dating range

Radiometric Age Dating. Radiometric dating calculates an age in years for geologic materials by measuring the presence of a short-life radioactive element, e.g., carbon-14, or a long-life radioactive element plus its decay product, e.g., potassium-14/argon-40. The term applies to all methods of age determination based on nuclear decay of naturally occurring radioactive isotopes. Bates and Jackson (1984) To determine the ages in years of Earth materials and the timing of geologic events such as exhumation and subduction, geologists utilize the process of radiometric decay. Geologists use these dates to further define the boundaries of the geologic periods shown on the geologic time scale. Radiometric decay occurs when the nucleus of a radioactive atom spontaneously transforms into an atomic nucleus of a different, more stable isotope. This transformation happens via the emission of particles such as electrons (known as beta decay) and alpha particles. For instance, rubidium-87 (87Rb), an unstable element, becomes strontium-87 (87Sr), a stable element, via beta decay. As explained on WebGeology from the University of Tormsø, Norway: One neutron of the nucleus emits a beta particle, which is identical to an electron. In addition the neutron emits a neutral particle that is called an antineutrino. By emitting a beta particle, the neutron is transformed into a proton. This results in a nucleus composed of 38 protons and 49 neutrons, corresponding to strontium’s nucleus of 87 atomic particles. Energy is released during this process. The rubidium-strontium method has been a popular method to determine the absolute age of geological processes. When discussing decay rates, scientists refer to “half-lives”—the length of time it takes for one-half of the original atom of the radioactive isotope to decay into an atom of a new isotope. Because decay occurs at a fixed rate (this is the key point), scientists can measure the amount of decayed material in the sample, determine the ratio between original and decayed material, and then calculate the sample’s age. Depending on the half-life and the material being dated, various methods are used. For instance, geologists use the Sm-Nd (samarium-147/neodymium-143) method for determining the age of very old materials (e.g., meteorites and metamorphic rocks) or when a rock became crystallized (in the mantle) or metamorphosed (at a subduction zone). For young organic materials, the carbon-14 (radiocarbon) method is used. The effective of the carbon-14 method is between 100 and 50,000 years. Many rock-forming minerals (e.g., biotite, muscovite, amphibole, and Potassium feldspar) Whole crushed metamorphic or igneous rock. Many rock-forming minerals (e.g., biotite, muscovite, amphibole, and Potassium feldspar) Whole crushed volcanic rocks (e.g., lava flows and ash) Dating Techniques. Dating techniques are procedures used by scientists to determine the age of rocks, fossils, or artifacts. Relative dating methods tell only if one sample is older or younger than another; absolute dating methods provide an approximate date in years. The latter have generally been available only since 1947. Many absolute dating techniques take advantage of radioactive decay, whereby a radioactive form of an element decays into a non-radioactive product at a regular rate. Others, such as amino acid racimization and cation-ratio dating, are based on chemical changes in the organic or inorganic composition of a sample. In recent years, a few of these methods have come under close scrutiny as scientists strive to develop the most accurate dating techniques possible. Relative dating. Relative dating methods determine whether one sample is older or younger than another. They do not provide an age in years. Before the advent of absolute dating methods, nearly all dating was relative. The main relative dating method is stratigraphy. Stratigraphy. Stratigraphy is the study of layers of rocks or the objects embedded within those layers. It is based on the assumption (which nearly always holds true) that deeper layers were deposited earlier, and thus are older, than more shallow layers. The sequential layers of rock represent sequential intervals of time. Although these units may be sequential, they are not necessarily continuous due to erosional removal of some intervening. units. The smallest of these rock units that can be matched to a specific time interval is called a bed. Beds that are related are grouped together into members, and members are grouped into formations. Stratigraphy is the principle method of relative dating, and in the early years of dating studies was virtually the only method available to scientists. Seriation. Seriation is the ordering of objects according to their age. It is a relative dating method. In a landmark study, archaeologist James Ford used seriation to determine the chronological order of American Indian pottery styles in the Mississippi Valley. Artifact styles such as pottery types are seriated by analyzing their abundances through time. This is done by counting the number of pieces of each style of the artifact in each stratigraphic layer and then graphing the data. A layer with many pieces of a particular style will be represented by a wide band on the graph, and a layer with only a few pieces will be represented by a narrow band. The bands are arranged into battleship-shaped curves, with each style getting its own curve. The curves are then compared with one another, and from this the relative ages of the styles are determined. A limitation to this method is that it assumes all differences in artifact styles are the result of different periods of time, and are not due to the immigration of new cultures into the area of study. Faunal dating. The term faunal dating refers to the use of animal bones to determine the age of sedimentary layers or objects such as cultural artifacts embedded within those layers. Scientists can determine an approximate age for a layer by examining which species or genera of animals are buried in it. The technique works best if the animals belonged to species, which evolved quickly, expanded rapidly over a large area, or suffered a mass extinction. In addition to providing rough absolute dates for specimens buried in the same stratigraphic unit as the bones, faunal analysis can also provide relative ages for objects buried above or below the fauna-encasing layers. Pollen dating (palynology) Each year seed-bearing plants release large numbers of pollen grains. This process results in a “ rain ” of pollen that falls over many types of environments. Pollen that ends up in lakebeds or peat bogs is the most likely to be preserved, but pollen may also become fossilized in arid conditions if the soil is acidic or cool. Scientists can develop a pollen chronology, or calendar, by noting which species of pollen were deposited earlier in time, that is, residue in deeper sediment or rock layers, than others. The unit of the calendar is the pollen zone. A pollen zone is a period of time in which a particular species is much more abundant than any other species of the time. In most cases, this tells us about the climate of the period, because most plants only thrive in specific climatic conditions. Changes in pollen zones can also indicate changes in human activities such as massive deforestation or new types of farming. Pastures for grazing livestock are distinguishable from fields of grain, so changes in the use of the land over time are recorded in the pollen history. The dates when areas of North America were first settled by immigrants can be determined to within a few years by looking for the introduction of ragweed pollen. Pollen zones are translated into absolute dates by the use of radiocarbon dating. In addition, pollen dating provides relative dates beyond the limits of radiocarbon (40, 000 years), and can be used in some places where radiocarbon dates are unobtainable. Fluorine is found naturally in ground water. This water comes in contact with skeletal remains under ground. When this occurs, the fluorine in the water saturates the bone, changing the mineral composition. Over time, more and more fluorine incorporates itself into the bone. By comparing the relative amounts of fluorine composition of skeletal remains, one can determine whether the remains were buried at the same time. A bone with a higher fluorine composition has been buried for a longer period of time. Absolute dating. Absolute dating is the term used to describe any dating technique that tells how old a specimen is in years. These are generally analytical methods, and are carried out in a laboratory. Absolute dates are also relative dates, in that they tell which specimens are older or younger than others. Absolute dates must agree with dates from other relative methods in order to be valid. Amino acid racimization. This dating technique was first conducted by Hare and Mitterer in 1967, and was popular in the 1970s. It requires a much smaller sample than radiocarbon dating, and has a longer range, extending up to a few hundred thousand years. It has been used to date coprolites (fossilized feces) as well as fossil bones and shells. These types of specimens contain proteins embedded in a network of minerals such as calcium. Amino acid racimization is based on the principle that amino acids (except glycine, which is a very simple amino acid) exist in two mirror image forms called stereoisomers. Living organisms (with the exception of some microbes) synthesize and incorporate only the L-form into proteins. This means that the ratio of the D-form to the L-form is zero (D/L = 0). When these organisms die, the L-amino acids are slowly converted into D-amino acids in a process called racimization. This occurs because protons (H +) are removed from the amino acids by acids or bases present in the burial environment. The protons are quickly replaced, but will return to either side of the amino acid, not necessarily to the side from which they came. This may form a D-amino acid instead of an L-amino acid. The reversible reaction eventually creates equal amounts of Land D-forms (D/L = 1.0). The rate at which the reaction occurs is different for each amino acid; in addition, it depends upon the moisture, temperature, and pH of the postmortem conditions. The higher the temperature, the faster the reaction occurs, so the cooler the burial environment, the greater the . The burial conditions are not always known, however, and can be difficult to estimate. For this reason, and because some of the amino acid racimization dates have disagreed with dates achieved by other methods, the technique is no longer widely used. Cation-ratio dating. Cation-ratio dating is used to date rock surfaces such as stone artifacts and cliff and ground drawings. It can be used to obtain dates that would be unobtainable by more conventional methods such as radio-carbon dating. Scientists use cation-ratio dating to determine how long rock surfaces have been exposed. They do this by chemically analyzing the varnish that forms on these surfaces. The varnish contains cations, which are positively charged atoms or molecules. Different cations move throughout the environment at different rates, so the ratio of different cations to each other changes over time. Cation ratio dating relies on the principle that the cation ratio (K + + Ca 2+)/Ti 4+ decreases with increasing age of a sample. By calibrating these ratios with dates obtained from rocks from a similar microenvironment, a minimum age for the varnish can be determined. This technique can only be applied to rocks from desert areas, where the varnish is most stable. Although cation-ratio dating has been widely used, recent studies suggest it has many problems. Many of the dates obtained with this method are inaccurate due to improper chemical analyses. In addition, the varnish may not actually be stable over long periods of time. Finally, some scientists have recently suggested that the cation ratios may not even be directly related to the age of the sample. Thermoluminescence dating. Thermoluminescence dating is useful for determining the age of pottery. Electrons from quartz and other minerals in the pottery clay are bumped out of their normal positions (ground state) when the clay is exposed to radiation. This radiation may come from radioactive substances such as uranium, present in the clay or burial medium, or from cosmic radiation. When the ceramic is heated to a very high temperature (over 932 ° F [500 ° C]), these electrons fall back to the ground state, emitting light in the process and resetting the “ clock ” to zero. The longer the exposure to the radiation, the more electrons that are bumped into an excited state, and the more light that 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 piece 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, 000 – 200, 000 years. In addition, it can be used to date materials that cannot be dated with these other two methods. Optically stimulated luminescence has only been used since 1984. 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 a 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 500, 000 years old. 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. Tree-ring dating. This absolute dating method is also known as dendrochronology. It is based on the fact that trees produce one growth ring each year. Narrow rings grow in cold and/or dry years, and wide rings grow in warm years with plenty of moisture. 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. 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 1-10, 000 years or more. Radioactive decay dating. As previously mentioned, radioactive decay refers to the process in which a radioactive form of an element is converted into a nonradioactive 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. When volcanic rocks are heated to extremely high temperatures, they release any argon gas trapped in them. As the rocks cool, argon-40 (40 Ar) begins to accumulate. Argon-40 is formed in the rocks by the radioactive decay of potassium-40 (40 K). The amount of 40 Ar formed is proportional to the decay rate (half-life) of 40 K, which is 1.3 billion years. In other words, it takes 1.3 billions years for half of the 40 K originally present to be converted into 40 Ar. 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. Radiocarbon is used to date charcoal, wood, and other biological materials. The range of conventional radiocarbon dating is 30, 000 – 40, 000 years, but with sensitive instrumentation this range can be extended to 70, 000 years. Radiocarbon (14 C) is a radioactive form of the element carbon. It decays spontaneously into nitrogen-14 (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). Atoms of 14 C and of a non-radioactive form of carbon, 12 C, are equally likely to be incorporated into living organisms – there is no discrimination. While a plant or animal is alive, the ratio of 14 C/ 12 C in its body will be nearly the same as the 14 C/ 12 C ratio in the atmosphere. 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, 730 years. Scientists can tell how many years have elapsed since an organism died by comparing the 14 C/ 12 C ratio in the remains with the ratio in the atmosphere. This allows us to determine how much 14 C has formed since the death of the organism. A problem with radiocarbon dating is that diagenic (after death) contamination of a specimen from soil, water, etc. can add carbon to the sample and affect the measured ratios. This can lead to inaccurate dates. Another problem lies with the assumptions associated with radiocarbon dating. One assumption is that the 14 C/ 12 C ratio in the atmosphere is constant though time. This is not completely true. Although 14 C levels can be measured in tree rings and used to correct for the 14 C/ 12 C ratio in the atmosphere at the time the organism died, and can even be used to calibrate some dates directly, radiocarbon remains a more useful relative dating technique than an absolute one. Uranium series dating. 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 thousand million years. This provides a for the different uranium series of a few thousand years to 500, 000 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 lake beds. 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. Carbonates may be dated this way using, for example, the daughter/parent isotope pair protactinium-231/uranium-235 (231 Pa/ 235 U). 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-231 begins to accumulate via the decay of 235 U after the organism dies. Scientists can determine the age of the sample by measuring how much 231 Pa is present and calculating how long it would have taken that amount to form. In the case of a 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, 234 U dissolves more readily in water than its parent, 238 U, so lakes and oceans contain an excess of this daughter isotope. This excess is transferred to organisms such as mollusks or corals, and is the basis of 234 U/ 238 U dating. Fission track dating. Some volcanic minerals and glasses, such as obsidian, contain uranium-238 (238 U). Over time, these substances become “ scratched. ” The marks, called tracks, are the damage caused by the fission (splitting) of the uranium atoms. When an atom of 238 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 238 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 238 U is 4.47 × 10 9 years. 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 238 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 . Objects heated only a few decades ago may be dated if they contain relatively high levels of 238 U; conversely, some meteorites have been dated to over a billion years old with this method. Resources. BOOKS. PERIODICALS. Citation styles. Encyclopedia.com gives you the ability to cite reference entries and articles according to common styles from the Modern Language Association (MLA), The Chicago Manual of Style, and the American Psychological Association (APA). Within the “Cite this article” tool, pick a style to see how all available information looks when formatted according to that style. 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.com cannot guarantee each citation it generates. Therefore, it’s best to use Encyclopedia.com citations as a starting point before checking the style against your school or publication’s requirements and the most-recent information available at these sites: Modern Language Association. The Chicago Manual of Style. American Psychological Association. Notes: Most online reference entries and articles do not have page numbers. Therefore, that information is unavailable for most Encyclopedia.com content. However, the date of retrieval is often important. Refer to each style’s convention regarding the best way to format page numbers and retrieval dates. In addition to the MLA, Chicago, and APA styles, your school, university, publication, or institution may have its own requirements for citations. Therefore, be sure to refer to those guidelines when editing your bibliography or works cited list. Dating Methods. Dating methods. Dating techniques are procedures used by scientists to determine the age of a specimen. Relative dating methods tell only if one sample is older or younger than another sample; absolute dating methods provide a date in years. The latter have generally been available only since 1947. Many absolute dating techniques take advantage of radioactive decay, whereby a radioactive form of an element is converted into another radioactive isotope or non-radioactive product at a regular rate. Others, such as amino acid racimization and cation-ratio dating, are based on chemical changes in the organic or inorganic composition of a sample. In recent years, a few of these methods have undergone continual refinement as scientists strive to develop the most accurate dating techniques possible. Relative dating methods determine whether one sample is older or younger than another. They do not provide an age in years. Before the advent of absolute dating methods, nearly all dating was relative. The main relative dating method is stratigraphy. Stratigraphy is the study of layers of rocks or the objects embedded within those layers. It is based on the assumption (which, except at unconformities, nearly always holds true) that deeper layers were deposited earlier, and thus are older than more shallow layers. The sequential layers of rock represent sequential intervals of time. Although these units may be sequential, they are not necessarily continuous due to erosional removal of some intervening units. The smallest of these rock units that can be matched to a specific time interval is called a bed. Beds that are related are grouped together into members, and members are grouped into formations. Seriation is the ordering of objects according to their age. It is a relative dating method. In a landmark study, archaeologist James Ford used seriation to determine the chronological order of American Indian pottery styles in the Mississippi Valley. Artifact styles such as pottery types are seriated by analyzing their abundances through time. This is done by counting the number of pieces of each style of the artifact in each stratigraphic layer and then graphing the data. A layer with many pieces of a particular style will be represented by a wide band on the graph, and a layer with only a few pieces will be represented by a narrow band. The bands are arranged into battleship-shaped curves, with each style getting its own curve. The curves are then compared with one another, and from this the relative ages of the styles are determined. A limitation to this method is that it assumes all differences in artifact styles are the result of different periods of time, and are not due to the immigration of new cultures into the area of study. The term faunal dating refers to the use of animal bones to determine the age of sedimentary layers or objects such as cultural artifacts embedded within those layers. Scientists can determine an approximate age for a layer by examining which species or genera of animals are buried in it. The technique works best if the animals belonged to species that evolved quickly, expanded rapidly over a large area, or suffered a mass extinction. In addition to providing rough absolute dates for specimens buried in the same stratigraphic unit as the bones, faunal analysis can also provide relative ages for objects buried above or below the fauna-encasing layers. Each year seed-bearing plants release large numbers of pollen grains. This process results in a "rain" of pollen that falls over many types of environments. Pollen that ends up in lakebeds or peat bogs is the most likely to be preserved, but pollen may also become fossilized in arid conditions if the soil is acidic or cool. Scientists can develop a pollen chronology, or calendar, by noting which species of pollen were deposited earlier in time, that is, residue in deeper sediment or rock layers, than others. A pollen zone is a period of time in which a particular species is much more abundant than any other species of the time. In most cases, this also reveals much about the climate of the period, because most plants only thrive in specific climatic conditions. Changes in pollen zones can also indicate changes in human activities such as massive deforestation or new types of farming. Pastures for grazing livestock are distinguishable from fields of grain, so changes in the use of the land over time are recorded in the pollen history. The dates when areas of North America were first settled by immigrants can be determined to within a few years by looking for the introduction of ragweed pollen. Pollen zones are translated into absolute dates by the use of radiocarbon dating. In addition, pollen dating provides relative dates beyond the limits of radiocarbon (40,000 years), and can be used in some places where radiocarbon dates are unobtainable. Fluorine is found naturally in ground water. This water comes in contact with skeletal remains under ground. When this occurs, the fluorine in the water saturates the bone, changing the mineral composition. Over time, more and more fluorine incorporates itself into the bone. By comparing the relative amounts of fluorine composition of skeletal remains, one can determine whether the remains were buried at the same time. A bone with a higher fluorine composition has been buried for a longer period of time. Absolute dating is the term used to describe any dating technique that tells how old a specimen is in years. These are generally analytical methods, and are carried out in a laboratory. Absolute dates are also relative dates, in that they tell which specimens are older or younger than others. Absolute dates must agree with dates from other relative methods in order to be valid. This dating technique of amino acid racimization was first conducted by Hare and Mitterer in 1967, and was popular in the 1970s. It requires a much smaller sample than radiocarbon dating, and has a longer range, extending up to a few hundred thousand years. It has been used to date coprolites (fossilized feces) as well as fossil bones and shells. These types of specimens contain proteins embedded in a network of minerals such as calcium. Amino acid racimization is based on the principle that amino acids (except glycine, a very simple amino acid) exist in two mirror image forms called stereoisomers. Living organisms (with the exception of some microbes) synthesize and incorporate only the L-form into proteins. This means that the ratio of the D-form to the L-form is zero (D/L=0). When these organisms die, the L-amino acids are slowly converted into D-amino acids in a process called racimization. This occurs because protons (H +) are removed from the amino acids by acids or bases present in the burial environment. The protons are quickly replaced, but will return to either side of the amino acid, not necessarily to the side from which they came. This may form a D-amino acid instead of an L – amino acid. The reversible reaction eventually creates equal amounts of L – and D-forms (D/L=1.0). The rate at which the reaction occurs is different for each amino acid; in addition, it depends upon the moisture, temperature, and pH of the postmortem conditions. The higher the temperature, the faster the reaction occurs, so the cooler the burial environment, the greater the . The burial conditions are not always known, however, and can be difficult to estimate. For this reason, and because some of the amino acid racimization dates have disagreed with dates achieved by other methods, the technique is no longer widely used. Cation-ratio dating is used to date rock surfaces such as stone artifacts and cliff and ground drawings. It can be used to obtain dates that would be unobtainable by more conventional methods such as radiocarbon dating. Scientists use cation-ratio dating to determine how long rock surfaces have been exposed. They do this by chemically analyzing the varnish that forms on these surfaces. The varnish contains cations, which are positively charged atoms or molecules. Different cations move throughout the environment at different rates, so the ratio of different cations to each other changes over time. Cation ratio dating relies on the principle that the cation ratio (K + +Ca 2+)/Ti 4+ decreases with increasing age of a sample. By calibrating these ratios with dates obtained from rocks from a similar microenvironment, a minimum age for the varnish can be determined. This technique can only be applied to rocks from desert areas, where the varnish is most stable. Although cation-ratio dating has been widely used, recent studies suggest it has potential errors. Many of the dates obtained with this method are inaccurate due to improper chemical analyses. In addition, the varnish may not actually be stable over long periods of time. Thermoluminescence dating is very useful for determining the age of pottery. Electrons from quartz and other minerals in the pottery clay are bumped out of their normal positions (ground state) when the clay is exposed to radiation. This radiation may come from radioactive substances such as uranium, present in the clay or burial medium, or from cosmic radiation. When the ceramic is heated to a very high temperature (over 932 ° F [500 ° C]), these electrons fall back to the ground state, emitting light in the process and resetting the "clock" to zero. 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 40,000 – 200,000 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 1984. 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 500,000 years old. 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. Narrow rings grow in cold and/or dry years, and wide rings grow in warm years with plenty of moisture. 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. 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 10,000 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 (40 Ar) begins to accumulate. Argon-40 is formed in the rocks by the radioactive decay of potassium-40 (40 K). The amount of 40 Ar formed is proportional to the decay rate (half-life) of 40 K, which is 1.3 billion years. In other words, it takes 1.3 billions years for half of the 40 K originally present to be converted into 40 Ar. 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 30,000 – 40,000 years, but with sensitive instrumentation, this range can be extended to 70,000 years. Radiocarbon (14 C) is a radioactive form of the element carbon. It decays spontaneously into nitrogen-14 (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. While a plant or animal is alive, the ratio of 14 C/ 12 C in its body will be nearly the same as the 14 C/ 12 C ratio in the atmosphere. 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 5,730 years. Scientists can estimate how many years have elapsed since an organism died by comparing the 14 C/ 12 C ratio in the remains with the ratio in the atmosphere. 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. Accurate radiocarbon dating is that diagenic (after death) demands consideration regarding potential contamination of a specimen and a proper application of changes in the 14 C/ 12 C ratio in the atmosphere over time. 14 C levels can be measured in tree rings and used to correct for the 14 C/ 12 C ratio in the atmosphere at the time the organism died, and can even be used to calibrate some dates directly. Although the magnitude of change of the 14 C/ 12 C ratio sometimes stirs controversy, with proper calibration and correction, radiocarbon dating correlates well with other dating techniques and consistently proves to be an accurate dating technique — especially for Pleistocene and Holocene period analysis. 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 for the different uranium series of a few thousand years to 500,000 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. Carbonates may be dated this way using, for example, the daughter/parent isotope pair protactinium-231/uranium-235 (231 Pa/ 235 U). 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-231 begins to accumulate via the decay of 235 U after the organism dies. Scientists can determine the age of the sample by measuring how much 231 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, 234 U dissolves more readily in water than its parent, 238 U, so lakes and oceans contain an excess of this daughter isotope. This excess is transferred to organisms such as mollusks or corals, and is the basis of 234 U/ 238 U dating. Some volcanic minerals and glasses, such as obsidian, contain uranium-238 (238 U). Over time, these substances become "scratched." The marks, called tracks, are the damage caused by the fission (splitting) of the uranium atoms. When an atom of 238 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 238 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 238 U is 4.47x10 9 years. 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 238 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 . Objects heated only a few decades ago may be dated if they contain relatively high levels of 238 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. Dude, She’s (Exactly 25 Percent) Out of Your League. A massive new study of online dating finds that everyone dates aspirationally—and that a woman’s desirability peaks 32 years before a man’s does. You’re at a party and you see someone cute across the room. They glance at you, maybe even smile for a second, then carry on with their conversation. You feel the room shrink, your heart rate quicken, your face go red: You’re crushing on this stranger, hard. But then the sensible part of your brain tells you to forget it: That person’s way, way out of your league. At this point, Elizabeth Bruch, a professor of sociology at the University of Michigan, crashes in to your thought process (and this news article). Yep, she says. Leagues do seem to exist. But you’re not alone in trying to escape yours: “Three-quarters, or more, of people are dating aspirationally,” she says. And according to a new study, users of online-dating sites spend most of their time trying to contact people “out of their league.” Bruch would know. She’s spent the past few years studying how people make decisions and pursue partners on online-dating sites, using exclusive data from the dating sites themselves. “There’s so much folk wisdom about dating and courtship, and very little scientific evidence,” she told me recently. “My research comes out of realizing that with these large-scale data sets, we can shed light on a lot of these old dating aphorisms.” Recommended Reading. When You Fall in Love, This Is What Facebook Sees. A Psychologist's Guide to Online Dating. Why Is Dating in the App Era Such Hard Work?

Recommended Reading. When You Fall in Love, This Is What Facebook Sees. A Psychologist's Guide to Online Dating. Why Is Dating in the App Era Such Hard Work?

In the new study, published Wednesday in the journal Science Advances, Bruch and her colleagues analyzed thousands of messages exchanged on a “popular, free online-dating service” between more than 186,000 straight men and women. They looked only at four metro areas—New York, Boston, Chicago, and Seattle—and only at messages from January 2014. Imagine for a second that you are one of the users Bruch and her colleagues studied—in fact, imagine that you are a very desirable user. Your specific desirability rank would have been generated by two figures: whether other desirable people contacted you, and whether other desirable people responded when you contacted them. If you contacted a much less desirable person, their desirability score would rise; if they contacted you and you replied, then your score would fall. The team had to analyze both first messages and first replies, because, well, men usually make the first move. “A defining feature of heterosexual online dating is that, in the vast majority of cases, it is men who establish the first contact—more than 80 percent of first messages are from men in our data set,” the study says. But “women reply very selectively to the messages they receive from men—their average reply rate is less than 20 percent—so women’s replies … can give us significant insight about who they are interested in.” The team combined all that data by using the PageRank algorithm, the same software that helps inform Google’s search results. It found that—insofar as dating “leagues” are not different tiers of hotness, but a single ascending hierarchy of desirability—then they do seem to exist in the data. But people do not seem universally locked into them—and they can occasionally find success escaping from theirs. “Reply rates [to the average message] are between zero percent and 10 percent,” she told me. Her advice: People should note those extremely low reply rates and send out more greetings. Michael Rosenfeld, a professor of sociology at Stanford University who was not connected to this study, agreed that persistence was a good strategy. “The idea that persistence pays off makes sense to me, as the online-dating world has a wider choice set of potential mates to choose from,” he told me in an email. “The greater choice set pays dividends to people who are willing to be persistent in trying to find a mate.” Yet what also emerges from the data is a far more depressing idea of “leagues” than many joking friends would suppose. Across the four cities and the thousands of users, consistent patterns around age, race, and education level emerge. White men and Asian women are consistently more desired than other users, while black women rank anomalously lower. Bruch said that race and gender stereotypes often get mixed up, with a race acquiring gendered connotations. “ Asian is coded as female, so that’s why Asian women get so much market power and Asian men get so little,” she told me. “For black men and women, it’s the opposite.” But “what we are seeing is overwhelmingly the effect of white preferences,” she cautioned. “This site is predominantly white, 70 percent white. If this was a site that was 20 percent white, we may see a totally different desirability hierarchy.” “Other people have done research using data from online-dating sites, and found similar racial and gender hierarchies,” said Rosenfeld, the Stanford professor. And Bruch emphasized that the hierarchy did not just depend on race, age, and education level: Because it is derived from user behavior, it “captures whatever traits people are responding to when they pursue partners. This will include traits like wittiness, genetic factors, or whatever else drives people to message,” she said. - In the study, men’s desirability peaks at age 50. But women’s desirability starts high at age 18 and falls throughout their lifespan. How Age Affects Online-Dating Desirability Among Heterosexual Men and Women. “I mean, everybody knows—and as a sociologist, it’s been shown—that older women have a harder time in the dating market. But I hadn’t expected to see their desirability drop off from the time they’re 18 to the time they’re 65,” Bruch told me. “But I was also surprised to see how flat men’s desirability was over the age distribution,” she said. “For men, it peaks around age 40 or 50. Especially in New York.” “New York is a special case for men,” Bruch told me. “It’s the market with the highest fraction of women. But it’s also about it being an incredibly dense market.” “Seattle presents the most unfavorable dating climate for men, with as many as two men for every woman in some segments,” the study says. Across all four cities, men and women generally tended to send longer messages to people who were more desirable than them. Women, especially, deployed this strategy. But the only place it paid off—and the only people for whom it worked with statistically significant success—were men in Seattle. The longest messages in the study were sent by Seattle men, the study says,“and only Seattle men experience a payoff to writing longer messages.” A more educated man is almost always more desirable, on average: Men with postgraduate degrees outperform men with bachelor’s degrees; men with bachelor’s degrees beat high-school graduates. “But for women, an undergraduate degree is most desirable,” the study says. “Postgraduate education is associated with decreased desirability among women.” How Education Affects Online-Dating Desirability Among Heterosexual Men and Women. Across all four cities, men tended to use less positive language when messaging more desirable women. They may have stumbled upon this strategy through trial and error because “in all four cities, men experience slightly lower reply rates when they write more positively worded messages.” Most people seem to know their position on the hierarchy because they most contact people who rank the same. “The most common behavior for both men and women is to contact members of the opposite sex who on average have roughly the same ranking as themselves,” Bruch and her colleagues write. But the overall distribution is skewed because “a majority of both sexes tend to contact partners who are more desirable than themselves on average—and hardly any users contact partners who are significantly less desirable.” “The most popular individual in our four cities, a 30-year-old woman living in New York, received 1504 messages during the period of observation,” the study says. This is “equivalent to one message every 30 min, day and night, for the entire month.” Yikes. Radiometric Age Dating. Radiometric dating calculates an age in years for geologic materials by measuring the presence of a short-life radioactive element, e.g., carbon-14, or a long-life radioactive element plus its decay product, e.g., potassium-14/argon-40. The term applies to all methods of age determination based on nuclear decay of naturally occurring radioactive isotopes. Bates and Jackson (1984) To determine the ages in years of Earth materials and the timing of geologic events such as exhumation and subduction, geologists utilize the process of radiometric decay. Geologists use these dates to further define the boundaries of the geologic periods shown on the geologic time scale. Radiometric decay occurs when the nucleus of a radioactive atom spontaneously transforms into an atomic nucleus of a different, more stable isotope. This transformation happens via the emission of particles such as electrons (known as beta decay) and alpha particles. For instance, rubidium-87 (87Rb), an unstable element, becomes strontium-87 (87Sr), a stable element, via beta decay. As explained on WebGeology from the University of Tormsø, Norway: One neutron of the nucleus emits a beta particle, which is identical to an electron. In addition the neutron emits a neutral particle that is called an antineutrino. By emitting a beta particle, the neutron is transformed into a proton. This results in a nucleus composed of 38 protons and 49 neutrons, corresponding to strontium’s nucleus of 87 atomic particles. Energy is released during this process. The rubidium-strontium method has been a popular method to determine the absolute age of geological processes. When discussing decay rates, scientists refer to “half-lives”—the length of time it takes for one-half of the original atom of the radioactive isotope to decay into an atom of a new isotope. Because decay occurs at a fixed rate (this is the key point), scientists can measure the amount of decayed material in the sample, determine the ratio between original and decayed material, and then calculate the sample’s age. Depending on the half-life and the material being dated, various methods are used. For instance, geologists use the Sm-Nd (samarium-147/neodymium-143) method for determining the age of very old materials (e.g., meteorites and metamorphic rocks) or when a rock became crystallized (in the mantle) or metamorphosed (at a subduction zone). For young organic materials, the carbon-14 (radiocarbon) method is used. The effective of the carbon-14 method is between 100 and 50,000 years. Many rock-forming minerals (e.g., biotite, muscovite, amphibole, and Potassium feldspar) Whole crushed metamorphic or igneous rock. Many rock-forming minerals (e.g., biotite, muscovite, amphibole, and Potassium feldspar) Whole crushed volcanic rocks (e.g., lava flows and ash) Peak Non-Creepy Dating Pool. As you get older, the percentage of people your age who are married increases and the percentage who have never married decreases. This must mean your dating pool gets smaller with time, right?

Well, this assumes you marry someone who is your age. What if you marry someone who is older or younger than you?

Thanks to the internet, we have a concrete dateable range given your age: the “half-your-age-plus-seven” rule. It’s on Wikipedia, so it’s basically law. ic xkcd describes it as the standard creepiness rule: Dating Pool Based On Creepiness Rule. Huh, that’s interesting. So if we account for the range and actual demographics counted by the U.S. Census Bureau, I wonder when your dating pool peaks. Does the peak change between different groups? In the charts that follow I look for the ages at which your dating pool is the largest, based on demographics and the standard creepiness rule (which only applies to those 14 and older). Considering Only Age. Looking for Single People. Looking for Single Men. Looking for Single Women. In contrast to looking for single men, the peak age increases just past 50 if you want to date a single woman. Women live longer than men. Keep Looking For Single… Employment. Race or Origin. The difference in dating pool for men and women surprised me the most. However, it makes sense. Men die earlier, so while the available age range increases, the population available decreases. In contrast, women live longer, which means for those looking for single women, the age range increases and the population decreases slower. This method is not without its caveats though. Aside from the validity of the standard creepiness rule, there are limitations to the data, which comes from the Census Bureau’s five-year American Community Survey 2016. The survey does not ask sexual orientation, so there is no way to see how that would shift the distributions. Also, age is only available as whole numbers, so I could not count on a continuous scale. That’s why the curves are jagged. I thought about averaging counts for the odd-numbered ages but then felt like I wouldn’t be showing the data correctly. So I went with what the data provides. Dating Techniques. Dating techniques are procedures used by scientists to determine the age of rocks, fossils, or artifacts. Relative dating methods tell only if one sample is older or younger than another; absolute dating methods provide an approximate date in years. The latter have generally been available only since 1947. Many absolute dating techniques take advantage of radioactive decay, whereby a radioactive form of an element decays into a non-radioactive product at a regular rate. Others, such as amino acid racimization and cation-ratio dating, are based on chemical changes in the organic or inorganic composition of a sample. In recent years, a few of these methods have come under close scrutiny as scientists strive to develop the most accurate dating techniques possible. Relative dating. Relative dating methods determine whether one sample is older or younger than another. They do not provide an age in years. Before the advent of absolute dating methods, nearly all dating was relative. The main relative dating method is stratigraphy. Stratigraphy. Stratigraphy is the study of layers of rocks or the objects embedded within those layers. It is based on the assumption (which nearly always holds true) that deeper layers were deposited earlier, and thus are older, than more shallow layers. The sequential layers of rock represent sequential intervals of time. Although these units may be sequential, they are not necessarily continuous due to erosional removal of some intervening. units. The smallest of these rock units that can be matched to a specific time interval is called a bed. Beds that are related are grouped together into members, and members are grouped into formations. Stratigraphy is the principle method of relative dating, and in the early years of dating studies was virtually the only method available to scientists. Seriation. Seriation is the ordering of objects according to their age. It is a relative dating method. In a landmark study, archaeologist James Ford used seriation to determine the chronological order of American Indian pottery styles in the Mississippi Valley. Artifact styles such as pottery types are seriated by analyzing their abundances through time. This is done by counting the number of pieces of each style of the artifact in each stratigraphic layer and then graphing the data. A layer with many pieces of a particular style will be represented by a wide band on the graph, and a layer with only a few pieces will be represented by a narrow band. The bands are arranged into battleship-shaped curves, with each style getting its own curve. The curves are then compared with one another, and from this the relative ages of the styles are determined. A limitation to this method is that it assumes all differences in artifact styles are the result of different periods of time, and are not due to the immigration of new cultures into the area of study. Faunal dating. The term faunal dating refers to the use of animal bones to determine the age of sedimentary layers or objects such as cultural artifacts embedded within those layers. Scientists can determine an approximate age for a layer by examining which species or genera of animals are buried in it. The technique works best if the animals belonged to species, which evolved quickly, expanded rapidly over a large area, or suffered a mass extinction. In addition to providing rough absolute dates for specimens buried in the same stratigraphic unit as the bones, faunal analysis can also provide relative ages for objects buried above or below the fauna-encasing layers. Pollen dating (palynology) Each year seed-bearing plants release large numbers of pollen grains. This process results in a “ rain ” of pollen that falls over many types of environments. Pollen that ends up in lakebeds or peat bogs is the most likely to be preserved, but pollen may also become fossilized in arid conditions if the soil is acidic or cool. Scientists can develop a pollen chronology, or calendar, by noting which species of pollen were deposited earlier in time, that is, residue in deeper sediment or rock layers, than others. The unit of the calendar is the pollen zone. A pollen zone is a period of time in which a particular species is much more abundant than any other species of the time. In most cases, this tells us about the climate of the period, because most plants only thrive in specific climatic conditions. Changes in pollen zones can also indicate changes in human activities such as massive deforestation or new types of farming. Pastures for grazing livestock are distinguishable from fields of grain, so changes in the use of the land over time are recorded in the pollen history. The dates when areas of North America were first settled by immigrants can be determined to within a few years by looking for the introduction of ragweed pollen. Pollen zones are translated into absolute dates by the use of radiocarbon dating. In addition, pollen dating provides relative dates beyond the limits of radiocarbon (40, 000 years), and can be used in some places where radiocarbon dates are unobtainable. Fluorine is found naturally in ground water. This water comes in contact with skeletal remains under ground. When this occurs, the fluorine in the water saturates the bone, changing the mineral composition. Over time, more and more fluorine incorporates itself into the bone. By comparing the relative amounts of fluorine composition of skeletal remains, one can determine whether the remains were buried at the same time. A bone with a higher fluorine composition has been buried for a longer period of time. Absolute dating. Absolute dating is the term used to describe any dating technique that tells how old a specimen is in years. These are generally analytical methods, and are carried out in a laboratory. Absolute dates are also relative dates, in that they tell which specimens are older or younger than others. Absolute dates must agree with dates from other relative methods in order to be valid. Amino acid racimization. This dating technique was first conducted by Hare and Mitterer in 1967, and was popular in the 1970s. It requires a much smaller sample than radiocarbon dating, and has a longer range, extending up to a few hundred thousand years. It has been used to date coprolites (fossilized feces) as well as fossil bones and shells. These types of specimens contain proteins embedded in a network of minerals such as calcium. Amino acid racimization is based on the principle that amino acids (except glycine, which is a very simple amino acid) exist in two mirror image forms called stereoisomers. Living organisms (with the exception of some microbes) synthesize and incorporate only the L-form into proteins. This means that the ratio of the D-form to the L-form is zero (D/L = 0). When these organisms die, the L-amino acids are slowly converted into D-amino acids in a process called racimization. This occurs because protons (H +) are removed from the amino acids by acids or bases present in the burial environment. The protons are quickly replaced, but will return to either side of the amino acid, not necessarily to the side from which they came. This may form a D-amino acid instead of an L-amino acid. The reversible reaction eventually creates equal amounts of Land D-forms (D/L = 1.0). The rate at which the reaction occurs is different for each amino acid; in addition, it depends upon the moisture, temperature, and pH of the postmortem conditions. The higher the temperature, the faster the reaction occurs, so the cooler the burial environment, the greater the . The burial conditions are not always known, however, and can be difficult to estimate. For this reason, and because some of the amino acid racimization dates have disagreed with dates achieved by other methods, the technique is no longer widely used. Cation-ratio dating. Cation-ratio dating is used to date rock surfaces such as stone artifacts and cliff and ground drawings. It can be used to obtain dates that would be unobtainable by more conventional methods such as radio-carbon dating. Scientists use cation-ratio dating to determine how long rock surfaces have been exposed. They do this by chemically analyzing the varnish that forms on these surfaces. The varnish contains cations, which are positively charged atoms or molecules. Different cations move throughout the environment at different rates, so the ratio of different cations to each other changes over time. Cation ratio dating relies on the principle that the cation ratio (K + + Ca 2+)/Ti 4+ decreases with increasing age of a sample. By calibrating these ratios with dates obtained from rocks from a similar microenvironment, a minimum age for the varnish can be determined. This technique can only be applied to rocks from desert areas, where the varnish is most stable. Although cation-ratio dating has been widely used, recent studies suggest it has many problems. Many of the dates obtained with this method are inaccurate due to improper chemical analyses. In addition, the varnish may not actually be stable over long periods of time. Finally, some scientists have recently suggested that the cation ratios may not even be directly related to the age of the sample. Thermoluminescence dating. Thermoluminescence dating is useful for determining the age of pottery. Electrons from quartz and other minerals in the pottery clay are bumped out of their normal positions (ground state) when the clay is exposed to radiation. This radiation may come from radioactive substances such as uranium, present in the clay or burial medium, or from cosmic radiation. When the ceramic is heated to a very high temperature (over 932 ° F [500 ° C]), these electrons fall back to the ground state, emitting light in the process and resetting the “ clock ” to zero. The longer the exposure to the radiation, the more electrons that are bumped into an excited state, and the more light that 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 piece 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, 000 – 200, 000 years. In addition, it can be used to date materials that cannot be dated with these other two methods. Optically stimulated luminescence has only been used since 1984. 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 a 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 500, 000 years old. 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. Tree-ring dating. This absolute dating method is also known as dendrochronology. It is based on the fact that trees produce one growth ring each year. Narrow rings grow in cold and/or dry years, and wide rings grow in warm years with plenty of moisture. 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. 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 1-10, 000 years or more. Radioactive decay dating. As previously mentioned, radioactive decay refers to the process in which a radioactive form of an element is converted into a nonradioactive 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. When volcanic rocks are heated to extremely high temperatures, they release any argon gas trapped in them. As the rocks cool, argon-40 (40 Ar) begins to accumulate. Argon-40 is formed in the rocks by the radioactive decay of potassium-40 (40 K). The amount of 40 Ar formed is proportional to the decay rate (half-life) of 40 K, which is 1.3 billion years. In other words, it takes 1.3 billions years for half of the 40 K originally present to be converted into 40 Ar. 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. Radiocarbon is used to date charcoal, wood, and other biological materials. The range of conventional radiocarbon dating is 30, 000 – 40, 000 years, but with sensitive instrumentation this range can be extended to 70, 000 years. Radiocarbon (14 C) is a radioactive form of the element carbon. It decays spontaneously into nitrogen-14 (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). Atoms of 14 C and of a non-radioactive form of carbon, 12 C, are equally likely to be incorporated into living organisms – there is no discrimination. While a plant or animal is alive, the ratio of 14 C/ 12 C in its body will be nearly the same as the 14 C/ 12 C ratio in the atmosphere. 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, 730 years. Scientists can tell how many years have elapsed since an organism died by comparing the 14 C/ 12 C ratio in the remains with the ratio in the atmosphere. This allows us to determine how much 14 C has formed since the death of the organism. A problem with radiocarbon dating is that diagenic (after death) contamination of a specimen from soil, water, etc. can add carbon to the sample and affect the measured ratios. This can lead to inaccurate dates. Another problem lies with the assumptions associated with radiocarbon dating. One assumption is that the 14 C/ 12 C ratio in the atmosphere is constant though time. This is not completely true. Although 14 C levels can be measured in tree rings and used to correct for the 14 C/ 12 C ratio in the atmosphere at the time the organism died, and can even be used to calibrate some dates directly, radiocarbon remains a more useful relative dating technique than an absolute one. Uranium series dating. 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 thousand million years. This provides a for the different uranium series of a few thousand years to 500, 000 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 lake beds. 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. Carbonates may be dated this way using, for example, the daughter/parent isotope pair protactinium-231/uranium-235 (231 Pa/ 235 U). 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-231 begins to accumulate via the decay of 235 U after the organism dies. Scientists can determine the age of the sample by measuring how much 231 Pa is present and calculating how long it would have taken that amount to form. In the case of a 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, 234 U dissolves more readily in water than its parent, 238 U, so lakes and oceans contain an excess of this daughter isotope. This excess is transferred to organisms such as mollusks or corals, and is the basis of 234 U/ 238 U dating. Fission track dating. Some volcanic minerals and glasses, such as obsidian, contain uranium-238 (238 U). Over time, these substances become “ scratched. ” The marks, called tracks, are the damage caused by the fission (splitting) of the uranium atoms. When an atom of 238 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 238 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 238 U is 4.47 × 10 9 years. 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 238 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 . Objects heated only a few decades ago may be dated if they contain relatively high levels of 238 U; conversely, some meteorites have been dated to over a billion years old with this method. Resources. BOOKS. PERIODICALS. Citation styles. Encyclopedia.com gives you the ability to cite reference entries and articles according to common styles from the Modern Language Association (MLA), The Chicago Manual of Style, and the American Psychological Association (APA). Within the “Cite this article” tool, pick a style to see how all available information looks when formatted according to that style. 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.com cannot guarantee each citation it generates. Therefore, it’s best to use Encyclopedia.com citations as a starting point before checking the style against your school or publication’s requirements and the most-recent information available at these sites: Modern Language Association. The Chicago Manual of Style. American Psychological Association. Notes: Most online reference entries and articles do not have page numbers. Therefore, that information is unavailable for most Encyclopedia.com content. However, the date of retrieval is often important. Refer to each style’s convention regarding the best way to format page numbers and retrieval dates. In addition to the MLA, Chicago, and APA styles, your school, university, publication, or institution may have its own requirements for citations. Therefore, be sure to refer to those guidelines when editing your bibliography or works cited list. Dating Methods. Dating methods. Dating techniques are procedures used by scientists to determine the age of a specimen. Relative dating methods tell only if one sample is older or younger than another sample; absolute dating methods provide a date in years. The latter have generally been available only since 1947. Many absolute dating techniques take advantage of radioactive decay, whereby a radioactive form of an element is converted into another radioactive isotope or non-radioactive product at a regular rate. Others, such as amino acid racimization and cation-ratio dating, are based on chemical changes in the organic or inorganic composition of a sample. In recent years, a few of these methods have undergone continual refinement as scientists strive to develop the most accurate dating techniques possible. Relative dating methods determine whether one sample is older or younger than another. They do not provide an age in years. Before the advent of absolute dating methods, nearly all dating was relative. The main relative dating method is stratigraphy. Stratigraphy is the study of layers of rocks or the objects embedded within those layers. It is based on the assumption (which, except at unconformities, nearly always holds true) that deeper layers were deposited earlier, and thus are older than more shallow layers. The sequential layers of rock represent sequential intervals of time. Although these units may be sequential, they are not necessarily continuous due to erosional removal of some intervening units. The smallest of these rock units that can be matched to a specific time interval is called a bed. Beds that are related are grouped together into members, and members are grouped into formations. Seriation is the ordering of objects according to their age. It is a relative dating method. In a landmark study, archaeologist James Ford used seriation to determine the chronological order of American Indian pottery styles in the Mississippi Valley. Artifact styles such as pottery types are seriated by analyzing their abundances through time. This is done by counting the number of pieces of each style of the artifact in each stratigraphic layer and then graphing the data. A layer with many pieces of a particular style will be represented by a wide band on the graph, and a layer with only a few pieces will be represented by a narrow band. The bands are arranged into battleship-shaped curves, with each style getting its own curve. The curves are then compared with one another, and from this the relative ages of the styles are determined. A limitation to this method is that it assumes all differences in artifact styles are the result of different periods of time, and are not due to the immigration of new cultures into the area of study. The term faunal dating refers to the use of animal bones to determine the age of sedimentary layers or objects such as cultural artifacts embedded within those layers. Scientists can determine an approximate age for a layer by examining which species or genera of animals are buried in it. The technique works best if the animals belonged to species that evolved quickly, expanded rapidly over a large area, or suffered a mass extinction. In addition to providing rough absolute dates for specimens buried in the same stratigraphic unit as the bones, faunal analysis can also provide relative ages for objects buried above or below the fauna-encasing layers. Each year seed-bearing plants release large numbers of pollen grains. This process results in a "rain" of pollen that falls over many types of environments. Pollen that ends up in lakebeds or peat bogs is the most likely to be preserved, but pollen may also become fossilized in arid conditions if the soil is acidic or cool. Scientists can develop a pollen chronology, or calendar, by noting which species of pollen were deposited earlier in time, that is, residue in deeper sediment or rock layers, than others. A pollen zone is a period of time in which a particular species is much more abundant than any other species of the time. In most cases, this also reveals much about the climate of the period, because most plants only thrive in specific climatic conditions. Changes in pollen zones can also indicate changes in human activities such as massive deforestation or new types of farming. Pastures for grazing livestock are distinguishable from fields of grain, so changes in the use of the land over time are recorded in the pollen history. The dates when areas of North America were first settled by immigrants can be determined to within a few years by looking for the introduction of ragweed pollen. Pollen zones are translated into absolute dates by the use of radiocarbon dating. In addition, pollen dating provides relative dates beyond the limits of radiocarbon (40,000 years), and can be used in some places where radiocarbon dates are unobtainable. Fluorine is found naturally in ground water. This water comes in contact with skeletal remains under ground. When this occurs, the fluorine in the water saturates the bone, changing the mineral composition. Over time, more and more fluorine incorporates itself into the bone. By comparing the relative amounts of fluorine composition of skeletal remains, one can determine whether the remains were buried at the same time. A bone with a higher fluorine composition has been buried for a longer period of time. Absolute dating is the term used to describe any dating technique that tells how old a specimen is in years. These are generally analytical methods, and are carried out in a laboratory. Absolute dates are also relative dates, in that they tell which specimens are older or younger than others. Absolute dates must agree with dates from other relative methods in order to be valid. This dating technique of amino acid racimization was first conducted by Hare and Mitterer in 1967, and was popular in the 1970s. It requires a much smaller sample than radiocarbon dating, and has a longer range, extending up to a few hundred thousand years. It has been used to date coprolites (fossilized feces) as well as fossil bones and shells. These types of specimens contain proteins embedded in a network of minerals such as calcium. Amino acid racimization is based on the principle that amino acids (except glycine, a very simple amino acid) exist in two mirror image forms called stereoisomers. Living organisms (with the exception of some microbes) synthesize and incorporate only the L-form into proteins. This means that the ratio of the D-form to the L-form is zero (D/L=0). When these organisms die, the L-amino acids are slowly converted into D-amino acids in a process called racimization. This occurs because protons (H +) are removed from the amino acids by acids or bases present in the burial environment. The protons are quickly replaced, but will return to either side of the amino acid, not necessarily to the side from which they came. This may form a D-amino acid instead of an L – amino acid. The reversible reaction eventually creates equal amounts of L – and D-forms (D/L=1.0). The rate at which the reaction occurs is different for each amino acid; in addition, it depends upon the moisture, temperature, and pH of the postmortem conditions. The higher the temperature, the faster the reaction occurs, so the cooler the burial environment, the greater the . The burial conditions are not always known, however, and can be difficult to estimate. For this reason, and because some of the amino acid racimization dates have disagreed with dates achieved by other methods, the technique is no longer widely used. Cation-ratio dating is used to date rock surfaces such as stone artifacts and cliff and ground drawings. It can be used to obtain dates that would be unobtainable by more conventional methods such as radiocarbon dating. Scientists use cation-ratio dating to determine how long rock surfaces have been exposed. They do this by chemically analyzing the varnish that forms on these surfaces. The varnish contains cations, which are positively charged atoms or molecules. Different cations move throughout the environment at different rates, so the ratio of different cations to each other changes over time. Cation ratio dating relies on the principle that the cation ratio (K + +Ca 2+)/Ti 4+ decreases with increasing age of a sample. By calibrating these ratios with dates obtained from rocks from a similar microenvironment, a minimum age for the varnish can be determined. This technique can only be applied to rocks from desert areas, where the varnish is most stable. Although cation-ratio dating has been widely used, recent studies suggest it has potential errors. Many of the dates obtained with this method are inaccurate due to improper chemical analyses. In addition, the varnish may not actually be stable over long periods of time. Thermoluminescence dating is very useful for determining the age of pottery. Electrons from quartz and other minerals in the pottery clay are bumped out of their normal positions (ground state) when the clay is exposed to radiation. This radiation may come from radioactive substances such as uranium, present in the clay or burial medium, or from cosmic radiation. When the ceramic is heated to a very high temperature (over 932 ° F [500 ° C]), these electrons fall back to the ground state, emitting light in the process and resetting the "clock" to zero. 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 40,000 – 200,000 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 1984. 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 500,000 years old. 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. Narrow rings grow in cold and/or dry years, and wide rings grow in warm years with plenty of moisture. 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. 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 10,000 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 (40 Ar) begins to accumulate. Argon-40 is formed in the rocks by the radioactive decay of potassium-40 (40 K). The amount of 40 Ar formed is proportional to the decay rate (half-life) of 40 K, which is 1.3 billion years. In other words, it takes 1.3 billions years for half of the 40 K originally present to be converted into 40 Ar. 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 30,000 – 40,000 years, but with sensitive instrumentation, this range can be extended to 70,000 years. Radiocarbon (14 C) is a radioactive form of the element carbon. It decays spontaneously into nitrogen-14 (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. While a plant or animal is alive, the ratio of 14 C/ 12 C in its body will be nearly the same as the 14 C/ 12 C ratio in the atmosphere. 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 5,730 years. Scientists can estimate how many years have elapsed since an organism died by comparing the 14 C/ 12 C ratio in the remains with the ratio in the atmosphere. 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. Accurate radiocarbon dating is that diagenic (after death) demands consideration regarding potential contamination of a specimen and a proper application of changes in the 14 C/ 12 C ratio in the atmosphere over time. 14 C levels can be measured in tree rings and used to correct for the 14 C/ 12 C ratio in the atmosphere at the time the organism died, and can even be used to calibrate some dates directly. Although the magnitude of change of the 14 C/ 12 C ratio sometimes stirs controversy, with proper calibration and correction, radiocarbon dating correlates well with other dating techniques and consistently proves to be an accurate dating technique — especially for Pleistocene and Holocene period analysis. 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 for the different uranium series of a few thousand years to 500,000 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. Carbonates may be dated this way using, for example, the daughter/parent isotope pair protactinium-231/uranium-235 (231 Pa/ 235 U). 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-231 begins to accumulate via the decay of 235 U after the organism dies. Scientists can determine the age of the sample by measuring how much 231 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, 234 U dissolves more readily in water than its parent, 238 U, so lakes and oceans contain an excess of this daughter isotope. This excess is transferred to organisms such as mollusks or corals, and is the basis of 234 U/ 238 U dating. Some volcanic minerals and glasses, such as obsidian, contain uranium-238 (238 U). Over time, these substances become "scratched." The marks, called tracks, are the damage caused by the fission (splitting) of the uranium atoms. When an atom of 238 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 238 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 238 U is 4.47x10 9 years. 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 238 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 . Objects heated only a few decades ago may be dated if they contain relatively high levels of 238 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. 6 Truths About Teens and Dating. Amy Morin, LCSW, is the Editor-in-Chief of Verywell Mind. She's also a psychotherapist, international bestselling author and host of the The Verywell Mind Podcast. Ann-Louise T. Lockhart, PsyD, ABPP, is a board-certified pediatric psychologist, parent coach, author, speaker, and owner of A New Day Pediatric Psychology, PLLC. The prospect of your teen starting to date is naturally unnerving. It's easy to fear your child getting hurt, getting in over their head, being manipulated or heartbroken, and especially, growing up and leaving the nest. But as uncomfortable or scary as it may feel to consider your child with a romantic life, remember that this is a normal, healthy, and necessary part of any young adult's emotional development. How Teen Dating Has Changed. But what exactly does teen dating even look like these days? The general idea may be the same as it's always been, but the way teens date has changed quite a bit from just a decade or so ago. Clearly, the explosion of social media and ever-present cellphones are two of the biggest influences on the changing world of teen dating—kids don't even need to leave their bedrooms to "hang out." Truths About Teen Dating. This quickly morphing social landscape makes it more challenging for parents to keep up, figure out how to talk with their teens about dating, and establish rules that will keep them safe. To help you navigate this unfamiliar territory, there are five essential truths every parent should know about the teen dating scene. Teen Romance Is Normal. While some teens will start dating earlier than others, romantic interests are normal and healthy during adolescence. Some kids are more overt or vocal about their interest in dating but most are paying attention and intrigued by the prospect of a romantic life, even if they keep it to themselves. According to the Department of Health and Human Services, dating helps teens build social skills and grow emotionally. Interestingly, teens "date" less now than they did in the past—perhaps in part due to the influx of cell phones and virtual social interactions. In 1991, only 14% of high school seniors did not date, while by 2013 that number had jumped to 38%. Of kids aged 13 to 17, around 35% have some experience with romantic relationships and 19% are in a relationship at any one time. But regardless of when it starts, the truth is that most teens, especially as they make their way through high school and college, are eventually going to be interested in dating. When they start dating, you’ll need to be ready by establishing expectations and opening a caring and supportive dialogue about these topics. Dating Builds Relationship Skills. Just like starting any new phase of life, entering the world of dating is both exciting and scary—for kids and their parents alike. Kids will need to put themselves out there by expressing romantic interest in someone else, risking rejection, figuring out how to be a dating partner, and what exactly that means. New skills in the realms of communication, caring, thoughtfulness, intimacy, and independence collide with a developing sexuality, limited impulse control, and the urge to push boundaries. Your teen may also have some unrealistic ideas about dating based on what they've seen online, in the movies, or read in books. Real-life dating doesn't mimic a teen Netflix or Disney movie—or porn. Instead, first dates may be awkward or they may not end in romance. Dates may be in a group setting or even via Snapchat—but the feelings are just as real. Today's teens spend a lot of time texting and messaging potential love interests on social media. For some, this approach can make dating easier because they can test the waters and get to know one another online first. For those teens who are shy, meeting in person can be more awkward, especially since kids spend so much time tied to their electronics at the expense of face-to-face communication. Understand that early dating is your teen's chance to work on these life skills. They may make mistakes and/or get hurt but ideally, they will also learn from those experiences. Your Teen Needs "The Talk" It's important to talk to your teen about a variety of dating topics, such as personal values, expectations, and peer pressure. Be open with your teen about everything from treating someone else with respect to your—and their—beliefs around sexual activity. It can be helpful to outline for your kids what early dating may be like for them. Even if your perspective is a bit outdated, sharing it can get the conversation started. Ask them what they have in mind about dating and what questions they may have. Possibly share some of your own experiences. Go over the topics of consent, feeling safe and comfortable, and honoring their own and the other person's feelings. Most importantly, tell them what you expect in terms of being respectful of their dating partner and vice versa. Talk about the basics too, like how to behave when meeting a date's parents or how to be respectful while you're on a date. Make sure your teen knows to show respect by being on time and not texting friends throughout the date. Talk about what to do if a date behaves disrespectfully. Talk to your child about safe sex. Additionally, don't assume you know (or should choose) the type (or gender) of the person your child will want to date. You might see your child with a sporty, clean-cut kid or a teen from their newspaper club, but they may express interest in someone else entirely. This is their time to experiment and figure out what and who they are interested in. Plus, we all know that the more you push, the more they'll pull. Your child may be interested in someone that you would never pick for them but aim to be as supportive as you can as long as it's a healthy, respectful relationship. Be open to the fact that sexuality and gender are a spectrum and many kids won't fall into the traditional boxes—or fit the exact expectations their parents have for them. Love your child no matter what. Privacy Is Essential. Your parenting values, your teen's maturity level, and the specific situation will help you determine how much chaperoning your teen needs. Having an eyes-on policy might be necessary and healthy in some circumstances but teens also need a growing amount of independence and the ability to make their own choices. Aim to offer your teen at least a little bit of privacy. Don't listen in on phone calls or eavesdrop on private chats, and don't read every social media message. Keep tabs on what you can, especially if you have any concerns about what is going on. You can certainly follow your child's public posts on social media. You'll need to follow your instincts on how closely to supervise what your child is doing. Inviting your child to bring their friends and dates to your house is another good strategy as you will get a better sense of the dynamic of the group or couple. Plus, if your child thinks you genuinely want to get to know their friends or romantic partners and aren't hostile to them, they are more likely to open up to you—and possibly, less likely to engage in questionable behavior. Your Teen Needs Guidance. While it's not healthy to get too wrapped up in your teen's dating life, there may be times when you'll have to intervene. If you overhear your teen saying mean comments or using manipulative tactics, speak up. Similarly, if your teen is on the receiving end of unhealthy behavior, it's important to step in and help out. There's a small window of time between when your teen begins dating and when they're going to be entering the adult world. Aim to provide guidance that can help them succeed in their future relationships. Whether they experience some serious heartbreak, or they're a heart breaker, adolescence is when teens begin to learn about romantic relationships firsthand. Expect that your child may feel uncomfortable talking about this stuff with you (and may even be explicitly resistant) but that doesn't mean that you shouldn't try. Offer advice, a caring ear, and an open shoulder. Make sure they understand that anything put online is forever and that sending a nude photo can easily backfire—and be shared with unintended recipients. Don't assume they've learned what they need to know from sex ed, movies, and their friends—tell them everything you think they should know, even the obvious stuff. They probably have questions (but may not ask them), and they've likely picked up misinformation along the way that needs to be corrected. Safety Rules Must Be Established. As a parent, your job is to keep your child safe and to help them learn the skills they need to navigate healthy relationships. As your teen matures, they should require fewer dating rules. But rules for your teen should be based on their behavior, not necessarily their age. If they aren't honest about their activities or don't abide by their curfew or other rules, they may lack the maturity to have more freedom (as long as your rules are reasonable). Tweens and younger teens will need more rules as they likely aren't able to handle the responsibilities of a romantic relationship yet. Get to know anyone your teen wants to date. Establish the expectation that you'll be introduced before a date, whatever you want that to look like. You can always start by meeting their date at your home a few times for dinner before allowing your teen to go out on a date alone. Make dating without a chaperone a privilege. For younger teens, inviting a romantic interest to the house may be the extent of dating. Or you can drive your teen and their date to the movies or a public place. Older teens are likely to want to go out on dates without a chauffeur. Make that a privilege that can be earned as long as your teen exhibits trustworthy behavior. Create clear guidelines about online romance. Many teens talk online, which can easily develop into a false sense of intimacy. Consequently, they're more likely to meet people they've chatted with, but never met because they don't view them as strangers. Create clear rules about online dating and stay up to date on any apps your teen might be tempted to use, like Tinder. Know your teen's itinerary. Make sure you have a clear itinerary for your teen’s date. Insist your teen contact you if the plan changes. If you feel it's needed, you can set up tracking apps on your child's phone so you'll always know where they are. Establish a clear curfew. Make it clear you need to know the details of who your teen will be with, where they will be going, and who will be there. Establish a clear curfew as well. Your child may rail against these rules but may also feel comforted by them—not that they will tell you that. Set age limits. In some states, teens can legally date anyone they want once they reach 16, but in other states, they don’t have that choice until they turn 18. But, legal issues aside, there’s usually a big difference in maturity level between a 14-year-old and an 18-year-old. So, set some rules about the acceptable dating age range. Know who is at home at the other person's house. If your teen is going to a date’s home, find out who will be home. Have a conversation with the date’s parents to talk about their rules. Discuss technology dangers, like sexting. Sometimes, teens are tempted to comply with a date’s request to send nude photos. Unfortunately, these photos can become public very quickly and unsuspecting teens can end up hurt, shamed, or embarrassed. Establish clear cellphone rules that will help your teen make good decisions. A Word From Verywell. Consider that how you parent your child during this new stage can have big ramifications on their future relationships (romantic and otherwise), the lifestyle choices they make, and the mature adult they become. The more open and supportive you can be with them, the better. After all, if something does go awry, you'll want them to know that you're always in their corner.

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