USGS/Cascades Volcano Observatory, Vancouver, Washington
REPORT:
Geology of Medicine Lake Volcano, Northern California, Cascade Range
--
Julie M. Donnelly-Nolan, 1990,
Geology of Medicine Lake Volcano, Northern California, Cascade Range:
Geothermal Resources Council TRANSACTIONS, v.14, Part II, August 1990,
p.1395-1396.
Abstract
Medicine Lake volcano (MLV) is located in an E-W extensional environment on the
Modoc Plateau just east of the main arc of the Cascades. It consists mainly of
mafic lavas, although drillhole data indicate that a larger volume of rhyolite
is present than is indicated by surface mapping. The most recent eruption was
rhyolitic and occurred about 900 years ago. At least seventeen eruptions have
occurred since 12,000 years ago, or between 1 and 2 eruptions per century on
average, although activity appears to be strongly episodic. The calculated
eruptive rate is about 0.6 cubic kilometers per thousand years during the entire
history of the volcano. Drillhole data indicate that the plateau surface
underlying the volcano has been downwarped by 0.5 kilometer under the center of
MLV. The volcano may be even larger than the estimated 600 cubic kilometers,
already the largest volcano by volume in the Cascades.
Introduction
Medicine Lake volcano (MLV) is a Pleistocene and Holocene shield volcano located
in the Cascade Range, east of the main arc and northeast of Mount Shasta. Lavas
from MLV cover about 2000 square kilometers. They range in composition from
basalt through rhyolite and include both tholeiitic and calc-alkaline types.
Basalt and basaltic andesite dominate the lower flanks of MLV. Higher on the
volcano, basaltic lavas are mostly absent, andesite dominates, and high-silica
lavas are present, including the spectacular late Holocene rhyolites of Glass
Mountain and Little Glass Mountain. Volume of the volcano is estimated at 600
cubic kilometers (Donnelly-Nolan, 1988), larger than Mount Shasta which is the
largest of the Cascade stratocones. The highest point on the rim of MLV's
caldera is 7913 feet. Lava flows reach elevations as low as 3360 feet although
most lavas are found above 4100 feet, the approximate elevation of the
surrounding Modoc Plateau. The low shield shape, central caldera, and dominance
of mafic lavas are similar to Newberry Volcano of central Oregon, also located
in an extensional tectonic environment east of the main Cascade arc.
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04/06/00, Lyn Topinka