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Geology & History

Lassen Volcanic Center is located at the southern edge of the Cascade Range, which is bounded on the west by the Sacramento Valley and the Klamath Mountains, on the south by the Sierra Nevada, and on the east by the Basin and Range geologic provinces. Volcanism in the Lassen segment is a result of subduction of the Juan de Fuca oceanic plate eastward beneath the North American continental plate.

The southern end of active volcanism in the Cascade Range has moved northward through time in conjunction with the northern progression of the San Andreas Fault system and the migration of the Mendocino Triple Junction off the coast of northern California. At 12 Ma, the south limit of Cascade volcanism was in the Sierran block approximately at the latitude of Lake Tahoe, 180 km (111 mi) southeast of the Lassen area. At 3 Ma, the south limit of active volcanism was in the area of the Yana Volcanic Center, 30 km south of Lassen Peak. At present, the south limit of active Cascade-related volcanism approximately corresponds to the south boundary of Lassen Volcanic National Park, and the youngest dated regional volcano in the area is Sifford Mountain at about 160 ka. These relations suggest a long-term migration rate for the southern end of Cascade Arc volcanism of 15 to 20 km per m.y. (1.5 to 2 cm per yr, about 1 in per yr) and a rate for at least the last 3 m.y. of about 10 km per m.y. (1 cm per yr, about 0.4 in per yr).

Regional volcanism in the Lassen area has been linked to two distinct originating magmas – one is the same type linked to the creation of the main Cascade Arc, and the other is magma associated with the Basin and Range geologic province to the east of Lassen. The Cascade Arc type magma, called calc-alkaline, dominates the area and erupts to build cinder cones and associated fields of lava flows as well as steep-sided cones or shield-shaped mounds with gentle slopes. There is less lava, by volume, associated with the Basin and Range type magmas, called low-potassium olivine tholeiitic basalt, but it has erupted from fissures between the higher-topography volcanoes and has filled in the valleys surrounding them. This latter eruption style is similar to the the eruptions that occur in Hawai`i forming sheets and tube-fed flows.