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Geology and History

Medicine Lake volcano lies in a strongly east-west extensional tectonic environment slightly east of the main Cascade Range arc axis on the Modoc Plateau in northern California.

Shaded relief map shows locations of Medicine Lake volcano (red out...
Shaded relief map shows locations of Medicine Lake volcano (red outline) and Mount Shasta with respect to the main axis of the Cascades Arc.

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.

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 600 km3 (143 mi3, already the largest volcano by volume in the Cascade Range. The highest point on the rim of Medicine Lake volcano's caldera is 2,412 m (7,913 ft). Lava flows reach elevations as low as 1,024 m (3,360 ft) although most lavas are found above 1,249 m (4,100 ft), 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 Cascade arc axis.

Lava Beds National Monument encompasses about 188 km2 (73 mi2) on the northern flank of Medicine Lake volcano and displays mostly basaltic and some andesitic lavas. It is in northeastern California about 50 km (31 mi) south of Klamath Falls, Oregon. The monument, established in 1925, includes the 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.