Methods of Geological Dating: Numerical and Relative Dating
Use them just like other courses to track progress, access quizzes and exams, and share content. These inclusions, called xenoliths meaning foreign rocks , consist primarily of olivine, a pale-green iron-magnesium silicate mineral. Index fossils help us to distinguish between rock strata from different time periods, so it's important that they don't cover too much historical ground. The fact is that there are a number of Bible-believing Christians who are involved in radiometric dating, and who can see its validity firsthand.
Dating Dinosaur Fossils
Leaching also occurs, releasing argon from rocks. This formation is approximately meters thick and consists of many layers of sedimentary rock. 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. Layering of Elec-trical Conductivity. A number of recent lava flows within the past few hundred years yield potassium-argon ages in the hundreds of thousands of years range.
Quite often this method is used in conjunction with the K-Ar and the Rb-Sr isochron methods to unravel the history of metamorphic rocks, because each of these methods responds differently to metamorphism and heating. For example, the U-Pb discordia age might give the age of initial formation of the rock, whereas the K-Ar method, which is especially sensitive to argon loss by heating, might give the age of the latest heating event. An example of a U-Pb discordia age is shown in Figure 5.
This example shows an age of 3. The K-Ar ages on rocks and minerals from this area in southwestern Minnesota also record this 1. This argument is specious and akin to concluding that all wristwatches do not work because you happen to find one that does not keep accurate time.
Like any complex procedure, radiometric dating does not work all the time under all circumstances. Each technique works only under a particular set of geologic conditions and occasionally a method is inadvertently misapplied.
There are, to be sure, inconsistencies, errors, and results that are poorly understood, but these are very few in comparison with the vast body of consistent and sensible results that clearly indicate that the methods do work and that the results, properly applied and carefully evaluated, can be trusted.
A few examples will demonstrate that their criticisms are without merit. The creationist author J. He claims that these examples cast serious doubt on the validity of radiometric dating. The use of radiometric dating in Geology involves a very selective acceptance of data.
Discrepant dates, attributed to open systems, may instead be evidence against the validity of radiometric dating. However, close examination of his examples, a few of which are listed in Table 2 , shows that he misrepresents both the data and their meaning.
The two ages from gulf coast localities Table 2 are from a report by Evernden and others These are K-Ar data obtained on glauconite, a potassium-bearing clay mineral that forms in some marine sediment. Woodmorappe fails to mention, however, that these data were obtained as part of a controlled experiment to test, on samples of known age, the applicability of the K-Ar method to glauconite and to illite, another clay mineral. He also neglects to mention that most of the 89 K-Ar ages reported in their study agree very well with the expected ages.
Evernden and others 43 found that these clay minerals are extremely susceptible to argon loss when heated even slightly, such as occurs when sedimentary rocks are deeply buried. As a result, glauconite is used for dating only with extreme caution. The ages from the Coast Range batholith in Alaska Table 2 are referenced by Woodmorappe to a report by Lanphere and others Whereas Lanphere and his colleagues referred to these two K-Ar ages of and million years, the ages are actually from another report and were obtained from samples collected at two localities in Canada, not Alaska.
There is nothing wrong with these ages; they are consistent with the known geologic relations and represent the crystallization ages of the Canadian samples. The Liberian example Table 2 is from a report by Dalrymple and others These authors studied dikes of basalt that intruded Precambrian crystalline basement rocks and Mesozoic sedimentary rocks in western Liberia.
The dikes cutting the Precambrian basement gave K-Ar ages ranging from to million years Woodmorappe erroneously lists this higher age as million years , whereas those cutting the Mesozoic sedimentary rocks gave K-Ar ages of from to million years.
Woodmorappe does not mention that the experiments in this study were designed such that the anomalous results were evident, the cause of the anomalous results was discovered, and the crystallization ages of the Liberian dikes were unambiguously determined. The Liberian study is, in fact, an excellent example of how geochronologists design experiments so that the results can be checked and verified.
The final example listed in Table 2 is a supposed 34 billion-year Rb-Sr isochron age on diabase of the Pahrump Group from Panamint Valley, California, and is referenced to a book by Faure and Powell Again, Woodmorappe badly misrepresents the facts. The data do not fall on any straight line and do not, therefore, form an isochron. The original data are from a report by Wasserburg and others , who plotted the data as shown but did not draw a billion-year isochron on the diagram.
As discussed above, one feature of the Rb-Sr isochron diagram is that, to a great extent, it is self-diagnostic. The scatter of the data in Figure 6 shows clearly that the sample has been an open system to 87 Sr and perhaps to other isotopes as well and that no meaningful Rb-Sr age can be calculated from these data.
This conclusion was clearly stated by both Wasserburg and others and by Faure and Powell There are two things wrong with this argument. First, the lead data that Kofahl and Segraves 77 cite, which come from a report by Oversby , are common lead measurements done primarily to obtain information on the genesis of the Reunion lavas and secondarily to estimate when the parent magma the lava was derived from was separated from primitive mantle material. These data cannot be used to calculate the age of the lava flows and no knowledgeable scientist would attempt to do so.
We can only speculate on where Kofahl and Segraves obtained their numbers. The data Morris 92 refers to were published by Evernden and others 44 , but include samples from different islands that formed at different times!
The age of 3. The approximate age of , years was the mean of the results from four samples from the Island of Hawaii, which is much younger than Kauai. Many of the rocks seem to have inherited Ar 40 from the magma from which the rocks were derived. Volcanic rocks erupted into the ocean definitely inherit Ar 40 and helium and thus when these are dated by the K 40 -Ar 40 clock, old ages are obtained for very recent flows.
For example, lavas taken from the ocean bottom off the island [sic] of Hawaii on a submarine extension of the east rift zone of Kilauea volcano gave an age of 22 million years, but the actual flow happened less than years ago. Slusher and Morris 92 advanced this argument in an attempt to show that the K-Ar method is unreliable, but the argument is a red herring. Two studies independently discovered that the glassy margins of submarine pillow basalts, so named because lava extruded under water forms globular shapes resembling pillows, trap 40 Ar dissolved in the melt before it can escape 36 , This effect is most serious in the rims of the pillows and increases in severity with water depth.
The excess 40 Ar content approaches zero toward pillow interiors, which cool more slowly and allow the 40 Ar to escape, and in water depths of less than about meters because of the lessening of hydrostatic pressure. The purpose of these two studies was to determine, in a controlled experiment with samples of known age, the suitability of submarine pillow basalts for dating, because it was suspected that such samples might be unreliable.
Such studies are not unusual because each different type of mineral and rock has to be tested carefully before it can be used for any radiometric dating technique. In the case of the submarine pillow basalts, the results clearly indicated that these rocks are unsuitable for dating, and so they are not generally used for this purpose except in special circumstances and unless there is some independent way of verifying the results.
The citation for this statement is to a report by Turner Turner, however, made no such comment about excess argon in lunar rocks, and there are no data in his report on which such a conclusion could be based. The statement by Rofahl and Segraves 77 is simply unjustifiable. Volcanic rocks produced by lava flows which occurred in Hawaii in the years were dated by the potassium-argon method.
Excess argon produced apparent ages ranging from million to 2. Similar modern rocks formed in near Hualalai, Hawaii, were found to give potassium-argon ages ranging from million years to 3 billion years. Kofahl and Segraves 77 and Morris 92 cite a study by Funkhouser and Naughton 51 on xenolithic inclusions in the flow from Hualalai Volcano on the Island of Hawaii.
The flow is unusual because it carries very abundant inclusions of rocks foreign to the lava. These inclusions, called xenoliths meaning foreign rocks , consist primarily of olivine, a pale-green iron-magnesium silicate mineral. They come from deep within the mantle and were carried upward to the surface by the lava. In the field, they look like large raisins in a pudding and even occur in beds piled one on top of the other, glued together by the lava.
The study by Funkhouser and Naughton 51 was on the xenoliths, not on the lava. The xenoliths, which vary in composition and range in size from single mineral grains to rocks as big as basketballs, do, indeed, carry excess argon in large amounts. Quite simply, xenoliths are one of the types of rocks that cannot be dated by the K-Ar technique.
Funkhouser and Naughton were able to determine that the excess gas resides primarily in fluid bubbles in the minerals of the xenoliths, where it cannot escape upon reaching the surface. Studies such as the one by Funkhouser and Naughton are routinely done to ascertain which materials are suitable for dating and which are not, and to determine the cause of sometimes strange results. They are part of a continuing effort to learn. Two extensive K-Ar studies on historical lava flows from around the world 31 , 79 showed that excess argon is not a serious problem for dating lava flows.
In nearly every case, the measured K-Ar age was zero, as expected if excess argon is uncommon. An exception is the lava from the Hualalai flow, which is so badly contaminated by the xenoliths that it is impossible to obtain a completely inclusion-free sample. There is really no valid way of determining what the initial amounts of Sr 87 in rocks were. As discussed above in the section on Rb-Sr dating the simplest form of Rb-Sr dating i.
Such samples are rare, and so nearly all modern Rb-Sr dating is done by the isochron method. The beauty of the Rb-Sr isochron method is that knowledge of the initial Sr isotopic composition is not necessary — it is one of the results obtained.
A second advantage of the isochron method is that it contains internal checks on reliability. Look again at the isochron for the meteorite Juvinas Figure 3. The data are straightforward albeit technically complex measurements that fall on a straight line, indicating that the meteorite has obeyed the closed-system requirement.
The decay constants used in the calculations were the same as those in use throughout the world in The age of 4. There is far too much Ar 40 in the earth for more than a small fraction of it to have been formed by radioactive decay of K This is true even if the earth were really 4. In the atmosphere of the earth, Ar 40 constitutes This is around times the amount that would be generated by radioactive decay over the hypothetical 4.
Certainly this is not produced by an influx from outer space. Thus it would seem that a large amount of Ar 40 was present in the beginning. Since geochronologists assume that errors due to presence of initial Ar 40 are small, their results are highly questionable.
This statement contains several serious errors. First, there is not more 40 Ar in the atmosphere than can be accounted for by radioactive decay of 40 K over 4. An amount of 40 Ar equivalent to all the 40 Ar now in the atmosphere could be generated in 4. Current estimates of the composition of the Earth indicate that the crust contains about 1.
The 40 Ar content of the atmosphere is well known and is 6. Thus, the Earth and the atmosphere now contain about equal amounts of 40 Ar, and the total could be generated if the Earth contained only ppm potassium and released half of its 40 Ar to the atmosphere.
Second, there have been sufficient tests to show that during their formation in the crust, igneous and metamorphic rocks nearly always release their entrapped 40 Ar, thus resetting the K-Ar clock. In addition, scientists typically design their experiments so that anomalous results, such as might be caused by the rare case of initial 40 Ar, are readily apparent.
The study of the Liberian diabase dikes, discussed above, is a good example of this practice. First, if it is assumed that there is a uniform distribution of Sr 87 in the rock, then it is assumed that there is also a uniform distribution of Rb It only requires that the Sr isotopic composition , i.
Even though the various minerals will incorporate different amounts of Sr as they cool and form, the Sr isotopic composition will be the same because natural processes do not significantly fractionate isotopes with so little mass difference as 87 Sr and 86 Sr. Second, Slusher has confused isotopes and elements. Rb and Sr are quite different elements and are incorporated into the various minerals in varying proportions according to the composition and structure of the minerals.
There is no way to correct for this natural isotopic variation since there is no way to determine it. This renders the Rb 87 -Sr 87 series useless as a clock. Slusher is wrong again. He has used an invalid analogy and come to an erroneous conclusion. Arndts and Overn 8 and Kramer and others 78 claim that Rb-Sr isochrons are the result of mixing, rather than of decay of 87 Rb over long periods:.
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. We want fossils of plants and animals that lived for a relatively short amount of time, like a few hundred thousand years or so.
I know that doesn't seem like a very short time span, but it is when we're talking about geologic time. An index fossil is a fossil representing a plant or animal that existed for a relatively short duration of time. These are the fossils that we want to use for relative dating.
Index fossils help us to distinguish between rock strata from different time periods, so it's important that they don't cover too much historical ground. We wouldn't want to use a horseshoe crab fossil, because horseshoe crabs have existed for over million years and are still alive today!
We'd want to use a more short-lived fossil, like the dodo bird. We also want our index fossils to be common, widely-distributed species that are easy for scientists to identify. Some of the scientists' favorite index fossils are trilobites, ammonites and scallop shells. So, how exactly is an index fossil used for relative dating of rocks? Well, let's go back to our surveyor, William Smith. He was often presented with the problem of finding two different rock outcrops from two different periods.
Let's say in the first outcrop, he found an upper rock layer containing ammonite fossils and a lower layer containing scallops. In the second outcrop, miles and miles away, he also found two layers; but these layers were different. The upper layer had scallop fossils, and the lower layer had trilobites. Smith would have brought these two arrangements together, overlapping the common scallop layer, to produce a larger succession of three rock strata! Now we have a more complete piece of geologic history: Index fossils can be used to correlate the relative ages of rocks that are separated by vast distances.
The cool thing is that we can even correlate rocks from different continents! For example, scientists found Barosaurus fossils inside a layer of Tendaguru rocks in East Africa.
They also found Hypsilophodon fossils inside a layer of Wealden rocks in Europe. Scientists didn't know how old either of the rocks were, or even which dinosaur was older than the other. But in North America, they found a big chunk of rock which contained both fossils.
Therefore, the Hypsilophodon had to be older than the Barosaurus. And, even though the rock types were different, scientists could assign relative ages to the other rocks based on their fossils. They could safely assume that the Tendaguru rocks in East Africa were older than the Wealden rocks in Europe.
When rocks are made up of distinct strata, we use stratigraphic succession to determine the relative ages of each of the layers in the rock. However, another form of relative dating is the use of fossil succession: In order to use fossils for relative dating, scientists focus their efforts on index fossils.
These fossils represent plants and animals that lived for a relatively short period of time. We use index fossils to identify periods of geologic history and to match up pieces of rock strata that have been separated by large distances.
When one outcrop contains two index fossils from two different time periods, it acts as a 'missing link' between other outcrops that have only one of the two fossils. To unlock this lesson you must be a Study. Did you know… We have over 95 college courses that prepare you to earn credit by exam that is accepted by over 2, colleges and universities.
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Explore over 4, video courses. Find a degree that fits your goals. Relative Dating with Fossils: Index Fossils as Indicators of Time You may already know how to date a fossil with a rock. But did you know that we can also date a rock with a fossil? Watch this video to find out how we use index fossils to establish the relative ages of rocks. An error occurred trying to load this video.
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Conditions of Fossil Preservation: Sea Floor Spreading and Polar Reversal. Major Eons, Eras, Periods and Epochs. How to Read Topographic and Geologic Maps.
Absolute Time in Geology. Interpreting Tables of Scientific Data: Theories of Geological Evolution: Prentice Hall Earth Science: Remedial High School Physical Science. Holt McDougal Earth Science:
Imsges: what is the main difference of both relative and radiometric dating
Beta decay involves the ejection of a beta particle an electron from the nucleus. Whiston added comets to Burnet's cosmogony as the source of the waters of the flood. Find a degree that fits your goals.
Prentice Hall Earth Science: Some gave virtually zero ages, although the geologic evidence suggested that devitrification took place shortly after the formation of a deposit. For example, Isotopic studies of the Cardenas Basalt and associated Proterozoic diabase sills and dikes have produced a geologic mystery.
One also has to know which isotopes to examine. I particularly want to thank Mark Isaak who supplied a number of references which were not available to park si hoo moon chae won dating, Chris Stassen for supplying the section on what is the main difference of both relative and radiometric dating history of radiometric dating, and Andrew MacRae who differsnce information about Hugh Miller's The Testimony of the Rocks. Potassium and are stable, but potassium is unstable, giving us the dating methods discussed above. I find this information very interesting, and thank him for it. 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.