Medicine Lake Highlands
- Excerpted from USGS and USFS Publications
The Medicine Lake Highlands were formed with the development of a broad shield-shaped volcano. The center block collapsed along fractured lines, creating an enclosed basin or caldera, 6 miles long by 4 miles wide. Lava then squeezed up the fracture lines forming rim volcanoes. The volcanoes discharged lava onto the caldera floor and down the outer edges of the original lake. While Mount Shasta provides a stunning visual display, it was actually the Medicine Lake volcano that created the hundreds of lava caves in the region.
Medicine Lake Highlands encompasses over 200 square miles and is one of North America's most unique geological areas. One of its most interesting volcanic formations is the Glass Mountain Obsidian Flow. Another feature, Medicine Lake, has no known outlets yet the water remains clean and clear. It lies within the volcanic caldera of the volcano.
Medicine Lake volcano is located at the intersection of major tectonic features including the northwestern extension of the Walker Lane fault zone. The volcano is positioned at a kink where northwest-trending faults entering the volcano from the south turn to the northeast, and then back to northwest. Therefore, it is located within a zone of crustal weakness that likely provides a preferred pathway for magmas to reach the surface; the regional fault trends are the primary control on vent locations.
Geologists examine tephra deposits to better understand the early eruptive behavior of the Medicine Lake Volcano, evidence of which can be found as far away as central Oregon. Widespread tephra deposits form stratigraphic markers that help decipher eruptive histories at other volcanoes in the region and are sometimes used by archeologists to help determine the age of prehistoric human activities. Photo by David Ramsey, USGS
The volcano is constructed mainly of mafic lavas, although drillhole data indicate that a larger volume of rhyolite is present than is indicated by surface mapping. Basalt and basaltic andesite dominate the lower flanks, but higher on the volcano, basaltic lavas are mostly absent, andesite dominates, and high-silica lavas are present.
Drillhole data indicate that the plateau surface underlying the volcano has been downwarped by 0.5 kilometer under its center. Therefore, the volcano may be even larger than the estimated 143 cubic miles - already the largest volcano by volume in the Cascade Range. The highest point on the rim of Medicine Lake volcano's caldera is 7,913 feet. Lava flows reach elevations as low as 3,360 feet, although most lavas are found above 4,100 feet - the approximate elevation of the surrounding Modoc Plateau.
Obsidian and pumice abound in the highlands and have been used by people through the ages. The area evidences thousands of years of Native American use. Glass Mountain was used as an obsidian quarry. Obsidian was prized for making sharp-edged tools, and was a popular trade item to other tribes from as far away as 100 miles.
Collecting obsidian anywhere in the Medicine Lake Highlands is prohibited by law. The Modoc National Forest has four obsidian mines in the Warner Mountains near Davis Creek, Calif. on the east side of the forest where it is legal to collect obsidian with a free collection permit available at Forest offices in Alturas and Cedarville. Photo by Matt Bowers, NSS
Obsidian in all forms covers hundreds of acres at Glass Mountain in the Medicine Lake Highlands. In the 1960's, NASA astronauts trained for future moon landings in the pumice fields of the highlands. Pumice has been quarried here for building materials and, more recently, used in creating the "stone washed" look of denim.
Lava Beds National Monument encompasses about 73 square miles on the northern flank of Medicine Lake volcano and displays mostly basaltic and some andesitic lavas. Established in 1925, the monument includes sites of many important battles of the Modoc Indian War of 1872-73. It is also known for scores of lava-tube caves and for well-preserved young volcanic features.
Medicine Lake Highlands is also an area of moderately sloping to steep mountains. Vegetation consists of sugar pine, red and white fir and at higher elevations lodgepole pine with an understory of bitterbrush, manzanita and snowbrush. Snow usually closes the Medicine Lake area from mid-November through mid-June. During this time, an average of 10 feet of snow prohibits access by all but over-the-snow vehicles and cross country skiers. The combined total of the trails in all three forests is over 200 miles.