USING RADIOISOTOPES TO DATE ROCKS
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Dating Rocks using Uranium Decay |
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The picture below shows how radioactive Uranium-238 atoms decay in a series of seven steps to stable lead atoms. |
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Uranium isotopes decay extremely slowly (long half-life) to stable isotopes of lead. Uranium-235 has a half-life of 700 million years. Uranium-238 has a half-life of 4.4 billion years. The relative amounts of Uranium and lead in an igneous rock can therefore be used to estimate the age of the rock. |
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Table showing the relative amounts of Uranium-238 and stable lead over time. |
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Time = Zero |
After 1 Half-life |
After 2 Half-lives |
After 3 Half-lives |
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Amount of Uranium-238 |
100 % |
50 % |
25 % |
12.5 % |
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Amount of Stable Lead |
0 % |
50 % |
75 % |
87.5 % |
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Ratio of Uranium : Lead |
1 : 0 |
1 : 1 |
1 : 3 |
1 : 7 |
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If a specimen of rock is found to have a ratio of Uranium to lead of 1 : 3 it means that a time of two half-lives of Uranium-238 have elapsed since the rock's formation. The age of the rock would therefore be 2 x 4.4 billion years = 8.8 billion years old. |
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The decay of the radioisotope Potassium-40 can be used to date rocks. Potassium-40 decays into Argon-40, which is a stable gas. To get an accurate result, however, no argon-40 gas must escape from the rock in the years following its formation. |
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Dating Materials using Carbon Decay |
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The picture below shows the cycle of the radioisotope carbon-14. |
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Carbon-14 has a half-life of 5730 years. It is found in all living material. When a plant, animal or human dies the Carbon-14 inside decays. By finding the ratio of Carbon-14 to stable Nitrogen-14 in a dead plant, animal or human the age of the specimen can be calculated. |
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Using the decay of Carbon-14 the Shroud of Turin (pictured below) was estimated to be only 600 years old. It could not have been the shroud that covered Jesus Christ. |
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