Geologic Age Dating Explained - Kids Discover
Isotopic dating methods help us determine the ages of rocks. The principle of using radioactive decay as a dating method is simple. Consider. There are two basic approaches: relative geologic age dating, and to represent a particular interval of time, or on radioactive decay of specific. The discovery of the natural radioactive decay of uranium in by Henry Becquerel, Precise dating has been accomplished since that relates radioactive decay to geologic time is called the age equation and is.
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.
DETERMINING AGE OF ROCKS AND FOSSILS
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.
- Radiometric Age Dating
- Radiometric dating
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.
Radiometric Age Dating - Geology (U.S. National Park Service)
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.
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.Radioactive Dating
The above equation makes use of information on the composition of parent and daughter isotopes at the time the material being tested cooled below its closure temperature. This is well-established for most isotopic systems. Plotting an isochron is used to solve the age equation graphically and calculate the age of the sample and the original composition.
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.
Samarium—neodymium dating method[ edit ] Main article: Samarium—neodymium dating This involves the alpha decay of Sm to Nd with a half-life of 1. Accuracy levels of within twenty million years in ages of two-and-a-half billion years are achievable. Potassium—argon dating This involves electron capture or positron decay of potassium to argon Potassium has a half-life of 1.
Rubidium—strontium dating method[ edit ] Main article: Rubidium—strontium dating This is based on the beta decay of rubidium to strontiumwith a half-life of 50 billion years.
Geologic Age Dating Explained
This scheme is used to date old igneous and metamorphic rocksand has also been used to date lunar samples. Closure temperatures are so high that they are not a concern. The Radiometric Decay Equation A constant-rate process such as radioactive decay is described by the simple equation: In order to use this equation for decay over a given time period, we will need the solution of a first-order differential equation. Obtaining such a solution is beyond the scope and requirements of this class, though with years of calculus, you to could do the impossible.
The equation above is known as the decay equation. It shows that at any time t, the number of parent atoms, N, is equal to the number of original parent atoms at time zero N0 gives the number of parent atoms at time zeromultiplied by the natural exponent raised to the negative power of the decay constant l multiplied by the time t.
Relationship of l to half-life The decay equation can be used to show the relationship of the decay constant l to the half-life of any unstable isotope. I use the term "appropriate" in the sense that the specimen to be dated must obviously contain isotopes of a well known radioactive decay series, and be suitable for precise chemical analysis. In the simplest ideal situation,the decay equation is utilized by making the following substitutions: Therefore, if we know the decay constant l and can accurately measure D and P, in principle, we can determine the absolute age.
Accurate measurement of either the absolute or relative abundance of trace quantities of radioactive isotopes requires sophisticated instruments, known as mass spectrometers, and instrument operators who really know what they are doing.
The technique appears to be simple and straightforward, but is actually very difficult and time-consuming. It is not a trivial task! Starting the Radiometric Clocks 1. Living organisms continually exchange carbon with the atmosphere through the process of photosynthesis. When the organism dies, however, exchange of carbon ceases and the carbon present in the organism becomes isolated.
This event death of the organism marks the effective starting of the C 14 clock. One of the most common types of material used in C 14 dating is charcoal e. These rocks form by the cooling and crystallization of hot silicate liquids magma or lava. As cooling proceeds from high temperatures ca.
A growing mineral may trap small amounts of a radioactive isotope within its crystal structure.