Medicine Lake Volcano

Medicine Lake Volcano
Medicine Lake volcano as seen fromLava Beds National Monument
Highest point
Elevation	7,921 ft (2,414 m)NAVD 88
Coordinates	41°36′39″N121°33′13″W
Geography
Location	Siskiyou County, California,U.S.
Parent range	Cascade Range
Topo map	USGSMedicine Lake
Geology
Age of rock	about 500,000 years
Mountain type	Shield volcano
Volcanic arc	Cascade Volcanic Arc
Last eruption	1080 ± 25 years

Medicine Lake
The lake with Mount Shasta in the background
Medicine Lake
Bathymetric map
Location	California
Coordinates	41°34′54″N121°35′56″W
Basincountries	United States
Max. length	0.6 mi (1 km)
Max. width	1.2 mi (2 km)
Surface area	0.64 sq mi (1.65 km2)
Average depth	24.0 ft (7.3 m)
Max. depth	152 ft (46.4 m)
Water volume	470,000,000 cu ft (13,400,000 m3)
Shore length1	20,384 ft (6,213 m)
References
1Shore length isnot a well-defined measure.

Medicine Lake Volcano, California, September 27, 2017.Sentinel-2true-colorsatellite image,scale 1:50,000.

Medicine Lake Volcanois a largeshield volcanoin northeasternCaliforniaabout 30 mi (50 km) northeast ofMount Shasta.Thevolcanois located in a zone of east-west crustal extension east of the main axis of theCascade Volcanic Arcand theCascade 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 km²). The underlying rock has downwarped by 0.3 mi (0.5 km) under the center of the volcano. The volcano is primarily composed ofbasaltandbasaltic andesitelava flows, and has a 4.3 by 7.5 mi (7 by 12 km)calderaat the center.

The Medicine Lake shield rises about 3,900 ft (1,200 m) above theModoc Plateauto an elevation of 7,795 ft (2,376 m).Lavasfrom Medicine Lake Volcano are estimated to be at least 140 cu mi (600 km³) involume,making Medicine Lake the largest volcano by volume in the Cascade Range (Newberry Volcanoin Oregon has the second largest volume).Lava Beds National Monumentlies on the northeast flank of the volcano.

Medicine Lake Volcano has been active for 500,000 years. The eruptions were gentle rather than explosive likeMount 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

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 ofandesitewas erupted from vents along the caldera rim. The distribution of latePleistocenevents, 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 producedashflowtuffoccurred 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 mostlymaficlava beginning early in the history of the volcano (perhaps in a manner similar to the formation ofKilaueacaldera inHawaii). Several small differentiated magma bodies may have been fed by and interspersed among a network ofdikesandsills.LateHoloceneandesitic to rhyolitic lavas were derived by fractionation, assimilation, and mi xing from high alumina basalt parental magma. The small lake from which Medicine Lake volcano derives its name lies within the central caldera.

Eruptive history

Early history

Medicine Lake Volcano began to grow about one million years ago inPleistocenetime, following the eruption of a large volume of tholeiitic high-aluminabasalt. Similar high-alumina basalt has continued to erupt around the volcano throughout its history. Althoughmaficlavas predominate on the volcano's flanks, all lava compositions from basalt torhyolitehave 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 km³) 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 km³) of lava ranging in composition from basalt to rhyolite. Late Holocene lava compositions include basalt and andesite, butsiliciclavas dominate.

Eruptive activity duringHolocenetime 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

Glass Mountain from Medicine Lake caldera rim. USGS photo by Julie Donnelly-Nolan.

The most recent eruption occurred around 1,000 years ago whenrhyoliteanddaciteerupted 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.No field evidence has been found to substantiate the 1910 eruption.

Glass Mountain consists of a spectacular, nearlytreeless,steep-sided rhyolite and daciteobsidianflow 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. Aradiocarbon datingage of 885±40 years before present (1990) was obtained on a deadincense-cedartree 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. Thetephradeposits 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 andLassen Peakdata.

See also

• Volcanic Legacy Scenic Byway

References

• Adapted fromUSGS textassumed to be in the public domain.
• Donnelly-Nolan, J.M. (1990)."Geology of Medicine Lake Volcano, Northern California, Cascade Range"(PDF).Transactions.14 Part II. Geothermal Resources Council: 1395–1396.

External links

• "Medicine Lake Vicinity, California, Lava Beds National Monument".Cascades Volcano ObservatoryUSGS.Retrieved2010-05-22.
• "Losing Sacred Ground".Sacred Land Film Project. Archived fromthe originalon 2011-07-18.Retrieved2010-05-22.