# How can relative dating be used in geology

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For example, in sedimentary rocks, it is common for from an older formation to be ripped up and included in a newer layer. Please note that none of the letters in this sequence may be reversed and still be correct. From the chart, which methods are best for older materials? For example, most limestones represent marine environments, whereas, sandstones with ripple marks might indicate a shoreline habitat or a riverbed.\
&#x20;![how can relative dating be used in geology](http://www.talkorigins.org/faqs/dating/bearpaw.gif)\\

Time factors of millions and billions of years is difficult even for adults to comprehend. The date for the Baculites reesidei zone is at least 0. Different species of ammonites lived at different times within the Mesozoic, so identifying a prime species can help narrow down when a rock was formed. The fossils within rock layer OXD i. Annual Review of Earth and Planetary Sciences. Dinosaurs disappeared about 65 million years ago. However, there are some smaller differences.

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. By matching partial sequences, the truly oldest layers with fossils can be worked out. The study of fossils and the exploration of what they tell scientists about past climates and environments on Earth can be an interesting study for students of all ages. The pertains to the formation of and the age of the sequences through which they cut.\
&#x20;![how can relative dating be used in geology](http://www.learner.org/courses/envsci/visual/img_lrg/geologic_crosssection.jpg)\\

**Geologic Age Dating Explained** - Although there might be some mineral differences due to the difference in source rock, most sedimentary rock deposited year after year look very similar to one another. You can have three different rock sequences from three different locations.\
&#x20;![how can relative dating be used in geology](http://3.bp.blogspot.com/-psI2xSf0PKg/VG0I4A93E9I/AAAAAAAAEuk/APXi7XJpmpw/s1600/Geological%2BTable.png)\\

It is not about the theory behind radiometric dating methods, it is about their application, and it therefore assumes the reader has some familiarity with the technique already refer to for more information. As an example of how they are used, radiometric dates from geologically simple, fossiliferous Cretaceous rocks in western North America are compared to the geological time scale. To get to that point, there is also a historical discussion and description of non-radiometric dating methods. A common form of criticism is to cite geologically complicated situations where the application of radiometric dating is very challenging. These are often characterised as the norm, rather than the exception. I thought it would be useful to present an example where the geology is simple, and unsurprisingly, the method does work well, to show the quality of data that would have to be invalidated before a major revision of the geologic time scale could be accepted by conventional scientists. Geochronologists do not claim that radiometric dating is foolproof no scientific method is , but it does work reliably for most samples. This document is partly based on a prior posting composed in reply to. My thanks to both him and other critics for motivating me. Background Stratigraphic Principles and Relative Time Much of the Earth's geology consists of successional layers of different rock types, piled one on top of another. The most common rocks observed in this form are sedimentary rocks derived from what were formerly sediments , and extrusive igneous rocks e. Fundamental to stratigraphy are a set of simple principles, based on elementary geometry, empirical observation of the way these rocks are deposited today, and gravity. Most of these principles were formally proposed by Nicolaus Steno Niels Steensen, Danish , in 1669, although some have an even older heritage that extends as far back as the authors of the Bible. A few principles were recognized and specified later. Note that these are principles. In no way are they meant to imply there are no exceptions. For example, the principle of superposition is based, fundamentally, on gravity. In order for a layer of material to be deposited, something has to be beneath it to support it. It can't float in mid-air, particularly if the material involved is sand, mud, or molten rock. The principle of superposition therefore has a clear implication for the relative age of a vertical succession of strata. There are situations where it potentially fails -- for example, in cave deposits. In this situation, the cave contents are younger than both the bedrock below the cave and the suspended roof above. Cave deposits also often have distinctive structures of their own e. These geological principles are not assumptions either. Each of them is a testable hypothesis about the relationships between rock units and their characteristics. In the last 200 or more years of their application, they are often valid, but geologists do not assume they are. Using these principles, it is possible to construct an interpretation of the sequence of events for any geological situation, even on other planets e. Despite this, the can be used to determine the sequence of deposition, folds, and faults based on their intersections -- if folds and faults deform or cut across the sedimentary layers and surfaces, then they obviously came after deposition of the sediments. You can't deform a structure e. Even in complex situations of multiple deposition, deformation, erosion, deposition, and repeated events, it is possible to reconstruct the sequence of events. No matter what the geologic situation, these basic principles reliably yield a reconstructed history of the sequence of events, both depositional, erosional, deformational, and others, for the geology of a region. This reconstruction is tested and refined as new field information is collected, and can be and often is done completely independently of anything to do with other methods e. These simple techniques have widely and successfully applied since at least the early 1700s, and by the early 1800s, geologists had recognized that many obvious similarities existed in terms of the independently-reconstructed sequence of geologic events observed in different parts of the world. The latter two subdivisions, in an emended form, are still used today by geologists. Another observation was the similarity of the fossils observed within the succession of strata, which leads to the next topic. Biostratigraphy As geologists continued to reconstruct the Earth's geologic history in the 1700s and early 1800s, they quickly recognized that the distribution of fossils within this history was not random -- fossils occurred in a consistent order. This was true at a regional, and even a global scale. 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. 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 1815. 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. These zones could then be traced over large regions, and eventually globally. By the end of the 1830s, 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 1830s, 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 1859. Fossil succession and the geologic time scale are constrained by the observed order of the stratigraphy -- basically geometry -- not by evolutionary theory. Radiometric Dating: Calibrating the Relative Time Scale For almost the next 100 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 1700s to scientifically estimate the age of the Earth, and, later, to use this to calibrate the relative time scale to numeric values refer to 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 100 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 , it can provide a numerical calibration for what would otherwise be only an ordering of events -- i. Given the background above, the information used for a geologic time scale can be related like this: Figure 2. How relative dating of events and radiometric numeric dates are combined to produce a calibrated geological time scale. 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. 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 others , the first appearance of new types of organisms, the presence of geochemical anomalies notably iridium , and 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. 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. 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 scale , and of the global geological time scale itself. 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 , and see below. 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. 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. It happens in all sciences. However, this statistical likelihood is not assumed, it is tested, usually by using other methods e. 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. 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 dating , and 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. This is the inevitable nature of rocks that have experienced millions of years of history: not all of them will preserve their age of origin intact, not every rock will have appropriate chemistry and mineralogy, no sample is perfect, and there is no dating method that can effectively date rocks of any age or rock type. 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. Among other problems documented in , many 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. Consider this stratigraphic section from the Bearpaw Formation of Saskatchewan, Canada : Figure 3. 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. 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. It should therefore be older than the results from. 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: Figure 5. Comparison of newer data with the time scale. 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: for intervals that should be about 70-80 million years old, radiometric dates do not yield for example 100 or 30 million years, let alone 1000 years, 100 000 years or 1 billion. 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? Specifically, he proposes an age of 71. The other dates are completely consistent with a lower boundary for the Campanian of 83±1 million years ago, as suggested by which Obradovich revises to 83. 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. 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. For example, estimates of the age of boundaries in the Tertiary regularly varied by 20-30% in the 1930s to 1970s. Since that time, they have varied by much smaller amounts, rarely approaching 5% again refer to. The same trend can be observed for other time periods. The latter includes an excellent diagram summarizing comparisons between earlier time scales. Since 1990, there have been still more revisions by other authors, such as for the Cretaceous Period, and for the entire Mesozoic. A recent geological time scale, based on As another example, and present radiometric dates that bracket the ages of Late Cretaceous fossil occurrences i. 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 , for some of the methods. 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. 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: testing hypotheses with data. 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. For potential critics: 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. 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. A radiometric age for the Cretaceous-Tertiary boundary based on K-Ar, Rb-Sr, and U-Pb ages of bentonites from Alberta, Saskatchewan, and Montana. 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 first radiometric ages from the Judith River Formation Upper Cretaceous , Hill County, Montana. Canadian Journal of Earth Sciences, v. A Triassic, Jurassic and Cretaceous time scale. Society of Economic Paleontologists and Mineralogists, Special Publication No. A Geologic Time Scale: 1982 edition. Cambridge University Press: Cambridge, 131p. A Geologic Time Scale, 1989 edition. Cambridge University Press: Cambridge, p. ISBN 0-521-38765-5 Harper, C. Relative age inference in paleontology. Bones of Contention: A Creationist Assessment of Human Fossils. Baker Book House: Grand Rapids. A Cretaceous time scale. Evolution of the Western Interior Basin. Geological Association of Canada, Special Paper 39, p. The Decade of North American Geology 1983 Geologic Time Scale. Encyclopaedia Britannica 10, p. Canadian Journal of Earth Sciences, v. Creation Research Society Quarterly, v. It therefore assumes the reader has some familiarity with radiometric dating. For a technical introduction to the methods, I highly recommend these two books: Berry, W. Growth of a Prehistoric Time Scale. Blackwell Scientific Publications: Boston, 202p. And a good summary is in by Richard Harter and Chris Stassen. A profusion of terms is applied to the different concepts, and, confusingly to the uninitiated, to the names applied to subdivisions of them e. The semantic difference exists to distinguish between the different but relatable types of observations and interpretation that go into them. These issues are explained in much more detail in the particularly. Acknowledgements This is my third revision of a FAQ on the application of dating methods. It benefits from the comments of several informal reviewers. Unfortunately, some were so long ago that I no longer have all their names :- But my thanks goes to all of them anyway, and to four recent ones I do remember: Stanley Friesen, Chris Stassen, Mark Isaak, and Martyne Brotherton. My thanks also to Brett Vickers for maintaining the talk.
