Radiometric dating or radioactive dating is a technique used to date materials such as rocks or . The age that can be calculated by radiometric dating is thus the time at which the rock or mineral cooled to closure temperature. . To be able to distinguish the relative ages of rocks from such old material, and to get a better .
Table of contents
- Radiometric dating
- Radioactive Decay and Absolute Age Determinations
- Dating Rocks: Absolute Age Determinations
- DETERMINING AGE OF ROCKS AND FOSSILS
To summarize, the key piece of information that needs to be determined from a mineral specimen in order to determine its absolute age is its age in number of half lives. This can be mathematically determined by solving for y in this equation:. Let's work through a hypothetical example problem. Suppose you analyzed a mineral sample and found that it contained 33, parent atoms and 14, daughter atoms. Further, suppose that the half-life of the parent atom is 2. How old is the mineral sample? First, we know that: So, we conclude that 0. As noted above, a radiometric date tells us when a system became closed, for example when a mineral containing radioactive parent elements first crystalized.
An individual mineral grain may have a long history after it first forms. For example, it may erode out of an igneous rock and then be transported long distances and over long periods of time before it is finally deposited, becoming one grain among billions in a layer of sedimentary rock e. Further, heating mineral grains to great temperatures can cause them to leak parent and daughter material, resetting their radiometric clocks.
The melting involved with metamorphic change can reset the radiometric clock. For example, suppose an igneous rock formed 2. If it were subjected to metamorphism 1.
Radiometric dating
As noted above, the rate at which a given radioactive isotope decays into its daughter product is constant. This rate, however, varies considerably among different radioactive isotopes. Further, many radioactive isotopes undergo a series of transformations--some of which have half-lives that persist for only very short amounts of time--before they are converted into their final daughter products.
Below are some of the decay series that are commonly used in radiometric dating of geological samples. Note the great variations in their half-lives. Note that the half-life for the rubidium to strontium series is 50 billion years! Since the entire universe is At the other end of the spectrum, note the very short half-life of carbon The is the isotope that is used in "carbon dating. Both it and carbon which is stable, meaning that it does not undergo radioactive decay are incorporated into the tissues of plants as they grow.
After a plant dies, the carbon in its tissues remains stable, but the carbon decays into nitrogen The ratio of carbon relative to carbon in a sample, therefore, may be used to determine the age of organic matter derived from plant tissues.
Radioactive Decay and Absolute Age Determinations
Because of its short half-life, carbon can only be used to date materials that are up to about 70, years old beyond this point, the amount of carbon remaining becomes so small that it is difficult to measure. Because of its precision, it is nevertheless very useful for dating organic matter from the near recent geological past, especially archeological materials from the Holocene epoch. At the beginning of this chapter , you learned that the Earth is 4.
As it turns out, the oldest dated mineral--a grain of zircon from the Jack Hills of Western Australia--is 4. A single grain of zircon, imaged using a scanning electron microscope. A sample of 4. If the oldest mineral grain is 4. The decay of radioactive materials can be shown with a graph Figure In the process of radiometric dating , several isotopes are used to date rocks and other materials.
Using several different isotopes helps scientists to check the accuracy of the ages that they calculate. Earth's atmosphere contains three isotopes of carbon. Carbon is stable and accounts for Carbon is also stable and accounts for 1. Carbon is radioactive and is found in tiny amounts.
Carbon is produced naturally in the atmosphere when cosmic rays interact with nitrogen atoms. The amount of carbon produced in the atmosphere at any particular time has been relatively stable through time. Radioactive carbon decays to stable nitrogen by releasing a beta particle.
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The nitrogen atoms are lost to the atmosphere, but the amount of carbon decay can be estimated by measuring the proportion of radioactive carbon to stable carbon As a substance ages, the relative amount of carbon decreases. Carbon is removed from the atmosphere by plants during the process of photosynthesis.
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Animals consume this carbon when they eat plants or other animals that have eaten plants. Therefore carbon dating can be used to date plant and animal remains. Examples include timbers from an old building, bones, or ashes from a fire pit. Carbon dating can be effectively used to find the age of materials between and 50, years old. Potassium decays to argon with a half-life of 1.
Because argon is a gas, it can escape from molten magma or lava. Therefore any argon that is found in a crystal probably formed as a result of the decay of potassium Measuring the ratio of potassium to argon will yield a good estimate of the age of the sample. Potassium is a common element found in many minerals such as feldspar, mica, and amphibole. The technique can be used to date igneous rocks from , years to over a billion years old.
Because it can be used to date geologically young materials, the technique has been useful in estimating the age of deposits containing the bones of human ancestors. Two isotopes of uranium are used for radiometric dating. Uranium decays to form lead with a half-life of 4. Uranium decays to form lead with a half-life of million years.
Uranium-lead dating is usually performed on crystals of the mineral zircon Figure When zircon forms in an igneous rock, the crystals readily accept atoms of uranium but reject atoms of lead. Therefore, if any lead is found in a zircon crystal, it can be assumed that it was produced from the decay of uranium.
Uranium-lead dating can be used to date igneous rocks from 1 million years to around 4. Some of the oldest rocks on Earth have been dated using this method, including zircon crystals from Australia that are 4. Radiometric dating can only be used on materials that contain measurable amounts of radioactive materials and their daughter products. This includes organic remains which compared to rocks are relatively young, less than , years old and older rocks. Ideally, several different radiometric techniques will be used to date the same rock.
Agreement between these values indicates that the calculated age is accurate. In general, radiometric dating works best for igneous rocks and is not very useful for determining the age of sedimentary rocks. To estimate the age of a sedimentary rock deposit, geologists search for nearby or interlayered igneous rocks that can be dated.
For example, if a sedimentary rock layer is sandwiched between two layers of volcanic ash, its age is between the ages of the two ash layers. September 30, by Beth Geiger. Dinosaurs disappeared about 65 million years ago. That corn cob found in an ancient Native American fire pit is 1, years old. How do scientists actually know these ages? Geologic age dating—assigning an age to materials—is an entire discipline of its own.
Dating Rocks: Absolute Age Determinations
In a way this field, called geochronology, is some of the purest detective work earth scientists do. There are two basic approaches: Here is an easy-to understand analogy for your students: Absolute age dating is like saying you are 15 years old and your grandfather is 77 years old. To determine the relative age of different rocks, geologists start with the assumption that unless something has happened, in a sequence of sedimentary rock layers, the newer rock layers will be on top of older ones.
This is called the Rule of Superposition. This rule is common sense, but it serves as a powerful reference point.
DETERMINING AGE OF ROCKS AND FOSSILS
Geologists draw on it and other basic principles http: Relative age dating also means paying attention to crosscutting relationships. Say for example that a volcanic dike, or a fault, cuts across several sedimentary layers, or maybe through another volcanic rock type. Pretty obvious that the dike came after the rocks it cuts through, right? With absolute age dating, you get a real age in actual years.