|MadSci Network: Earth Sciences|
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
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