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radiometric dating

radiometric dating(radioactive dating) The most precise method of dating rocks, in which the relative percentages of ‘parent’ and ‘daughter’ isotopes of a given radioactive element are estimated. Early methods relied on uranium and thorium minerals, but potassium–argon, rubidium–strontium, samarium–neodymium, and carbon-14–carbon-12 are now of considerable importance. Uranium-238 decays to lead-206 with a half-life of 4.5 billion (109) years, rubidium-87 decays to strontium-87 with a half-life of 50.0 billion years, and potassium-40 decays to argon-40 with a half-life of 1.5 billion years. For carbon-14 the half-life is a mere 5730 ± 30 years (see radiocarbon dating). It is important that the radioactive isotope be contained within the sample being dated. Carbon-14 is contained within plant material, but potassium-40, argon-40, and uranium-238 are contained satisfactorily only within crystals. Igneous rocks are the most suitable for dating. Fossils occur mostly in sedimentary rocks, however, so absolute dates can be calculated for them less commonly than might be supposed. The only exceptions are fossils occurring in glauconite, a clay mineral containing potassium and argon which forms authigenically on the bottom of shelf seas.

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radiometric dating

radiometric dating (radioactive dating) The most precise method of dating rocks, in which the relative percentages of ‘parent’ and ‘daughter’ isotopes of a given radioactive element are estimated. Early methods relied on uranium and thorium minerals (see URANIUM-LEAD DATING), but potassium-argon, rubidium-strontium, samarium-neodymium, and carbon-14—carbon-12 are now of considerable importance. Uranium-238 decays to lead-206 with a half-life of 4.5 billion (109) years, rubidium-87 decays to strontium-87 with a half-life of 50.0 billion years, and potassium-40 decays to argon-40 with a half-life of 1.5 billion years. For carbon-14 the half-life is a mere 5730 ± 30 years (see RADIOCARBON DATING), and beyond about 70 000 years the amount of carbon-14 remaining in organic matter is beyond accurate measurement. Compare INITIAL STRONTIUM RATIO.

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dating, radioactive

dating, radioactive (radiometric dating) Any of several methods using radioactive decay to assess the ages of archaeological remains, fossils and rocks. The specimens must contain a very long-lived radioisotope of known half-life (time taken for one half of its nuclei to decay), which, with a measurement of the ratio of radioisotope to a stable isotope (usually the decay product), gives the age. In potassium-argon dating, the ratio of potassium-40 to its stable decay product argon-40 gives ages more than 10 million years. In rubidium-strontium dating, the ratio of rubidium-87 to its stable product strontium-87 gives ages to several thousand million years. In carbon dating, the proportion of carbon-14 (half-life 5730 years) to stable carbon-12 absorbed into once-living matter gives ages to several thousand years.

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radiometric dating

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radioactive dating

radioactive dating See radiometric dating.

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radioactive dating

radioactive dating: see dating.

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Radioactive Dating

Radioactive Dating

In the nineteenth century, prominent scientists such as Charles Lyell, Charles Darwin, Sir William Thomson (Lord Kelvin), and Thomas Huxley, were in continual debate about the age of the earth. The discovery of the radioactive properties of uranium in 1896 by Henri Becquerel subsequently revolutionized the way scientists measured the age of artifacts and supported the theory that Earth was considerably older than what some scientists believed.

There are several methods of determining the actual or relative age of Earths crust: examination of fossil remains of plants and animals, relating the magnetic field of ancient days to the current magnetic field of Earth, and examination of artifacts from past civilizations. However, one of the most widely used and accepted method is radioactive dating. All radioactive dating is based on the fact that a radioactive substance, through its characteristic disintegration, eventually transmutes into a stable nuclide. When the rate of decay of a radioactive substance is known, the age of a specimen can be determined from the relative proportions of the remaining radioactive material and the product of its decay.

In 1907, the American chemist Bertram Boltwood demonstrated that he could determine the age of a rock containing uranium-238 and thereby proved to the scientific community that radioactive dating was a reliable method. Uranium-238, whose half-life is 4.5 billion years, transmutes into lead-206, a stable end-product. Boltwood explained that by studying a rock containing uranium-238, one can determine the age of the rock by measuring the remaining amount of uranium-238 and the relative amount of lead-206. The more lead the rock contains, the older it is.

The long half-life of uranium-238 makes it possible to date only the oldest rocks. This method is not reliable for measuring the age of rocks less than 10 million years old because so little of the uranium will have decayed within that period of time. This method is also very limited because uranium is not found in every old rock. It is rarely found in sedimentary or metamorphic rocks, and is not found in all igneous rocks. Another method for dating the rocks of Earths crust is the rubidium-87/strontium-87 method. Although the half-life of rubidium-87 is even longer than uranium-238 (49 billion years or 10 times the age of Earth), it is useful because it can be found in almost all igneous rocks. Perhaps the best method for dating rocks is the potassium-40/argon-40 method. Potassium is a very common mineral and is found in sedimentary, metamorphic, and igneous rock. Also, the half-life of potassium-40 is only 1.3 billion years, so it can be used to date rocks as young as 50,000 years old.

In 1947, a radioactive dating method for determining the age of organic materials, was developed by Willard Frank Libby, who received the Nobel Prize in chemistry in 1960 for his radiocarbon research. All living plants and animals contain carbon, and while most of the total carbon is carbon-12, a very small amount of the total carbon is radioactive carbon-14. Libby found that the amount of carbon-14 remains constant in a living plant or animal and is in equilibrium with the environment, however once the organism dies, the carbon-14 within it diminishes according to its rate of decay. This is because living organisms utilize carbon from the environment for metabolism. Libby, and his team of researchers, measured the amount of carbon-14 in a piece of acacia wood from an Egyptian tomb dating 2700-2600 BC. Based on the half-life of carbon-14 (5,568 years), Libby predicted that the concentration of carbon-14 would be about 50% of that found in a living tree. His prediction was correct.

Radioactive dating is also used to study the effects of pollution on an environment. Scientists are able to study recent climactic events by measuring the amount of a specific radioactive nuclide that is known to have attached itself to certain particles that have been incorporated into Earths surface. For example, during the 1960s, when many above-ground tests of nuclear weapons occurred, Earth was littered by cesium-137 (half-life of 30.17 years) particle fallout from the nuclear weapons. By collecting samples of sediment, scientists are able to obtain various types of kinetic information based on the concentration of cesium-137 found in the samples. Lead-210, a naturally occurring radionuclide with a half-life of 21.4 years, is also used to obtain kinetic information about Earth. Radium-226, a grandparent of lead-210, decays to radon-222, the radioactive gas that can be found in some basements. Because it is a gas, radon-222 exists in the atmosphere. Radon-222 decays to polonium-218, which attaches to particles in the atmosphere and is consequently rained outfalling into and traveling through streams, rivers, and lakes.

Radioactive dating has proved to be an invaluable tool in many scientific fields, including geology, archeology, paleoclimatology, atmospheric science, oceanography, hydrology, and biomedicine. This method of dating has also been used to study artifacts that have received a great deal of public attention, such as the Shroud of Turin (with highly controversial and disputed results), the Dead Sea Scrolls, Egyptian tombs, and Stonehenge. Since the discovery of radioactive dating, there have been several improvements in the equipment used to measure radioactive residuals in samples. For example, with the invention of accelerator mass spectometry, scientists have been able to date samples very accurately.

See also Radioactive decay.

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Radioactive Dating

Radioactive dating

In the nineteenth century, prominent scientists such as Charles Lyell, Charles Darwin, Sir William Thomson (Lord Kelvin), and Thomas Huxley, were in continual debate about the age of the earth . The discovery of the radioactive properties of uranium in 1896 by Henri Becquerel subsequently revolutionized the way scientists measured the age of artifacts and supported the theory that the earth was considerably older than what some scientists believed.

There are several methods of determining the actual or relative age of the earth's crust: examination of fossil remains of plants and animals, relating the magnetic field of ancient days to the current magnetic field of the earth, and examination of artifacts from past civilizations. However, one of the most widely used and accepted method is radioactive dating. All radioactive dating is based on the fact that a radioactive substance, through its characteristic disintegration, eventually transmutes into a stable nuclide. When the rate of decay of a radioactive substance is known, the age of a specimen can be determined from the relative proportions of the remaining radioactive material and the product of its decay.

In 1907, the American chemist Bertram Boltwood demonstrated that he could determine the age of a rock containing uranium-238 and thereby proved to the scientific community that radioactive dating was a reliable method. Uranium-238, whose half-life is 4.5 billion years, transmutes into lead-206, a stable end-product. Boltwood explained that by studying a rock containing uranium-238, one can determine the age of the rock by measuring the remaining amount of uranium-238 and the relative amount of lead-206. The more lead the rock contains, the older it is.

The long half-life of uranium-238 makes it possible to date only the oldest rocks . This method is not reliable for measuring the age of rocks less than 10 million years old because so little of the uranium will have decayed within that period of time. This method is also very limited because uranium is not found in every old rock. It is rarely found in sedimentary or metamorphic rocks, and is not found in all igneous rocks . Another method for dating the rocks of the earth's crust is the rubidium-87/strontium-87 method. Although the half-life of rubidium-87 is even longer than uranium-238 (49 billion years or 10 times the age of the earth), it is useful because it can be found in almost all igneous rocks. Perhaps the best method for dating rocks is the potassium-40/argon-40 method. Potassium is a very common mineral and is found in sedimentary, metamorphic, and igneous rock. Also, the half-life of potassium-40 is only 1.3 billion years, so it can be used to date rocks as young as 50,000 years old.

In 1947, a radioactive dating method for determining the age of organic materials, was developed by Willard Frank Libby, who received the Nobel Prize in Chemistry in 1960 for his radiocarbon research. All living plants and animals contain carbon , and while most of the total carbon is carbon-12, a very small amount of the total carbon is radioactive carbon-14. Libby found that the amount of carbon-14 remains constant in a living plant or animal and is in equilibrium with the environment, however once the organism dies, the carbon-14 within it diminishes according to its rate of decay. This is because living organisms utilize carbon from the environment for metabolism . Libby, and his team of researchers, measured the amount of carbon-14 in a piece of acacia wood from an Egyptian tomb dating 2700-2600 b.c. Based on the half-life of carbon-14 (5,568 years), Libby predicted that the concentration of carbon-14 would be about 50% of that found in a living tree . His prediction was correct.

Radioactive dating is also used to study the effects of pollution on an environment. Scientists are able to study recent climactic events by measuring the amount of a specific radioactive nuclide that is known to have attached itself to certain particles that have been incorporated into the earth's surface. For example, during the 1960s, when many above-ground tests of nuclear weapons occurred, the earth was littered by cesium-137 (half-life of 30.17 years) particle fallout from the nuclear weapons. By collecting samples of sediment, scientists are able to obtain various types of kinetic information based on the concentration of cesium-137 found in the samples. Lead-210, a naturally occurring radionuclide with a half-life of 21.4 years, is also used to obtain kinetic information about the earth. Radium-226, a grandparent of lead-210, decays to radon-222, the radioactive gas that can be found in some basements. Because it is a gas, radon-222 exists in the atmosphere. Radon-222 decays to polonium-218, which attaches to particles in the atmosphere and is consequently rained out—falling into and traveling through streams, rivers , and lakes.

Radioactive dating has proved to be an invaluable tool and has been used in many scientific fields, including geology , archeology, paleoclimatology, atmospheric science, oceanography , hydrology , and biomedicine. This method of dating has also been used to study artifacts that have received a great deal of public attention, such as the Shroud of Turin, the Dead Sea Scrolls, Egyptian tombs, and Stonehenge. Since the discovery of radioactive dating, there have been several improvements in the equipment used to measure radioactive residuals in samples. For example, with the invention of accelerator mass spectometry, scientists have been able to date samples very accurately.

See also Radioactive decay.

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Radioactive Dating

Radioactive Dating


Radioactive dating is a method of determining the approximate age of an old object by measuring the amount of a known radioactive element it contains. Rocks as well as fossil plants and animals can be dated by this process. It has given paleontologists (a person specializing in the study of fossils) as well as geologists (a person specializing in the study of the origin, history, and structure of Earth) a powerful way of dating ancient objects.

Until the discovery of radioactive dating, scientists had no way of approximating how old any part of Earth was. Once the principle behind this method was discovered, however, it became possible to gather reliable information about the age of Earth and its rocks and fossils. Radioactive dating was not possible until 1896, when the radioactive properties of uranium (a radioactive metallic element) were discovered by French physicist (a person specializing in the study of energy and matter), Antoine Henri Becquerel (1852–1908). When a substance is described as radioactive, it means that at the subatomic (relating to parts of an atom) level, some parts of it are unstable. When a substance is described as unstable, it means that it has a tendency to break down or decay. During this decay, one substance actually changes into another and radiation is released.

As long ago as 1907, the American chemist Bertram B. Boltwood (1870–1927) suggested that knowledge of radioactivity might be used to determine the age of Earth's crust. He suggested this because he knew that the end product of the decay of uranium was a form of lead. Since each radioactive element decays at a known rate, it can be thought of as a ticking clock. Boltwood explained that by studying a rock containing uranium, its age could be determined by measuring its amounts of uranium and lead. The more lead the rock contained, the older it was.

Although this was a major breakthrough, Boltwood's dating method made it possible to date only the oldest rocks. This is because uranium decayed or changed into lead at such a slow rate that it was not reliable for measuring the age of rocks that were younger than 10,000,000 years old. Another drawback was that uranium is not found in every rock. A later method that used rubidium (which changes into strontium) proved more useful because it is found in nearly all rocks, although it still was not useful for younger specimens. Perhaps the best method for rock dating is the potassium-argon method. This method proved useful to date rocks as young as 50,000 years old.

In 1947 another dating breakthrough occurred. The American chemist Willard F. Libby (1908–1980) discovered the radiocarbon method for determining the age of organic materials. Called the carbon-14 dating technique, this ingenious method used the simple knowledge that all living plants and animals contain carbon (a nonmetallic element that occurs in all plants and animals). Libby also knew that while most of this carbon is a common, stable form called carbon-12, a very small amount of the total carbon is radioactive carbon-14. All plants absorb carbon during photosynthesis (the process in which plants use light energy to create food), and animals absorb this carbon by eating plants or eating other animals that ate plants. Libby also found that as long as an organism remains alive, its supply of carbon-14 remains the same. However, once the organism dies, the supply stops and the carbon-14 in its body begins to decrease according to its own rate of decay. Libby realized that this could be a practical dating tool. He eventually designed a device that used Geiger counters (which measure radiation) to accurately measure the amount of carbon-14 left in an organic substance. Libby won the 1960 Nobel Prize in chemistry for his discovery. The discovery allowed him to correctly date a piece of wood from an Egyptian tomb that was known to be about 4,600 years old.

In the last 40 years, radiocarbon dating has been used on more than 100,000 samples in 80 different laboratories. Besides dating plant and animal life, this method has been used to verify the age of such different artifacts as the Dead Sea Scrolls (2,100 years), a charcoal sample from an ancient South Dakota campsite (7,000 years), and a pair of sandals from an Oregon cave (9,300 years). Improvements have raised its accuracy to nearly 70,000 years, with an uncertainty of plus-or-minus 10 percent.

[See alsoFossil; Paleontology ]

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