Determining a rock age using volcanic ash layer?

Question

I wanted to know if volcanic ash deposits found in the geologic record are most useful in correlating the age of rock layers if the volcanic ash was distributed over a large area during a short period of time or large area during a long period of timeI was thinking over a long period of time because it helps more with determining a rocks age.

Answer 1

Age is best determined when an ash layer is deposited over a large area in a short amount of time to ensure that Zircon populations in the deposit are homogeneous for U/Pb dating. If radiometric dating is not available, shorter events can be used as lithostratigraphic marker beds and dated using biostratigraphy. A famous example of such an ash bed is the "X" Bentonite from the Graneros Shale (Cretaceous Western Interior Seaway).