The geology of the Lassen volcanic area
presents a record of sedimentation and volcanic activity in the area in and around Lassen Volcanic National Park
in Northern California
. The park is located in the southernmost part of the Cascade Mountain Range
in the Pacific Northwest
region of the United States
. Oceanic tectonic plates
have plunged below the North American Plate
in this part of North America
for hundreds of millions of years. Heat from these subducting
plates have fed scores of volcanoes
and British Columbia
over at least the past 30 million years (see Geology of the Pacific Northwest
) and is also responsible for activities in the Lassen volcanic area.
Between 3 and 4 million years ago, volcanic-derived mud flows called lahars streamed down several major mountains that included nearby but now extinct Mount Yana and Mount Maidu to become the Tuscan Formation. Basaltic and later andesitic to dacitic flows of lava covered increasingly larger areas of this formation to eventually form the lava plateau that the park sits on. About 600,000 years ago, Mount Tehama started to rise as a stratovolcano in the southwestern corner of the park, eventually reaching an estimated 11,000 feet (3,350 m) in height. Following a series of eruptions approximately 350,000 years ago, its cone collapsed into itself to form a two-mile (3 km) wide caldera.