Geologic Age Dating Explained - Kids Discover
May 20, Geologists often need to know the age of material that they find. They use absolute dating methods, sometimes called numerical dating, to give. Oct 26, These include radiometric dating of volcanic layers above or below the fossils Stratigraphy, 0 to > 4 billion, fossils and other objects found in. Numerous radioactive isotopes exist. One system that has been very successful in dating the ages of fossils is potassium-argon dating. Potassium is an.
If the new data have a large inconsistency by "large" I mean orders of magnitudeit is far more likely to be a problem with the new data, but geologists are not satisfied until a specific geological explanation is found and tested. An inconsistency often means something geologically interesting is happening, and there is always a tiny possibility that it could be the tip of a revolution in understanding about geological history.
Admittedly, this latter possibility is VERY unlikely. There is almost zero chance that the broad understanding of geological history e. The amount of data supporting that interpretation is immense, is derived from many fields and methods not only radiometric datingand a discovery would have to be found that invalidated practically all previous data in order for the interpretation to change greatly.
So far, I know of no valid theory that explains how this could occur, let alone evidence in support of such a theory, although there have been highly fallacious attempts e. When Radiometric Dating "Just Works" or not A poor example There are many situations where radiometric dating is not possible, or where a dating attempt will be fraught with difficulty. This is the inevitable nature of rocks that have experienced millions of years of history: The real question is what happens when conditions are ideal, versus when they are marginal, because ideal samples should give the most reliable dates.
If there are good reasons to expect problems with a sample, it is hardly surprising if there are! It contains a mixture of minerals from a volcanic eruption and detrital mineral grains eroded from other, older rocks.
If the age of this unit were not so crucial to important associated hominid fossils, it probably would not have been dated at all because of the potential problems. After some initial and prolonged troubles over many years, the bed was eventually dated successfully by careful sample preparation that eliminated the detrital minerals.
Radiometric Dating and the Geological Time Scale
Lubenow's work is fairly unique in characterising the normal scientific process of refining a difficult date as an arbitrary and inappropriate "game", and documenting the history of the process in some detail, as if such problems were typical. Another example is "John Woodmorappe's" paper on radiometric datingwhich adopts a "compilation" approach, and gives only superficial treatment to the individual dates.
Among other problems documented in an FAQ by Steven Schimmrichmany of Woodmorappe's examples neglect the geological complexities that are expected to cause problems for some radiometrically-dated samples. A good example By contrast, the example presented here is a geologically simple situation -- it consists of several primary i.
It demonstrates how consistent radiometric data can be when the rocks are more suitable for dating. For most geological samples like this, radiometric dating "just works". Consider this stratigraphic section from the Bearpaw Formation of Saskatchewan, Canada Baadsgaard et al. Modified from Baadsgaard et al. The section is measured in metres, starting with 0m at the bottom oldest.
This section is important because it places a limit on the youngest age for a specific ammonite shell -- Baculites reesidei -- which is used as a zonal fossil in western North America.
It consistently occurs below the first occurrence of Bacultes jenseni and above the occurrence of Baculites cuneatus within the upper part of the Campanian, the second to last "stage" of the Cretaceous Period in the global geological time scale. The biostratigraphic situation can be summarized as a vertically-stacked sequence of "zones" defined by the first appearance of each ammonite species: About 40 of these ammonite zones are used to subdivide the upper part of the Cretaceous Period in this area.
Dinosaurs and many other types of fossils are also found in this interval, and in broad context it occurs shortly before the extinction of the dinosaurs, and the extinction of all ammonites. The Bearpaw Formation is a marine unit that occurs over much of Alberta and Saskatchewan, and it continues into Montana and North Dakota in the United States, although it adopts a different name in the U.
The numbers above are just summary values. Other examples yield similar results - i. The results are therefore highly consistent given the analytical uncertainties in any measurement. Eberth and Braman described the vertebrate paleontology and sedimentology of the Judith River Formation, a dinosaur-bearing unit that occurs stratigraphically below the Baculites reesidei zone the Judith River Formation is below the Bearpaw Formation.
It should therefore be older than the results from Baadsgaard et al. An ash bed near the top of the Judith River Fm. Again, this is compatible with the age determined for the Baculites reesidei zone and its relative stratigraphic position, and even with the relative position of the two samples within the same formation. How do these dates compare to the then current geological time scale? Here are the numbers they applied to the geological boundaries in this interval, compared to the numbers in the newer studies: Comparison of newer data with the Harland et al.
As you can see, the numbers in the rightmost column are basically compatible. Skeptics of radiometric dating procedures sometimes claim these techniques should not work reliably, or only infrequently, but clearly the results are similar: Most of the time, the technique works exceedingly well to a first approximation. However, there are some smaller differences.
The date for the Baculites reesidei zone is at least 0. Well, standard scientific procedure is to collect more data to test the possible explanations -- is it the time scale or the data that are incorrect? Obradovich has measured a large number of high-quality radiometric dates from the Cretaceous Period, and has revised the geological time scale for this interval.
Specifically, he proposes an age of This is completely compatible with the data in Baadsgaard et al. Conclusions Skeptics of conventional geology might think scientists would expect, or at least prefer, every date to be perfectly consistent with the current geological time scale, but realistically, this is not how science works. The age of a particular sample, and a particular geological time scale, only represents the current understanding, and science is a process of refinement of that understanding.
In support of this pattern, there is an unmistakable trend of smaller and smaller revisions of the time scale as the dataset gets larger and more precise Harland et al. If something were seriously wrong with the current geologic time scale, one would expect inconsistencies to grow in number and severity, but they do not. The same trend can be observed for other time periods.
Dating dinosaurs and other fossils - Australian Museum
Palmer and Harland et al. The latter includes an excellent diagram summarizing comparisons between earlier time scales Harland et al. Sincethere have been still more revisions by other authors, such as Obradovich for the Cretaceous Period, and Gradstein et al.
A recent geological time scale, based on Harland et al. This is not uncommon. Besides the papers mentioned here, there are hundreds, if not thousands, of similar papers providing bracketing ranges for fossil occurrences.
The synthesis of work like this by thousands of international researchers over many decades is what defines geological time scales in the first place refer to Harland et al. Although geologists can and do legitimately quibble over the exact age of a particular fossil or formation e. The data do not support such an interpretation. The methods work too well most of the time. In addition, evidence from other aspects of geology e.
Prior to the availability of radiometric dating, and even prior to evolutionary theory, the Earth was estimated to be at least hundreds of millions of years old see above.
Radiometric dating has simply made the estimates more precise, and extended it into rocks barren of fossils and other stratigraphic tools. The geological time scale and the techniques used to define it are not circular. They rely on the same scientific principles as are used to refine any scientific concept: There are innumerable independent tests that can identify and resolve inconsistencies in the data. This makes the geological time scale no different from other aspects of scientific study.
Refuting the conventional geological time scale is not an exercise in collecting examples of the worst samples possible. A critique of conventional geologic time scale should address the best and most consistent data available, and explain it with an alternative interpretation, because that is the data that actually matters to the current understanding of geologic time.
References also refer to " Other sources " Baadsgaard, H. Multimethod radiometric age for a bentonite near the top of the Baculites reesidei Zone of southwestern Saskatchewan Campanian-Maastrichtian stage boundary? Canadian Journal of Earth Sciences, v.
Stratigraphy, sedimentology, and vertebrate paleontology of the Judith River Formation Campanian near Muddy Lake, west-central Saskatchewan.
Bulletin of Canadian Petroleum Geology, v. 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.
Half-life simply means the amount of time it takes for half of a remaining particular isotope to decay to a daughter product. Good discussion from the US Geological Survey: There are a couple catches, of course. Not all rocks have radioactive elements. Sedimentary rocks in particular are notoriously radioactive-free zones. So to date those, geologists look for layers like volcanic ash that might be sandwiched between the sedimentary layers, and that tend to have radioactive elements.
You might have noticed that many of the oldest age dates come from a mineral called zircon. Each radioactive isotope works best for particular applications. The half-life of carbon 14, for example, is 5, years.
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On the other hand, the half-life of the isotope potassium 40 as it decays to argon is 1. National Museum Smithsonian Institution. Provides brief overview of 1 relative dating and stratigraphic methods, 2 absolute dating and radiometric dating, including a table with parent to daughter isotopes and half lives of those isotopes commonly used in radiometric dating, 3 paleomagnetics and 4 geologic time.
Pamela Gore, Georgia Perimeter College. Short discussion of radioactive dating and stratigraphic principles. Includes tables of common radioactive parent isotopes and their stable daughter products, and half lives of common radioactive isotopes.
Radiocarbon Dating - Kids History
Geologic timeU. This online version of their informative booklet contains short, content explanations about relative time, major geologic time divisions, index fossils for use in age dating, radiometric dating and the age of the earth.
Content information about absolute and relative dating methods used by the U. Geological Survey Radiometric datingWikipedia. Content information from on-line encyclopedia that explains various.