Techniques include tree rings in timbers, radiocarbon dating of wood or bones, and trapped-charge dating methods such as thermoluminescence dating of glazed ceramics. In historical geology , the primary methods of absolute dating involve using the radioactive decay of elements trapped in rocks or minerals, including isotope systems from very young radiocarbon dating with 14 C to systems such as uranium—lead dating that allow acquisition of absolute ages for some of the oldest rocks on earth. Radiometric dating is based on the known and constant rate of decay of radioactive isotopes into their radiogenic daughter isotopes.
- Absolute dating - Wikipedia.
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- make the best samples for the k/ar dating method.
Particular isotopes are suitable for different applications due to the types of atoms present in the mineral or other material and its approximate age. For example, techniques based on isotopes with half lives in the thousands of years, such as carbon, cannot be used to date materials that have ages on the order of billions of years, as the detectable amounts of the radioactive atoms and their decayed daughter isotopes will be too small to measure within the uncertainty of the instruments.
One of the most widely used and well-known absolute dating techniques is carbon or radiocarbon dating, which is used to date organic remains. This is a radiometric technique since it is based on radioactive decay. Carbon moves up the food chain as animals eat plants and as predators eat other animals.
With death, the uptake of carbon stops. It takes 5, years for half the carbon to change to nitrogen; this is the half-life of carbon After another 5, years only one-quarter of the original carbon will remain. After yet another 5, years only one-eighth will be left. By measuring the carbon in organic material , scientists can determine the date of death of the organic matter in an artifact or ecofact.
- What is Radiometric Dating?.
- Use of Index Fossils.
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The relatively short half-life of carbon, 5, years, makes dating reliable only up to about 50, years. The technique often cannot pinpoint the date of an archeological site better than historic records, but is highly effective for precise dates when calibrated with other dating techniques such as tree-ring dating. An additional problem with carbon dates from archeological sites is known as the "old wood" problem. It is possible, particularly in dry, desert climates, for organic materials such as from dead trees to remain in their natural state for hundreds of years before people use them as firewood or building materials, after which they become part of the archaeological record.
Thus dating that particular tree does not necessarily indicate when the fire burned or the structure was built. For this reason, many archaeologists prefer to use samples from short-lived plants for radiocarbon dating. The development of accelerator mass spectrometry AMS dating, which allows a date to be obtained from a very small sample, has been very useful in this regard. Other radiometric dating techniques are available for earlier periods. One of the most widely used is potassium—argon dating K—Ar dating. Potassium is a radioactive isotope of potassium that decays into argon The half-life of potassium is 1.
Potassium is common in rocks and minerals, allowing many samples of geochronological or archeological interest to be dated. Argon , a noble gas, is not commonly incorporated into such samples except when produced in situ through radioactive decay. The date measured reveals the last time that the object was heated past the closure temperature at which the trapped argon can escape the lattice. K—Ar dating was used to calibrate the geomagnetic polarity time scale. Thermoluminescence testing also dates items to the last time they were heated.
Results from different techniques, often measured in rival labs, continually confirm each other. Every few years, new geologic time scales are published, providing the latest dates for major time lines. Older dates may change by a few million years up and down, but younger dates are stable. For example, it has been known since the s that the famous Cretaceous-Tertiary boundary, the line marking the end of the dinosaurs, was 65 million years old.
Repeated recalibrations and retests, using ever more sophisticated techniques and equipment, cannot shift that date. It is accurate to within a few thousand years.
The fossil record is fundamental to an understanding of evolution. Fossils document the order of appearance of groups and they tell us about some of the amazing plants and animals that died out long ago. Fossils can also show us how major crises, such as mass extinctions, happened, and how life recovered after them. If the fossils, or the dating of the fossils, could be shown to be inaccurate, all such information would have to be rejected as unsafe.
Geologists and paleontologists are highly self-critical, and they have worried for decades about these issues. Repeated, and tough, regimes of testing have confirmed the broad accuracy of the fossils and their dating, so we can read the history of life from the rocks with confidence. Educators have permission to reprint articles for classroom use; other users, please contact editor actionbioscience. Currently, he is studying certain basal dinosaurs from the Late Triassic and the quality of different segments of the fossil record.
He holds the Chair in Vertebrate Paleontology at the University of Bristol, UK, in addition to chairing the Masters program in paleobiology at the university.
Dating Fossils – How Are Fossils Dated?
Your one-stop source for information on evolution. Michael Benton wrote another article, Evidence of Evolutionary Transitions , for this website which explains how fossils support the stages of evolutionary history. Data bases and software for studying the quality of the fossil record.
Michael Benton has written over 30 books on dinosaurs and paleobiology. Two suggested readings are provided — the first for adults, the second for children:.
An online directory of dinosaur exhibits fro around the world. Many natural history museums and universities worldwide offer public participation programs in dinosaur events, such as fossil hunting or fossil cataloguing. No experience needed in most cases!
If we know from radiometric dating that an animal only existed for a particular period of a few million years, we know that whenever we see that animal in a rock face, that part of the rock must have that particular age. This also tells us that any other animals in the same rock, or the same layer, must have died around the same time.
One thing that makes index fossils more useful is the way sedimentary rock is created. Many of the rock faces we look at were created layer by layer as sediments settled on a sea or lake bed. Because of this, when we look at a rock-face, we know that the bottom layers are the oldest, and that fossils parallel to each other along the layers of sediment are likely to be from the same era.
These rock faces often look striped, because different layers contain different materials as local conditions changed over millions of years.
The most useful index fossils are from animals that existed for only short periods of time, and that are found over a wide area. They're useful because they give us a more exact date for a rock, and they can be used to date more rocks since they're common. Ammonites are a great example of an especially useful index fossil.
Ammonites changed over time, so that ones from a few million years apart are never quite the same. These fossils are found all over the place! Other examples of good index fossils include tropites, which existed only from to million years ago, and trilobites, which are useful in the same way as ammonites -- due to the large number of variations over time, even though they existed for a full million years.
Tropites were similar to squid and octopus, except with a hard shell, and trilobites were a type of sea-based arthropod. Radiometric dating and index fossils are two methods we can use to figure out how old fossil-containing chunks and layers of rock are. Sedimentary rock forms layers as particles settle on sea beds over millions of years. When we look at a rock face, we know that the lower layers are colder. But we can get more detailed information using these two methods. Radiometric dating is where we look at the amount of certain radioactive isotopes to figure out how old a rock is.
We know the amount of radioactive substance when the rock was formed, and we know how fast it decays into other non-radioactive substances, so we can look at the amounts in each rock and use it to calculate how old the rock is. Reference fossils are the remnants of dead animals and plants that we know existed at a particular time in history. If we see one of these fossils in a layer of rock, this tells us how old that rock must be. An ideal reference fossil would be an animal or plant that only lived for a short time, but yet is found all over.
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