Radioactive dating is a method of dating rocks and minerals using radioactive isotopes. This method is useful for igneous and metamorphic rocks, which cannot be dated by the stratigraphic correlation method used for sedimentary rocks. Over naturally-occurring isotopes are known. Some do not change with time and form stable isotopes i. The unstable or more commonly known radioactive isotopes break down by radioactive decay into other isotopes. Radioactive decay is a natural process and comes from the atomic nucleus becoming unstable and releasing bits and pieces. These are released as radioactive particles there are many types.
It has become increasingly clear that these radiometric dating techniques agree with each other and as a whole, present a coherent picture in which the Earth was created a very long time ago. Further evidence comes from the complete agreement between radiometric dates and other dating methods such as counting tree rings or glacier ice core layers.
Many Christians have been led to distrust radiometric dating and are completely unaware of the great number of laboratory measurements that have shown these methods to be consistent. Many are also unaware that Bible-believing Christians are among those actively involved in radiometric dating. This paper describes in relatively simple terms how a number of the dating techniques work, how accurately the half-lives of the radioactive elements and the rock dates themselves are known, and how dates are checked with one another.
In the process the paper refutes a number of misconceptions prevalent among Christians today. This paper is available on the web via the American Scientific Affiliation and related sites to promote greater understanding and wisdom on this issue, particularly within the Christian community.
Doubters Still Try Apparent Age? Arguments over the age of the Earth have sometimes been divisive for people who regard the Bible as God's word. Even though the Earth's age is never mentioned in the Bible, it is an issue because those who take a strictly literal view of the early chapters of Genesis can calculate an approximate date for the creation by adding up the life-spans of the people mentioned in the genealogies.
Assuming a strictly literal interpretation of the week of creation, even if some of the generations were left out of the genealogies, the Earth would be less than ten thousand years old. Radiometric dating techniques indicate that the Earth is thousands of times older than that-approximately four and a half billion years old. Many Christians accept this and interpret the Genesis account in less scientifically literal ways. However, some Christians suggest that the geologic dating techniques are unreliable, that they are wrongly interpreted, or that they are confusing at best.
Unfortunately, much of the literature available to Christians has been either inaccurate or difficult to understand, so that confusion over dating techniques continues.
The next few pages cover a broad overview of radiometric dating techniques, show a few examples, and discuss the degree to which the various dating systems agree with each other.
The goal is to promote greater understanding on this issue, particularly for the Christian community. Many people have been led to be skeptical of dating without knowing much about it. For example, most people don't realize that carbon dating is only rarely used on rocks. God has called us to be "wise as serpents" Matt. In spite of this, differences still occur within the church. A disagreement over the age of the Earth is relatively minor in the whole scope of Christianity; it is more important to agree on the Rock of Ages than on the age of rocks.
But because God has also called us to wisdom, this issue is worthy of study. Rocks are made up of many individual crystals, and each crystal is usually made up of at least several different chemical elements such as iron, magnesium, silicon, etc. Most of the elements in nature are stable and do not change. However, some elements are not completely stable in their natural state. Some of the atoms eventually change from one element to another by a process called radioactive decay.
If there are a lot of atoms of the original element, called the parent element, the atoms decay to another element, called the daughter element, at a predictable rate. The passage of time can be charted by the reduction in the number of parent atoms, and the increase in the number of daughter atoms.
Something calcium radiometric dating excellent
Radiometric dating can be compared to an hourglass. When the glass is turned over, sand runs from the top to the bottom.
Measuring the age of corals can provide insights for paleoclimatology studies. For deep sea corals, a combination of radiocarbon dating and uranium-thorium dating can be used. The carbon date represents the age of the coral and the water, whereas the uranium-thorium date reflects the coral itself. This can provide information on past deep. Radioactive dating is a method of dating rocks and minerals using radioactive isotopes. This method is useful for igneous and metamorphic rocks, which cannot be dated by the stratigraphic correlation method used for sedimentary rocks. Over naturally-occurring isotopes are known. Uranium-thorium dating, also called thorium dating, uranium-series disequilibrium dating or uranium-series dating, is a radiometric dating technique established in the s which has been used since the s to determine the age of calcium carbonate materials such as speleothem or coral. Unlike other commonly used radiometric dating techniques such as rubidium-strontium or uranium.
Radioactive atoms are like individual grains of sand-radioactive decays are like the falling of grains from the top to the bottom of the glass. You cannot predict exactly when any one particular grain will get to the bottom, but you can predict from one time to the next how long the whole pile of sand takes to fall.
Have calcium radiometric dating think, you
Once all of the sand has fallen out of the top, the hourglass will no longer keep time unless it is turned over again. Similarly, when all the atoms of the radioactive element are gone, the rock will no longer keep time unless it receives a new batch of radioactive atoms.
Figure 1. The rate of loss of sand from from the top of an hourglass compared to exponential type of decay of radioactive elements. In exponential decay the amount of material decreases by half during each half-life. After two half-lives one-fourth remains, after three half-lives, one-eighth, etc.
Unlike the hourglass, where the amount of sand falling is constant right up until the end, the number of decays from a fixed number of radioactive atoms decreases as there are fewer atoms left to decay see Figure 1. If it takes a certain length of time for half of the atoms to decay, it will take the same amount of time for half of the remaining atoms, or a fourth of the original total, to decay.
In the next interval, with only a fourth remaining, only one eighth of the original total will decay. By the time ten of these intervals, or half-lives, has passed, less than one thousandth of the original number of radioactive atoms is left.
The equation for the fraction of parent atoms left is very simple. The type of equation is exponential, and is related to equations describing other well-known phenomena such as population growth. No deviations have yet been found from this equation for radioactive decay.
Also unlike the hourglass, there is no way to change the rate at which radioactive atoms decay in rocks. If you shake the hourglass, twirl it, or put it in a rapidly accelerating vehicle, the time it takes the sand to fall will change.
Radiometric or Absolute Rock Dating
But the radioactive atoms used in dating techniques have been subjected to heat, cold, pressure, vacuum, acceleration, and strong chemical reactions to the extent that would be experienced by rocks or magma in the mantle, crust, or surface of the Earth or other planets without any significant change in their decay rate. In only a couple of special cases have any decay rates been observed to vary, and none of these special cases apply to the dating of rocks as discussed here. These exceptions are discussed later.
An hourglass will tell time correctly only if it is completely sealed. If it has a hole allowing the sand grains to escape out the side instead of going through the neck, it will give the wrong time interval. Similarly, a rock that is to be dated must be sealed against loss or addition of either the radioactive daughter or parent. If it has lost some of the daughter element, it will give an inaccurately young age.
As will be discussed later, most dating techniques have very good ways of telling if such a loss has occurred, in which case the date is thrown out and so is the rock! An hourglass measures how much time has passed since it was turned over.
Actually it tells when a specific amount of time, e. Radiometric dating of rocks also tells how much time has passed since some event occurred. For igneous rocks the event is usually its cooling and hardening from magma or lava. For some other materials, the event is the end of a metamorphic heating event in which the rock gets baked underground at generally over a thousand degrees Fahrenheitthe uncovering of a surface by the scraping action of a glacier, the chipping of a meteorite off of an asteroid, or the length of time a plant or animal has been dead.
There are now well over forty different radiometric dating techniques, each based on a different radioactive isotope. The term isotope subdivides elements into groups of atoms that have the same atomic weight. For example carbon has isotopes of weight 12, 13, and 14 times the mass of a nucleon, referred to as carbon, carbon, or carbon abbreviated as 12 C, 13 C, 14 C.
It is only the carbon isotope that is radioactive.
Radiometric dating is possible if a rock contains a measurable amount of . both daughter and parent atoms. Calcium Carbonate and Silica. Decreasing order of grain size: Gravel, sand, silt, clay. Fluids in metamorphism are important because: They speed up chemical reactions. Radiometric dating of rocks and minerals using naturally occurring, long-lived radioactive isotopes is troublesome for young-earth creationists because the techniques have provided overwhelming evidence of the antiquity of the earth and life. Some so-called creation scientists have attempted to show that radiometric dating does not work on. The geochronologist considers the Ca40 of little practical use in radiometric dating since common calcium is such an abundant element and the radiogenic Ca40 has the same atomic mass as common calcium. "Juggling" is also performed by geochronologists in this K-Ar system.
This will be discussed further in a later section. A partial list of the parent and daughter isotopes and the decay half-lives is given in Table I. Notice the large range in the half-lives.
Isotopes with long half-lives decay very slowly, and so are useful for dating. Table 1. Some Naturally Occurring Radioactive Isotopes and their half-lives. Parent Product. Daughter Half-Life. Years Samarium Neodymium billion Rubidium Strontium Isotopes with shorter half-lives cannot date very ancient events because all of the atoms of the parent isotope would have already decayed away, like an hourglass left sitting with all the sand at the bottom.
Isotopes with relatively short half-lives are useful for dating correspondingly shorter intervals, and can usually do so with greater accuracy, just as you would use a stopwatch rather than a grandfather clock to time a meter dash.
On the other hand, you would use a calendar, not a clock, to record time intervals of several weeks or more. The half-lives have all been measured directly either by using a radiation detector to count the number of atoms decaying in a given amount of time from a known amount of the parent material, or by measuring the ratio of daughter to parent atoms in a sample that originally consisted completely of parent atoms.
Work on radiometric dating first started shortly after the turn of the 20th century, but progress was relatively slow before the late. However, by now we have had over fifty years to measure and re-measure the half-lives for many of the dating techniques.
Very precise counting of the decay events or the daughter atoms can be done, so while the number of, say, rhenium atoms decaying in 50 years is a very small fraction of the total, the resulting osmium atoms can be very precisely counted.
For example, recall that only one gram of material contains over 10 21 1 with 21 zeros behind atoms. Even if only one trillionth of the atoms decay in one year, this is still millions of decays, each of which can be counted by a radiation detector! The uncertainties on the half-lives given in the table are all very small. There is no evidence of any of the half-lives changing over time. In fact, as discussed below, they have been observed to not change at all over hundreds of thousands of years.
Examples of Dating Methods for Igneous Rocks. Now let's look at how the actual dating methods work. Igneous rocks are good candidates for dating.
Recall that for igneous rocks the event being dated is when the rock was formed from magma or lava. When the molten material cools and hardens, the atoms are no longer free to move about.
Daughter atoms that result from radioactive decays occurring after the rock cools are frozen in the place where they were made within the rock. These atoms are like the sand grains accumulating in the bottom of the hourglass. Determining the age of a rock is a two-step process. First one needs to measure the number of daughter atoms and the number of remaining parent atoms and calculate the ratio between them. Then the half-life is used to calculate the time it took to produce that ratio of parent atoms to daughter atoms.
However, there is one complication. One cannot always assume that there were no daughter atoms to begin with. It turns out that there are some cases where one can make that assumption quite reliably. But in most cases the initial amount of the daughter product must be accurately determined. Most of the time one can use the different amounts of parent and daughter present in different minerals within the rock to tell how much daughter was originally present.
Each dating mechanism deals with this problem in its own way. Some types of dating work better in some rocks; others are better in other rocks, depending on the rock composition and its age. Let's examine some of the different dating mechanisms now. Potassium is an abundant element in the Earth's crust. One isotope, potassium, is radioactive and decays to two different daughter products, calcium and argon, by two different decay methods.
This is not a problem because the production ratio of these two daughter products is precisely known, and is always constant: It is possible to date some rocks by the potassium-calcium method, but this is not often done because it is hard to determine how much calcium was initially present.
Argon, on the other hand, is a gas. Whenever rock is melted to become magma or lava, the argon tends to escape. Once the molten material hardens, it begins to trap the new argon produced since the hardening took place. In this way the potassium-argon clock is clearly reset when an igneous rock is formed.
In its simplest form, the geologist simply needs to measure the relative amounts of potassium and argon to date the rock. The age is given by a relatively simple equation:.
However, in reality there is often a small amount of argon remaining in a rock when it hardens. This is usually trapped in the form of very tiny air bubbles in the rock. One percent of the air we breathe is argon.
Any extra argon from air bubbles may need to be taken into account if it is significant relative to the amount of radiogenic argon that is, argon produced by radioactive decays.
This would most likely be the case in either young rocks that have not had time to produce much radiogenic argon, or in rocks that are low in the parent potassium. One must have a way to determine how much air-argon is in the rock. This is rather easily done because air-argon has a couple of other isotopes, the most abundant of which is argon The ratio of argon to argon in air is well known, at Thus, if one measures argon as well as argon, one can calculate and subtract off the air-argon to get an accurate age.
One of the best ways of showing that an age-date is correct is to confirm it with one or more different dating. Although potassium-argon is one of the simplest dating methods, there are still some cases where it does not agree with other methods. When this does happen, it is usually because the gas within bubbles in the rock is from deep underground rather than from the air.
This gas can have a higher concentration of argon escaping from the melting of older rocks. This is called parentless argon because its parent potassium is not in the rock being dated, and is also not from the air.
In these slightly unusual cases, the date given by the normal potassium-argon method is too old. However, scientists in the mids came up with a way around this problem, the argon-argon method, discussed in the next section. Even though it has been around for nearly half a century, the argon-argon method is seldom discussed by groups critical of dating methods.
This method uses exactly the same parent and daughter isotopes as the potassium-argon method. In effect, it is a different way of telling time from the same clock. Instead of simply comparing the total potassium with the non-air argon in the rock, this method has a way of telling exactly what and how much argon is directly related to the potassium in the rock.
In the argon-argon method the rock is placed near the center of a nuclear reactor for a period of hours. A nuclear reactor emits a very large number of neutrons, which are capable of changing a small amount of the potassium into argon Argon is not found in nature because it has only a year half-life. This half-life doesn't affect the argon-argon dating method as long as the measurements are made within about five years of the neutron dose.
The rock is then heated in a furnace to release both the argon and the argon representing the potassium for analysis. The heating is done at incrementally higher temperatures and at each step the ratio of argon to argon is measured. If the argon is from decay of potassium within the rock, it will come out at the same temperatures as the potassium-derived argon and in a constant proportion. On the other hand, if there is some excess argon in the rock it will cause a different ratio of argon to argon for some or many of the heating steps, so the different heating steps will not agree with each other.
Figure 2 is an example of a good argon-argon date. The fact that this plot is flat shows that essentially all of the argon is from decay of potassium within the rock. The potassium content of the sample is found by multiplying the argon by a factor based on the neutron exposure in the reactor. When this is done, the plateau in the figure represents an age date based on the decay of potassium to argon There are occasions when the argon-argon dating method does not give an age even if there is sufficient potassium in the sample and the rock was old enough to date.
This most often occurs if the rock experienced a high temperature usually a thousand degrees Fahrenheit or more at some point since its formation. If that occurs, some of the argon gas moves around, and the analysis does not give a smooth plateau across the extraction temperature steps. An example of an argon-argon analysis that did not yield an age date is shown in Figure 3. Notice that there is no good plateau in this plot.
In some instances there will actually be two plateaus, one representing the formation age, and another representing the time at which the heating episode occurred. But in most cases where the system has been disturbed, there simply is no date given. The important point to note is that, rather than giving wrong age dates, this method simply does not give a date if the system has been disturbed. This is also true of a number of other igneous rock dating methods, as we will describe below.
Figure 3. In nearly all of the dating methods, except potassium-argon and the associated argon-argon method, there is always some amount of the daughter product already in the rock when it cools.
Using these methods is a little like trying to tell time from an hourglass that was turned over before all of the sand had fallen to the bottom. One can think of ways to correct for this in an hourglass: One could make a mark on the outside of the glass where the sand level started from and then repeat the interval with a stopwatch in the other hand to calibrate it.
Or if one is clever she or he could examine the hourglass' shape and determine what fraction of all the sand was at the top to start with. By knowing how long it takes all of the sand to fall, one could determine how long the time interval was. Similarly, there are good ways to tell quite precisely how much of the daughter product was already in the rock when it cooled and hardened. Figure 4 is an important type of plot used in rubidium-strontium dating.
Figure 5. This works because if there were no rubidium in the sample, the strontium composition would not change. The slope of the line is used to determine the age of the sample. As the rock starts to age, rubidium gets converted to strontium. The amount of strontium added to each mineral is proportional to the amount of rubidium present. The solid line drawn through the samples will thus progressively rotate from the horizontal to steeper and steeper slopes.
From that we can determine the original daughter strontium in each mineral, which is just what we need to know to determine the correct age. It also turns out that the slope of the line is proportional to the age of the rock. The older the rock, the steeper the line will be. If the slope of the line is m and the half-life is hthe age t in years is given by the equation. For a system with a very long half-life like rubidium-strontium, the actual numerical value of the slope will always be quite small.
To give an example for the above equation, if the slope of a line in a plot similar to Fig. Several things can on rare occasions cause problems for the rubidium-strontium dating method.
One possible source of problems is if a rock contains some minerals that are older than the main part of the rock. This can happen when magma inside the Earth picks up unmelted minerals from the surrounding rock as the magma moves through a magma chamber. Usually a good geologist can distinguish these "xenoliths" from the younger minerals around them.
If he or she does happen to use them for dating the rock, the points represented by these minerals will lie off the line made by the rest of the points. Another difficulty can arise if a rock has undergone metamorphism, that is, if the rock got very hot, but not hot enough to completely re-melt the rock. In these cases, the dates look confused, and do not lie along a line. Some of the minerals may have completely melted, while others did not melt at all, so some minerals try to give the igneous age while other minerals try to give the metamorphic age.
In these cases there will not be a straight line, and no date is determined. In a few very rare instances the rubidium-strontium method has given straight lines that give wrong ages.
This can happen when the rock being dated was formed from magma that was not well mixed, and which had two distinct batches of rubidium and strontium. One magma batch had rubidium and strontium compositions near the upper end of a line such as in Fig. In this case, the. This is called a two-component mixing line. It is a very rare occurrence in these dating mechanisms, but at least thirty cases have been documented among the tens of thousands of rubidium-strontium dates made. The agreement of several dating methods is the best fail-safe way of dating rocks.
All of these methods work very similarly to the rubidium-strontium method. They all use three-isotope diagrams similar to Figure 4 to determine the age. The samarium-neodymium method is the most-often used of these three. It uses the decay of samarium to neodymium, which has a half-life of billion years. The ratio of the daughter isotope, neodymium, to another neodymium isotope, neodymium, is plotted against the ratio of the parent, samarium, to neodymium If different minerals from the same rock plot along a line, the slope is determined, and the age is given by the same equation as above.
The samarium-neodymium method may be preferred for rocks that have very little potassium and rubidium, for which the potassium-argon, argon-argon, and rubidium-strontium methods might be difficult. The samarium-neodymium method has also been shown to be more resistant to being disturbed or re-set by metamorphic heating events, so for some metamorphosed rocks the samarium-neodymium method is preferred.
For a rock of the same age, the slope on the neodymium-samarium plots will be less than on a rubidium-strontium plot because the half-life is longer. However, these isotope ratios are usually measured to extreme accuracy-several parts in ten thousand-so accurate dates can be obtained even for ages less than one fiftieth of a half-life, and with correspondingly small slopes.
The lutetium-hafnium method uses the 38 billion year half-life of lutetium decaying to hafnium This dating system is similar in many ways to samarium-neodymium, as the elements tend to be concentrated in the same types of minerals. Since samarium-neodymium dating is somewhat easier, the lutetium-hafnium method is used less often. The rhenium-osmium method takes advantage of the fact that the osmium concentration in most rocks and minerals is very low, so a small amount of the parent rhenium can produce a significant change in the osmium isotope ratio.
The half-life for this radioactive decay is 42 billion years. The non-radiogenic stable isotopes, osmium orare used as the denominator in the ratios on the three-isotope plots. This method has been useful for dating iron meteorites, and is now enjoying greater use for dating Earth rocks due to development of easier rhenium and osmium isotope measurement techniques. Uranium-Lead and related techniques.
The uranium-lead method is the longest-used dating method. It was first used inabout a century ago. The uranium-lead system is more complicated than other parent-daughter systems; it is actually several dating methods put together. Natural uranium consists primarily of two isotopes, U and U, and these isotopes decay with different half-lives to produce lead and lead, respectively.
In addition, lead is produced by thorium Only one isotope of lead, lead, is not radiogenic. The uranium-lead system has an interesting complication: none of the lead isotopes is produced directly from the uranium and thorium. Each decays through a series of relatively short-lived radioactive elements that each decay to a lighter element, finally ending up at lead.
Since these half-lives are so short compared to U, U, and thorium, they generally do not affect the overall dating scheme. The result is that one can obtain three independent estimates of the age of a rock by measuring the lead isotopes and their parent isotopes. Long-term dating based on the U, U, and thorium will be discussed briefly here; dating based on some of the shorter-lived intermediate isotopes is discussed later.
The uranium-lead system in its simpler forms, using U, U, and thorium, has proved to be less reliable than many of the other dating systems.
This is because both uranium and lead are less easily retained in many of the minerals in which they are found. Yet the fact that there are three dating systems all in one allows scientists to easily determine whether the system has been disturbed or not. Using slightly more complicated mathematics, different combinations of the lead isotopes and parent isotopes can be plotted in such a way as to.
One of these techniques is called the lead-lead technique because it determines the ages from the lead isotopes alone. Some of these techniques allow scientists to chart at what points in time metamorphic heating events have occurred, which is also of significant interest to geologists.
Radiometric dating is one subset of the many dating methods used in geology. Stalactite A cylindrical or conical deposit of minerals, generally calcite or aragonite (forms of calcium carbonate), hanging from the roof of a cavern, and generally formed by precipitation (or crystallization) of carbonates from water dripping from the roof. Radiometric dating is a means of determining the age of very old objects, including the Earth itself. Radiometric dating depends on the decay of isotopes, which are different forms of the same element that include the same number of protons but different numbers of neutrons in their atoms. More Bad News for Radiometric Dating and calcium, because these elements are found in the earliest formed minerals. But at the same time, it will be enriched in the elements contained in the later forming minerals, namely sodium and potassium. Further, the silicon content of the melt becomes enriched toward the latter stages of.
The Age of the Earth. We now turn our attention to what the dating systems tell us about the age of the Earth. The most obvious constraint is the age of the oldest rocks. These have been dated at up to about four billion years. But actually only a very small portion of the Earth 's rocks are that old. From satellite data and other measurements we know that the Earth's surface is constantly rearranging itself little by little as Earth quakes occur.
Such rearranging cannot occur without some of the Earth's surface disappearing under other parts of the Earth's surface, re-melting some of the rock. So it appears that none of the rocks have survived from the creation of the Earth without undergoing remelting, metamorphism, or erosion, and all we can say-from this line of evidence-is that the Earth appears to be at least as old as the four billion year old rocks.
When scientists began systematically dating meteorites they learned a very interesting thing: nearly all of the meteorites had practically identical ages, at 4. These meteorites are chips off the asteroids.
When the asteroids were formed in space, they cooled relatively quickly some of them may never have gotten very warmso all of their rocks were formed within a few million years. The asteroids' rocks have not been remelted ever since, so the ages have generally not been disturbed.
Meteorites that show evidence of being from the largest asteroids have slightly younger ages. The moon is larger than the largest asteroid. Most of the rocks we have from the moon do not exceed 4. The samples thought to be the oldest are highly pulverized and difficult to date, though there are a few dates extending all the way to 4. Most scientists think that all the bodies in the solar system were created at about the same time.
Evidence from the uranium, thorium, and lead isotopes links the Earth's age with that of the meteorites. This would make the Earth 4. Figure 6. There is another way to determine the age of the Earth. If we see an hourglass whose sand has run out, we know that it was turned over longer ago than the time interval it measures.
Similarly, if we find that a radioactive parent was once abundant but has since run out, we know that it too was set longer ago than the time interval it measures. There are in fact many, many more parent isotopes than those listed in Table 1.
However, most of them are no longer found naturally on Earth-they have run out. Their half-lives range down to times shorter than we can measure. Every single element has radioisotopes that no longer exist on Earth! Many people are familiar with a chart of the elements Fig. Nuclear chemists and geologists use a different kind of figure to show all of the isotopes. It is called a chart of the nuclides. Figure 7 shows a portion of this chart. It is basically a plot of the number of protons vs.
Recall that an element is defined by how many protons it has. Each element can have a number of different isotopes, that is. Figure 7.
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-lifeyears 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. Cosmogenic Radionuclides: Carbon, Beryllium, Chlorine Extinct Isotope Half-Life.
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 contaminate 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.
Really. And calcium radiometric dating pity, that now
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. Figure 9. 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.
Calcium radiometric dating
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 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 pastyears 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. Ice Cores. 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 asyears. 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 towardsyears. Meese et al. Recently, absolute ages have been determined to 75, years for at least one location using cosmogenic radionuclides chlorine and beryllium G.
Wagner et al. 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.
Table III. 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 toyears 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.
He is afraid to do this with all of the chocolate chips, so instead, each day, he swipes half of the number of remaining chocolate chips and puts raisins in their place, never quite completing his diabolical transformation of your dessert, but getting closer and closer.
Final, sorry, calcium radiometric dating seems
Say a second friend who is aware of this arrangement visits and notices that your carton of ice cream contains 70 raisins and 10 chocolate chips. She declares, "I guess you went shopping about three days ago. Because your roommate eats half of the chips on any given day, and not a fixed number, the carton must have held 20 chips the day before, 40 the day before that, and 80 the day before that.
Calculations involving radioactive isotopes are more formal but follow the same basic principle: If you know the half-life of the radioactive element and can measure how much of each isotope is present, you can figure out the age of the fossil, rock or other entity it comes from.
Elements that have half-lives are said to obey a first-order decay process. They have what is known as a rate constant, usually denoted by k.
The relationship between the number of atoms present at the start N 0the number present at the time of measurement N the elapsed time t, and the rate constant k can be written in two mathematically equivalent ways:. In addition, you may wish to know the activity A of a sample, typically measured in disintegrations per second or dps. This is expressed simply as:. You don't need to know how these equations are derived, but you should be prepared to use them so solve problems involving radioactive isotopes.
Scientists interested in figuring out the age of a fossil or rock analyze a sample to determine the ratio of a given radioactive element's daughter isotope or isotopes to its parent isotope in that sample. With the element's decay rate, and hence its half-life, known in advance, calculating its age is straightforward. The trick is knowing which of the various common radioactive isotopes to look for.
This in turn depends in the approximate expected age of the object because radioactive elements decay at enormously different rates. Also, not all objects to be dated will have each of the elements commonly used; you can only date items with a given dating technique if they include the needed compound or compounds.
Uranium-lead U-Pb dating: Radioactive uranium comes in two forms, uranium and uranium The number refers to the number of protons plus neutrons. Uranium's atomic number is 92, corresponding to its number of protons. The half-life of uranium is 4. Because these differ by a factor of almost seven recall that a billion is 1, times a millionit proves a "check" to make sure you're calculating the age of the rock or fossil properly, making this among the most precise radiometric dating methods.
The long half-lives make this dating technique suitable for especially old materials, from about 1 million to 4. U-Pb dating is complex because of the two isotopes in play, but this property is also what makes it so precise.
The method is also technically challenging because lead can "leak" out of many types of rocks, sometimes making the calculations difficult or impossible.
U-Pb dating is often used to date igneous volcanic rocks, which can be hard to do because of the lack of fossils; metamorphic rocks; and very old rocks. All of these are hard to date with the other methods described here. Rubidium-strontium Rb-Sr dating: Radioactive rubidium decays into strontium with a half -life of Not surprisingly, Ru-Sr dating is used to date very old rocks as old as the Earth, in fact, since the Earth is "only" around 4.
Strontium exists in other stable i. But because rubidium is abundant in the Earth's crust, the concentration of strontium is much higher than that of the other isotopes of strontium. Scientists can then compare the ratio of the strontium to the total amount of stable strontium isotopes to calculate the level of decay that produces the detected concentration of strontium This technique is often used to date igneous rocks and very old rocks.
Potassium-argon K-Ar dating: The radioactive potassium isotope is K, which decays into both calcium Ca and argon Ar in a ratio of Argon is a noble gas, which means that it is nonreactive and would not be a part of the initial formation of any rocks or fossils. Any argon found in a rocks or fossils therefore has to be the result of this kind of radioactive decay.
The half-life of potassium is 1. Potassium is very abundant in the Earth, making it great for dating because it is found in some levels in most kinds of samples. It is good for dating igneous rocks volcanic rocks. Carbon C dating: Carbon enters organisms from the atmosphere. When the organism dies, no more of the carbon isotope can enter the organism, and it will begin to decay starting at that point.