gone.
This is just like finding hourglasses measuring a long time interval still going, while hourglasses measuring shorter intervals have run out. Years Plutonium 82 million Iodine 16 million Palladium 6. Unlike the radioactive isotopes discussed above, these isotopes are constantly being replenished in small amounts in one of two ways. The bottom two entries, uranium and thorium, are replenished as the long-lived uranium atoms decay.
These will be discussed in the next section. The other three, Carbon, beryllium, and chlorine are produced by cosmic rays--high energy particles and photons in space--as they hit the Earth's upper atmosphere.
Very small amounts of each of these isotopes are present in the air we breathe and the water we drink. As a result, living things, both plants and animals, ingest very small amounts of carbon, and lake and sea sediments take up small amounts of beryllium and chlorine The cosmogenic dating clocks work somewhat differently than the others.
Carbon in particular is used to date material such as bones, wood, cloth, paper, and other dead tissue from either plants or animals. To a rough approximation, the ratio of carbon to the stable isotopes, carbon and carbon, is relatively constant in the atmosphere and living organisms, and has been well calibrated.
Once a living thing dies, it no longer takes in carbon from food or air, and the amount of carbon starts to drop with time. Since the half-life of carbon is less than 6, years, it can only be used for dating material less than about 45, years old.
Dinosaur bones do not have carbon unless contaminated , as the dinosaurs became extinct over 60 million years ago. But some other animals that are now extinct, such as North American mammoths, can be dated by carbon Also, some materials from prehistoric times, as well as Biblical events, can be dated by carbon The carbon dates have been carefully cross-checked with non-radiometric age indicators.
For example growth rings in trees, if counted carefully, are a reliable way to determine the age of a tree. Each growth ring only collects carbon from the air and nutrients during the year it is made. To calibrate carbon, one can analyze carbon from the center several rings of a tree, and then count the rings inward from the living portion to determine the actual age. This has been done for the "Methuselah of trees", the bristlecone pine trees, which grow very slowly and live up to 6, years.
Scientists have extended this calibration even further. These trees grow in a very dry region near the California-Nevada border. Dead trees in this dry climate take many thousands of years to decay. Growth ring patterns based on wet and dry years can be correlated between living and long dead trees, extending the continuous ring count back to 11, years ago. An effort is presently underway to bridge the gaps so as to have a reliable, continuous record significantly farther back in time.
The study of tree rings and the ages they give is called "dendrochronology". Calibration of carbon back to almost 50, years ago has been done in several ways. One way is to find yearly layers that are produced over longer periods of time than tree rings. In some lakes or bays where underwater sedimentation occurs at a relatively rapid rate, the sediments have seasonal patterns, so each year produces a distinct layer.
Such sediment layers are called "varves", and are described in more detail below. Varve layers can be counted just like tree rings. If layers contain dead plant material, they can be used to calibrate the carbon ages. Another way to calibrate carbon farther back in time is to find recently-formed carbonate deposits and cross-calibrate the carbon in them with another short-lived radioactive isotope.
Where do we find recently-formed carbonate deposits? If you have ever taken a tour of a cave and seen water dripping from stalactites on the ceiling to stalagmites on the floor of the cave, you have seen carbonate deposits being formed. Since most cave formations have formed relatively recently, formations such as stalactites and stalagmites have been quite useful in cross-calibrating the carbon record.
What does one find in the calibration of carbon against actual ages? If one predicts a carbon age assuming that the ratio of carbon to carbon in the air has stayed constant, there is a slight error because this ratio has changed slightly.
Figure 9 shows that the carbon fraction in the air has decreased over the last 40, years by about a factor of two. This is attributed to a strengthening of the Earth's magnetic field during this time. A stronger magnetic field shields the upper atmosphere better from charged cosmic rays, resulting in less carbon production now than in the past.
Changes in the Earth's magnetic field are well documented. Complete reversals of the north and south magnetic poles have occurred many times over geologic history. A small amount of data beyond 40, years not shown in Fig.
What change does this have on uncalibrated carbon ages? The bottom panel of Figure 9 shows the amount. Ratio of atmospheric carbon to carbon, relative to the present-day value top panel. Tree-ring data are from Stuiver et al. The offset is generally less than years over the last 10, years, but grows to about 6, years at 40, years before present.
Uncalibrated radiocarbon ages underestimate the actual ages. Note that a factor of two difference in the atmospheric carbon ratio, as shown in the top panel of Figure 9, does not translate to a factor of two offset in the age.
Rather, the offset is equal to one half-life, or 5, years for carbon The initial portion of the calibration curve in Figure 9 has been widely available and well accepted for some time, so reported radiocarbon dates for ages up to 11, years generally give the calibrated ages unless otherwise stated.
The calibration curve over the portions extending to 40, years is relatively recent, but should become widely adopted as well. It is sometimes possible to date geologically young samples using some of the long-lived methods described above. These methods may work on young samples, for example, if there is a relatively high concentration of the parent isotope in the sample. In that case, sufficient daughter isotope amounts are produced in a relatively short time.
As an example, an article in Science magazine vol. There are other ways to date some geologically young samples. Besides the cosmogenic radionuclides discussed above, there is one other class of short-lived radionuclides on Earth. These are ones produced by decay of the long-lived radionuclides given in the upper part of Table 1. As mentioned in the Uranium-Lead section, uranium does not decay immediately to a stable isotope, but decays through a number of shorter-lived radioisotopes until it ends up as lead.
While the uranium-lead system can measure intervals in the millions of years generally without problems from the intermediate isotopes, those intermediate isotopes with the longest half-lives span long enough time intervals for dating events less than several hundred thousand years ago.
Note that these intervals are well under a tenth of a percent of the half-lives of the long-lived parent uranium and thorium isotopes discussed earlier. Two of the most frequently-used of these "uranium-series" systems are uranium and thorium These are listed as the last two entries in Table 1, and are illustrated in Figure A schematic representation of the uranium decay chain, showing the longest-lived nuclides.
Half-lives are given in each box. Solid arrows represent direct decay, while dashed arrows indicate that there are one or more intermediate decays, with the longest intervening half-life given below the arrow. Like carbon, the shorter-lived uranium-series isotopes are constantly being replenished, in this case, by decaying uranium supplied to the Earth during its original creation.
Following the example of carbon, you may guess that one way to use these isotopes for dating is to remove them from their source of replenishment. This starts the dating clock. In carbon this happens when a living thing like a tree dies and no longer takes in carbonladen CO 2. For the shorter-lived uranium-series radionuclides, there needs to be a physical removal from uranium. The chemistry of uranium and thorium are such that they are in fact easily removed from each other.
Uranium tends to stay dissolved in water, but thorium is insoluble in water. So a number of applications of the thorium method are based on this chemical partition between uranium and thorium. Sediments at the bottom of the ocean have very little uranium relative to the thorium.
Because of this, the uranium, and its contribution to the thorium abundance, can in many cases be ignored in sediments. Thorium then behaves similarly to the long-lived parent isotopes we discussed earlier. It acts like a simple parent-daughter system, and it can be used to date sediments. On the other hand, calcium carbonates produced biologically such as in corals, shells, teeth, and bones take in small amounts of uranium, but essentially no thorium because of its much lower concentrations in the water.
This allows the dating of these materials by their lack of thorium. A brand-new coral reef will have essentially no thorium As it ages, some of its uranium decays to thorium While the thorium itself is radioactive, this can be corrected for. Comparison of uranium ages with ages obtained by counting annual growth bands of corals proves that the technique is. The method has also been used to date stalactites and stalagmites from caves, already mentioned in connection with long-term calibration of the radiocarbon method.
In fact, tens of thousands of uranium-series dates have been performed on cave formations around the world. Previously, dating of anthropology sites had to rely on dating of geologic layers above and below the artifacts.
But with improvements in this method, it is becoming possible to date the human and animal remains themselves. Work to date shows that dating of tooth enamel can be quite reliable. However, dating of bones can be more problematic, as bones are more susceptible to contamination by the surrounding soils. As with all dating, the agreement of two or more methods is highly recommended for confirmation of a measurement. If the samples are beyond the range of radiocarbon e. We will digress briefly from radiometric dating to talk about other dating techniques.
It is important to understand that a very large number of accurate dates covering the past , years has been obtained from many other methods besides radiometric dating. We have already mentioned dendrochronology tree ring dating above.
Dendrochronology is only the tip of the iceberg in terms of non-radiometric dating methods. Here we will look briefly at some other non-radiometric dating techniques. One of the best ways to measure farther back in time than tree rings is by using the seasonal variations in polar ice from Greenland and Antarctica.
There are a number of differences between snow layers made in winter and those made in spring, summer, and fall.
These seasonal layers can be counted just like tree rings. The seasonal differences consist of a visual differences caused by increased bubbles and larger crystal size from summer ice compared to winter ice, b dust layers deposited each summer, c nitric acid concentrations, measured by electrical conductivity of the ice, d chemistry of contaminants in the ice, and e seasonal variations in the relative amounts of heavy hydrogen deuterium and heavy oxygen oxygen in the ice.
These isotope ratios are sensitive to the temperature at the time they fell as snow from the clouds. The heavy isotope is lower in abundance during the colder winter snows than it is in snow falling in spring and summer. So the yearly layers of ice can be tracked by each of these five different indicators, similar to growth rings on trees.
The different types of layers are summarized in Table III. Ice cores are obtained by drilling very deep holes in the ice caps on Greenland and Antarctica with specialized drilling rigs. As the rigs drill down, the drill bits cut around a portion of the ice, capturing a long undisturbed "core" in the process. These cores are carefully brought back to the surface in sections, where they are catalogued, and taken to research laboratories under refrigeration. A very large amount of work has been done on several deep ice cores up to 9, feet in depth.
Several hundred thousand measurements are sometimes made for a single technique on a single ice core. A continuous count of layers exists back as far as , years. In addition to yearly layering, individual strong events such as large-scale volcanic eruptions can be observed and correlated between ice cores. A number of historical eruptions as far back as Vesuvius nearly 2, years ago serve as benchmarks with which to determine the accuracy of the yearly layers as far down as around meters.
As one goes further down in the ice core, the ice becomes more compacted than near the surface, and individual yearly layers are slightly more difficult to observe.
For this reason, there is some uncertainty as one goes back towards , years. Recently, absolute ages have been determined to 75, years for at least one location using cosmogenic radionuclides chlorine and beryllium G.
These agree with the ice flow models and the yearly layer counts. Note that there is no indication anywhere that these ice caps were ever covered by a large body of water, as some people with young-Earth views would expect. Polar ice core layers, counting back yearly layers, consist of the following:. Visual Layers Summer ice has more bubbles and larger crystal sizes Observed to 60, years ago Dust Layers Measured by laser light scattering; most dust is deposited during spring and summer Observed to , years ago Layering of Elec-trical Conductivity Nitric acid from the stratosphere is deposited in the springtime, and causes a yearly layer in electrical conductivity measurement Observed through 60, years ago Contaminant Chemistry Layers Soot from summer forest fires, chemistry of dust, occasional volcanic ash Observed through 2, years; some older eruptions noted Hydrogen and Oxygen Isotope Layering Indicates temperature of precipitation.
Heavy isotopes oxygen and deuterium are depleted more in winter. Yearly layers observed through 1, years; Trends observed much farther back in time Varves. Another layering technique uses seasonal variations in sedimentary layers deposited underwater. The two requirements for varves to be useful in dating are 1 that sediments vary in character through the seasons to produce a visible yearly pattern, and 2 that the lake bottom not be disturbed after the layers are deposited.
These conditions are most often met in small, relatively deep lakes at mid to high latitudes. Shallower lakes typically experience an overturn in which the warmer water sinks to the bottom as winter approaches, but deeper lakes can have persistently thermally stratified temperature-layered water masses, leading to less turbulence, and better conditions for varve layers.
Varves can be harvested by coring drills, somewhat similar to the harvesting of ice cores discussed above. Overall, many hundreds of lakes have been studied for their varve patterns. Each yearly varve layer consists of a mineral matter brought in by swollen streams in the spring. Regular sequences of varves have been measured going back to about 35, years.
The thicknesses of the layers and the types of material in them tells a lot about the climate of the time when the layers were deposited.
For example, pollens entrained in the layers can tell what types of plants were growing nearby at a particular time. Other annual layering methods. Besides tree rings, ice cores, and sediment varves, there are other processes that result in yearly layers that can be counted to determine an age.
Annual layering in coral reefs can be used to date sections of coral. Coral generally grows at rates of around 1 cm per year, and these layers are easily visible. As was mentioned in the uranium-series section, the counting of annual coral layers was used to verify the accuracy of the thorium method. There is a way of dating minerals and pottery that does not rely directly on half-lives. Thermoluminescence dating, or TL dating, uses the fact that radioactive decays cause some electrons in a material to end up stuck in higher-energy orbits.
The number of electrons in higher-energy orbits accumulates as a material experiences more natural radioactivity over time. If the material is heated, these electrons can fall back to their original orbits, emitting a very tiny amount of light. If the heating occurs in a laboratory furnace equipped with a very sensitive light detector, this light can be recorded.
The term comes from putting together thermo , meaning heat, and luminescence , meaning to emit light. By comparison of the amount of light emitted with the natural radioactivity rate the sample experienced, the age of the sample can be determined. TL dating can generally be used on samples less than half a million years old.
TL dating and its related techniques have been cross calibrated with samples of known historical age and with radiocarbon and thorium dating. While TL dating does not usually pinpoint the age with as great an accuracy as these other conventional radiometric dating, it is most useful for applications such as pottery or fine-grained volcanic dust, where other dating methods do not work as well.
Electron spin resonance ESR. Also called electron paramagnetic resonance, ESR dating also relies on the changes in electron orbits and spins caused by radioactivity over time. However, ESR dating can be used over longer time periods, up to two million years, and works best on carbonates, such as in coral reefs and cave deposits.
It has also seen extensive use in dating tooth enamel. This dating method relies on measuring certain isotopes produced by cosmic ray impacts on exposed rock surfaces. Because cosmic rays constantly bombard meteorites flying through space, this method has long been used to date the ' flight time' of meteorites--that is the time from when they were chipped off a larger body like an asteroid to the time they land on Earth.
The cosmic rays produce small amounts of naturally-rare isotopes such as neon and helium-3, which can be measured in the laboratory.
The cosmic-ray exposure ages of meteorites are usually around 10 million years, but can be up to a billion years for some iron meteorites. In the last fifteen years, people have also used cosmic ray exposure ages to date rock surfaces on the Earth. This is much more complicated because the Earth's magnetic field and atmosphere shield us from most of the cosmic rays. Cosmic ray exposure calibrations must take into. Nevertheless, terrestrial cosmic-ray exposure dating has been shown to be useful in many cases.
We have covered a lot of convincing evidence that the Earth was created a very long time ago. The agreement of many different dating methods, both radiometric and non-radiometric, over hundreds of thousands of samples, is very convincing.
Yet, some Christians question whether we can believe something so far back in the past. My answer is that it is similar to believing in other things of the past. It only differs in degree. Why do you believe Abraham Lincoln ever lived? Because it would take an extremely elaborate scheme to make up his existence, including forgeries, fake photos, and many other things, and besides, there is no good reason to simply have made him up.
Well, the situation is very similar for the dating of rocks, only we have rock records rather than historical records. The last three points deserve more attention. Some Christians have argued that something may be slowly changing with time so all the ages look older than they really are.
The only two quantities in the exponent of a decay rate equation are the half-life and the time. So for ages to appear longer than actual, all the half-lives would have to be changing in sync with each other. One could consider that time itself was changing if that happened remember that our clocks are now standardized to atomic clocks! Beyond this, scientists have now used a "time machine" to prove that the half-lives of radioactive species were the same millions of years ago.
This time machine does not allow people to actually go back in time, but it does allow scientists to observe ancient events from a long way away. The time machine is called the telescope. Because God's universe is so large, images from distant events take a long time to get to us.
Telescopes allow us to see supernovae exploding stars at distances so vast that the pictures take hundreds of thousands to millions of years to arrive at the Earth. So the events we see today actually occurred hundreds of thousands to millions of years ago.
And what do we see when we look back in time? Much of the light following a supernova blast is powered by newly created radioactive parents. So we observe radiometric decay in the supernova light. The half-lives of decays occurring hundreds of thousands of years ago are thus carefully recorded! These half-lives completely agree with the half-lives measured from decays occurring today. We must conclude that all evidence points towards unchanging radioactive half-lives. Some individuals have suggested that the speed of light must have been different in the past, and that the starlight has not really taken so long to reach us.
However, the astronomical evidence mentioned above also suggests that the speed of light has not changed, or else we would see a significant apparent change in the half-lives of these ancient radioactive decays. Some doubters have tried to dismiss geologic dating with a sleight of hand by saying that no rocks are completely closed systems that is, that no rocks are so isolated from their surroundings that they have not lost or gained some of the isotopes used for dating.
Speaking from an extreme technical viewpoint this might be true--perhaps 1 atom out of 1,,,, of a certain isotope has leaked out of nearly all rocks, but such a change would make an immeasurably small change in the result.
The real question to ask is, "is the rock sufficiently close to a closed system that the results will be same as a really closed system? These books detail experiments showing, for a given dating system, which minerals work all of the time, which minerals work under some certain conditions, and which minerals are likely to lose atoms and give incorrect results. Understanding these conditions is part of the science of geology. Geologists are careful to use the most reliable methods whenever possible, and as discussed above, to test for agreement between different methods.
Some people have tried to defend a young Earth position by saying that the half-lives of radionuclides can in fact be changed, and that this can be done by certain little-understood particles such as neutrinos, muons, or cosmic rays. This is stretching it. While certain particles can cause nuclear changes, they do not change the half-lives. The nuclear changes are well understood and are nearly always very minor in rocks. In fact the main nuclear changes in rocks are the very radioactive decays we are talking about.
There are only three quite technical instances where a half-life changes, and these do not affect the dating methods we have discussed. Only one technical exception occurs under terrestrial conditions, and this is not for an isotope used for dating. According to theory, electron-capture is the most likely type of decay to show changes with pressure or chemical combination, and this should be most pronounced for very light elements.
The artificially-produced isotope, beryllium-7 has been shown to change by up to 1. In another experiment, a half-life change of a small fraction of a percent was detected when beryllium-7 was subjected to , atmospheres of pressure, equivalent to depths greater than miles inside the Earth Science , , All known rocks, with the possible exception of diamonds, are from much shallower depths.
In fact, beryllium-7 is not used for dating rocks, as it has a half-life of only 54 days, and heavier atoms are even less subject to these minute changes, so the dates of rocks made by electron-capture decays would only be off by at most a few hundredths of a percent. Physical conditions at the center of stars or for cosmic rays differ very greatly from anything experienced in rocks on or in the Earth.
Yet, self-proclaimed "experts" often confuse these conditions. Cosmic rays are very, very high-energy atomic nuclei flying through space. The electron-capture decay mentioned above does not take place in cosmic rays until they slow down. This is because the fast-moving cosmic ray nuclei do not have electrons surrounding them, which are necessary for this form of decay.
Another case is material inside of stars, which is in a plasma state where electrons are not bound to atoms. In the extremely hot stellar environment, a completely different kind of decay can occur. This has been observed for dysprosium and rhenium under very specialized conditions simulating the interior of stars Phys. All normal matter, such as everything on Earth, the Moon, meteorites, etc.
As an example of incorrect application of these conditions to dating, one young-Earth proponent suggested that God used plasma conditions when He created the Earth a few thousand years ago. This writer suggested that the rapid decay rate of rhenium under extreme plasma conditions might explain why rocks give very old ages instead of a young-Earth age.
This writer neglected a number of things, including: More importantly, b rocks and hot gaseous plasmas are completely incompatible forms of matter! The material would have to revert back from the plasma state before it could form rocks. In such a scenario, as the rocks cooled and hardened, their ages would be completely reset to zero as described in previous sections.
That is obviously not what is observed. The last case also involves very fast-moving matter. It has been demonstrated by atomic clocks in very fast spacecraft. These atomic clocks slow down very slightly only a second or so per year as predicted by Einstein's theory of relativity.
No rocks in our solar system are going fast enough to make a noticeable change in their dates. These cases are very specialized, and all are well understood.
None of these cases alter the dates of rocks either on Earth or other planets in the solar system. The conclusion once again is that half-lives are completely reliable in every context for the dating of rocks on Earth and even on other planets. The Earth and all creation appears to be very ancient. It would not be inconsistent with the scientific evidence to conclude that God made everything relatively recently, but with the appearance of great age, just as Genesis 1 and 2 tell of God making Adam as a fully grown human which implies the appearance of age.
This idea was captured by Phillip Henry Gosse in the book, " Omphalos: The idea of a false appearance of great age is a philosophical and theological matter that we won't go into here. The main drawback--and it is a strong one--is that this makes God appear to be a deceiver.
Certainly whole civilizations have been incorrect deceived? Whatever the philosophical conclusions, it is important to note that an apparent old Earth is consistent with the great amount of scientific evidence. As Christians it is of great importance that we understand God's word correctly. Yet from the middle ages up until the s people insisted that the Bible taught that the Earth, not the Sun, was the center of the solar system.
It wasn't that people just thought it had to be that way; they actually quoted scriptures: I am afraid the debate over the age of the Earth has many similarities. But I am optimistic. Today there are many Christians who accept the reliability of geologic dating, but do not compromise the spiritual and historical inerrancy of God's word. While a full discussion of Genesis 1 is not given here, references are given below to a few books that deal with that issue.
There are a number of misconceptions that seem especially prevalent among Christians. Most of these topics are covered in the above discussion, but they are reviewed briefly here for clarity. Radiometric dating is based on index fossils whose dates were assigned long before radioactivity was discovered.
This is not at all true, though it is implied by some young-Earth literature.
Imsges: radiometric dating is accurate
Today, all the constants for the isotopes used in radiometric dating are known to better than 1 percent.
As a result, simple U-Pb ages are often discordant. This estimate was actually reduced over his lifetime to between 20 Ma and 40 Ma and eventually to less than 10 Ma. In these cases, the dates look confused, and do not lie along a line.
The existence of isotopes is confirmed. This would most likely be the case in either young rocks that have not had time acucrate produce much radiogenic argon, or in rocks that are low in the parent potassium. The ratio lovers matchmaking agency radiometric dating is accurate to argon in air is well known, at This results in the formation of a water rich hydration rind that increases in depth with time. It gives rise to Thorium
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