FOSSILS: how fossils are datedChronological dating , or simply dating , is the process of attributing to an object or event a date in the past, allowing such object or event to be located in a previously established chronology. This usually requires what is commonly known as a "dating method". Several dating methods exist, depending on different criteria and techniques, and some very well known examples of disciplines using such techniques are, for example, history , archaeology , geology , paleontology , astronomy and even forensic science , since in the latter it is sometimes necessary to investigate the moment in the past in which the death of a cadaver occurred. Dating methods are most commonly classified following two criteria: relative dating and absolute dating. Relative dating methods are unable to determine the absolute age of an object or event, but can determine the impossibility of a particular event happening before or after another event of which the absolute date is well known. In this relative dating method, Latin terms ante quem and post quem are usually used to indicate both the oldest and the most recent possible moments when an event occurred or an artifact was left in a stratum.
The technique was first applied in the American Southwest and later extended to other parts of the world. Tree-ring dating is relatively simple. Trees add a new layer of cambium the layer right under the bark every year. The thickness of the layer depends on local weather and climate. In years with plenty of rain, the layer will be thick and healthy. Over the lifetime of the tree, these rings accumulate, and the rings form a record of regional variation in climate that may extend back hundreds of years.
Since all of the trees in a region experience the same climate variations, they will have similar growth patterns and similar tree ring patterns. One tree usually does not cover a period sufficiently long to be archaeologically useful. However, patterns of tree ring growth have been built up by "overlapping" ring sequences from different trees so that the tree ring record extends back several thousand years in many parts of the world.
The process starts with examination of the growth ring patterns of samples from living trees. Then older trees are added to the sequence by overlapping the inner rings of a younger sample with the outer rings of an older sample.
Older trees are recovered from old buildings, archaeological sites, peat bogs, and swamps. Eventually, a regional master chronology is constructed. When dendrochronology can be used, it provides the most accurate dates of any technique.
This dating method is also known as “Archaeological Dating” or “Historical But, even when the scientific methods of absolute dating are available, this method of .. Finds of index fossils in the tool bearing geological deposits are sufficient to. Archaeological dating techniques can assure buyers that their item is not a fake by providing scientific reassurance of the artefact's likely age. Chronological dating, or simply dating, is the process of attributing to an object or event a date in the past, allowing such object or event to be located in a previously established chronology. This usually requires what is commonly known as a "dating method". . Dating material drawn from the archaeological record can be made by a.
In the American Southwest, the accuracy and precision of dendrochronology has enabled the development of one of the most. Often events can be dated to within a decade. This precision has allowed archaeologists working in the American Southwest to reconstruct patterns of village growth and subsequent abandonment with a fineness of detail unmatched in most of the world.
Radiometric dating methods are more recent than dendrochronology. However, dendrochronology provides an important calibration technique for radiocarbon dating techniques.
All radiometric-dating techniques are based on the well-established principle from physics that large samples of radioactive isotopes decay at precisely known rates. The rate of decay of a radioactive isotope is usually given by its half-life. The decay of any individual nucleus is completely random. The half-life is a measure of the probability that a given atom will decay in a certain time. The shorter the half-life, the more likely the atom will decay. This probability does not increase with time.
If an atom has not decayed, the probability that it will decay in the future remains exactly the same.
This means that no matter how many atoms are in a sample, approximately one-half will decay in one half-life. The remaining atoms have exactly the same decay probability, so in another half-life, one half of the remaining atoms will decay. The amount of time required for one-half of a radioactive sample to decay can be precisely determined. The particular radioisotope used to determine the age of an object depends on the type of object and its age.
Radiocarbon is the most common and best known of radiometric dating techniques, but it is also possibly the most misunderstood. It was developed at the University of Chicago in by a group of American scientists led by Willard F. Radiocarbon dating has had an enormous impact on archaeology. In the last 50 years, radiocarbon dating has provided the basis for a worldwide cultural chronology.
Recognizing the importance of this technique, the Nobel Prize committee awarded the Prize in Chemistry to Libby in The physics behind radiocarbon dating is straightforward. Earth 's atmosphere is constantly bombarded with cosmic rays from outer space. Cosmic-ray neutrons collide with atoms of nitrogen in the upper atmosphere, converting them to atoms of radioactive carbon The carbon atom quickly combines with an oxygen molecule to form carbon dioxide.
This radioactive carbon dioxide spreads throughout Earth's atmosphere, where it is taken up by plants along with normal carbon As long as the plant is alive, the relative amount ratio of carbon to carbon remains constant at about one carbon atom for every one trillion carbon atoms. Some animals eat plants and other animals eat the plant-eaters. As long as they are alive, all living organisms have the same ratio of carbon to carbon as in the atmosphere because the radioactive carbon is continually replenished, either through photosynthesis or through the food animals eat.
However, when the plant or animal dies, the intake of carbon stops and the ratio of carbon to carbon immediately starts to decrease. The half-life of carbon is 5, years. After 5, years, about one-half of the carbon atoms will have decayed. After another 5, years, one-half of the remaining atoms will have decayed. So after 11, years, only one-fourth will remain. After 17, years, one-eighth of the original carbon will remain.
After 22, years, one-sixteenth will remain. Radiocarbon dating has become the standard technique for determining the age of organic remains those remains that contain carbon. There are many factors that must be taken into account when determining the age of an object.
The best objects are bits of charcoal that have been preserved in completely dry environments. The worst candidates are bits of wood that have been saturated with sea water, since sea water contains dissolved atmospheric carbon dioxide that may throw off the results. Radiocarbon dating can be used for small bits of clothing or other fabric, bits of bone, baskets, or anything that contains organic material. There are well over labs worldwide that do radiocarbon dating.
In the early twenty-first century, the dating of objects up to about 10 half-lives, or up to about 50, years old, is possible. However, objects less than years old cannot be reliably dated because of the widespread burning of fossil fuels, which began in the nineteenth century, and the production of carbon from atmospheric testing of nuclear weapons in the s and s. Another problem with radiocarbon dating is that the production of carbon in the atmosphere has not been constant, due to variation in solar activity.
For example, in the s, solar activity dropped a phenomenon called the "Maunder Minimum"so carbon production also decreased during this period.
To achieve the highest level of accuracy, carbon dates must be calibrated by comparison to dates obtained from dendrochronology. Calibration of Radiocarbon Dates. Samples of Bristlecone pine, a tree with a very long life span, have been dated using both dendrochronology and radiocarbon dating.
The results do not agree, but the differences are consistent. That is, the radiocarbon dates were always wrong by the same number of years. Consequently, tree-ring chronologies have been used to calibrate radiocarbon dates to around 12, years ago. When radiocarbon dating was first put into use, it was decided that dates would always be reported as B. That way, dates reported in magazine articles and books do not have to be adjusted as the years pass.
So if a lab determines that an object has a radiocarbon age of 1, years inits age will be given as B. Calibrated dates are given using the actual date, such as c. Potassium-Argon Dating. If an object is too old to be dated by radiocarbon dating, or if it contains no organic material, other methods must be used.
One of these is potassium-argon dating. All naturally occurring rocks contain potassium.
Geological periods: Periods of time characterized by successive deposits of animal and plants remains 2. Stratigraphy: study and interpretation. So, how do we know how old a fossil is? There are two main methods determining a fossils age, relative dating and absolute dating. Relative dating is used to. Using relative and radiometric dating methods, geologists are able to answer the Third, magnetism in rocks can be used to estimate the age of a fossil site. .. P.R., Swisher, C.C. 40Ar/39Ar dating in paleoanthropology and archaeology.
Some of the potassium in rocks is the radioactive isotope potassium Potassium gradually decays to the stable isotope argon, which is a gas. When the rock is melted, as in a volcano, any argon gas trapped in the rock escapes. When the rock cools, the argon will begin to build up. So this method can be used to measure the age of any volcanic rock, fromyears up to around 5 billion years old. This method is not widely used in archaeology, since most archaeological deposits are not associated with volcanic activity.
However, Louis and Mary Leakey successfully used the method to determine the ages of fossils in Olduvai Gorge in Tanzania by examining rocks from lava flows above and below the fossils.
They were able to establish an absolute chronology for humans and human ancestors extending back two million years. At Laetolli, in Tanzania, volcanic ash containing early hominid footprints was dated by this method at 3. Other Methods. Uranium is present in most rocks. This isotope of uranium spontaneously undergoes fission. The fission fragments have a lot of energy, and they plow through the rock, leaving a track that can be made visible by treating the rock.
So by counting fission tracks, the age of the rock can be determined.
Like potassium-argon datingthis can only be used to determine the age of the rock, not the age of the artifact itself. Thermoluminescence is a recently developed technique that uses the property of some crystals to "store" light. Sometimes an electron will be knocked out of its position in a crystal and will "stick" somewhere else in the crystal.
These displaced electrons will accumulate over time. If the sample is heated, the electrons will fall back to their normal positions, emitting a small flash of light. By measuring the light emitted, the time that has passed since the artifact was heated can be determined. This method should prove to be especially useful in determining the age of ceramics, rocks that have been used to build fire rings, and samples of chert and flint that have been deliberately heated to make them easier to flake into a projectile point.
Science continues to develop new methods to determine the age of objects. As our knowledge of past chronologies improves, archaeologists will be better able to understand how cultures change over time, and how different cultures interact with each other. As a result, this knowledge will enable us to achieve a progressively better understanding of our own culture. Baillie, M. London U. Taylor, R. Radiocarbon Dating : An Archaeological Perspective. Orlando, FL: Academic Press, Long, and R.
New York : Springer-Verlag, Wood, Michael. In Search of the Trojan War. New York : New American Library, Dating techniques are procedures used by scientists to determine the age of an object or a series of events. The two main types of dating methods are relative and absolute. Relative dating methods are used to determine only if one sample is older or younger than another. Absolute dating methods are used to determine an actual date in years for the age of an object.
Before the advent of absolute dating methods in the twentieth century, nearly all dating was relative. The main relative dating method is stratigraphy pronounced stra-TI-gra-feewhich is the study of layers of rocks or the objects embedded within those layers.
This method is based on the assumption which nearly always holds true that deeper layers of rock were deposited earlier in Earth 's history, and thus are older than more shallow layers.
The successive layers of rock represent successive intervals of time. Since certain species of animals existed on Earth at specific times in history, the fossils or remains of such animals embedded within those successive layers of rock also help scientists determine the age of the layers.
Methods of dating fossils and archeology
Similarly, pollen grains released by seed-bearing plants became fossilized in rock layers. If a certain kind of pollen is found in an archaeological site, scientists can check when the plant that produced that pollen lived to determine the relative age of the site. Absolute dating methods are carried out in a laboratory. The most widely used and accepted form of absolute dating is radioactive decay dating.
Radioactive decay dating. Radioactive decay refers to the process in which a radioactive form of an element is converted into a nonradioactive product at a regular rate. The nucleus of every radioactive element such as radium and uranium spontaneously disintegrates over time, transforming itself into the nucleus of an atom of a different element.
Dating in Archaeology
In the process of disintegration, the atom gives off radiation energy emitted in the form of waves. Hence the term radioactive decay. Each element decays at its own rate, unaffected by external physical conditions. By measuring the amount of original and transformed atoms in an object, scientists can determine the age of that object.
Cosmic rays: Invisible, high-energy particles that constantly bombard Earth from all directions in space.
Morris ( ) talks about methods used to date fossils. He claims rocks and for dating geologic events exactly is furnished by the fossils. Owing to. Dating methods Dating techniques are procedures used by scientists to determine the Timbers can be used to date buildings and archaeological sites. been used to date volcanic layers above and below fossils and artifacts in east Africa. Methods of dating fossils in archaeology - Men looking for a man - Women looking for a man. Is the number one destination for online dating with more.
Dendrochronology: Also known as tree-ring dating, the science concerned with determining the age of trees by examining their growth rings. Half-life: Measurement of the time it takes for one-half of a radioactive substance to decay. Radioactive decay: The predictable manner in which a population of atoms of a radioactive element spontaneously disintegrate over time.
The age of the remains of plants, animals, and other organic material can be determined by measuring the amount of carbon contained in that material. Carbon, a radioactive form of the element carbon, is created in the atmosphere by cosmic rays invisible, high-energy particles that constantly bombard Earth from all directions in space.
When carbon falls to Earth, it is absorbed by plants. These plants are eaten by animals who, in turn, are eaten by even larger animals.
Eventually, the entire ecosystem community of plants and animals of the planet, including humans, is filled with a concentration of carbon As long as an organism is alive, the supply of carbon is replenished. When the organism dies, the supply stops, and the carbon contained in the organism begins to spontaneously decay into nitrogen The time it takes for one-half of the carbon to decay a period called a half-life is 5, years.
By measuring the amount of carbon remaining, scientists can pinpoint the exact date of the organism's death. The range of conventional radiocarbon dating is 30, to 40, years.
With sensitive instrumentation, this range can be extended to 70, years. In addition to the radiocarbon dating technique, scientists have developed other dating methods based on the transformation of one element into another. These include the uranium-thorium method, the potassium-argon method, and the rubidium-strontium method. Thermoluminescence pronounced ther-moeloo-mi-NES-ence dating is very useful for determining the age of pottery.
The older the pottery, the brighter the light that will be emitted. Using thermoluminescence, pottery pieces as old asyears can be dated with precision. Tree-ring dating.
Known as dendrochronology pronounced den-dro-crow-NOL-o-geetree-ring dating is based on the fact that trees produce one growth ring each year. Narrow rings grow in cold or dry years, and wide rings grow in warm or wet years. Thus, the growth pattern of a tree of a known age can be used as a standard to determine the age of similar trees.
The ages of buildings and archaeological sites can also be determined by examining the ring patterns of the trees used in their construction. Dendrochronology has a range of 1 to 10, years or more. Relative dating techniques date specimens in relation to one another; for example, stratigraphy is used to establish the succession of fossils. Absolute or chronometric techniques give an absolute estimate of the age and fall into two main groups. The first depends on the existence of something that develops at a seasonally varying rate, as in dendrochronology and varve dating.
The other uses some measurable change that occurs at a known rate, as in chemical datingradioactive or radiometric dating see carbon dating ; fission-track dating ; potassium—argon dating ; rubidium—strontium dating ; uranium—lead datingand thermoluminescence.
Depositional rates of sediments have also been employed as a dating method, but only recently has absolute dating been made possible through the use of radioactive isotopes. Of the various methods the last is obviously the most precise, but fossilslithologiesand cross-cutting relationships do enable the geologist to give an approximate relative age in field studies.
A relative time scale, constructed in the last century, is based on correlations between palaeontological and stratigraphic data.
The rate at which sediments accumulate can also be used for dating see varve. Absolute dating relies on the decay of radioactive isotopes of elements present in the material to be dated see decay constant ; decay curve ; decay series ; isotopic dating; radiocarbon dating ; and radiometric dating. 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. Many absolute dating techniques take advantage of radioactive decaywhereby a radioactive form of an element decays into a non-radioactive product at a regular rate. In recent years, a few of these methods have come under close scrutiny as scientists strive to develop the most accurate dating techniques possible. 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.
Although these units may be sequential, they are not necessarily continuous due to erosional removal of some intervening. Stratigraphy is the principle method of relative dating, and in the early years of dating studies was virtually the only method available to scientists.
The technique works best if the animals belonged to species, which evolved quickly, expanded rapidly over a large area, or suffered a mass extinction. The unit of the calendar is the pollen zone. In most cases, this tells us about the climate of the period, because most plants only thrive in specific climatic conditions.
This dating technique was first conducted by Hare and Mitterer inand was popular in the s. 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. This may form a D-amino acid instead of an L-amino acid.Minecraft Mod Showcase: Fossils And Archeology Mod (Create your own dinosaurs!)
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. It can be used to obtain dates that would be unobtainable by more conventional methods such as radio-carbon dating.
Although cation-ratio dating has been widely used, recent studies suggest it has many problems. Finally, some scientists have recently suggested that the cation ratios may not even be directly related to the age of the sample.
Thermoluminescence dating is useful for determining the age of pottery. This radiation may come from radioactive substances such as uranium, present in the clay or burial medium, or from cosmic radiation. 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. 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.
Optically stimulated luminescence has only been used since 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 absolute dating method is also known as dendrochronology. Dendrochronology has a range ofyears or more. 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.
When volcanic rocks are heated to extremely high temperatures, they release any argon gas trapped in them. Radiocarbon is used to date charcoal, wood, and other biological materials. The range of conventional radiocarbon dating is 30, — 40, years, but with sensitive instrumentation this range can be extended to 70, years. 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.
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. This can lead to inaccurate dates. Another problem lies with the assumptions associated with radiocarbon dating. This is not completely true. 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.
In the case of a daughter excess, a larger amount of the daughter is initially deposited than the parent. Some volcanic minerals and glasses, such as obsidian, contain uranium U. See also Pollen analysis ; Strata. Dickin, Alan P. Radiogenic Isotope Geology. Balter, Michael. Guilderson, Tom P. Turney, Chris S. Relative dating methods tell only if one sample is older or younger than another; absolute dating methods provide a date in years.
Many absolute dating techniques take advantage of radioactive decaywhereby a radioactive form of an element is converted into a non-radioactive product at a regular rate. The technique works best if the animals belonged to species which evolved quickly, expanded rapidly over a large area, or suffered a mass extinction.
Pollen that ends up in lake beds 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. The varnish contains cations, which are positivelycharged atoms or molecules. This radiation may come from radioactive substances such as uraniumpresent in the clay or burial medium, or from cosmic radiation. Thermoluminescence dating has the advantage of covering the time interval between radiocarbon and potassium-argon datingor 40,—, years.
As the rocks cool, argon 40Ar begins to accumulate. Argon is formed in the rocks by the radioactive decay of potassium 40K. The amount of 40Ar formed is proportional to the decay rate half-life of 40K, which is 1. The reason such old material is required is that it takes a very long time to accumulate enough 40Ar to be measured accurately.
The range of conventional radiocarbon dating is 30,—40, years, but with sensitive instrumentation this range can be extended to 70, years. Radiocarbon 14C is a radioactive form of the element carbon. It decays spontaneously into nitrogen 14N. Atoms of 14C and of a non-radioactive form of carbon, 12C, are equally likely to be incorporated into living organisms—there is no discrimination.
The ratio will then begin to change as the 14C in the dead organism decays into 14N. This is the time required for half of the 14C to decay into 14N. The half-life of 14C is 5, years. This allows us to determine how much 14C has formed since the death of the organism.
The "parent" isotopes have half-lives of several thousand million years. Geyh, Mebus A. Absolute Age Determination. Oberhofer, and D. Regulla, eds. Scientific Dating Methods. Boston: Kluwer Academic Publishers, Lewis, C.
Fission-Track Dating. Print this article Print all entries for this topic Cite this article. Dating methods Dating techniques are procedures used by scientists to determine the age of a specimen.
Learn more about citation styles Citation styles Encyclopedia. Dating Techniques Movies and television have presented a romantic vision of archaeology as adventure in far-away and exotic locations. Dating techniques Dating techniques are procedures used by scientists to determine the age of an object or a series of events. Relative dating Before the advent of absolute dating methods in the twentieth century, nearly all dating was relative.
Absolute dating Absolute dating methods are carried out in a laboratory. Dating Techniques Relative dating Stratigraphy Seriation Faunal dating Pollen dating palynology Absolute dating Amino acid racimization Cation-ratio dating Thermoluminescence dating Tree-ring dating Radioactive decay dating Potassium-argon dating Radiocarbon dating Uranium series dating Fission track dating Resources Dating techniques are procedures used by scientists to determine the age of rocks, fossils, or artifacts.
For those researchers working in the field of human history, the chronology of events remains a major element of reflection. Archaeologists have access to various techniques for dating archaeological sites or the objects found on those sites. There are two main categories of dating methods in archaeology : indirect or relative dating and absolute dating.
Relative dating includes methods that rely on the analysis of comparative data or the context eg, geological, regional, cultural in which the object one wishes to date is found. This approach helps to order events chronologically but it does not provide the absolute age of an object expressed in years. Relative dating includes different techniques, but the most commonly used are soil stratigraphy analysis and typology. On the other hand, absolute dating includes all methods that provide figures about the real estimated age of archaeological objects or occupations.
These methods usually analyze physicochemical transformation phenomena whose rate are known or can be estimated relatively well. This is the only type of techniques that can help clarifying the actual age of an object.
Absolute dating methods mainly include radiocarbon dating, dendrochronology and thermoluminescence.
Stratigraphy Inspired by geologystratigraphy uses the principle of the superposition of strata which suggests that, in a succession of undisturbed SOILSthe upper horizons are newer than the lower ones.
Generally, each stratum is isolated in a separate chronological unit that incorporates artifacts. However, this method is sometimes limited because the reoccupation of an area may require excavation to establish the foundation of a building, for instance, that goes through older layers.
In this case, even if the foundation of the building is found in the same stratigraphic level as the previous occupation, the two events are not contemporary. Stratigraphic dating remains very reliable when it comes to dating objects or events in undisturbed stratigraphic levels. For example, the oldest human remains known to date in Canada, found at Gore Creekhave been dated using soil stratification. The bones were buried under and are therefore older a layer of ash that resulted from a volcanic eruption dating back to years BP Before Present; "present" indicates c.
Subsequently, radiocarbon dating, an absolute dating technique, was used to date the bones directly and provided a date of BP, showing how useful the combined used of relative and absolute dating can be. Moreover, stratigraphic dating is sometimes based on the objects that are found within the soil strata.
Indeed, some items whose exact or approximate age is known are called "diagnostic artifacts. Their presence on archaeological sites is used to date the soil layers and the objects and events they are associated with and thus contributes to refine the chronology of sites. Typology Typology is a method that compares reference objects in order to classify them according to their similarity or dissimilarity and link them to a specific context or period.
This technique is frequently used when it is impossible to make use of absolute dating methods; it generally allows archaeologists to identify the period to which a cultural site or object belongs, without specifying the date of occupation.
This method is primarily applied to projectile points and ceramic vessels. These present many characteristics that are used for comparing them, such as morphology and raw materials in the case of stone tools, and decorative techniques and motifs in the case of ceramics.
Radiocarbon Dating Radiocarbon dating is the most widely used dating technique in archaeology. It relies on a natural phenomenon that is the foundation of life on earth.
Indeed, carbon 14 14C is formed from the reaction caused by cosmic rays that convert nitrogen into carbon 14 and then carbon dioxide by combining with carbon 12 12C and carbon 13 13Cwhich are stable carbon isotopes.
Following the death of an organism, any exchange ceases and the carbon 14, which is radioactive and therefore unstable, slowly begins to disintegrate at a known rate half-life of years, ie, after this period only half of the total carbon 14 present at the time of death remains. A sample requires 10 to 20 grams of matter and usually consists of charred organic material, mainly charcoal, but bones see zooarchaeology and shells can also be dated using this technique.
An initial reading dates the specimen which is then calibrated by considering this date and its correspondence with the measurable level of carbon 14 stored over time in the growth rings of certain tree species, including redwood and pine bristol.
Subsequently, the calibration of that date provides a time interval where the event or object being dated can be situated eg, AD. Radiocarbon dating, however, can only be used for dating objects that are less than 50 years. Dendrochronology Dendrochronology is a method that studies the rings of tree trunks to define characteristic sequences by analyzing the morphology of growth rings for a given species. This method is based on the principle that the variation in tree growth from one year to another is influenced by the degree of precipitation, sunshine, temperature, soil type and all ambient conditions and that, consequently, reference patterns can be distinguished.
Several sets of rings from different trees are matched to build an average sequence.