Re: How did scientists determine that the oldest ash deposits around Mt.

The ages of rocks can be determined in a number of ways, as you may know.  
In this case, though, the ages are determined by the layers of sediment 
that underlie and overlie the volcanic rocks.  According to a summary at 
the Volcano World web site:
 http://volcano.und.nodak.edu/vwdocs/msh/llc/cs/eh.html

"The first stratigraphic evidence of the existence of Mount St. Helens 
consists of voluminous dacitic deposits of slightly
vesicular to pumiceous air-fall tephra and pyroclastic flows, and at least 
one pumice-bearing lahar. These deposits overlie
extensively weathered glacial drift formed during the next-to-last alpine 
glaciation of the Cascade Range. The volcanic
deposits were formed during at least four episodes, separated by intervals 
during which very weak soils developed. The
entire eruptive period may have extended over a time span as long as 5,000 
yr."

This summary is taken from a book on Mt St Helens by the USGS:

Mullineaux, D.R., and Crandell, D.R., 1981, The Eruptive History of Mount 
St. Helens, in Lipman, P.W., and Mullineaux,
D.R., (eds.), The 1980 eruptions of Mount St. Helens, Washington, U.S. 
Geological Survey Professional Paper 1250, p.
3-15.

From this quote, you can see that the earliest deposits from St Helens 
(known as the Ape Canyon eruptive period) lie on top on top of glacial 
deposits whose age is known from the region.  I do not know exactly how the 
age of these glacial deposits is known, but they are too old for effective 
radiocarbon dating. The volcanic rocks must be younger than the glacial 
sediments they sit on top of.  The Ape Canyon deposits have soils developed 
on them at several different levels which makes it possible to estimate how 
long they were lying exposed to the weather before another eruption or 
another source of sediment such as glaciation covered them up.  I do not 
know if any radiometric dates have been obtained from these volcanic units. 
 Radioactive potassium in the minerals in the rock would break down into 
argon gas, which can be measured to determine an age.  It has only been 
possible to accurately date very young rocks, such as these for about 
fifteen years, so they may or may not have been done.

Dave Smith