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- Dating Fossils – How Are Fossils Dated?
- How Do Scientists Determine the Age of Dinosaur Bones? | HowStuffWorks
The element's half-life is the amount of time it takes for half the parent atoms in a sample to become daughters.
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To read the time on this radioactive clock, scientists use a device called a mass spectrometer to measure the number of parent and daughter atoms. The ratio of parents to daughters can tell the researcher how old the specimen is. The more parent isotopes there are -- and the fewer daughter isotopes -- the younger the sample. The half-life of the isotope being measured determines how useful it is at dating very old samples.
Once all the parents have become daughters, there's no more basis for comparison between the two isotopes. Scientists can't tell whether the clock ran down a few days or millions of years ago. This means that isotopes with a short half-life won't work to date dinosaur bones. The short half-life is only part of the problem when dating dinosaur bones -- researchers also have to find enough of the parent and daughter atoms to measure. Geologists assert that generally speaking, older dates are found deeper down in the geologic column, which they take as evidence that radiometric dating is giving true ages, since it is apparent that rocks that are deeper must be older.
But even if it is true that older radiometric dates are found lower down in the geologic column which is open to question , this can potentially be explained by processes occurring in magma chambers which cause the lava erupting earlier to appear older than the lava erupting later. Lava erupting earlier would come from the top of the magma chamber, and lava erupting later would come from lower down. A number of processes could cause the parent substance to be depleted at the top of the magma chamber, or the daughter product to be enriched, both of which would cause the lava erupting earlier to appear very old according to radiometric dating, and lava erupting later to appear younger.
Other possible confounding variables are the mechanisms that can alter daughter-to-parent ratios. We can see that many varieties of minerals are produced from the same magma by the different processes of crystallization, and these different minerals may have very different compositions.
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It is possible that the ratio of daughter to parent substances for radiometric dating could differ in the different minerals. Clearly, it is important to have a good understanding of these processes in order to evaluate the reliability of radiometric dating. Other confounding factors such as contamination and fractionation issues are frankly acknowledged by the geologic community, but are not taken into consideration when the accuracy and validity of these dating methods are examined.
The following quotation from Elaine G. Kennedy addresses this problem.
Contamination and fractionation issues are frankly acknowledged by the geologic community. For example, if a magma chamber does not have homogeneously mixed isotopes, lighter daughter products could accumulate in the upper portion of the chamber.
Dating Fossils – How Are Fossils Dated?
If this occurs, initial volcanic eruptions would have a preponderance of daughter products relative to the parent isotopes. Such a distribution would give the appearance of age. As the magma chamber is depleted in daughter products, subsequent lava flows and ash beds would have younger dates. Such a scenario does not answer all of the questions or solve all of the problems that radiometric dating poses for those who believe the Genesis account of Creation and the Flood. It does suggest at least one aspect of the problem that could be researched more thoroughly.
The problems inherent in radiometric dating often cause them to be so unreliable that they contradict one another rather than validating each other. It would really be nice if geologists would just do a double blind study sometime to find out what the distributions of the ages are. In practice, geologists carefully select what rocks they will date, and have many explanations for discordant dates, so it's not clear how such a study could be done, but it might be a good project for creationists.
There is also evidence that many anomalies are never reported. There are so many complicated phenomena to consider like this that it calls the whole radiometric dating scheme into question. Only then can you gauge the accuracy and validity of that race. We need to observe when the race begins, how the race is run are there variations from the course, is the runner staying within the course, are they taking performance enhancing drugs, etc.
All bases must be covered if we are going to accurately time the race. This is the major flaw in radiometric dating, e. Secondly, you must have an observable time span so we can be certain nothing has affected the amount of the radioactive element being measured, e. Finally, we need to be certain about the end or finish point. This is some finite point in the future. Without an accurate starting time, an observable span in between, and an observable finish, our measurement cannot be deemed accurate.
How Do Scientists Determine the Age of Dinosaur Bones? | HowStuffWorks
There are two basic approaches: Here is an easy-to understand analogy for your students: Absolute age dating is like saying you are 15 years old and your grandfather is 77 years old. To determine the relative age of different rocks, geologists start with the assumption that unless something has happened, in a sequence of sedimentary rock layers, the newer rock layers will be on top of older ones. This is called the Rule of Superposition. This rule is common sense, but it serves as a powerful reference point. Geologists draw on it and other basic principles http: Relative age dating also means paying attention to crosscutting relationships.
Say for example that a volcanic dike, or a fault, cuts across several sedimentary layers, or maybe through another volcanic rock type. Pretty obvious that the dike came after the rocks it cuts through, right? With absolute age dating, you get a real age in actual years. Based on the Rule of Superposition, certain organisms clearly lived before others, during certain geologic times.
The narrower a range of time that an animal lived, the better it is as an index of a specific time. No bones about it, fossils are important age markers. But the most accurate forms of absolute age dating are radiometric methods. This method works because some unstable radioactive isotopes of some elements decay at a known rate into daughter products. This rate of decay is called a half-life.