USGS/Cascades Volcano Observatory, Vancouver, Washington
REPORT:
Post-11,000-Year Volcanism
at Medicine Lake Volcano, Cascade Range, Northern California
--
Julie M. Donnelly-Nolan, Duane E. Champion
C. Dan Miller, Timothy L. Grove, and Deborah A. Trimble, 1990,
Post-11,000-Year Volcanism
at Medicine Lake Volcano, Cascade Range, Northern California:
Journal of Geophysical Research, v.95, p.19,693-19,704
Abstract
Eruptive activity during the past 11,000 years at Medicine Lake
volcano has been episodic. Eight eruptions produced about 5.3 km3 of
basaltic lava during an interval of a few hundred years about 10,500 yr
B.P. After a hiatus of about 6000 years, eruptive activity resumed with a
small andesite eruption at about 4300 yr B.P. Approximately 2.5 km3 of
lava with compositions ranging from basalt to rhyolite vented in nine
eruptions during an interval of about 3400 years in late Holocene time.
The most recent eruption occurred about 900 yr B.P. A compositional gap in
SiO2 values of erupted lavas occurs between 58 and 63%. The gap is spanned
by chilled magmatic inclusions in late Holocene silicic lavas. Late
Holocene andesitic to rhyolitic lavas were probably derived by
fractionation, assimilation, and mixing from high-alumina basalt parental
magma, possibly from basalt intruded into the volcano during the early
mafic episode. Many basaltic to andesitic lavas contain iron-rich crystals
and have high FeO*/MgO--characteristics caused by mixing of high-alumina
basalt with ferrobasalt liquid produced by fractionation of parental
high-alumina basalt. When ferrobasalt and high-alumina basalt are
contaminated with a granitic crustal component, a calc-alkaline trend is
produced. Some eruptions have produced both tholeiitic and calc-alkaline
compositions. The eruptive activity is probably driven by intrusions of
basalt that occur during east-west stretching of the crust in an
extensional tectonic environment. Vents are typically aligned parallel or
subparallel to major structural features, most commonly within 30 degrees of
north. Intruded magma should provide adequate heat for commercial
geothermal development if sufficient fluids can be found. The nature and
timing of future volcanic activity cannot be predicted from the observed
pattern, but eruptions high on the edifice could produce high-silica
products that might be accompanied by explosive activity, whereas eruptions
lower on the flanks are likely to vent more fluid mafic lavas.
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06/06/01, Lyn Topinka