Have any individual North American volcanoes shown geochemical trends
over time?
Dee Edwards
Dear Dee,
The answer to your question depends on the time frame you select. For
example, the climactic eruption of Mount Mazama about 6,845 year ago was
the immediate precursor to the formation of the Crater Lake caldera and
produced a widespread ash fall deposit called the Wineglass Welded Tuff.
The tuff is a spectacular example of a compositionally zoned volcanic
deposit. At its base the tuff is rhyodacite (70% SiO2). The middle
layers of the tuff are andesite (<61% SiO2). Mafic scoria (about 49%
SiO2) caps the deposit. The compositional zonation also shows in the
color of the tuff, which changes from buff to gray to black from base to
top. The compositional zonation of the deposit reflects the
progressively deeper tapping of a stratified magma chamber with
rhyodacite on top and mafic magma at the base. The eruption that
produced the Wineglass Tuff probably lasted days to weeks.
On a much longer time scale, Condie and Swenson (1973, Bulletin
volcanologique, v. 37, p. 205-230) did not find systematic compositional
trends in lavas as a function of stratigraphic height (over time) for
Mounts Jefferson, Rainier, and Shasta in the Cascades. Portions of the
layers of volcanic rocks do exhibit compositional coherency and probably
represent batches of magma erupted during short intervals compared to the
lifetime of the volcano.
In general, the Cascade stratovolcanoes do have pronounced long term
chemical zonation in that many start out as basaltic shield volcanoes.
Continuing eruptions build a typical stratovolcano with more silicic
composition and often the final stage of activity is a highly silicic
plug or dome. Examples of such chemical evolution are Mt. Baker which
has fluid basal lava (including basalts) but the main cone is andesite.
In Oregon, Mt. Hood is largely andesite, but the eruptions of the last
15,00 years are dacites.
We see by your email address that you are at the Open University. Peter
Francis and other volcanologists there can provide a lot more information
on this topic!
Sorry for the long delay in answering your question.
Steve Mattox and Chuck Wood, University of North Dakota
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