Medicine Lake
Introduction
The Medicine Lake Shield volcano is located in the rain-shadow north-east-east of Mt. Shasta (Latitude: 41.58 N, Longitude: 121.57 W). While it is still considered part of the Cascade chain of volcanoes, most of the others are stratovolcanoes, having steeper slopes and a higher elevation. The region is filled with remnants of past eruptions, which include Lava Beds National Monument, Glass Mountain, numerous smaller cinder cones, and Medicine Lake itself, from which Medicine Lake volcano derives its name.
Medicine Lake Shield Volcano
The Medicine Lake shield volcano is the largest volume volcano in the Cascade range (Ritter and Evans, 1997). This is a bit surprising since it practically sits in the shadow of Mt. Shasta and is significantly smaller in elevation; Mt. Shasta is 4,317 meters above sea level while the Medicine Lake shield volcano is only 2,412 meters. But the Medicine Lake shield volcano has broad, gentle slopes built from the eruption of basalt that flows easily across the ground while Mt. Shasta is a stratovolcano, also called a composite volcano, formed by alternating layers of pyroclastics and lava that have accumulated over thousands of years to form a tall cone. The total area of the flows from the Medicine Lake shield volcano is much greater than those of the stratovolcanoes of the Cascade range, including Mt. Shasta.
Lava flows from the Medicine Lake volcano cover an area of approximately 2000 km2, approximately 40 km east to west and 50 north to south (Ritter and Evans, 1997). The flows are approximately one kilometer thick. The flows are composed chiefly of rhyolitic, basaltic, basaltic andesitic, and andisitic rock.
Most eruptions initially occurred during the Pleistocene (1.8 million years ago to 10,000 years ago) and later in the the Holocene (10,000 years ago to present) with the most recent eruptions, forming Glass Mountain, occurring approximately 1000 years ago (Ritter and Evans, 1997). The volcano rests on top of a Tertiary and early Quaternary volcanic plateau (Condie and Hayslip, 1974).
Looking at a geologic map of the region, one can see the vast flows emanating from the caldera rim, made up of the chief compounds described above. The most notable features when looking at the region are the 100+ Quaternary and Pliocene cinder cones that formed in the region. A cinder cone is a steep sided volcano (sometimes 40 degrees) and usually not more than a few 100 ft high. These cinder cones were formed from pyroclastics (rock fragments or congealed lava) spewed from a single vent in the earth. The fragments fall around the base of the vent, forming the cone shaped mound. Many of these cinder cones are now covered in vegetation, but the shape and size provide a clue to their identity.
Medicine Lake
Medicine Lake sits in the caldera, the depression near the eruption site. The lakes has an area of 1.65 square kilometers, a volume of 10,860 acre feet, a maximum depth of 46 meters and a median depth of 5 meters. The lake bed is composed of relatively course rhyolitic material producing a high pitch scraping sound when trampled.
The rim of the caldera is roughly 7 x 12 kilometers. It is speculated that no single eruption caused the formation of the caldera, but rather it collapsed as a result of a series of lava extrusions early in the volcano's history.
The lake has cultural significance to several Native American tribes. For example, the Pit River people believe the Creator's spirit is in the lake, providing powers to heal and renew oneself. For this reason, the area has been a training ground for medicine men. Although it has been designated as a Traditional Cultural District, this is being threatened by the construction of a geothermal plant that is to be constructed one mile from the lake, an initiative approved by the Bush administration.
Little Glass Mountain
The eruption that created Little Glass Mountain occurred less than 1000 years ago, just west of the caldera rim of the Medicine Lake shield volcano. The flows are composed mainly of rhyolite and dacite, which include extremely large obsidian, pumice, and olivine boulders. Many of the olivine specimens have many pore spaces that are produced by gasses that escape as the lava cools. Obsidian has a a glassy texture, and it has cooled quickly preventing the formation of crystals. Pumice, an ultra light porous glass which can sometimes float in water (Wyckoff, 1999).
Because of the composition of rocks in the area, one needs to be careful while hiking around this flow. Obsidian can be very sharp. One slip could quite easily produce a knife like cut. Because of this property, obsidian is one of the main rock types used by Native Americans in the creation of arrow heads and stone tools. Little Glass Mountain is a rich source of obsidian, and it was likely exploited by many groups in the region.
Sources
- Condie, Kent C., Hayslip, Dave L. (1975). Young bimodal volcanism at Medicine Lake volcanic center, northern California, Geochimica et Cosmochimica, Vol. 39, pp. 1165-1178, 1975.
- Dzurisin, et.al., 1991, Crustal Subsidence, Seismicity, and Structure Near Medicine Lake Volcano, California: Journal of Geophysical Research, v.96, no.B10.
- Ritter, Joachim R.R., Evans, John R. (1997). Deep Structures of Medicine Lake volcano, California, Tectonophysics, Vol. 275, pp. 221-241, 1997.
- Wicander, Reed, Monroe, James S. (1989). Historical Geology: Evolution of the Earth and Life Through Time, Revised Edition. West Publishing Company.
- Wyckoff, Jerome (1999). Reading the Earth: Landforms in the Making. Adastra West Inc. Publishers.