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Radiometric dating , radioactive dating or radioisotope dating is a technique which is used to date materials such as rocks or carbon , in which trace radioactive impurities were selectively incorporated when they were formed. The method compares the abundance of a naturally occurring radioactive isotope within the material to the abundance of its decay products, which form at a known constant rate of decay. Together with stratigraphic principles , radiometric dating methods are used in geochronology to establish the geologic time scale. By allowing the establishment of geological timescales, it provides a significant source of information about the ages of fossils and the deduced rates of evolutionary change. Radiometric dating is also used to date archaeological materials, including ancient artifacts.
From these assumptions, he calculated that the Earth was million years old.
It provided a way to find the absolute age of a rock. To understand how this is done, it is necessary to review some facts about atoms. Atoms contain three particles: protons, neutrons, and electrons. Protons and neutrons are located in the nucleus, while electrons orbit around the nucleus.
For example, all atoms of carbon have six protons, all atoms of oxygen have eight protons, and all atoms of gold have 79 protons. The number of neutrons, however, is variable. For example, the isotope carbon contains 6 neutrons in its nucleus, while the isotope carbon has 7 neutrons.
Relative Vs. Absolute Dating: The Ultimate Face-off
This means the atom will spontaneously change from an unstable form to a stable form. There are two forms of nuclear decay that are relevant in how geologists can date rocks Table If an element decays by losing an alpha particle, it will lose 2 protons and 2 neutrons.
Sep 30, Geologic age dating is an entire discipline of its own. In a way, this field, called geochronology, is some of the purest detective work earth scientists do. There are two basic approaches: relative geologic age dating, and absolute geologic age dating. The science of absolute age dating is known as geochronology and the fundamental method of geochronology is called radiometric dating. Scholars and naturalists, understandably, have long been interested in knowing the absolute age of the Earth, as well as other important geological events. Absolute Dating It determines the age of a rock/object using radiometric techniques. Absolute dating is quantitative. This technique helps determine the exact age of the remains. It is more specific than relative dating. Absolute dating is expensive and time-consuming. It works .
If an atom decays by losing a beta particle, it loses just one electron. So what does this have to do with the age of Earth? Radioactive materials decay at known rates. As time passes, the proportion of radioactive isotopes will decrease and the proportion of daughter isotopes will increase. A rock with a relatively high proportion of radioactive isotopes is probably very young, while a rock with a high proportion of daughter products is probably very old. The half-life of a radioactive substance is the amount of time, on average, it takes for half of the atoms to decay.
For example, imagine a radioactive substance with a half-life of one year. When a rock is formed, it contains a certain number of radioactive atoms. After the third year three half-lives After four years four half-lives6. If you find a rock whose radioactive material has a half life of one year and measure 3.
Sep 30, Relative age dating has given us the names we use for the major and minor geologic time periods we use to split up the history of Earth and all the other planets. When you talk about the Precambrian, Paleozoic, Mesozoic, and Cenozoic on Earth, or the Noachian, Hesperian, and Amazonian for Mars, these are all relative ages. Relative age dating has to do with determining the temporal ordering of events in Earth's past. Geologists employ a handful of simple principles in relative age dating; two of the most important of these are are the principles of superposition and cross-cutting relationships. A third key principle-faunal succession-is reviewed in Section 3. Geologists often need to know the age of material that they find. They use absolute dating methods, sometimes called numerical dating, to give rocks an actual date, or date range, in number of years. This is different to relative dating, which only puts geological events in time order.
The decay of radioactive materials can be shown with a graph Figure Using several different isotopes helps scientists to check the accuracy of the ages that they calculate. Carbon is stable and accounts for Carbon is also stable and accounts for 1. Carbon is radioactive and is found in tiny amounts.
Carbon is produced naturally in the atmosphere when cosmic rays interact with nitrogen atoms. The amount of carbon produced in the atmosphere at any particular time has been relatively stable through time. Radioactive carbon decays to stable nitrogen by releasing a beta particle. The nitrogen atoms are lost to the atmosphere, but the amount of carbon decay can be estimated by measuring the proportion of radioactive carbon to stable carbon As a substance ages, the relative amount of carbon decreases.
Carbon is removed from the atmosphere by plants during the process of photosynthesis. Animals consume this carbon when they eat plants or other animals that have eaten plants. Therefore carbon dating can be used to date plant and animal remains. Examples include timbers from an old building, bones, or ashes from a fire pit. Carbon dating can be effectively used to find the age of materials between and 50, years old. Potassium decays to argon with a half-life of 1.
Because argon is a gas, it can escape from molten magma or lava.
1. Relative age dating
Therefore any argon that is found in a crystal probably formed as a result of the decay of potassium Measuring the ratio of potassium to argon will yield a good estimate of the age of the sample. Potassium is a common element found in many minerals such as feldspar, mica, and amphibole.
The technique can be used to date igneous rocks fromyears to over a billion years old. Because it can be used to date geologically young materials, the technique has been useful in estimating the age of deposits containing the bones of human ancestors.
Two isotopes of uranium are used for radiometric dating.
Uranium decays to form lead with a half-life of 4. Uranium decays to form lead with a half-life of million years. Uranium-lead dating is usually performed on crystals of the mineral zircon Figure When zircon forms in an igneous rock, the crystals readily accept atoms of uranium but reject atoms of lead. Therefore, if any lead is found in a zircon crystal, it can be assumed that it was produced from the decay of uranium.
Uranium-lead dating can be used to date igneous rocks from 1 million years to around 4. Some of the oldest rocks on Earth have been dated using this method, including zircon crystals from Australia that are 4.
Radiometric dating can only be used on materials that contain measurable amounts of radioactive materials and their daughter products. This includes organic remains which compared to rocks are relatively young, less thanyears old and older rocks. This is done by multiplying the number of half-lives that have passed by the half-life decay constant of the parent atom again, this value is determined in a laboratory. To summarize, the key piece of information that needs to be determined from a mineral specimen in order to determine its absolute age is its age in number of half lives.
This can be mathematically determined by solving for y in this equation:. Let's work through a hypothetical example problem. Suppose you analyzed a mineral sample and found that it contained 33, parent atoms and 14, daughter atoms.
Further, suppose that the half-life of the parent atom is 2. How old is the mineral sample? So, we conclude that 0. As noted above, a radiometric date tells us when a system became closed, for example when a mineral containing radioactive parent elements first crystalized.
An individual mineral grain may have a long history after it first forms. For example, it may erode out of an igneous rock and then be transported long distances and over long periods of time before it is finally deposited, becoming one grain among billions in a layer of sedimentary rock e.
A Brief History of Geologic Time
Further, heating mineral grains to great temperatures can cause them to leak parent and daughter material, resetting their radiometric clocks. The melting involved with metamorphic change can reset the radiometric clock. For example, suppose an igneous rock formed 2. If it were subjected to metamorphism 1. As noted above, the rate at which a given radioactive isotope decays into its daughter product is constant.
This rate, however, varies considerably among different radioactive isotopes. Further, many radioactive isotopes undergo a series of transformations-some of which have half-lives that persist for only very short amounts of time-before they are converted into their final daughter products. Below are some of the decay series that are commonly used in radiometric dating of geological samples. Note the great variations in their half-lives.
Note that the half-life for the rubidium to strontium series is 50 billion years! Since the entire universe is At the other end of the spectrum, note the very short half-life of carbon 5, years. The is the isotope that is used in "carbon dating. Both it and carbon which is stable, meaning that it does not undergo radioactive decay are incorporated into the tissues of plants as they grow.
After a plant dies, the carbon in its tissues remains stable, but the carbon decays into nitrogen The ratio of carbon relative to carbon in a sample, therefore, may be used to determine the age of organic matter derived from plant tissues. Because of its short half-life, carbon can only be used to date materials that are up to about 70, years old beyond this point, the amount of carbon remaining becomes so small that it is difficult to measure.
Because of its precision, it is nevertheless very useful for dating organic matter from the near recent geological past, especially archeological materials from the Holocene epoch. At the beginning of this chapteryou learned that the Earth is 4. As it turns out, the oldest dated mineral-a grain of zircon from the Jack Hills of Western Australia-is 4.
A single grain of zircon, imaged using a scanning electron microscope. A sample of 4. If the oldest mineral grain is 4. The answer is radiometric dating of meteorite specimens, which we presume to have formed around the same time as the Earth, Sun, and other planetary bodies in our solar system. One such dated meteorite comes from Meteor Crater in Arizona. The Holsinger Meteorite, which is a piece of the meteor that crashed in ancient Arizona, forming Meteor Crater.
Samples from this meteor were used by Clair Patterson to determine the age of the Earth. True or False: It is generally not possible to use carbon dating to date samples older than 70, years.
After three half-lives, what percentage of the original radioactive parent isotope will remain in a sample? What key discovery allowed scientists to begin measuring the absolute ages of rock samples? True or False: Different isotopes of the same element vary in their numbers of protons.
True or False: The age of the Earth was determined by dating a rock sample found at the bottom of the Grand Canyon.
If you know the number of radioactive parent atoms remaining in a sample, as well as the number originally present, what additional key piece of information is needed to calculate the age of the sample? True or False: Radioactive isotopes of different elements decay at the same rate. Adding the number of protons and the number of neutrons in an atom gives you what value?
Radiometric dating Hypotheses of absolute ages of rocks as well as the events that they represent are determined from rates of radioactive decay of some isotopes of elements that occur naturally in rocks. Elements and isotopes In chemistry, an element is a particular kind of atom that is defined by the number of protons that it has in its nucleus.
What is absolute geologic age dating
Consider, for example, the three different isotopes of Carbon: Carbon 6 protons, 6 neutrons Carbon 6 protons, 7 neutrons Carbon 6 protons, 8 neutrons Most isotopes are stable, meaning that they do not change.
Calculating radiometric dates By counting the numbers of parent atoms remaining in a sample relative to the number originally present, it is possible to determine the number of half-lives that have passed since the initial formation of a mineral grain that is, when it became a "closed system" that prevented parent and daughter atoms from escaping.
Variation in half-lives among different isotopes As noted above, the rate at which a given radioactive isotope decays into its daughter product is constant. Parent isotope Final decay product Half-life Uranium Lead 4. Age of the Earth At the beginning of this chapteryou learned that the Earth is 4. Next Section: Geological time scale Previous section: Relative age dating.
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