Volcanic ash layer dating
They build up over time so that that the layers at the bottom of the pile are older than the ones at the top.
Geologists call this simple observation the Principle of Superposition, and it is most important way of working out the order of rocks in time.
The older, Messinian-age ash bed, sampled at Maccarone, yields a corrected glass fission-track age of 5.38±0.42 Ma. Angelo and at two localities at Bellante, yields corrected glass ages of ∼2.1 Ma, similar to ages previously determined on Pliocene–Pleistocene ash collected along the Ionic coast and on the Tyrrhenian side of Italy.
Once you have an idea how old an ash layer is in an outcrop, that’s one of the tools you can use to help estimate the age of sedimentary layers as well as the age of fossils that may be present in the sedimentary layers.
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 time I was thinking over a long period of time because it helps more with determining a rocks age. dating sediments using volcanic ash layer) is specifically its instantaneity (relatively to geological timescale of course). An example would be the Kawakawa/Oruanui tephra from New Zealand which is a good isochronous marker bed at 26.5 ka, spread over 1500km, but represents probably only a few months of volcanic activity (see e. Manville & Wilson 2006) Important factors vis-à-vis the usefulness of a specific ash layer however are its geographical extent (a volcanic event that will spread an ash layer over a whole basin would be more useful in that it will be used to correlate a large number of sites together) and maybe its volume (you need a minimum amount of material to work with, I'll assume).
Geochronology is the science of dating and determining the time sequence of events in the history of the Earth.
This web page provides an overview of selected geochronology methods used by USGS scientists.
New dating methods are invented all the time, however, most have practical limitations.