This predictability allows the relative abundances of related nuclides to be used as a clock to measure the time from the incorporation of the original nuclides into a material to the present. The basic equation of radiometric dating requires that neither the parent nuclide nor the daughter product can enter or leave the material after its formation. The possible confounding effects of contamination of parent and daughter isotopes have to be considered, as do the effects of any loss or gain of such isotopes since the sample was created.
It is therefore essential to have as much information as possible about the material being dated and to check for possible signs of alteration. Alternatively, if several different minerals can be dated from the same sample and are assumed to be formed by the same event and were in equilibrium with the reservoir when they formed, they should form an isochron. This can reduce the problem of contamination. In uranium—lead dating , the concordia diagram is used which also decreases the problem of nuclide loss.
Finally, correlation between different isotopic dating methods may be required to confirm the age of a sample. For example, the age of the Amitsoq gneisses from western Greenland was determined to be 3. Accurate radiometric dating generally requires that the parent has a long enough half-life that it will be present in significant amounts at the time of measurement except as described below under "Dating with short-lived extinct radionuclides" , the half-life of the parent is accurately known, and enough of the daughter product is produced to be accurately measured and distinguished from the initial amount of the daughter present in the material.
The procedures used to isolate and analyze the parent and daughter nuclides must be precise and accurate. This normally involves isotope-ratio mass spectrometry. The precision of a dating method depends in part on the half-life of the radioactive isotope involved.
For instance, carbon has a half-life of 5, years. After an organism has been dead for 60, years, so little carbon is left that accurate dating cannot be established. On the other hand, the concentration of carbon falls off so steeply that the age of relatively young remains can be determined precisely to within a few decades. If a material that selectively rejects the daughter nuclide is heated, any daughter nuclides that have been accumulated over time will be lost through diffusion , setting the isotopic "clock" to zero.
The temperature at which this happens is known as the closure temperature or blocking temperature and is specific to a particular material and isotopic system. These temperatures are experimentally determined in the lab by artificially resetting sample minerals using a high-temperature furnace. As the mineral cools, the crystal structure begins to form and diffusion of isotopes is less easy. At a certain temperature, the crystal structure has formed sufficiently to prevent diffusion of isotopes. This temperature is what is known as closure temperature and represents the temperature below which the mineral is a closed system to isotopes.
Thus an igneous or metamorphic rock or melt, which is slowly cooling, does not begin to exhibit measurable radioactive decay until it cools below the closure temperature. The age that can be calculated by radiometric dating is thus the time at which the rock or mineral cooled to closure temperature. This field is known as thermochronology or thermochronometry.
The mathematical expression that relates radioactive decay to geologic time is  . The equation is most conveniently expressed in terms of the measured quantity N t rather than the constant initial value N o. The above equation makes use of information on the composition of parent and daughter isotopes at the time the material being tested cooled below its closure temperature.
This is well-established for most isotopic systems. Plotting an isochron is used to solve the age equation graphically and calculate the age of the sample and the original composition. Radiometric dating has been carried out since when it was invented by Ernest Rutherford as a method by which one might determine the age of the Earth. In the century since then the techniques have been greatly improved and expanded. The mass spectrometer was invented in the s and began to be used in radiometric dating in the s.
It operates by generating a beam of ionized atoms from the sample under test. The ions then travel through a magnetic field, which diverts them into different sampling sensors, known as " Faraday cups ", depending on their mass and level of ionization. On impact in the cups, the ions set up a very weak current that can be measured to determine the rate of impacts and the relative concentrations of different atoms in the beams.
Uranium—lead radiometric dating involves using uranium or uranium to date a substance's absolute age. This scheme has been refined to the point that the error margin in dates of rocks can be as low as less than two million years in two-and-a-half billion years. Uranium—lead dating is often performed on the mineral zircon ZrSiO 4 , though it can be used on other materials, such as baddeleyite , as well as monazite see: Zircon has a very high closure temperature, is resistant to mechanical weathering and is very chemically inert.
Zircon also forms multiple crystal layers during metamorphic events, which each may record an isotopic age of the event. One of its great advantages is that any sample provides two clocks, one based on uranium's decay to lead with a half-life of about million years, and one based on uranium's decay to lead with a half-life of about 4. This can be seen in the concordia diagram, where the samples plot along an errorchron straight line which intersects the concordia curve at the age of the sample. This involves the alpha decay of Sm to Nd with a half-life of 1.
Accuracy levels of within twenty million years in ages of two-and-a-half billion years are achievable. This involves electron capture or positron decay of potassium to argon Potassium has a half-life of 1. This is based on the beta decay of rubidium to strontium , with a half-life of 50 billion years. This scheme is used to date old igneous and metamorphic rocks , and has also been used to date lunar samples.
Closure temperatures are so high that they are not a concern. Rubidium-strontium dating is not as precise as the uranium-lead method, with errors of 30 to 50 million years for a 3-billion-year-old sample. A relatively short-range dating technique is based on the decay of uranium into thorium, a substance with a half-life of about 80, years.
It is accompanied by a sister process, in which uranium decays into protactinium, which has a half-life of 32, years. While uranium is water-soluble, thorium and protactinium are not, and so they are selectively precipitated into ocean-floor sediments , from which their ratios are measured. The scheme has a range of several hundred thousand years.
A related method is ionium—thorium dating , which measures the ratio of ionium thorium to thorium in ocean sediment.
Say for example that a volcanic dike, or a fault, cuts across several sedimentary layers, or maybe through another volcanic rock type. Pretty obvious that the dike came after the rocks it cuts through, right? With absolute age dating, you get a real age in actual years. Based on the Rule of Superposition, certain organisms clearly lived before others, during certain geologic times.
The narrower a range of time that an animal lived, the better it is as an index of a specific time. No bones about it, fossils are important age markers. But the most accurate forms of absolute age dating are radiometric methods. This method works because some unstable radioactive isotopes of some elements decay at a known rate into daughter products. This rate of decay is called a half-life. Half-life simply means the amount of time it takes for half of a remaining particular isotope to decay to a daughter product.
Good discussion from the US Geological Survey: So geochronolgists just measure the ratio of the remaining parent atom to the amount of daughter and voila, they know how long the molecule has been hanging out decaying. There are a couple catches, of course. Not all rocks have radioactive elements. Sedimentary rocks in particular are notoriously radioactive-free zones. So to date those, geologists look for layers like volcanic ash that might be sandwiched between the sedimentary layers, and that tend to have radioactive elements. You might have noticed that many of the oldest age dates come from a mineral called zircon.
The longer the radiation exposure, the more electrons get bumped into an excited state. With more electrons in an excited state, more light is emitted upon heating. The process of displacing electrons begins again after the object cools. Scientists can determine how many years have passed since a ceramic was fired by heating it in the laboratory and measuring how much light is given off. Thermoluminescence dating has the advantage of covering the time interval between radiocarbon and potassium-argon dating , or 40, — , years. In addition, it can be used to date materials that cannot be dated with these other two methods.
Optically stimulated luminescence OSL has only been used since It is very similar to thermoluminescence dating, both of which are considered "clock setting" techniques. Minerals found in sediments are sensitive to light. Electrons found in the sediment grains leave the ground state when exposed to light, called recombination. To determine the age of sediment, scientists expose grains to a known amount of light and compare these grains with the unknown sediment. This technique can be used to determine the age of unheated sediments less than , years old.
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A disadvantage to this technique is that in order to get accurate results, the sediment to be tested cannot be exposed to light which would reset the "clock" , making sampling difficult. The absolute dating method utilizing tree ring growth is known as dendrochronology. It is based on the fact that trees produce one growth ring each year. The rings form a distinctive pattern, which is the same for all members in a given species and geographical area. The patterns from trees of different ages including ancient wood are overlapped, forming a master pattern that can be used to date timbers thousands of years old with a resolution of one year.
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Timbers can be used to date buildings and archaeological sites. In addition, tree rings are used to date changes in the climate such as sudden cool or dry periods. Dendrochronology has a range of one to 10, years or more. As previously mentioned, radioactive decay refers to the process in which a radioactive form of an element is converted into a decay product at a regular rate.
Radioactive decay dating is not a single method of absolute dating but instead a group of related methods for absolute dating of samples. Potassium-argon dating relies on the fact that when volcanic rocks are heated to extremely high temperatures, they release any argon gas trapped in them. As the rocks cool, argon 40 Ar begins to accumulate. Argon is formed in the rocks by the radioactive decay of potassium 40 K.
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The amount of 40 Ar formed is proportional to the decay rate half-life of 40 K, which is 1. In other words, it takes 1. This method is generally only applicable to rocks greater than three million years old, although with sensitive instruments, rocks several hundred thousand years old may be dated. The reason such old material is required is that it takes a very long time to accumulate enough 40 Ar to be measured accurately.
Potassium-argon dating has been used to date volcanic layers above and below fossils and artifacts in east Africa. Radiocarbon dating is used to date charcoal, wood, and other biological materials. The range of conventional radiocarbon dating is 30, — 40, years, but with sensitive instrumentation, this range can be extended to 70, years. Radiocarbon 14 C is a radioactive form of the element carbon.
It decays spontaneously into nitrogen 14 N. Plants get most of their carbon from the air in the form of carbon dioxide , and animals get most of their carbon from plants or from animals that eat plants. Relative to their atmospheric proportions, atoms of 14 C and of a non-radioactive form of carbon, 12 C, are equally likely to be incorporated into living organisms.
When the organism dies, however, its body stops incorporating new carbon. The ratio will then begin to change as the 14 C in the dead organism decays into 14 N. The rate at which this process occurs is called the half-life. This is the time required for half of the 14 C to decay into 14 N. The half-life of 14 C is 5, years. This allows them to determine how much 14 C has formed since the death of the organism.
One of the most familiar applications of radioactive dating is determining the age of fossilized remains, such as dinosaur bones. Radioactive dating is also used to authenticate the age of rare archaeological artifacts. Because items such as paper documents and cotton garments are produced from plants, they can be dated using radiocarbon dating. Without radioactive dating , a clever forgery might be indistinguishable from a real artifact.
There are some limitations, however, to the use of this technique.
Samples that were heated or irradiated at some time may yield by radioactive dating an age less than the true age of the object. Because of this limitation, other dating techniques are often used along with radioactive dating to ensure accuracy. Uranium series dating techniques rely on the fact that radioactive uranium and thorium isotopes decay into a series of unstable, radioactive "daughter" isotopes; this process continues until a stable non-radioactive lead isotope is formed.
The daughters have relatively short half-lives ranging from a few hundred thousand years down to only a few years. The "parent" isotopes have half-lives of several billion years. This provides a dating range for the different uranium series of a few thousand years to , years. Uranium series have been used to date uranium-rich rocks, deep-sea sediments, shells, bones, and teeth, and to calculate the ages of ancient lakebeds.
The two types of uranium series dating techniques are daughter deficiency methods and daughter excess methods. In daughter deficiency situations, the parent radioisotope is initially deposited by itself, without its daughter the isotope into which it decays present. Through time, the parent decays to the daughter until the two are in equilibrium equal amounts of each. The age of the deposit may be determined by measuring how much of the daughter has formed, providing that neither isotope has entered or exited the deposit after its initial formation.
Living mollusks and corals will only take up dissolved compounds such as isotopes of uranium, so they will contain no protactinium, which is insoluble. Protactinium begins to accumulate via the decay of U after the organism dies. Scientists can determine the age of the sample by measuring how much Pa is present and calculating how long it would have taken that amount to form. In the case of daughter excess, a larger amount of the daughter is initially deposited than the parent. Non-uranium daughters such as protactinium and thorium are insoluble, and precipitate out on the bottoms of bodies of water, forming daughter excesses in these sediments.
Over time, the excess daughter disappears as it is converted back into the parent, and by measuring the extent to which this has occurred, scientists can date the sample. If the radioactive daughter is an isotope of uranium, it will dissolve in water, but to a different extent than the parent; the two are said to have different solubilities. For example, U dissolves more readily in water than its parent, U, so lakes and oceans contain an excess of this daughter isotope. Some volcanic minerals and glasses, such as obsidian , contain uranium U.
Over time, these substances become "scratched. When an atom of U splits, two "daughter" atoms rocket away from each other, leaving in their wake tracks in the material in which they are embedded. The rate at which this process occurs is proportional to the decay rate of U. The decay rate is measured in terms of the half-life of the element, or the time it takes for half of the element to split into its daughter atoms. The half-life of U is 4. When the mineral or glass is heated, the tracks are erased in much the same way cut marks fade away from hard candy that is heated.
This process sets the fission track clock to zero, and the number of tracks that then form are a measure of the amount of time that has passed since the heating event. Scientists are able to count the tracks in the sample with the aid of a powerful microscope.
The sample must contain enough U to create enough tracks to be counted, but not contain too much of the isotope, or there will be a jumble of tracks that cannot be distinguished for counting. One of the advantages of fission track dating is that it has an enormous dating range. Objects heated only a few decades ago may be dated if they contain relatively high levels of U; conversely, some meteorites have been dated to over a billion years old with this method.
Although certain dating techniques are accurate only within certain age ranges, whenever possible, scientists attempt to use multiple methods to date specimens. Correlation of dates via different dating methods provides a highest degree of confidence in dating.
See also Evolution, evidence of; Fossil record; Fossils and fossilization; Geologic time; Historical geology. Cite this article Pick a style below, and copy the text for your bibliography. Retrieved January 12, from Encyclopedia. Then, copy and paste the text into your bibliography or works cited list.
Because each style has its own formatting nuances that evolve over time and not all information is available for every reference entry or article, Encyclopedia. Movies and television have presented a romantic vision of archaeology as adventure in far-away and exotic locations. A more realistic picture might show researchers digging in smelly mud for hours under the hot sun while battling relentless mosquitoes.
This type of archaeological research produces hundreds of small plastic bags containing pottery shards, animal bones, bits of worked stone, and other fragments. These findings must be classified, which requires more hours of tedious work in a stuffy tent. At its best, archaeology involves a studious examination of the past with the goal of learning important information about the culture and customs of ancient or not so ancient peoples. Much archaeology in the early twenty-first century investigates the recent past, a sub-branch called "historical archaeology.
Archaeology is the study of the material remains of past human cultures. It is distinguished from other forms of inquiry by its method of study, excavation. Most archaeologists call this "digging. That sort of unscientific digging destroys the archaeological information. Archaeological excavation requires the removal of material layer by layer to expose artifacts in place. The removed material is carefully sifted to find small artifacts , tiny animal bones, and other remains.
Archaeologists even examine the soil in various layers for microscopic material, such as pollen. Excavations, in combination with surveys, may yield maps of a ruin or collections of artifacts. Time is important to archaeologists. There is rarely enough time to complete the work, but of even greater interest is the time that has passed since the artifact was created. An important part of archaeology is the examination of how cultures change over time. It is therefore essential that the archaeologist is able to establish the age of the artifacts or other material remains and arrange them in a chronological sequence.
The archaeologist must be able to distinguish between objects that were made at the same time and objects that were made at different times. When objects that were made at different times are excavated, the archaeologist must be able to arrange them in a sequence from the oldest to the most recent. Before scientific dating techniques such as dendrochronology and radiocarbon dating were introduced to archaeology, the discipline was dominated by extensive discussions of the chronological sequence of events. Most of those questions have now been settled and archaeologists have moved on to other issues.
Scientific dating techniques have had a huge impact on archaeology. Archaeologists use many different techniques to determine the age of an object. Usually, several different techniques are applied to the same object. Relative dating arranges artifacts in a chronological sequence from oldest to most recent without reference to the actual date. For example, by studying the decorations used on pottery, the types of materials used in the pottery, and the types and shapes of pots, it is often possible to arrange them into a sequence without knowing the actual date.
In absolute dating , the age of an object is determined by some chemical or physical process without reference to a chronology. The most common and widely used relative dating technique is stratigraphy. The principle of superposition borrowed from geology states that higher layers must be deposited on top of lower layers. Thus, higher layers are more recent than lower layers. This only applies to undisturbed deposits.
Rodent burrows, root action, and human activity can mix layers in a process known as bioturbation. However, the archaeologist can detect bioturbation and allow for its effects. Discrete layers of occupation can often be determined. For example, Hisarlik, which is a hill in Turkey , is thought by some archaeologists to be the site of the ancient city of Troy. However, Hisarlik was occupied by many different cultures at various times both before and after the time of Troy, and each culture built on top of the ruins of the previous culture, often after violent conquest.
Consequently, the layers in this famous archaeological site represent many different cultures. An early excavator of Hisarlik, Heinrich Schleimann, inadvertently dug through the Troy layer into an earlier occupation and mistakenly assigned the gold artifacts he found there to Troy. Other sites have been continuously occupied by the same culture for a long time and the different layers represent gradual changes.
In both cases, stratigraphy will apply. A chronology based on stratigraphy often can be correlated to layers in other nearby sites. For example, a particular type or pattern of pottery may occur in only one layer in an excavation. If the same pottery type is found in another excavation nearby, it is safe to assume that the layers are the same age. Archaeologists rarely make these determinations on the basis of a single example.
Usually, a set of related artifacts is used to determine the age of a layer. Seriation simply means ordering. This technique was developed by the inventor of modern archaeology, Sir William Matthew Flinders Petrie. Seriation is based on the assumption that cultural characteristics change over time. For example, consider how automobiles have changed in the last 50 years a relatively short time in archaeology. Automobile manufacturers frequently introduce new styles about every year, so archaeologists thousands of years from now will have no difficulty identifying the precise date of a layer if the layer contains automobile parts.
Cultural characteristics tend to show a particular pattern over time. The characteristic is introduced into the culture for example, using a certain type of projectile point for hunting or wearing low-riding jeans , becomes progressively more popular, then gradually wanes in popularity. The method of seriation uses this distinctive pattern to arrange archaeological materials into a sequence.
However, seriation only works when variations in a cultural characteristic are due to rapid and significant change over time. It also works best when a characteristic is widely shared among many different members of a group. Even then, it can only be applied to a small geographic area, because there is also geographic variation in cultural characteristics.
For example, 50 years ago American automobiles changed every year while the Volkswagen Beetle hardly changed at all from year to year. Cross dating is also based on stratigraphy. It uses the principle that different archaeological sites will show a similar collection of artifacts in layers of the same age. Sir Flinders Petrie used this method to establish the time sequence of artifacts in Egyptian cemeteries by identifying which burials contained Greek pottery vessels. These same Greek pottery styles could be associated with monuments in Greece whose construction dates were fairly well known.
Since absolute dating techniques have become common, the use of cross dating has decreased significantly. Pollen grains also appear in archaeological layers. They are abundant and they survive very well in archaeological contexts. As climates change over time, the plants that grow in a region change as well. People who examine pollen grains the study of which is known as pollen analysis can usually determine the genus , and often the exact species producing a certain pollen type. Archaeologists can then use this information to determine the relative ages of some sites and layers within sites.
However, climates do not change rapidly, so this type of analysis is best for archaeological sites dating back to the last ice age. Absolute dating methods produce an actual date, usually accurate to within a few years.