Shield volcano in northeastern California, United States
Medicine Lake Volcano
is a large
shield volcano
in northeastern
California
about 30 mi (50 km) northeast of
Mount Shasta
. The
volcano
is located in a zone of east-west crustal extension east of the main axis of the
Cascade Volcanic Arc
and the
Cascade Range
. The 0.6 mi (1 km) thick shield is 22 mi (35 km) from east to west and 28 to 31 mi (45 to 50 km) from north to south, and covers more than 770 sq mi (2,000 km
2
). The underlying rock has downwarped by 0.3 mi (0.5 km) under the center of the volcano. The volcano is primarily composed of
basalt
and
basaltic andesite
lava flows, and has a 4.3 by 7.5 mi (7 by 12 km)
caldera
at the center.
The Medicine Lake shield rises about 3,900 ft (1,200 m) above the
Modoc Plateau
to an elevation of 7,795 ft (2,376 m).
Lavas
from Medicine Lake Volcano are estimated to be at least 140 cu mi (600 km
3
) in
volume
, making Medicine Lake the largest volcano by volume in the Cascade Range (
Newberry Volcano
in Oregon has the second largest volume).
Lava Beds National Monument
lies on the northeast flank of the volcano.
Medicine Lake Volcano has been active for 500,000 years. The eruptions were gentle rather than explosive like
Mount St. Helens
, coating the volcano's sides with flow after flow of basaltic lava. Medicine Lake is part of the old caldera, a bowl-shaped depression in the mountain. It is believed that the Medicine Lake volcano is unique, having many small magma chambers rather than one large one.
Caldera
[
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]
Medicine Lake is in the caldera of the volcano, which measures 4.3 by 7.5 mi (7 by 12 km). The caldera may have formed by collapse after a large volume of
andesite
was erupted from vents along the caldera rim. The distribution of late
Pleistocene
vents, mostly concentrated along the rim, suggests that ring faults already existed when most of the andesite erupted. No single large eruption has been related to caldera formation. The only eruption recognized to have produced
ash
flow
tuff
occurred in late Pleistocene time, and this eruption was too small to account for formation of the caldera. Later conclusions were that Medicine Lake caldera formed by collapse in response to repeated extrusions of mostly
mafic
lava beginning early in the history of the volcano (perhaps in a manner similar to the formation of
Kilauea
caldera in
Hawaii
). Several small differentiated magma bodies may have been fed by and interspersed among a network of
dikes
and
sills
. Late
Holocene
andesitic to rhyolitic lavas were derived by fractionation, assimilation, and mixing from high alumina basalt parental magma. The small lake from which Medicine Lake volcano derives its name lies within the central caldera.
Eruptive history
[
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]
Early history
[
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]
Medicine Lake Volcano began to grow about one million years ago in
Pleistocene
time, following the eruption of a large volume of tholeiitic high-
alumina
basalt. Similar high-alumina basalt has continued to erupt around the volcano throughout its history. Although
mafic
lavas predominate on the volcano's flanks, all lava compositions from basalt to
rhyolite
have erupted during Pleistocene time. The lower flanks consist of mostly basaltic and some andesitic lavas. Basalt is mostly absent at higher elevation, where andesite dominates and rhyolite and small volumes of dacite are present. During the past 11,000 years, eruptive activity at Medicine Lake Volcano has been episodic. Eight eruptions produced about 1.3 cu mi (5.3 km
3
) of basaltic lava during a time interval of a few hundred years about 10,500 years ago. That eruptive episode was followed by a hiatus that ended with a small andesitic eruption about 4,300 years ago. During the most recent eruptive episode between 3000 and 900 years ago, eight eruptions produced approximately 0.6 cu mi (2.5 km
3
) of lava ranging in composition from basalt to rhyolite. Late Holocene lava compositions include basalt and andesite, but
silicic
lavas dominate.
Eruptive activity during
Holocene
time has included numerous rhyolite and dacite lava flows erupted at high elevations inside and outside the caldera; cinder cones and associated lava flows of basalt and basaltic andesite have resulted from eruptions at vents on the flanks of the shield. Most vents are aligned along zones of crustal weakness that trend northeast to northwest.
Glass Mountain
[
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]
The most recent eruption occurred around 1,000 years ago when
rhyolite
and
dacite
erupted at Glass Mountain and associated vents near the caldera's eastern rim. Fitch cites reports that a light ash fall that occurred in 1910 may have come from a small eruption at Glass Mountain.
[6]
[7]
No field evidence has been found to substantiate the 1910 eruption.
[8]
Glass Mountain consists of a spectacular, nearly
treeless
, steep-sided rhyolite and dacite
obsidian
flow that erupted just outside the eastern caldera rim and flowed down the steep eastern flank of Medicine Lake Volcano. Ten additional small domes of Glass Mountain rhyolite and rhyodacite lava lie on a N25degreesW trend to the north and one to the south. The age of Glass Mountain and its preceding pumice deposits has been a matter of discussion for some time. A
radiocarbon dating
age of 885±40 years before present (1990) was obtained on a dead
incense-cedar
tree without limbs or bark that is preserved in the edge of one of the distal tongues of the flow. The dated material consisted of a piece of exterior wood containing about 30 annual growth rings. This age may be too old, because some of the outside of the tree is missing. The
tephra
deposits that precede the flow and domes may be somewhat older but are constrained to be less than about 1,050 years before present (1990) by the Little Glass Mountain and
Lassen Peak
data.
See also
[
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]
References
[
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]
- ^
a
b
"Hoffman"
.
NGS Data Sheet
.
National Geodetic Survey
,
National Oceanic and Atmospheric Administration
,
United States Department of Commerce
. Retrieved
2009-08-08
.
- ^
a
b
"Medicine Lake Shield Volcano: The Sleeping Giant"
.
Lava Beds National Monument
.
National Park Service
. Retrieved
2008-09-22
.
- ^
a
b
c
d
Julie M. Donnelly-Nolan; Manuel Nathenson; Duane E. Champion; David W. Ramsey; Jacob B. Lowenstern; John W. Ewert (2007).
Volcano Hazards Assessment for Medicine Lake Volcano, Northern California - Scientific Investigations Report 2007?5174-A
(PDF)
(Report).
United States Geological Survey
. Retrieved
2008-09-25
.
- ^
"Medicine Lake"
.
Global Volcanism Program
.
Smithsonian Institution
. Retrieved
2008-08-25
.
- ^
a
b
c
d
e
f
Jonathan R. Childs; Jacob B. Lowenstern; R. Lawrence Phillips; Patrick Hart; James J. Rytuba; John A. Barron; Scott W. Starratt; Sarah Spaulding (2000).
Bathymetric, geophysical and geologic sample data from Medicine Lake, Siskiyou County, northern California - Open File Report OF 00-043
(Report). Version 2.0.
United States Geological Survey
. Retrieved
2008-09-25
.
- ^
Fitch, R. H. (1928). "The Lassen Report: No. 14".
The Volcano Letter
.
161
: 1.
- ^
Macdonald, Gordon A. (1966).
"Geology of the Cascade Range and Modoc Plateau"
.
Geology of Northern California
.
190
. USGS: 65?95
. Retrieved
July 27,
2015
.
- ^
Wood, Charles A; Kienle, Jurgen, eds. (1992).
Volcanoes of North America: United States and Canada
. Cambridge University Press. p. 212.
ISBN
9780521438117
.
External links
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