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geo hw 3

# geo hw 3 - HOMEWORK#3 GEOLOGY 1020 Spring 2008 Use the...

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HOMEWORK #3, GEOLOGY 1020, Spring 2008 Use the graph of 40 K - 40 Ar decay shown below to answer questions 12-14. ( 40 K is the parent, 40 Ar is the daughter) The half-life of the 40 K - 40 Ar decay is 1,250 million years. 0 10 20 30 40 50 60 70 80 90 100 0 0.5 1 1.5 2 2.5 3 3.5 4 4.5 5 5.5 6 Number of Half Lives Elapsed # P a r e n t s R e m a i n i n g % 1. If 25% of the original number of atoms of 40 K in a feldspar crystal have decayed to 40 Ar, what is the age of the sample? A. 312.5 million years B. 500 million years 25% of parent gone means 75% of parent remains. C. 1,250 million years Enter the y-axis at 75%, go out to the curve and then D. 2,500 million years down to the x-axis to note that this means 0.4 half lives have passed. 1,250 million years per half * 0.4 half lives = 500 million yr 2. Based on that age, in what geologic period was the sample formed? A. Pennsylvanian B. Ordovician Lecture images and text (p. 12) show the geologic periods C. Cambrian and the absolute ages of their boundaries. 500 million years D. Precambrian ago was the latest part of the Cambrian 3. What is the age and period of formation of a crystal which contains only 10% of the original 40 K atoms? A. ~125 million years, Cretaceous B. ~250 million years, Permian C. ~4250 million years, Precambrian (Archean eon) 10% of parents remain after 3.4 D. 12,500 million years, Precambrian (Hadean eon) half lives, so 1250 * 3.4 = 4250

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Use the following figure to answer the next three questions. dashes = shale, block pattern = limestone, dots = sandstone, black is basalt
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