Radioactive dating geology

How do geologists date rocks? Radiometric dating! Radioactive elements were incorporated into the Earth when the Solar System formed. All rocks and.
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An early summary of them is found in Charles Lyell's Principles of Geology , published in , and does not differ greatly from a modern formulation:. Note that these are principles.

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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. However, note that because of the " principle of cross-cutting relationships" , careful examination of the contact between the cave infill and the surrounding rock will reveal the true relative age relationships, as will the "principle of inclusion" if fragments of the surrounding rock are found within the infill.

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. They are applied by geologists in the same sense that a "null hypothesis" is in statistics -- not necessarily correct, just testable.

In the last or more years of their application, they are often valid, but geologists do not assume they are. They are the "initial working hypotheses" to be tested further by data. Using these principles, it is possible to construct an interpretation of the sequence of events for any geological situation, even on other planets e.

Radiometric Dating and the Geological Time Scale

The simplest situation for a geologist is a "layer cake" succession of sedimentary or extrusive igneous rock units arranged in nearly horizontal layers. In such a situation, the " principle of superposition" is easily applied, and the strata towards the bottom are older, those towards the top are younger. For example, wave ripples have their pointed crests on the "up" side, and more rounded troughs on the "down" side. Many other indicators are commonly present, including ones that can even tell you the angle of the depositional surface at the time "geopetal structures" , "assuming" that gravity was "down" at the time, which isn't much of an assumption: In more complicated situations, like in a mountain belt, there are often faults, folds, and other structural complications that have deformed and "chopped up" the original stratigraphy.

Despite this, the "principle of cross cutting relationships" 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.

Even if the folding is so intense that some of the strata is now upside down, this fact can be recognized with "way up" indicators. 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.

The reconstructed history of events forms a "relative time scale", because it is possible to tell that event A occurred prior to event B, which occurred prior to event C, regardless of the actual duration of time between them. Sometimes this study is referred to as "event stratigraphy", a term that applies regardless of the type of event that occurs biologic, sedimentologic, environmental, volcanic, magnetic, diagenetic, tectonic, etc.

Radioactivity: A steady but unpredictable (spontaneous) process

These simple techniques have widely and successfully applied since at least the early s, and by the early s, 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. One of the earliest relative time scales based upon this observation was the subdivision of the Earth's stratigraphy and therefore its history , into the "Primary", "Secondary", "Tertiary", and later "Quaternary" strata based mainly on characteristic rock types in Europe.

The latter two subdivisions, in an emended form, are still used today by geologists. The earliest, "Primary" is somewhat similar to the modern Paleozoic and Precambrian, and the "Secondary" is similar to the modern Mesozoic. Another observation was the similarity of the fossils observed within the succession of strata, which leads to the next topic.

Essential Concepts

As geologists continued to reconstruct the Earth's geologic history in the s and early s, 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.

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.

Radiometric Dating: Methods, Uses & the Significance of Half-Life

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.

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

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 time , it 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: 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 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. 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 scale , and of the global geological time scale itself.