Why would you not use the uranium-lead isotope pair method to determine the age of rock that is less than 1 million years old?

Answer 1

Because it won't work very well. The radiometric dating methods associated with uranium-lead cannot measure changes as "short" as 1 million years with much accuracy. There won't have been much radioactive decay of uranium in "just" 1 million years to make the test a good one and allow us to do an accurate assessment. Oh, we can count atoms fairly accurately. But remember that a half-life is the amount of time it takes half an amount of an unstable nuclide to decay, and the smaller the sample of the unstable nuclide, the "less accurate" the half-life figure is. Let's look. A whole bunch of radioactive material, 120Wm has a half life of 1 year. (Yes, that's a made-up element.) That means in a year, half of the material has decayed. But if you have just two atoms at the start, they could both be gone in a month, or they could both still be here two or three years from now. You just don't know. Half-lives are based on "gross quantities" of material. And because the amounts of material we trace are so small, accuracy falls of due to the (small) quantity of the counted material faster than the accuracy falls off for our ability to count it. Perhaps a rough comparison would be like using a stop watch to measure the speed of a bullet. The stop watch can clock us running the 100 meters pretty well, but it is "slower" than the bullet by quite a bit, and it cannot help determine the speed of the projectile with much accuracy.