High School Earth Science/Absolute Ages of Rocks - Wikibooks, open books for an open world
Earth sciences - Radiometric dating: In , shortly after the discovery of After estimating the rate of this radioactive change, he calculated that the absolute ages of his Experimental petrology, both at the low-temperature range explored by van 't . Articles from Britannica Encyclopedias for elementary and high school. Radiometric dating is rooted in the rates of radioactive decay of various Each method has its own particular range of applicability, which derives from the by a simple exponential formula, taught in many high school math classes: whose slope m is simply related to the age of the specimen by the formula m = eL t - 1. Radiometric dating and the age of the Earth by Ralph W. Matthews, Ph.D. [Click The values they assumed were based on the lead isotope ratios observed for . the centres of Interdisciplinary Studies, Glassboro State College, Glassboro.
Rather, the offset is equal to one half-life, or 5, years for carbon The initial portion of the calibration curve in Figure 9 has been widely available and well accepted for some time, so reported radiocarbon dates for ages up to 11, years generally give the calibrated ages unless otherwise stated. The calibration curve over the portions extending to 40, years is relatively recent, but should become widely adopted as well. These methods may work on young samples, for example, if there is a relatively high concentration of the parent isotope in the sample.
In that case, sufficient daughter isotope amounts are produced in a relatively short time.
As an example, an article in Science magazine vol. There are other ways to date some geologically young samples. Besides the cosmogenic radionuclides discussed above, there is one other class of short-lived radionuclides on Earth. These are ones produced by decay of the long-lived radionuclides given in the upper part of Table 1. As mentioned in the Uranium-Lead section, uranium does not decay immediately to a stable isotope, but decays through a number of shorter-lived radioisotopes until it ends up as lead.
While the uranium-lead system can measure intervals in the millions of years generally without problems from the intermediate isotopes, those intermediate isotopes with the longest half-lives span long enough time intervals for dating events less than several hundred thousand years ago. Note that these intervals are well under a tenth of a percent of the half-lives of the long-lived parent uranium and thorium isotopes discussed earlier.
Two of the most frequently-used of these "uranium-series" systems are uranium and thorium These are listed as the last two entries in Table 1, and are illustrated in Figure A schematic representation of the uranium decay chain, showing the longest-lived nuclides.
Half-lives are given in each box. Solid arrows represent direct decay, while dashed arrows indicate that there are one or more intermediate decays, with the longest intervening half-life given below the arrow.
Earlham College - Geology - Radiometric Dating
Like carbon, the shorter-lived uranium-series isotopes are constantly being replenished, in this case, by decaying uranium supplied to the Earth during its original creation. Following the example of carbon, you may guess that one way to use these isotopes for dating is to remove them from their source of replenishment.
This starts the dating clock. In carbon this happens when a living thing like a tree dies and no longer takes in carbonladen CO2. For the shorter-lived uranium-series radionuclides, there needs to be a physical removal from uranium.
The chemistry of uranium and thorium are such that they are in fact easily removed from each other. Uranium tends to stay dissolved in water, but thorium is insoluble in water.
- Radioactive dating
- Radiometric dating
- Geologic Age Dating Explained
So a number of applications of the thorium method are based on this chemical partition between uranium and thorium. Sediments at the bottom of the ocean have very little uranium relative to the thorium.
Because of this, the uranium, and its contribution to the thorium abundance, can in many cases be ignored in sediments. Thorium then behaves similarly to the long-lived parent isotopes we discussed earlier. It acts like a simple parent-daughter system, and it can be used to date sediments. On the other hand, calcium carbonates produced biologically such as in corals, shells, teeth, and bones take in small amounts of uranium, but essentially no thorium because of its much lower concentrations in the water.
This allows the dating of these materials by their lack of thorium. A brand-new coral reef will have essentially no thorium As it ages, some of its uranium decays to thorium While the thorium itself is radioactive, this can be corrected for.
Comparison of uranium ages with ages obtained by counting annual growth bands of corals proves that the technique is page. The method has also been used to date stalactites and stalagmites from caves, already mentioned in connection with long-term calibration of the radiocarbon method. In fact, tens of thousands of uranium-series dates have been performed on cave formations around the world. Previously, dating of anthropology sites had to rely on dating of geologic layers above and below the artifacts.
But with improvements in this method, it is becoming possible to date the human and animal remains themselves. Work to date shows that dating of tooth enamel can be quite reliable. However, dating of bones can be more problematic, as bones are more susceptible to contamination by the surrounding soils. As with all dating, the agreement of two or more methods is highly recommended for confirmation of a measurement.
If the samples are beyond the range of radiocarbon e. Non-Radiometric Dating Methods for the PastYears We will digress briefly from radiometric dating to talk about other dating techniques. It is important to understand that a very large number of accurate dates covering the pastyears has been obtained from many other methods besides radiometric dating. We have already mentioned dendrochronology tree ring dating above. Dendrochronology is only the tip of the iceberg in terms of non-radiometric dating methods.
Here we will look briefly at some other non-radiometric dating techniques. One of the best ways to measure farther back in time than tree rings is by using the seasonal variations in polar ice from Greenland and Antarctica. There are a number of differences between snow layers made in winter and those made in spring, summer, and fall. These seasonal layers can be counted just like tree rings.
The seasonal differences consist of a visual differences caused by increased bubbles and larger crystal size from summer ice compared to winter ice, b dust layers deposited each summer, c nitric acid concentrations, measured by electrical conductivity of the ice, d chemistry of contaminants in the ice, and e seasonal variations in the relative amounts of heavy hydrogen deuterium and heavy oxygen oxygen in the ice.
These isotope ratios are sensitive to the temperature at the time they fell as snow from the clouds. The heavy isotope is lower in abundance during the colder winter snows than it is in snow falling in spring and summer. So the yearly layers of ice can be tracked by each of these five different indicators, similar to growth rings on trees. The different types of layers are summarized in Table III.
Page 17 Ice cores are obtained by drilling very deep holes in the ice caps on Greenland and Antarctica with specialized drilling rigs. As the rigs drill down, the drill bits cut around a portion of the ice, capturing a long undisturbed "core" in the process. These cores are carefully brought back to the surface in sections, where they are catalogued, and taken to research laboratories under refrigeration.
A very large amount of work has been done on several deep ice cores up to 9, feet in depth. Several hundred thousand measurements are sometimes made for a single technique on a single ice core. A continuous count of layers exists back as far asyears. In addition to yearly layering, individual strong events such as large-scale volcanic eruptions can be observed and correlated between ice cores.
High School Earth Science/Absolute Ages of Rocks
A number of historical eruptions as far back as Vesuvius nearly 2, years ago serve as benchmarks with which to determine the accuracy of the yearly layers as far down as around meters. As one goes further down in the ice core, the ice becomes more compacted than near the surface, and individual yearly layers are slightly more difficult to observe. For this reason, there is some uncertainty as one goes back towardsyears. Recently, absolute ages have been determined to 75, years for at least one location using cosmogenic radionuclides chlorine and beryllium G.
These agree with the ice flow models and the yearly layer counts. 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.
Radiometric dating - RationalWiki
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. 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.
The Iceman is a very famous frozen body found in northern Italy in Samples of his bones, grass boot, leather and hair were dated, the results showed that he lived almost years ago BCduring the age when people first began using copper in Europe. Radiocarbon dating was tremendously important in dating the precise age of the Iceman.
How, in your opinion, did the use of radiocarbon dating change the way scientists are able to interpret and understand history? Beforewhen radiocarbon dating was first developed by scientists from the US, archaeologists had no way of knowing precisely how old in numbers of years an archaeological site or artefact was.
In some parts of the world, where historic records extended back far enough in time, such as in the Mediterranean, archaeologists had dated artefacts by comparison with material from other sites which could be historically dated. This method was called "relative dating" and it is still used today.
Radiocarbon dating enabled archaeologists and other scientists to verify the ages of carbon-bearing materials ndependently and almost overnight revolutionised the approach of dating the past. The reason was that now any samples could be dated, so long as they were once living organisms.
Radiocarbon dating is one of the critical discoveries in 20th century science and it provided one of the most important tools for archaeologists in their quest to uncover the past.
Instead of spending large amounts of time solving the problem of "when" something happened, archaeologists could now concentrate on investigating "how" and "why" things happened. What if any arguments were provoked because of the use of radio-carbon dating? One of the most controversial examples of the use of radiocarbon dating was the analysis of the Turin Shroud, the supposed burial cloth of Jesus. The shroud itself appears to show a person who was crucified and is an object of some veneration because of its supposed association with Christ.
Its history dates back at least as far as the mid 14th century AD. The first photograph of the shroud showed the man as a negative image, a kind of three dimensional picture. This, along with other discoveries, such as the supposed presence of pollen spores from Israel on the cloth have suggested the shroud might be an important and genuine relic.
In the s, the Archbishop of Turin gave permission to a group of scientists to date small pieces of fabric sampled from the shroud. Radiocarbon laboratories at Tucson USOxford England and Zurich Switzerland dated the samples, along with 3 control samples of varying ages. The results were very consistent and showed the shroud dated between AD.
This fits closely with its first appearance in the historical record and suggests strongly that it is a medieval artefact, rather than a genuine year-old burial cloth. You can read the original scientific paper on the age of the Shroud here. Can you find the age of rocks by using radiocarbon dating or are they generally too old? If a rock was shot from a volcano and isn't that old, can we use radiocarbon dating?
Samples of rock are not able to be dated using radiocarbon, because rocks contain no organic carbon from living organisms that are of recent enough age. Most rocks formed hundreds of thousands if not millions of years ago. Geologic deposits of coal and lignite formed from the compressed remains of plants contain no remaining radiocarbon so they cannot be dated.
Radiocarbon dating is limited to the period 0 - 60 years, because the 'half-life' of radiocarbon is about years, so to date rocks scientists must use other methods. There is a number of different techniques available. We can date volcanic rocks using a method called argon-argon dating for instance. This method uses principles of isotopic decay like radiocarbon, but different isotopes argon and argon 40 which have a longer halflife million years. This means scientists can date rock which is many millions of years old.
The technique can date materials the size of one grain of volcanic ash, using a laser. There are other methods which can be used as well which operate using different radiochemistries.
The only way to date a volcanic ash layer using radiocarbon dating is to find ash within a lake sediment or peat layer and then date the organic carbon from above and below it, and therefore fix an age for the ash event.
This is a commonly used approach to date volcanic events over the past 60 years around the world.