RELATIVE VS. ABSOLUTE DATING by Terasa Hodson on Prezi
Absolute or Chronometric Dating . In archaeology or paleoanthropology, a technique for relative dating based on the fact that the amount of fluorine in bones is. Archaeological scientists have two primary ways of telling the age of artefacts and the sites from which they came: relative dating and absolute. Chronology: Relative and Absolute Dating methods According to the Dictionary of Anthropology, the word chronology means the science of computing dates.
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. 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 datingor 40,—, years.
In addition, it can be used to date materials that cannot be dated with these other two methods. Optically stimulated luminescence OSL has only been used since It is very similar to thermoluminescence dating, both of which are considered "clock setting" techniques. Minerals found in sediments are sensitive to light.
Electrons found in the sediment grains leave the ground state when exposed to light, called recombination. To determine the age of sediment, scientists expose grains to a known amount of light and compare these grains with the unknown sediment.
This technique can be used to determine the age of unheated sediments less thanyears old.
Dating Methods in Prehistory
A disadvantage to this technique is that in order to get accurate results, the sediment to be tested cannot be exposed to light which would reset the "clock"making sampling difficult. The absolute dating method utilizing tree ring growth is known as dendrochronology.
It is based on the fact that trees produce one growth ring each year. The rings form a distinctive pattern, which is the same for all members in a given species and geographical area.
The patterns from trees of different ages including ancient wood are overlapped, forming a master pattern that can be used to date timbers thousands of years old with a resolution of one year.
Timbers can be used to date buildings and archaeological sites. In addition, tree rings are used to date changes in the climate such as sudden cool or dry periods.
Dendrochronology has a range of one to 10, years or more. As previously mentioned, radioactive decay refers to the process in which a radioactive form of an element is converted into a decay product at a regular rate.
Radioactive decay dating is not a single method of absolute dating but instead a group of related methods for absolute dating of samples. Potassium-argon dating relies on the fact that when volcanic rocks are heated to extremely high temperatures, they release any argon gas trapped in them. As the rocks cool, argon 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. In other words, it takes 1. This method is generally only applicable to rocks greater than three million years old, although with sensitive instruments, rocks several hundred thousand years old may be dated. The reason such old material is required is that it takes a very long time to accumulate enough 40Ar to be measured accurately. Potassium-argon dating has been used to date volcanic layers above and below fossils and artifacts in east Africa.
Radiocarbon dating is used to date charcoal, wood, and other biological materials. The range of conventional radiocarbon dating is 30,—40, years, but with sensitive instrumentation, this range can be extended to 70, years. Radiocarbon 14C is a radioactive form of the element carbon. It decays spontaneously into nitrogen 14N. Plants get most of their carbon from the air in the form of carbon dioxideand animals get most of their carbon from plants or from animals that eat plants.
Relative to their atmospheric proportions, atoms of 14C and of a non-radioactive form of carbon, 12C, are equally likely to be incorporated into living organisms. When the organism dies, however, its body stops incorporating new carbon. The ratio will then begin to change as the 14C in the dead organism decays into 14N. The rate at which this process occurs is called the half-life. This is the time required for half of the 14C to decay into 14N.
The half-life of 14C is 5, years. This allows them to determine how much 14C 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 datinga clever forgery might be indistinguishable from a real artifact. There are some limitations, however, to the use of this technique. Samples that were heated or irradiated at some time may yield by radioactive dating an age less than the true age of the object.
Because of this limitation, other dating techniques are often used along with radioactive dating to ensure accuracy. Uranium series dating techniques rely on the fact that radioactive uranium and thorium isotopes decay into a series of unstable, radioactive "daughter" isotopes; this process continues until a stable non-radioactive lead isotope is formed.
The daughters have relatively short half-lives ranging from a few hundred thousand years down to only a few years. The "parent" isotopes have half-lives of several billion years.
This provides a dating range for the different uranium series of a few thousand years toyears.
Relative dating - Wikipedia
Uranium series have been used to date uranium-rich rocks, deep-sea sediments, shells, bones, and teeth, and to calculate the ages of ancient lakebeds. The two types of uranium series dating techniques are daughter deficiency methods and daughter excess methods.
In daughter deficiency situations, the parent radioisotope is initially deposited by itself, without its daughter the isotope into which it decays present. Through time, the parent decays to the daughter until the two are in equilibrium equal amounts of each. The age of the deposit may be determined by measuring how much of the daughter has formed, providing that neither isotope has entered or exited the deposit after its initial formation.
Living mollusks and corals will only take up dissolved compounds such as isotopes of uranium, so they will contain no protactinium, which is insoluble. Protactinium begins to accumulate via the decay of U after the organism dies. Scientists can determine the age of the sample by measuring how much Pa is present and calculating how long it would have taken that amount to form.
In the case of daughter excess, a larger amount of the daughter is initially deposited than the parent.
Non-uranium daughters such as protactinium and thorium are insoluble, and precipitate out on the bottoms of bodies of water, forming daughter excesses in these sediments. Over time, the excess daughter disappears as it is converted back into the parent, and by measuring the extent to which this has occurred, scientists can date the sample.
If the radioactive daughter is an isotope of uranium, it will dissolve in water, but to a different extent than the parent; the two are said to have different solubilities. For example, U dissolves more readily in water than its parent, U, so lakes and oceans contain an excess of this daughter isotope.
Some volcanic minerals and glasses, such as obsidiancontain uranium U. Over time, these substances become "scratched. When an atom of U splits, two "daughter" atoms rocket away from each other, leaving in their wake tracks in the material in which they are embedded. The rate at which this process occurs is proportional to the decay rate of U.
The decay rate is measured in terms of the half-life of the element, or the time it takes for half of the element to split into its daughter atoms. The half-life of U is 4. When the mineral or glass is heated, the tracks are erased in much the same way cut marks fade away from hard candy that is heated.
This process sets the fission track clock to zero, and the number of tracks that then form are a measure of the amount of time that has passed since the heating event. Scientists are able to count the tracks in the sample with the aid of a powerful microscope.
The sample must contain enough U to create enough tracks to be counted, but not contain too much of the isotope, or there will be a jumble of tracks that cannot be distinguished for counting.
Sediment will continue to be transported to an area and it will eventually be deposited. However, the layer of that material will become thinner as the amount of material lessens away from the source.
Often, coarser-grained material can no longer be transported to an area because the transporting medium has insufficient energy to carry it to that location. In its place, the particles that settle from the transporting medium will be finer-grained, and there will be a lateral transition from coarser- to finer-grained material. The lateral variation in sediment within a stratum is known as sedimentary facies. If sufficient sedimentary material is available, it will be deposited up to the limits of the sedimentary basin.
Often, the sedimentary basin is within rocks that are very different from the sediments that are being deposited, in which the lateral limits of the sedimentary layer will be marked by an abrupt change in rock type.
- Dating methods
- Dating Techniques In Archaeology
- Principles of Prehistoric Archaeology. Chronology: Relative and Absolute Dating methods
Inclusions of igneous rocks[ edit ] Multiple melt inclusions in an olivine crystal. Individual inclusions are oval or round in shape and consist of clear glass, together with a small round vapor bubble and in some cases a small square spinel crystal.
The black arrow points to one good example, but there are several others. The occurrence of multiple inclusions within a single crystal is relatively common Melt inclusions are small parcels or "blobs" of molten rock that are trapped within crystals that grow in the magmas that form igneous rocks.
In many respects they are analogous to fluid inclusions. Melt inclusions are generally small — most are less than micrometres across a micrometre is one thousandth of a millimeter, or about 0. Nevertheless, they can provide an abundance of useful information. Using microscopic observations and a range of chemical microanalysis techniques geochemists and igneous petrologists can obtain a range of useful information from melt inclusions.
Two of the most common uses of melt inclusions are to study the compositions of magmas present early in the history of specific magma systems. This is because inclusions can act like "fossils" — trapping and preserving these early melts before they are modified by later igneous processes. In addition, because they are trapped at high pressures many melt inclusions also provide important information about the contents of volatile elements such as H2O, CO2, S and Cl that drive explosive volcanic eruptions.
Sorby was the first to document microscopic melt inclusions in crystals. The study of melt inclusions has been driven more recently by the development of sophisticated chemical analysis techniques. Scientists from the former Soviet Union lead the study of melt inclusions in the decades after World War II Sobolev and Kostyuk,and developed methods for heating melt inclusions under a microscope, so changes could be directly observed.
Although they are small, melt inclusions may contain a number of different constituents, including glass which represents magma that has been quenched by rapid coolingsmall crystals and a separate vapour-rich bubble. They occur in most of the crystals found in igneous rocks and are common in the minerals quartzfeldsparolivine and pyroxene. The formation of melt inclusions appears to be a normal part of the crystallization of minerals within magmas, and they can be found in both volcanic and plutonic rocks.
Included fragments[ edit ] The law of included fragments is a method of relative dating in geology. Essentially, this law states that clasts in a rock are older than the rock itself. Another example is a derived fossilwhich is a fossil that has been eroded from an older bed and redeposited into a younger one.
These foreign bodies are picked up as magma or lava flowsand are incorporated, later to cool in the matrix. As a result, xenoliths are older than the rock which contains them Many of the same principles are applied.