Radiometric dating in geology - IOPscience
Adapted from The Age of the Earth, by the Branch of Isotope Geology, United have been dated by four independent radiometric dating methods at Nineteenth century geologists recognized that rocks formed slowly as mountains Scientists measure the ages of rock layers on Earth using radiometric dating. In a related article on geologic ages (Ages), we presented a chart with the various geologic eras and their ages. In a separate article (Radiometric dating), we.
The use of different dating methods on the same rock is an excellent way to check the accuracy of age results. If two or more radiometric clocks based on different elements and running at different rates give the same age, that's powerful evidence that the ages are probably correct.
Along this line, Roger Wiens, a scientist at the Los Alamos National Laboratory, asks those who are skeptical of radiometric dating to consider the following quoted in several cases from [ Wiens ]: There are well over forty different radiometric dating methods, and scores of other methods such as tree rings and ice cores.
All of the different dating methods agree--they agree a great majority of the time over millions of years of time. Some [skeptics] make it sound like there is a lot of disagreement, but this is not the case. The disagreement in values needed to support the position of young-earth proponents would require differences in age measured by orders of magnitude e.
The differences actually found in the scientific literature are usually close to the margin of error, usually a few percent, not orders of magnitude! Vast amounts of data overwhelmingly favor an old Earth. Several hundred laboratories around the world are active in radiometric dating. Their results consistently agree with an old Earth. Over a thousand papers on radiometric dating were published in scientifically recognized journals in the last year, and hundreds of thousands of dates have been published in the last 50 years.
Essentially all of these strongly favor an old Earth. Radioactive decay rates have been measured for over sixty years now for many of the decay clocks without any observed changes. And it has been close to a hundred years since the uranium decay rate was first determined. A recent survey of the rubidium-strontium method found only about 30 cases, out of tens of thousands of published results, where a date determined using the proper procedures was subsequently found to be in error.
Both long-range and short-range dating methods have been successfully verified by dating lavas of historically known ages over a range of several thousand years. The mathematics for determining the ages from the observations is relatively simple.
Rates of radioactivity One question that sometimes arises here is how can scientists assume that rates of radioactivity have been constant over the great time spans involved. Creationist Henry Morris, for example, criticizes this type of "uniformitarian" assumption [ Morrispg. But numerous experiments have been conducted to detect any change in radioactivity as a result of chemical activity, exceedingly high heat, pressure, or magnetic field. None of these experiments has detected any significant deviation for any isotope used in geologic dating [ Dalrymplepg.
Scientists have also performed very exacting experiments to detect any change in the constants or laws of physics over time, but various lines of evidence indicate that these laws have been in force, essentially the same as we observe them today, over the multi-billion-year age of the universe.
Note, for instance, that light coming to Earth from distant stars which in some cases emanated billions of years ago reflects the same patterns of atomic spectra, based in the laws of quantum mechanics, that we see today.
What's more, in observed supernova events that we observe in telescopes today, most of which occurred many millions of years ago, the patterns of light and radiation are completely consistent with the half-lives of radioactive isotopes that we measure today [ Isaakpg.
As another item of evidence, researchers studying a natural nuclear reactor in Africa have concluded that a certain key physical constant "alpha" has not changed measurably in hundreds of millions of years [ Barrowpg. Finally, researchers have just completed a study of the proton-electron mass ratio approximately Thus scientists are on very solid ground in asserting that rates of radioactivity have been constant over geologic time. The issue of the "uniformitarian" assumption is discussed in significantly greater detail at Uniformitarian.Creation v. Evolution: How Carbon Dating Works
Responses to specific creationist claims Wiens' online article, mentioned above, is an excellent resource for countering claims of creationists on the reliability of geologic dating. In an appendix to this article, Wiens addresses and responds to a number of specific creationist criticisms.
Here is a condensed summary of these items, quoted from Wiens' article [ Wiens ]: Radiometric dating is based on index fossils whose dates were assigned long before radioactivity was discovered. This is not at all true, though it is implied by some young-earth literature. Radiometric dating is based on the half-lives of the radioactive isotopes.
These half-lives have been measured over the last years. They are not calibrated by fossils. No one has measured the decay rates directly; we only know them from inference. Decay rates have been directly measured over the last years. In some cases a batch of the pure parent material is weighed and then set aside for a long time and then the resulting daughter material is weighed. In many cases it is easier to detect radioactive decays by the energy burst that each decay gives off.
For this a batch of the pure parent material is carefully weighed and then put in front of a Geiger counter or gamma-ray detector. The scheme has a range of several hundred thousand years. A related method is ionium—thorium datingwhich measures the ratio of ionium thorium to thorium in ocean sediment. Radiocarbon dating method[ edit ] Main article: Carbon is a radioactive isotope of carbon, with a half-life of 5, years,   which is very short compared with the above isotopes and decays into nitrogen.
Carbon, though, is continuously created through collisions of neutrons generated by cosmic rays with nitrogen in the upper atmosphere and thus remains at a near-constant level on Earth. The carbon ends up as a trace component in atmospheric carbon dioxide CO2.
A carbon-based life form acquires carbon during its lifetime. Plants acquire it through photosynthesisand animals acquire it from consumption of plants and other animals. When an organism dies, it ceases to take in new carbon, and the existing isotope decays with a characteristic half-life years. The proportion of carbon left when the remains of the organism are examined provides an indication of the time elapsed since its death. This makes carbon an ideal dating method to date the age of bones or the remains of an organism.
The carbon dating limit lies around 58, to 62, years. However, local eruptions of volcanoes or other events that give off large amounts of carbon dioxide can reduce local concentrations of carbon and give inaccurate dates. The releases of carbon dioxide into the biosphere as a consequence of industrialization have also depressed the proportion of carbon by a few percent; conversely, the amount of carbon was increased by above-ground nuclear bomb tests that were conducted into the early s.
Also, an increase in the solar wind or the Earth's magnetic field above the current value would depress the amount of carbon created in the atmosphere. Fission track dating method[ edit ] Main article: This involves inspection of a polished slice of a material to determine the density of "track" markings left in it by the spontaneous fission of uranium impurities. The uranium content of the sample has to be known, but that can be determined by placing a plastic film over the polished slice of the material, and bombarding it with slow neutrons.
This causes induced fission of U, as opposed to the spontaneous fission of U. The fission tracks produced by this process are recorded in the plastic film.
The uranium content of the material can then be calculated from the number of tracks and the neutron flux. This scheme has application over a wide range of geologic dates. For dates up to a few million years micastektites glass fragments from volcanic eruptionsand meteorites are best used.
Older materials can be dated using zirconapatitetitaniteepidote and garnet which have a variable amount of uranium content. The technique has potential applications for detailing the thermal history of a deposit. The residence time of 36Cl in the atmosphere is about 1 week. Thus, as an event marker of s water in soil and ground water, 36Cl is also useful for dating waters less than 50 years before the present.
Radiometric Age Dating - Geology (U.S. National Park Service)
Luminescence dating methods[ edit ] Main article: Luminescence dating Luminescence dating methods are not radiometric dating methods in that they do not rely on abundances of isotopes to calculate age.
Instead, they are a consequence of background radiation on certain minerals.
Over time, ionizing radiation is absorbed by mineral grains in sediments and archaeological materials such as quartz and potassium feldspar. The radiation causes charge to remain within the grains in structurally unstable "electron traps". Exposure to sunlight or heat releases these charges, effectively "bleaching" the sample and resetting the clock to zero.
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The trapped charge accumulates over time at a rate determined by the amount of background radiation at the location where the sample was buried.
Stimulating these mineral grains using either light optically stimulated luminescence or infrared stimulated luminescence dating or heat thermoluminescence dating causes a luminescence signal to be emitted as the stored unstable electron energy is released, the intensity of which varies depending on the amount of radiation absorbed during burial and specific properties of the mineral.
These methods can be used to date the age of a sediment layer, as layers deposited on top would prevent the grains from being "bleached" and reset by sunlight.
Furthermore, fossil organisms were more unique than rock types, and much more varied, offering the potential for a much more precise subdivision of the stratigraphy and events within it. The recognition of the utility of fossils for more precise "relative dating" is often attributed to William Smith, a canal engineer who observed the fossil succession while digging through the rocks of southern England.
But scientists like Albert Oppel hit upon the same principles at about about the same time or earlier. In Smith's case, by using empirical observations of the fossil succession, he was able to propose a fine subdivision of the rocks and map out the formations of southern England in one of the earliest geological maps Other workers in the rest of Europe, and eventually the rest of the world, were able to compare directly to the same fossil succession in their areas, even when the rock types themselves varied at finer scale.
For example, everywhere in the world, trilobites were found lower in the stratigraphy than marine reptiles. Dinosaurs were found after the first occurrence of land plants, insects, and amphibians.
Spore-bearing land plants like ferns were always found before the occurrence of flowering plants. The observation that fossils occur in a consistent succession is known as the "principle of faunal and floral succession". The study of the succession of fossils and its application to relative dating is known as "biostratigraphy".
Each increment of time in the stratigraphy could be characterized by a particular assemblage of fossil organisms, formally termed a biostratigraphic "zone" by the German paleontologists Friedrich Quenstedt and Albert Oppel.
These zones could then be traced over large regions, and eventually globally. Groups of zones were used to establish larger intervals of stratigraphy, known as geologic "stages" and geologic "systems". The time corresponding to most of these intervals of rock became known as geologic "ages" and "periods", respectively.
By the end of the s, most of the presently-used geologic periods had been established based on their fossil content and their observed relative position in the stratigraphy e. These terms were preceded by decades by other terms for various geologic subdivisions, and although there was subsequent debate over their exact boundaries e. By the s, fossil succession had been studied to an increasing degree, such that the broad history of life on Earth was well understood, regardless of the debate over the names applied to portions of it, and where exactly to make the divisions.
All paleontologists recognized unmistakable trends in morphology through time in the succession of fossil organisms.
This observation led to attempts to explain the fossil succession by various mechanisms. Perhaps the best known example is Darwin's theory of evolution by natural selection. Note that chronologically, fossil succession was well and independently established long before Darwin's evolutionary theory was proposed in Fossil succession and the geologic time scale are constrained by the observed order of the stratigraphy -- basically geometry -- not by evolutionary theory.
Calibrating the Relative Time Scale For almost the next years, geologists operated using relative dating methods, both using the basic principles of geology and fossil succession biostratigraphy. Various attempts were made as far back as the s to scientifically estimate the age of the Earth, and, later, to use this to calibrate the relative time scale to numeric values refer to "Changing views of the history of the Earth" by Richard Harter and Chris Stassen.
Most of the early attempts were based on rates of deposition, erosion, and other geological processes, which yielded uncertain time estimates, but which clearly indicated Earth history was at least million or more years old. A challenge to this interpretation came in the form of Lord Kelvin's William Thomson's calculations of the heat flow from the Earth, and the implication this had for the age -- rather than hundreds of millions of years, the Earth could be as young as tens of million of years old.
This evaluation was subsequently invalidated by the discovery of radioactivity in the last years of the 19th century, which was an unaccounted for source of heat in Kelvin's original calculations.
With it factored in, the Earth could be vastly older. Estimates of the age of the Earth again returned to the prior methods. 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.
Geologic Age Dating Explained - Kids Discover
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. 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.
Radiometric Age Dating
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. 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.