Radiometric dating definition
Radiometric dating calculates an age in years for geologic materials by measuring the presence of a short-life radioactive element, e. The term applies to all methods of age determination based on nuclear decay of naturally occurring radioactive isotopes. Bates and Jackson To determine the ages in years of Earth materials and the timing of geologic events such as exhumation and subduction, geologists utilize the process of radiometric decay. Geologists use these dates to further define the boundaries of the geologic periods shown on the geologic time scale. Radiometric decay occurs when the nucleus of a radioactive atom spontaneously transforms into an atomic nucleus of a different, more stable isotope.SEE VIDEO BY TOPIC: Radiometric or Absolute Rock Dating
Science in Christian Perspective. Radiometric Dating. A Christian Perspective. Roger C. Wiens has a PhD in Physics, with a minor in Geology. His PhD thesis was on isotope ratios in meteorites, including surface exposure dating. Radiometric dating--the process of determining the age of rocks from the decay of their radioactive elements--has been in widespread use for over half a century. There are over forty such techniques, each using a different radioactive element or a different way of measuring them.
It has become increasingly clear that these radiometric dating techniques agree with each other and as a whole, present a coherent picture in which the Earth was created a very long time ago. Further evidence comes from the complete agreement between radiometric dates and other dating methods such as counting tree rings or glacier ice core layers. Many Christians have been led to distrust radiometric dating and are completely unaware of the great number of laboratory measurements that have shown these methods to be consistent.
Many are also unaware that Bible-believing Christians are among those actively involved in radiometric dating. This paper describes in relatively simple terms how a number of the dating techniques work, how accurately the half-lives of the radioactive elements and the rock dates themselves are known, and how dates are checked with one another.
In the process the paper refutes a number of misconceptions prevalent among Christians today. This paper is available on the web via the American Scientific Affiliation and related sites to promote greater understanding and wisdom on this issue, particularly within the Christian community. Doubters Still Try Apparent Age? Arguments over the age of the Earth have sometimes been divisive for people who regard the Bible as God's word. Even though the Earth's age is never mentioned in the Bible, it is an issue because those who take a strictly literal view of the early chapters of Genesis can calculate an approximate date for the creation by adding up the life-spans of the people mentioned in the genealogies.
Assuming a strictly literal interpretation of the week of creation, even if some of the generations were left out of the genealogies, the Earth would be less than ten thousand years old. Radiometric dating techniques indicate that the Earth is thousands of times older than that--approximately four and a half billion years old.
Many Christians accept this and interpret the Genesis account in less scientifically literal ways. However, some Christians suggest that the geologic dating techniques are unreliable, that they are wrongly interpreted, or that they are confusing at best.
Unfortunately, much of the literature available to Christians has been either inaccurate or difficult to understand, so that confusion over dating techniques continues. The next few pages cover a broad overview of radiometric dating techniques, show a few examples, and discuss the degree to which the various dating systems agree with each other.
The goal is to promote greater understanding on this issue, particularly for the Christian community. Many people have been led to be skeptical of dating without knowing much about it. For example, most people don't realize that carbon dating is only rarely used on rocks.
God has called us to be "wise as serpents" Matt. In spite of this, differences still occur within the church. A disagreement over the age of the Earth is relatively minor in the whole scope of Christianity; it is more important to agree on the Rock of Ages than on the age of rocks. But because God has also called us to wisdom, this issue is worthy of study. Rocks are made up of many individual crystals, and each crystal is usually made up of at least several different chemical elements such as iron, magnesium, silicon, etc.
Most of the elements in nature are stable and do not change. However, some elements are not completely stable in their natural state. Some of the atoms eventually change from one element to another by a process called radioactive decay. If there are a lot of atoms of the original element, called the parent element, the atoms decay to another element, called the daughter element, at a predictable rate.
The passage of time can be charted by the reduction in the number of parent atoms, and the increase in the number of daughter atoms. Radiometric dating can be compared to an hourglass. When the glass is turned over, sand runs from the top to the bottom. Radioactive atoms are like individual grains of sand--radioactive decays are like the falling of grains from the top to the bottom of the glass.
You cannot predict exactly when any one particular grain will get to the bottom, but you can predict from one time to the next how long the whole pile of sand takes to fall. Once all of the sand has fallen out of the top, the hourglass will no longer keep time unless it is turned over again. Similarly, when all the atoms of the radioactive element are gone, the rock will no longer keep time unless it receives a new batch of radioactive atoms.
Figure 1. The rate of loss of sand from from the top of an hourglass compared to exponential type of decay of radioactive elements. In exponential decay the amount of material decreases by half during each half-life. After two half-lives one-fourth remains, after three half-lives, one-eighth, etc. Unlike the hourglass, where the amount of sand falling is constant right up until the end, the number of decays from a fixed number of radioactive atoms decreases as there are fewer atoms left to decay see Figure 1.
If it takes a certain length of time for half of the atoms to decay, it will take the same amount of time for half of the remaining atoms, or a fourth of the original total, to decay.
In the next interval, with only a fourth remaining, only one eighth of the original total will decay. By the time ten of these intervals, or half-lives, has passed, less than one thousandth of the original number of radioactive atoms is left. The equation for the fraction of parent atoms left is very simple. The type of equation is exponential, and is related to equations describing other well-known phenomena such as population growth.
No deviations have yet been found from this equation for radioactive decay. Also unlike the hourglass, there is no way to change the rate at which radioactive atoms decay in rocks. If you shake the hourglass, twirl it, or put it in a rapidly accelerating vehicle, the time it takes the sand to fall will change.
But the radioactive atoms used in dating techniques have been subjected to heat, cold, pressure, vacuum, acceleration, and strong chemical reactions to the extent that would be experienced by rocks or magma in the mantle, crust, or surface of the Earth or other planets without any significant change in their decay rate. In only a couple of special cases have any decay rates been observed to vary, and none of these special cases apply to the dating of rocks as discussed here.
These exceptions are discussed later. An hourglass will tell time correctly only if it is completely sealed. If it has a hole allowing the sand grains to escape out the side instead of going through the neck, it will give the wrong time interval. Similarly, a rock that is to be dated must be sealed against loss or addition of either the radioactive daughter or parent. If it has lost some of the daughter element, it will give an inaccurately young age.
As will be discussed later, most dating techniques have very good ways of telling if such a loss has occurred, in which case the date is thrown out and so is the rock! An hourglass measures how much time has passed since it was turned over.
Actually it tells when a specific amount of time, e. Radiometric dating of rocks also tells how much time has passed since some event occurred. For igneous rocks the event is usually its cooling and hardening from magma or lava. For some other materials, the event is the end of a metamorphic heating event in which the rock gets baked underground at generally over a thousand degrees Fahrenheit , the uncovering of a surface by the scraping action of a glacier, the chipping of a meteorite off of an asteroid, or the length of time a plant or animal has been dead.
There are now well over forty different radiometric dating techniques, each based on a different radioactive isotope. The term isotope subdivides elements into groups of atoms that have the same atomic weight.
For example carbon has isotopes of weight 12, 13, and 14 times the mass of a nucleon, referred to as carbon, carbon, or carbon abbreviated as 12 C, 13 C, 14 C.
It is only the carbon isotope that is radioactive. This will be discussed further in a later section. A partial list of the parent and daughter isotopes and the decay half-lives is given in Table I. Notice the large range in the half-lives. Isotopes with long half-lives decay very slowly, and so are useful for dating. Table 1. Some Naturally Occurring Radioactive Isotopes and their half-lives. Parent Product. Daughter Half-Life. Years Samarium Neodymium billion Rubidium Strontium Isotopes with shorter half-lives cannot date very ancient events because all of the atoms of the parent isotope would have already decayed away, like an hourglass left sitting with all the sand at the bottom.
Isotopes with relatively short half-lives are useful for dating correspondingly shorter intervals, and can usually do so with greater accuracy, just as you would use a stopwatch rather than a grandfather clock to time a meter dash.
On the other hand, you would use a calendar, not a clock, to record time intervals of several weeks or more. The half-lives have all been measured directly either by using a radiation detector to count the number of atoms decaying in a given amount of time from a known amount of the parent material, or by measuring the ratio of daughter to parent atoms in a sample that originally consisted completely of parent atoms.
Work on radiometric dating first started shortly after the turn of the 20th century, but progress was relatively slow before the late. However, by now we have had over fifty years to measure and re-measure the half-lives for many of the dating techniques. Very precise counting of the decay events or the daughter atoms can be done, so while the number of, say, rhenium atoms decaying in 50 years is a very small fraction of the total, the resulting osmium atoms can be very precisely counted.
For example, recall that only one gram of material contains over 10 21 1 with 21 zeros behind atoms. Even if only one trillionth of the atoms decay in one year, this is still millions of decays, each of which can be counted by a radiation detector!
The uncertainties on the half-lives given in the table are all very small. There is no evidence of any of the half-lives changing over time. In fact, as discussed below, they have been observed to not change at all over hundreds of thousands of years. Examples of Dating Methods for Igneous Rocks. Now let's look at how the actual dating methods work. Igneous rocks are good candidates for dating. Recall that for igneous rocks the event being dated is when the rock was formed from magma or lava.
When the molten material cools and hardens, the atoms are no longer free to move about.
Science in Christian Perspective. Radiometric Dating. A Christian Perspective.
In , shortly after the discovery of radioactivity , the American chemist Bertram Boltwood suggested that lead is one of the disintegration products of uranium, in which case the older a uranium-bearing mineral the greater should be its proportional part of lead. Analyzing specimens whose relative geologic ages were known, Boltwood found that the ratio of lead to uranium did indeed increase with age. After estimating the rate of this radioactive change, he calculated that the absolute ages of his specimens ranged from million to 2. Though his figures were too high by about 20 percent, their order of magnitude was enough to dispose of the short scale of geologic time proposed by Lord Kelvin. Versions of the modern mass spectrometer were invented in the early s and s, and during World War II the device was improved substantially to help in the development of the atomic bomb.
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This page has been archived and is no longer updated. Despite seeming like a relatively stable place, the Earth's surface has changed dramatically over the past 4. Mountains have been built and eroded, continents and oceans have moved great distances, and the Earth has fluctuated from being extremely cold and almost completely covered with ice to being very warm and ice-free. These changes typically occur so slowly that they are barely detectable over the span of a human life, yet even at this instant, the Earth's surface is moving and changing.
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.
Radiocarbon dating also referred to as carbon dating or carbon dating is a method for determining the age of an object containing organic material by using the properties of radiocarbon , a radioactive isotope of carbon. The method was developed in the late s at the University of Chicago by Willard Libby , who received the Nobel Prize in Chemistry for his work in It is based on the fact that radiocarbon 14 C is constantly being created in the atmosphere by the interaction of cosmic rays with atmospheric nitrogen.
Also called radioactive dating. Origin of radiometric dating First recorded in — Words nearby radiometric dating radioluminescence , radiolysis , radioman , radiometeorograph , radiometer , radiometric dating , radiomicrometer , radiomimetic , radionecrosis , radioneuritis , radionics. Words related to radiometric dating dating , thermoluminescence. A method for determining the age of an object based on the concentration of a particular radioactive isotope contained within it. For inorganic materials, such as rocks containing the radioactive isotope rubidium, the amount of the isotope in the object is compared to the amount of the isotope's decay products in this case strontium.
Dating Rocks and Fossils Using Geologic Methods
Carbon dating , also called radiocarbon dating , method of age determination that depends upon the decay to nitrogen of radiocarbon carbon Radiocarbon present in molecules of atmospheric carbon dioxide enters the biological carbon cycle : it is absorbed from the air by green plants and then passed on to animals through the food chain. Radiocarbon decays slowly in a living organism, and the amount lost is continually replenished as long as the organism takes in air or food. Once the organism dies, however, it ceases to absorb carbon, so that the amount of the radiocarbon in its tissues steadily decreases. Because carbon decays at this constant rate, an estimate of the date at which an organism died can be made by measuring the amount of its residual radiocarbon. The carbon method was developed by the American physicist Willard F. Libby about
Rachel Wood does not work for, consult, own shares in or receive funding from any company or organisation that would benefit from this article, and has disclosed no relevant affiliations beyond their academic appointment. Radiocarbon dating has transformed our understanding of the past 50, years. Professor Willard Libby produced the first radiocarbon dates in and was later awarded the Nobel Prize for his efforts.
Geologists use radiometric dating to estimate how long ago rocks formed, and to infer the ages of fossils contained within those rocks. Radioactive elements decay The universe is full of naturally occurring radioactive elements. Radioactive atoms are inherently unstable; over time, radioactive "parent atoms" decay into stable "daughter atoms. When molten rock cools, forming what are called igneous rocks, radioactive atoms are trapped inside.
Radiometric Age Dating
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