how would you be able to ascertain the relative ages of several volcanic ash falls exposed in an outcrop?

Answer 1

Carbon Dating!

"Radiocarbon dating is a radiometric dating method that uses the naturally occurring isotope carbon-14 to determine the age of carbonaceous materials up to ca 60,000 years. Within archaeology it is considered an absolute dating technique. The technique was discovered by Willard Frank Libby and his colleagues in 1949 during his tenure as a professor at the University of Chicago. Libby estimated that the steady state radioactivity concentration of exchangeable 14C would be about 14 disintegrations per minute (dpm) per gram carbon (ca. 230 mBq/g). In 1960, Libby was awarded the Nobel Prize in chemistry for his method to use carbon-14 for age determination."

Answer 2

NOT with carbon dating!!! Why would there be any carbon in an ash fall that was from biogenic sources????? Mineral carbon will not have C-14 in it so it can't be used for this.

Having done quite a bit of this type of stratigraphy in Africa:

1) Basic stratigraphy work will tell you the relative ages. If the ash beds are still in their original position, then this is relatively simple. If there has been structural deformation, then you will have to map that out first, finding faults, fault throw, and being cautious that there are not overturned beds. If you are dealing with layer-cake stratigraphy, then the relative age will be younger as you move up the section. Otherwise, it becomes a geometric puzzle that you must put together.

Often an ash bed will appear in one outcrop and not in the same outcrop as other ash beds. To correlate these types of problems, the best approach is to walk out the bed. Follow it as far as you can until you can measure up section to another ash bed. Then walk that bed out, moving across the outcrop as far as you have to find the place where you can measure vertically up-section to the next younger ash bed.

2) Since you are only seeking relative ages, that first step should completely answer your question. If you want to go further, I would recommend K-Ar dating. You will have to dig far enough into the outcrop to find unweathered samples, and should collect multiple samples. This will give a radiometric date once it is analyzed in a lab.

3) You can verify or build on your radiometric date by searching sediment samples above and below the ash beds for fossils. These can give you additional evidence of the age, but are not needed for relative ages.

4)If you really want to be thorough, you can take paleomag samples up and down through the section. These often have resolution of 20-50K years, and based on the larger framework of the ash flow radiometric dates you can be quite precise on the age of deposits. Here's a reference text:
http://www.geo.arizona.edu/Paleomag/book/main.htm

Here are the field geology manuals I recommend:
For a new field geologist this book is the best; this book is in a hard plastic looseleaf binder and is small enough to fit easily in a pack or a pocket:
Freeman, Tom. 1992, Procedures in Field Geology, Columbia, Missouri: Friendship Publications, University of Missouri, Columbia MO 65211 Phone: [phone] (95 pp)

A classic with lots of good map symbol references:
Compton, Robert R., 1985, Geology in the Field, New York: John Wiley & Sons. 398 pp. ISBN: 0471829021

Great little book for sedimentary field geology:
Tucker, Maurice E., 1996, Sedimentary Rocks in the Field, New York: John Wiley & Sons, 153 pp. ISBN: 0471962155

A true collector’s book (yeah I actually collect stuff like this):
Lahee, Frederic H (Chief Geologist of Sun Oil Company)., 1941, Field Geology. New York: McGraw-Hill. 853 pp.

Answer 3

As suggested above you could use radiometric dating - but not carbon.

Radiocarbon is just one of many radiometric dating methods, but is rarely used by geologists. It would be highly unlikely that volcanic ash could be dated by radiocarbon methods because of insufficient carbon, unless there were some plant or animal remains in the ash strata. Also radiocarbon dating is not very good beyond about 20,000 years.

Other radioelement pairs such as U-Pb, K-Ar, and Rb-Sr would be a better choice, and that is called "absolute" dating.

But your question said "relative" - which means determining which layer is older/younger than an adjoining layer. Assuming that the ash strata have not been deformed and overturned, the younger layers will always be above the older layers.

Answer 4

Cheaper way is through stratigraphy. If you know the age of a layer above or below...