Radiocarbon dating is used to find the age of once-living materials between 100 and 50,000 years old. This range is especially useful for determining ages of human fossils and habitation sites (Figure 1.1). The atmosphere contains three isotopes of carbon: carbon-12, carbon-13 and carbon-14. Only carbon-14 is radioac- tive; it has a half-life of 5,730 years. The amount of carbon-14 in the atmosphere is tiny and has been relatively stable through time. Plants remove all three isotopes of carbon from the atmosphere during photosynthesis. Animals consume this carbon when they eat plants or other animals that have eaten plants. After the organisms death, the carbon-14 decays to stable nitrogen-14 by releasing a beta particle. The nitrogen atoms are lost to the atmosphere, but the amount of carbon-14 that has decayed can be estimated by measuring the proportion of radioactive carbon-14 to stable carbon- 12. As time passes, the amount of carbon-14 decreases relative to the amount of carbon-12. Carbon isotopes from the black material in these cave paintings places their cre- ating at about 26,000 to 27,000 years BP (before present).
Potassium-40 decays to argon-40 with a half-life of 1.26 billion years. Argon is a gas so it can escape from molten magma, meaning that any argon that is found in an igneous crystal probably formed as a result of the decay of potassium-40. Measuring the ratio of potassium-40 to argon-40 yields a good estimate of the age of that crystal. Potassium is common in many minerals, such as feldspar, mica, and amphibole. With its half-life, the technique is used to date rocks from 100,000 years to over a billion years old. The technique has been useful for dating fairly young geological materials and deposits containing the bones of human ancestors.
radiometric dating of rocks
Radiometric dating is the process of using the concentrations of radioactive substances and daughter products to estimate the age of a material. Different isotopes are used to date materials of different ages. Using more than one isotope helps scientists to check the accuracy of the ages that they calculate.
using radiometric ages to date other materials
As youve learned, radiometric dating can only be done on certain materials. But these important numbers can still be used to get the ages of other materials! How would you do this? One way is to constrain a material that cannot be dated by one or more that can. For example, if sedimentary rock A is below volcanic rock B and the age of volcanic rock B is 2.0 million years, then you know that sedimentary rock A is older than 2.0 million years. If sedimentary rock A is above volcanic rock C and its age is 2.5 million years then you know that sedimentary rock A is between 2.0 and 2.5 million years. In this way, geologists can figure out the approximate ages of many different rock formations.
limitations of radiometric dating
Radiometric dating is a very useful tool for dating geological materials but it does have limits: 1. The material being dated must have measurable amounts of the parent and/or the daughter isotopes. Ideally, different radiometric techniques are used to date the same sample; if the calculated ages agree, they are thought to be accurate. 2. Radiometric dating is not very useful for determining the age of sedimentary rocks. To estimate the age of a sedimentary rock, geologists find nearby igneous rocks that can be dated and use relative dating to constrain the age of the sedimentary rock.
Two uranium isotopes are used for radiometric dating. Uranium-238 decays to lead-206 with a half-life of 4.47 billion years. Uranium-235 decays to form lead-207 with a half-life of 704 million years. Uranium-lead dating is usually performed on zircon crystals (Figure 1.2). When zircon forms in an igneous rock, the crystals readily accept atoms of uranium but reject atoms of lead. 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 is useful for dating igneous rocks from 1 million years to around 4.6 billion years old. Zircon crystals from Australia are 4.4 billion years old, among the oldest rocks on the planet.
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radiometric dating is used to estimate the age of a material by
a) counting the amount of parent isotope. b) counting the amount of daughter isotope. c) knowing or calculating the half life of parent to daughter. --> d) all of these.
radiometric dating only works for materials of these ages because
a) young materials; the amount of parent isotope gets too low to count. b) old materials; the amount of daughter isotope gets too low to count. --> c) all ages; different isotope pairs have different half lives. d) none of these.
radiometric dating uses the rate of decay of unstable isotopes to calculate the absolute ages of fossils and rocks.
--> a) true b) false
carbon-14 dating is used for dating human remains and artifacts because
a) the half life is right for dating materials of those ages. b) human fossils and materials earlier humans used often contain carbon. c) the method is useful for materials that are between 100 and 50,000 years old. --> d) all of these.
carbon-14 decays to carbon-12 with a half-life of 5,730 years.
a) true --> b) false
carbon dating measures when the organism died because only then does
a) carbon -14 decay to carbon-12. --> b) carbon-14 decay to nitrogen-14 c) carbon-12 decay to carbon-14. d) none of these.
in potassium-argon dating
--> a) potassium-40 decays to argon-40 b) the parent isotope as a half-life of 1.26 million years c) the technique can be used only in organic materials d) all of the above
which is a limitation of radiometric dating?
a) radiometric dating can be done only on sedimentary rock and plant fossils. b) there must be carbon-12 in the material. --> c) both the amount of parent and daughter must be measureable. d) all of the above.
potassium-argon and uranium-lead dating are useful for very young materials.
a) true --> b) false
uranium-lead has been used to calculate the ages of zircon crystals that are 4.4 billion years old.
--> a) true b) false
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