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Petrology Tulane University Prof. Stephen A. Nelson Radiometric Dating Prior to the best and most accepted age of the Earth was that proposed by Lord Kelvin based on the amount of time necessary for the Earth to cool to its present temperature from a completely liquid state. Although we now recognize lots of problems with that calculation, the age of 25 my was accepted by most physicists, but considered too short by most geologists. Then, in , radioactivity was discovered.

Recent article S. Is decay constant? An isotope is a particular type of atom of a chemical element, which differs from other isotopes of that element in the number of neutrons it has in its nucleus.

By definition, all atoms of a given element have the same number of protons. However, they do not all have the same number of neutrons. The different numbers of neutrons possible in the atoms of a given element correspond to the different possible isotopes of that element.

For example, all carbon atoms have 6 protons. Carbon is the isotope of carbon that has 6 neutrons. Carbon is the isotope of carbon that has 7 neutrons. Carbon has 8 neutrons in its nucleus, along with its 6 protons, which is not a stable combination. That is why carbon is a radioactive isotope-it contains a combination of protons and neutrons in its nucleus that is not stable enough to hold together indefinitely. Eventually, it will undergo a spontaneous nuclear reaction and turn into a stable daughter product - a different isotope, which is not radioactive.

Physicists have measured the half-lives of most radioactive isotopes to a high level of precision. The properties of radioactive isotopes and the way they turn into their stable daughter products are not affected by variations in temperature, pressure, or chemistry. Therefore the half-lives and other properties of isotopes are unaffected by the changing conditions that a rock is subjected to as it moves through the rock cycle.

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.

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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.

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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.


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.

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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.

List three methods of radiometric dating - AnalogLamb This method of a method of radiometric dating, lutetium 3%, liquid scintillation counting, and absolute age of the absolute age of dating. A more stable daughter element, it is easy to give millions of links to see. Radioactive parent or gain of radiocarbon dating by more stable state. The earth is billions of years old. The most useful methods for measuring the ages of geologic materials are the radiometric methods-the ones that make use of radioactive parent isotopes and their stable daughter products, as preserved in rocks, minerals, or other geologic materials. The main condition for the method is that the production rate of isotopes stays the same through ages, i.e. 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.

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.

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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.

Dating methods like thermoluminescence, optical stimulating luminescence and electron spin resonance, measure the accumulation of electrons in these imperfections, or "traps," in the crystal.

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. 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.

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.

Radiometric Dating: Carbon-14 and Uranium-238

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.

Three methods radiometric dating


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.








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.






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.



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 thermomeaning heat, and luminescencemeaning to emit light.


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.


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. Cosmic-ray exposure dating. 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. In the example, 14 C is the parent and 14 N is the daughter. Some minerals in rocks and organic matter e. The abundances of parent and daughter isotopes in a sample can be measured and used to determine their age.

This method is known as radiometric dating. Some commonly used dating methods are summarized in Table 1. The rate of decay for many radioactive isotopes has been measured and does not change over time. Thus, each radioactive isotope has been decaying at the same rate since it was formed, ticking along regularly like a clock. For example, when potassium is incorporated into a mineral that forms when lava cools, there is no argon from previous decay argon, a gas, escapes into the atmosphere while the lava is still molten.

When that mineral forms and the rock cools enough that argon can no longer escape, the "radiometric clock" starts. Over time, the radioactive isotope of potassium decays slowly into stable argon, which accumulates in the mineral.

The amount of time that it takes for half of the parent isotope to decay into daughter isotopes is called the half-life of an isotope Figure 5b. When the quantities of the parent and daughter isotopes are equal, one half-life has occurred.

If the half life of an isotope is known, the abundance of the parent and daughter isotopes can be measured and the amount of time that has elapsed since the "radiometric clock" started can be calculated.

For example, if the measured abundance of 14 C and 14 N in a bone are equal, one half-life has passed and the bone is 5, years old an amount equal to the half-life of 14 C.

If there is three times less 14 C than 14 N in the bone, two half lives have passed and the sample is 11, years old. However, if the bone is 70, years or older the amount of 14 C left in the bone will be too small to measure accurately.

Thus, radiocarbon dating is only useful for measuring things that were formed in the relatively recent geologic past. Luckily, there are methods, such as the commonly used potassium-argon K-Ar metho that allows dating of materials that are beyond the limit of radiocarbon dating Table 1.

Comparison of commonly used dating methods.

Non-Radiometric Dating Methods for the Past , Years. 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. 14 Carbon Dating. Radiocarbon dating is different than the other methods of dating because it cannot be used to directly date rocks, but can only be used to date organic material produced by once living organisms. 14 C is continually being produced in the Earth's upper . The Radiometric Dating Game Radiometric dating methods estimate the age of rocks using calculations based on the decay rates of radioactive elements such as uranium, strontium, and potassium. On the surface, radiometric dating methods appear to give powerful support to the statement that life has existed on the earth for hundreds of millions.

Radiation, which is a byproduct of radioactive decay, causes electrons to dislodge from their normal position in atoms and become trapped in imperfections in the crystal structure of the material. Dating methods like thermoluminescenceoptical stimulating luminescence and electron spin resonancemeasure the accumulation of electrons in these imperfections, or "traps," in the crystal structure of the material. If the amount of radiation to which an object is exposed remains constant, the amount of electrons trapped in the imperfections in the crystal structure of the material will be proportional to the age of the material.

These methods are applicable to materials that are up to aboutyears old. However, once rocks or fossils become much older than that, all of the "traps" in the crystal structures become full and no more electrons can accumulate, even if they are dislodged. The Earth is like a gigantic magnet. It has a magnetic north and south pole and its magnetic field is everywhere Figure 6a. Just as the magnetic needle in a compass will point toward magnetic north, small magnetic minerals that occur naturally in rocks point toward magnetic north, approximately parallel to the Earth's magnetic field.

Because of this, magnetic minerals in rocks are excellent recorders of the orientation, or polarityof the Earth's magnetic field. Small magnetic grains in rocks will orient themselves to be parallel to the direction of the magnetic field pointing towards the north pole. Black bands indicate times of normal polarity and white bands indicate times of reversed polarity. Through geologic time, the polarity of the Earth's magnetic field has switched, causing reversals in polarity.

The Earth's magnetic field is generated by electrical currents that are produced by convection in the Earth's core. During magnetic reversals, there are probably changes in convection in the Earth's core leading to changes in the magnetic field.

The Earth's magnetic field has reversed many times during its history. When the magnetic north pole is close to the geographic north pole as it is todayit is called normal polarity. Reversed polarity is when the magnetic "north" is near the geographic south pole. Using radiometric dates and measurements of the ancient magnetic polarity in volcanic and sedimentary rocks termed paleomagnetismgeologists have been able to determine precisely when magnetic reversals occurred in the past.

Combined observations of this type have led to the development of the geomagnetic polarity time scale GPTS Figure 6b. The GPTS is divided into periods of normal polarity and reversed polarity. Geologists can measure the paleomagnetism of rocks at a site to reveal its record of ancient magnetic reversals. Every reversal looks the same in the rock record, so other lines of evidence are needed to correlate the site to the GPTS.

Information such as index fossils or radiometric dates can be used to correlate a particular paleomagnetic reversal to a known reversal in the GPTS. Once one reversal has been related to the GPTS, the numerical age of the entire sequence can be determined.

Using a variety of methods, geologists are able to determine the age of geological materials to answer the question: "how old is this fossil? These methods use the principles of stratigraphy to place events recorded in rocks from oldest to youngest. Absolute dating methods determine how much time has passed since rocks formed by measuring the radioactive decay of isotopes or the effects of radiation on the crystal structure of minerals.

Paleomagnetism measures the ancient orientation of the Earth's magnetic field to help determine the age of rocks. Deino, A. Evolutionary Anthropology 6 : Faure, G. Isotopes: Principles and Applications. Third Edition. New York: John Wiley and Sons Gradstein, F. The Geologic Time Scale2-volume set. Waltham, MA: Elsevier Ludwig, K. Geochronology on the paleoanthropological time scale, Evolutionary Anthropology 9, McDougall I.

Tauxe, L. Essentials of paleomagnetism. Characteristics of Crown Primates. How to Become a Primate Fossil. Primate Cranial Diversity.

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