How old is the dinosaur fossil footprint based on the following information? Consider the following scenario: I am a famous paleontologist and an expert in radioactive-dating techniques. I examine a dinosaur fossil footprint and discover that the fresh mud the dinosaur stepped in had just been covered with a thin layer of volcanic ash. I study the amount of potassium-40 and argon-40 in the ash. The ratio shows that 1/10 of one potassium-40 half-life has passed since the footprint was made. I know that the half-life of potassium-40 is 1.3 billion years. So,

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Well, a half-life is the amount of time it takes for a radioactive substance to decay by half (mass decays into another, more stable substance in series).  So if 1/10 (or 0.1) of the half-life of 1.3 billion years has passed, that's 1.3 X 0.1 = .13 billion years, or 130 million years since the ash was deposited.

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