With radiometric dating the absolute age of a rock can be found
- How do scientists date rocks and fossils? — Earth@Home
- How do you determine the age of a radioactive isotope?
- How do geologists determine the age of the Earth?
- How do scientists date igneous rocks?
- Why do Geologists use radiometric decay dates?
- How do scientists know the age of radioactive isotopes?
- Why do we use the half-life of a radioactive isotope?
- What is the useful range of a radioactive isotope?
- How can you tell the age of a carbon atom?
How do scientists date rocks and fossils? — Earth@Home
Absolute age dating deals with assigning actual dates in years before the present to rocks or geological events. Contrast this with relative age dating, which instead is concerned with determining the orders of events in Earth's past.
The science of absolute age dating is known as geochronology and the fundamental method of geochronology is called radiometric dating. Scholars and naturalists, understandably, have long been interested in knowing the absolute age of the Earth, as well as other important geological events. In 1650, Archbishop famously used the genealogy of the Old Testament of the Bible e. In the 1800's, practitioners of the young science of geology applied the uniformitarian views of Hutton and Lyell see the to this chapter to try to determine the age of the Earth.
For example, some geologists observed how long it took for a given amount of sediment say, a centimeter of sand to accumulate in a modern habitat, then applied this rate to the total known thickness of sedimentary rocks. When they did this, they estimated that the Earth is many millions of years old. But, unlike Ussher's calculation, this estimate was on the order of millions of years, rather than 6,000.
Geologists were beginning to accept the views of Hutton that the Earth is unimaginably ancient. The answer is radioactivity. Radiometric dating Hypotheses of absolute ages of rocks as well as the events that they represent are determined from rates of radioactive decay of some isotopes of elements that occur naturally in rocks. Elements and isotopes In chemistry, an element is a particular kind of atom that is defined by the number of protons that it has in its nucleus.
The number of protons equals the element's atomic number. Have a look at the periodic table of the elements below. Carbon's C atomic number is 6 because it has six protons in its nucleus; gold's Au atomic number is 79 because with radiometric dating the absolute age of a rock can be found has 79 atoms in its nucleus.
Even though individual elements always have the same number of protons, the number of neutrons in their nuclei sometimes varies. These variations are called isotopes. Isotopes of individual elements are defined by their mass numberwhich is simply the number of protons + the number of neutrons. Some isotopes are unstable, however, and with radiometric dating the absolute age of a rock can be found radioactive decay.
Radioactive decay Radioactive decay involves unstable isotopes shedding energy in the form of radiation, causing their numbers of protons and neutrons to change, in turn resulting in one element changing into another. As a matter of convention, we call the atomic nucleus that undergoes radioactive decay the parent and the resulting product the daughter product or, decay product.
The rate at which a particular parent isotope decays into its daughter product is constant. This rate is determined in a laboratory setting and is typically represented by its half-life. A half-life is the amount of time needed for half of the parent atoms in a sample to be changed into daughter products. This is illustrated in the chart below. At the start time zero half-lives passedthe sample consists of 100% parent atoms blue diamonds ; there are no daughter products red squares because no time has passed.
After the passage of one half-life, 50% of the parent atoms have become daughter products. After two half-lives, 75% of the original parent atoms have been transformed into daughter products thus, only 25% of the original parent atoms remain.
After three half-lives, only 12. As more half-lives pass, the number of parent atoms remaining approaches zero. Based on this principle, geologists can count the number of parent atoms relative to daughter products in a sample to determine how many half-lives have passed since a mineral grain first formed. Consider the example shown below. The left-most box in the figure above represents an initial state, with parent atoms distributed throughout molten rock magma.
As the magma cools, grains of different minerals begin to crystalize. Some of these minerals represented above as gray hexagons incorporate the radioactive parent atoms blue diamonds into their crystalline structures; this marks the initiation of the "half-life clock" i.
After one half-life has passed, half 50%, or four of the parent atoms in each mineral grain have been transformed with radiometric dating the absolute age of a rock can be found their daughter products red squares. After two half-lives have passed, 75% six of the original parent atoms in each grain have been transformed into daughter products.
How many parent atoms would remain if three half-lives passed? Calculating radiometric dates By counting the numbers of parent atoms remaining in a sample relative to the number originally present, it is possible to determine the number of half-lives that have passed since the initial formation of a mineral grain that is, when it became a "closed system" that prevented parent and daughter atoms from escaping.
You might be wondering how it is possible to know the number of parent atoms that were originally in a sample. This number is attained by simply adding the number of parent and daughter atoms currently in the sample because each daughter atom was once a parent atom.
The next step in radiometric dating involves converting the number of half-lives that have passed into an absolute i. This is done by multiplying the number of half-lives that have passed by the half-life decay constant of the parent atom again, this value is determined in a laboratory.
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.
Let's work through a hypothetical example problem. Suppose you analyzed a mineral sample and found that it contained 33,278 parent atoms and 14,382 daughter atoms. Further, suppose that the half-life of the parent atom is 2. How old is the mineral sample? 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. If a radiometric date were to be attained from this mineral grain, it would tell us when the mineral first formed, but not when the sedimentary rock formed it would, however, tell us the maximum possible age of the sedimentary rock layer.
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. With radiometric dating the absolute age of a rock can be found in half-lives among different isotopes As noted above, the rate at which a given radioactive isotope decays into its daughter product is constant.
This rate, with radiometric dating the absolute age of a rock can be found, 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. Parent isotope Final decay product Half-life Uranium-238 Lead-206 4. Since the entire universe is 13. At the other end of the spectrum, note the very short half-life of carbon-14: 5,730 years. The is the isotope that is used in "carbon dating.
Both it and carbon-12 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-12 in its tissues remains stable, but the carbon-14 decays into nitrogen-14.
The ratio of carbon-14 relative to carbon-12 in a sample, therefore, may be used to determine the age of organic matter derived from plant tissues. Because of its short half-life, carbon-14 can only be used to date materials with radiometric dating the absolute age of a rock can be found are up to about 70,000 years old beyond this point, the amount of carbon-14 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. Age of the Earth At theyou 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. If the oldest mineral grain is 4. The answer is radiometric dating of meteorite specimens, which we presume to have formed around the same time as the Earth, Sun, and other planetary bodies in our solar system.
One such dated meteorite comes from Meteor Crater in Arizona. The Digital Atlas of Ancient Life project is managed by theIthaca, New York. Development of this project was supported by the National Science Foundation. Any opinions, findings, and conclusions or recommendations expressed in this material are those of the author s and do not necessarily reflect the views of the National Science Foundation.
How do you determine the age of a radioactive isotope?
Radiometric dating calculates an age in years for geologic materials by measuring the presence of a short-life radioactive element, e.g., carbon-14, or a long-life radioactive element plus its decay product, e.g., potassium-14/argon-40.
How do geologists determine the age of the Earth?
The term applies to all methods of age determination based on nuclear decay of naturally occurring radioactive isotopes. Bates and Jackson (1984) To determine the ages in years of Earth materials and the timing of geologic events such as exhumation and subduction, geologists utilize the process of radiometric decay.
How do scientists date igneous rocks?
Scientists date igneous rock using elements that are slow to decay, such as uranium and potassium. By dating these surrounding layers, they can figure out the youngest and oldest that the fossil might be; this is known as “bracketing” the age of the sedimentary layer in which the fossils occur.
Why do Geologists use radiometric decay dates?
Geologists use these dates to further define the boundaries of the geologic periods shown on the geologic time scale. Radiometric decay occurs when the nucleus of a radioactive atom spontaneously transforms into an atomic nucleus of a different, more stable isotope.
How do scientists know the age of radioactive isotopes?
Scientists know how quickly radioactive isotopes decay into other elements over thousands, millions and even billions of years. Scientists calculate ages by measuring how much of the isotope remains in the substance.
Why do we use the half-life of a radioactive isotope?
So, we use the time in which half of any of these unstable nuclei will decay. The half-life of a radioactive isotope is the time taken for half the unstable nuclei in a sample to decay. Different isotopes have different half-lives. Plutonium-239 has a half-life of 24,100 years but plutonium-241 has a half-life of only 14.4 years.
What is the useful range of a radioactive isotope?
Their useful range is from about 1/10 their half-life (the time it takes for half of the radioactive element/isotope-- the parent, to convert into a non-radioactive element/isotope-- the daughter) to 10 times their half-life. For example, Potassium-40 decays to Argon-40.
How can you tell the age of a carbon atom?
Using half-life Half-life can be used to work out the age of fossils or wooden objects. Living things absorb carbon dioxide and other carbon compounds. Some of the carbon atoms are carbon-14, which is a radioactive isotope of carbon.