Absolute Dating — Mr. Mulroy's Earth Science
Using relative and radiometric dating methods, geologists are able to answer the Second, it is possible to determine the numerical age for fossils or earth. Describe how scientists know earth is billions of years old. Radiometric dating and other forms of absolute age dating allowed scientists to get an absolute age . Absolute dating is the process of determining an age on a specified chronology in archaeology and geology. Some scientists prefer the terms chronometric or calendar dating, as use of Cosmic radiation entering the earth's atmosphere produces carbon, and plants take in carbon as they fix carbon dioxide.
Additionally, elements may exist in different isotopeswith each isotope of an element differing in the number of neutrons in the nucleus. A particular isotope of a particular element is called a nuclide.
Some nuclides are inherently unstable. That is, at some point in time, an atom of such a nuclide will undergo radioactive decay and spontaneously transform into a different nuclide.
This transformation may be accomplished in a number of different ways, including alpha decay emission of alpha particles and beta decay electron emission, positron emission, or electron capture. Another possibility is spontaneous fission into two or more nuclides.
While the moment in time at which a particular nucleus decays is unpredictable, a collection of atoms of a radioactive nuclide decays exponentially at a rate described by a parameter known as the half-lifeusually given in units of years when discussing dating techniques. After one half-life has elapsed, one half of the atoms of the nuclide in question will have decayed into a "daughter" nuclide or decay product.
Radioactive Dating Game
In many cases, the daughter nuclide itself is radioactive, resulting in a decay chaineventually ending with the formation of a stable nonradioactive daughter nuclide; each step in such a chain is characterized by a distinct half-life. In these cases, usually the half-life of interest in radiometric dating is the longest one in the chain, which is the rate-limiting factor in the ultimate transformation of the radioactive nuclide into its stable daughter.
Isotopic systems that have been exploited for radiometric dating have half-lives ranging from only about 10 years e. It is not affected by external factors such as temperaturepressurechemical environment, or presence of a magnetic or electric field. For all other nuclides, the proportion of the original nuclide to its decay products changes in a predictable way as the original nuclide decays over time.
This predictability allows the relative abundances of related nuclides to be used as a clock to measure the time from the incorporation of the original nuclides into a material to the present. Accuracy of radiometric dating[ edit ] Thermal ionization mass spectrometer used in radiometric dating. The basic equation of radiometric dating requires that neither the parent nuclide nor the daughter product can enter or leave the material after its formation.
The possible confounding effects of contamination of parent and daughter isotopes have to be considered, as do the effects of any loss or gain of such isotopes since the sample was created. It is therefore essential to have as much information as possible about the material being dated and to check for possible signs of alteration.
Alternatively, if several different minerals can be dated from the same sample and are assumed to be formed by the same event and were in equilibrium with the reservoir when they formed, they should form an isochron. This can reduce the problem of contamination. In uranium—lead datingthe concordia diagram is used which also decreases the problem of nuclide loss.
Finally, correlation between different isotopic dating methods may be required to confirm the age of a sample. For example, the age of the Amitsoq gneisses from western Greenland was determined to be 3.
The procedures used to isolate and analyze the parent and daughter nuclides must be precise and accurate. This normally involves isotope-ratio mass spectrometry. For instance, carbon has a half-life of 5, years. After an organism has been dead for 60, years, so little carbon is left that accurate dating cannot be established. On the other hand, the concentration of carbon falls off so steeply that the age of relatively young remains can be determined precisely to within a few decades.
Radiometric dating - Wikipedia
Closure temperature If a material that selectively rejects the daughter nuclide is heated, any daughter nuclides that have been accumulated over time will be lost through diffusionsetting the isotopic "clock" to zero. The temperature at which this happens is known as the closure temperature or blocking temperature and is specific to a particular material and isotopic system.
These temperatures are experimentally determined in the lab by artificially resetting sample minerals using a high-temperature furnace. As the mineral cools, the crystal structure begins to form and diffusion of isotopes is less easy.
At a certain temperature, the crystal structure has formed sufficiently to prevent diffusion of isotopes. This temperature is what is known as closure temperature and represents the temperature below which the mineral is a closed system to isotopes. Thus an igneous or metamorphic rock or melt, which is slowly cooling, does not begin to exhibit measurable radioactive decay until it cools below the closure temperature. The age that can be calculated by radiometric dating is thus the time at which the rock or mineral cooled to closure temperature.
This field is known as thermochronology or thermochronometry. The age is calculated from the slope of the isochron line and the original composition from the intercept of the isochron with the y-axis.
The equation is most conveniently expressed in terms of the measured quantity N t rather than the constant initial value No. The above equation makes use of information on the composition of parent and daughter isotopes at the time the material being tested cooled below its closure temperature. This is well-established for most isotopic systems.Relative Vs Absolute Dating
Plotting an isochron is used to solve the age equation graphically and calculate the age of the sample and the original composition. Modern dating methods[ edit ] Radiometric dating has been carried out since when it was invented by Ernest Rutherford as a method by which one might determine the age of the Earth. In the century since then the techniques have been greatly improved and expanded. The mass spectrometer was invented in the s and began to be used in radiometric dating in the s.
It operates by generating a beam of ionized atoms from the sample under test. The ions then travel through a magnetic field, which diverts them into different sampling sensors, known as " Faraday cups ", depending on their mass and level of ionization.
On impact in the cups, the ions set up a very weak current that can be measured to determine the rate of impacts and the relative concentrations of different atoms in the beams. Uranium—lead dating method[ edit ] Main article: Uranium—lead dating A concordia diagram as used in uranium—lead datingwith data from the Pfunze BeltZimbabwe.
This scheme has been refined to the point that the error margin in dates of rocks can be as low as less than two million years in two-and-a-half billion years.
Zircon has a very high closure temperature, is resistant to mechanical weathering and is very chemically inert. Zircon also forms multiple crystal layers during metamorphic events, which each may record an isotopic age of the event.
This can be seen in the concordia diagram, where the samples plot along an errorchron straight line which intersects the concordia curve at the age of the sample. However, it can be used to confirm the antiquity of an item. Optically stimulated luminescence OSL [ edit ] Optically stimulated luminescence OSL dating constrains the time at which sediment was last exposed to light.
During sediment transport, exposure to sunlight 'zeros' the luminescence signal.
- Radiometric dating
Upon burial, the sediment accumulates a luminescence signal as natural ambient radiation gradually ionises the mineral grains. Careful sampling under dark conditions allows the sediment to be exposed to artificial light in the laboratory which releases the OSL signal. The amount of luminescence released is used to calculate the equivalent dose De that the sediment has acquired since deposition, which can be used in combination with the dose rate Dr to calculate the age. Dendrochronology The growth rings of a tree at Bristol ZooEngland.
Each ring represents one year; the outside rings, near the bark, are the youngest. Dendrochronology or tree-ring dating is the scientific method of dating based on the analysis of patterns of tree rings, also known as growth rings.
Absolute dating - Wikipedia
Dendrochronology can date the time at which tree rings were formed, in many types of wood, to the exact calendar year. Dendrochronology has three main areas of application: In some areas of the world, it is possible to date wood back a few thousand years, or even many thousands. Currently, the maximum for fully anchored chronologies is a little over 11, years from present.
Amino acid dating Amino acid dating is a dating technique      used to estimate the age of a specimen in paleobiologyarchaeologyforensic sciencetaphonomysedimentary geology and other fields. This technique relates changes in amino acid molecules to the time elapsed since they were formed.
All biological tissues contain amino acids.