The narrower a range of time that an animal lived, the better it is as an index of a specific time.
No bones about it, fossils are important age markers.
Unlike people, you can’t really guess the age of a rock from looking at it.
Yet, you’ve heard the news: Earth is 4.6 billion years old. That corn cob found in an ancient Native American fire pit is 1,000 years old. Geologic age dating—assigning an age to materials—is an entire discipline of its own.
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).
New Zealand Journal of Geology and Geophysics, 47: 525-547.
Are there repairs or cracks in the sidewalk that came after the sidewalk was built?What’s more, if the whole rock is badly weathered, it will be hard to find an intact mineral grain containing radioactive isotopes.You might have noticed that many of the oldest age dates come from a mineral called zircon.Pretty obvious that the dike came after the rocks it cuts through, right?With absolute age dating, you get a real age in actual years.
Are there repairs or cracks in the sidewalk that came after the sidewalk was built?
What’s more, if the whole rock is badly weathered, it will be hard to find an intact mineral grain containing radioactive isotopes.
You might have noticed that many of the oldest age dates come from a mineral called zircon.
Pretty obvious that the dike came after the rocks it cuts through, right?
With absolute age dating, you get a real age in actual years.
To determine the relative age of different rocks, geologists start with the assumption that unless something has happened, in a sequence of sedimentary rock layers, the newer rock layers will be on top of older ones. This rule is common sense, but it serves as a powerful reference point.