DETERMINING AGE OF ROCKS AND FOSSILS
Radiometric dating. Geologists use radiometric dating to estimate how long ago rocks formed, and to infer the ages of fossils contained within those rocks. The method of dating rocks and minerals is known as geochronology. Although in principle this term could be applied to estimation of relative ages according to. Adapted from The Age of the Earth, by the Branch of Isotope Geology, United have been dated by four independent radiometric dating methods at
U is the parent isotope of Pb, which is the daughter isotope. Many rocks contain small amounts of unstable isotopes and the daughter isotopes into which they decay.
Where the amounts of parent and daughter isotopes can be accurately measured, the ratio can be used to determine how old the rock is, as shown in the following activities.
That chance of decay is very small, but it is always present and it never changes. In other words, the nuclei do not "wear out" or get "tired". If the nucleus has not yet decayed, there is always that same, slight chance that it will change in the near future.
Atomic nuclei are held together by an attraction between the large nuclear particles protons and neutrons that is known as the "strong nuclear force", which must exceed the electrostatic repulsion between the protons within the nucleus.
In general, with the exception of the single proton that constitutes the nucleus of the most abundant isotope of hydrogen, the number of neutrons must at least equal the number of protons in an atomic nucleus, because electrostatic repulsion prohibits denser packing of protons.
But if there are too many neutrons, the nucleus is potentially unstable and decay may be triggered. This happens at any time when addition of the fleeting "weak nuclear force" to the ever-present electrostatic repulsion exceeds the binding energy required to hold the nucleus together. In other words, during million years, half the U atoms that existed at the beginning of that time will decay to Pb This is known as the half life of U- Many elements have some isotopes that are unstable, essentially because they have too many neutrons to be balanced by the number of protons in the nucleus.
Each of these unstable isotopes has its own characteristic half life. Some half lives are several billion years long, and others are as short as a ten-thousandth of a second. On a piece of notebook paper, each piece should be placed with the printed M facing down. This represents the parent isotope.
The candy should be poured into a container large enough for them to bounce around freely, it should be shaken thoroughly, then poured back onto the paper so that it is spread out instead of making a pile. This first time of shaking represents one half life, and all those pieces of candy that have the printed M facing up represent a change to the daughter isotope. Then, count the number of pieces of candy left with the M facing down.
These are the parent isotope that did not change during the first half life. The teacher should have each team report how many pieces of parent isotope remain, and the first row of the decay table Figure 2 should be filled in and the average number calculated.
Radiometric dating in geology - IOPscience
The same procedure of shaking, counting the "survivors", and filling in the next row on the decay table should be done seven or eight more times. Each time represents a half life. Each team should plot on a graph Figure 3 the number of pieces of candy remaining after each of their "shakes" and connect each successive point on the graph with a light line.
AND, on the same graph, each group should plot points where, after each "shake" the starting number is divided by exactly two and connect these points by a differently colored line. The discovery of radioactivity also had another side effect, although it was several more decades before its additional significance to geology became apparent and the techniques became refined. Because of the chemistry of rocks, it was possible to calculate how much radioactive decay had occurred since an appropriate mineral had formed, and how much time had therefore expired, by looking at the ratio between the original radioactive isotope and its product, if the decay rate was known.
Many geological complications and measurement difficulties existed, but initial attempts at the method clearly demonstrated that the Earth was very old. In fact, the numbers that became available were significantly older than even some geologists were expecting -- rather than hundreds of millions of years, which was the minimum age expected, the Earth's history was clearly at least billions of years long.
Radiometric dating provides numerical values for the age of an appropriate rock, usually expressed in millions of years. Therefore, by dating a series of rocks in a vertical succession of strata previously recognized with basic geologic principles see Stratigraphic principles and relative timeit can provide a numerical calibration for what would otherwise be only an ordering of events -- i.
The integration of relative dating and radiometric dating has resulted in a series of increasingly precise "absolute" i. Given the background above, the information used for a geologic time scale can be related like this: How relative dating of events and radiometric numeric dates are combined to produce a calibrated geological time scale. In this example, the data demonstrates that "fossil B time" was somewhere between and million years ago, and that "fossil A time" is older than million years ago.
Note that because of the position of the dated beds, there is room for improvement in the time constraints on these fossil-bearing intervals e.
A continuous vertical stratigraphic section will provide the order of occurrence of events column 1 of Figure 2. These are summarized in terms of a "relative time scale" column 2 of Figure 2.
Geologists can refer to intervals of time as being "pre-first appearance of species A" or "during the existence of species A", or "after volcanic eruption 1" at least six subdivisions are possible in the example in Figure 2. For this type of "relative dating" to work it must be known that the succession of events is unique or at least that duplicate events are recognized -- e.
Radiometric dating - Wikipedia
Unique events can be biological e. Ideally, geologists are looking for events that are unmistakably unique, in a consistent order, and of global extent in order to construct a geological time scale with global significance.
Some of these events do exist. For example, the boundary between the Cretaceous and Tertiary periods is recognized on the basis of the extinction of a large number of organisms globally including ammonites, dinosaurs, and othersthe first appearance of new types of organisms, the presence of geochemical anomalies notably iridiumand unusual types of minerals related to meteorite impact processes impact spherules and shocked quartz.
These types of distinctive events provide confirmation that the Earth's stratigraphy is genuinely successional on a global scale.
Even without that knowledge, it is still possible to construct local geologic time scales. Although the idea that unique physical and biotic events are synchronous might sound like an "assumption", it is not.
It can, and has been, tested in innumerable ways since the 19th century, in some cases by physically tracing distinct units laterally for hundreds or thousands of kilometres and looking very carefully to see if the order of events changes.
Geologists do sometimes find events that are "diachronous" i. Because any newly-studied locality will have independent fossil, superpositional, or radiometric data that have not yet been incorporated into the global geological time scale, all data types serve as both an independent test of each other on a local scaleand of the global geological time scale itself.
The test is more than just a "right" or "wrong" assessment, because there is a certain level of uncertainty in all age determinations. For example, an inconsistency may indicate that a particular geological boundary occurred 76 million years ago, rather than 75 million years ago, which might be cause for revising the age estimate, but does not make the original estimate flagrantly "wrong".
It depends upon the exact situation, and how much data are present to test hypotheses e. Whatever the situation, the current global geological time scale makes predictions about relationships between relative and absolute age-dating at a local scale, and the input of new data means the global geologic time scale is continually refined and is known with increasing precision.
This trend can be seen by looking at the history of proposed geologic time scales described in the first chapter of [Harland et al,p.
The unfortunate part of the natural process of refinement of time scales is the appearance of circularity if people do not look at the source of the data carefully enough. Most commonly, this is characterised by oversimplified statements like: When a geologist collects a rock sample for radiometric age dating, or collects a fossil, there are independent constraints on the relative and numerical age of the resulting data.
Stratigraphic position is an obvious one, but there are many others. There is no way for a geologist to choose what numerical value a radiometric date will yield, or what position a fossil will be found at in a stratigraphic section.
Every piece of data collected like this is an independent check of what has been previously studied. The data are determined by the rocks, not by preconceived notions about what will be found. Every time a rock is picked up it is a test of the predictions made by the current understanding of the geological time scale.
The time scale is refined to reflect the relatively few and progressively smaller inconsistencies that are found. This is not circularity, it is the normal scientific process of refining one's understanding with new data.
Geologic Age Dating Explained
It happens in all sciences. If an inconsistent data point is found, geologists ask the question: However, this statistical likelihood is not assumed, it is tested, usually by using other methods e. Geologists search for an explanation of the inconsistency, and will not arbitrarily decide that, "because it conflicts, the data must be wrong.
The continued revision of the time scale as a result of new data demonstrates that geologists are willing to question it and change it. The geological time scale is far from dogma. 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.
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.How Radiometric Dating Works: Relative not Absolute Ages
The numbers above are just summary values. Other examples yield similar results - i. 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. Chart of a few different isotope half lifes: If a rock has been partially melted, or otherwise metamorphosed, that causes complications for radiometric absolute age dating as well.
Good overview as relates to the Grand Canyon: Which are the youngest? I also like this simple exercise, a spin-off from an activity described on the USGS site above. Take students on a neighborhood walk and see what you can observe about age dates around you.
For example, which is older, the bricks in a building or the building itself? Are there repairs or cracks in the sidewalk that came after the sidewalk was built? Have students work alone or in pairs to find an article or paper that uses radiometric age dating.