Many unconformities exist in the walls of the Canyon, including the one below the Precambrian sediments and the one above those sediments. The idea of the river narrowing over time was the hypothesis that an interested tourist presented to one of the professors and a ranger at the Canyon a few years ago. When the professor asked whether the tourist would want to go out on a narrow point with a jackhammer, the tourist said no, because the rocks might fall off and slide down into the Canyon. When the professor pointed out the many places that rocks had fallen off and slid down, the quick-witted tourist figured out that the Canyon has been widened by such rockfalls as the river has cut downward. , Points Earned: , 1/1 Correct Answer: , B Your Response: , B 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. 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. , Points Earned: , 1/1 Correct Answer: , C Your Response: , C 7. , One practical radioactive system used to date lava flows involves: , A. , The solid potassium-40, which decays to the gas argon-40. B. , The gas argon-40, which decays to solid potassium-40. C. , The solid potassium-40, which decays to the solid grahamspanierum-41. D. , The gas argon-40, which decays to the gas potassium-40. E. , The solid potassium-40, which decays to solid argon-40. Potassium-40 is common in solid minerals, and decays to produce the gas argon-40. And despite his great contributions to humanity, no one has named an isotope after Penn State’s president. , Points Earned: , 1/1 Correct Answer: , A Your Response: , A 8., You start with 400 parent atoms of a particular radioactive type, which decays to give stable offspring. You wait just long enough for three half lives to pass. You should expect to have how many parent atoms remaining (on average):, A., 400. After one half-life, you’ve gone from 400 parents to 200; after a second half-life you go from 200 parents to 100, and after a third half-life you go from 100 parents to 50. (Typical studies of radioactive decay use many more atoms, to avoid statistical fluctuations, but the question says “on average”, so we asked you about 400 rather than 400,000,000,000,000 to make the math easier.) , Points Earned: , 1/1 Correct Answer: , E Your Response: , E
9. , In age dating, geologists use: , A. , Cross-cutting relationships for absolute ages, and uniformitarianism for relative ages.