6.Using only uniformitarian calculations from the thickness of known sedimentary rocks, likely rates at which those rocks accumulated, and features in and under those sedimentary rocks,geologists working two to three hundred years ago estimated that the Earth:
A) Is less than about one-hundred-million years old. B) Is about one-hundred-million years old.C) Is more than about one-hundred-million years old.D) Is 4.6 billion years old.E) Has been here forever.Feedback: Radiometric techniques reveal the Earth to be about 4.6 billion years old, but early geologists did not have the sophisticated instruments to measure the trace radioactive elements and their offspring. Working from the rocks, the geologists knew that the age must be in the neighborhood of 100 million years, plus extra time in unconformities and additional extra time in the oldest, metamorphic rocks. Table for Individual Question Feedback Points Earned:1.0/1.0Correct 7.You are dating a lava flow by the potassium-argon system. However, the offspring in this system are leaking out of the minerals. Which is accurate?
Points Earned:1.0/1.0Correct 8.You start with 200 parent atoms of a particular radioactive type, which decays in a single step to give a stable offspring, and you start with none of those stable offspring. You wait just long enough for two half lives to pass. You should expect to have how many offspring atoms (on average)(remember that the number ofparents and the number of offspring add up to 200, so if you have10 parents, you have 190 offspring because 10 and 190 add up to200, and if you have 20 parents you have 180 offspring, and so on): A) 200.B) 150.C) 100.D) 50.E) 25.Feedback: After one half-life, you’ve gone from 200 parents to 100, and 100 offspring have been made. In the second half-life, you go from 100 to 50 parents, and that makes another 50 offspring. Adding the additional 50 to the 100 from the previous half-life gives 150 offspring. (Typical studies of radioactive decay use many more atoms, to avoid statistical fluctuations, but the question says “on average”, so we asked you about 200 rather than 200,000,000,000,000 to make the math easier.) Table for Individual Question Feedback Points Earned: 0.0/1. 0
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- Fall '08