Valley Dating On a trip into a valley in Washington State, you find a cliff in w
ID: 288824 • Letter: V
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Valley Dating On a trip into a valley in Washington State, you find a cliff in which there are five different igneous rock layers (A-E) You collect a sample from eachlayer Later back in the laboratory, these five samples are examined to discover the amount of Lead-207 found in the minerals of the rock You know that Lead-207 forms from the radioactive decay of Uranium-235 and that you can work out the age of each of the igneous samples using the decay product curve. The lab results for the amounts of Lead-207 are in the table below. Using the decay curve on the next page, calculate the age of each sample and complete the table Sample Amount Age of Lead-207 ) 35 80 54 Layer E appears to be a feeder pipe that may have brought molten rock up from deep in the Earth's crust Do you think there is any link between L.ayer E and any of the other layers found in this valley, and if so, describe what you think may have happened in the past? The Geoleyical Soriety of mernica, 2003Explanation / Answer
1.
Sample Amount of Pb-207 Age
A 5% 0
B 35% 330 million years
C 55% 830 million years
D 80% 1625 million years
E 54% 796 million years
2. According to the Law of Cross-Cutting Relationships, any igneous body cutting across a series of rocks is younger than the series. So, the Layer E must have formed after the formation of Layers D-B.
3. According to the product decay curve, the half-life of U-235 is 690 million years.
4. For an accurate estimate, try analyzing the isotopic ratios of Rb and Sr, which give a half-life of around 48 billion years.
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