The Lassen volcanic area presents a geological record of sedimentation and volcanic activity in and around Lassen Volcanic National Park in Northern California, U.S. The park is located in the southernmost part of the Cascade Mountain Range in the Pacific Northwest region of the United States. Pacific Oceanic tectonic plates have plunged below the North American Plate in this part of North America for hundreds of millions of years. Heat and molten rock from these subducting plates has fed scores of volcanoes in California, Oregon, Washington and British Columbia over at least the past 30 million years, including these in the Lassen volcanic areas. 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 upon which the park is situated. 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 ft (3,400 m) in height. Roughly 27,000 years ago, a dacite lava dome quickly pushed its way through Tehama's former north-eastern flank, becoming the approximately 1,000 ft (300 m) shorter Lassen Peak. Lassen's shape was significantly altered by glacial erosion from 25,000 to 18,000 years ago during the Wisconsin glaciation. Since then, smaller dacite domes such as the 1,100-year-old Chaos Crags have formed around Lassen. Phreatic (steam explosion) eruptions, dacite and andesite lava flows along with cinder cone formation have persisted into modern times. Most notable of these is the eruption and formation of Cinder Cone in the mid- to late 17th century and the eruption of Lassen Peak in the early 20th century. The only activity since then has been the constant bubbling of mud pots and steaming of fumaroles from the various geothermal areas in Lassen Volcanic National Park. The potential exists for renewed vigorous volcanic activity that could threaten life and property in the area.
Regional geologic setting
Current setting The Lassen volcanic area lies at the southern extremity of the Cascade Range, which extends northward some 500 mi (800 km) from Lassen Peak within the park through Oregon and Washington and into British Columbia. Lassen Peak is one of the Cascade Volcanoes that form a segment of a ring of volcanoes that circle the Pacific Ocean known collectively as the 'Pacific Ring of Fire'. All four types of volcanoes found in the world—shield, composite, cinder cone, and plug dome—are represented in Lassen Volcanic National Park. The Cascade Volcanoes are fed by heat generated as the Gorda and Juan de Fuca tectonic plates are being subducted below the much larger but lighter North American Plate. Lying some 300 mi (480 km) offshore, the spreading center of the Gorda Plate pushes out about 1 in (2.54 cm) of new crust toward the coast of northernmost California and southern Oregon every year. The composition of the molten rock (magma) that feeds volcanism in the Lassen volcanic area ranges widely in its content of silica or SiO2; the higher the silica content, the greater the ability of the magma to trap and hold on to gas and water vapor. When high-silica (dacitic) magma rises to the Earth's surface, the trapped gases and vapors can erupt explosively to produce ash clouds and pyroclastic flows that consist of superheated gas, ash and volcanic fragments. Dacite magma that is extruded nonexplosively as lava forms domes because it is too viscous (sticky) to flow far away from its source. Low-silica (basaltic) magma is more fluid and usually erupts as lava in less explosive eruptions than dacite because gas and water vapor escape easily from it. Eruptions of basalt magma typically produce elongate lava flows, as well as build cinder cones (piles of small frothy lava fragments or 'cinders') around volcanic vents. Basaltic volcanism in the Lassen volcanic area occurs mainly along chains of vents aligned in a north or northwest direction, parallel to regional faults. Examples include Poison Buttes, Subglacial Buttes, Tumble Buttes, the Prospect Peak-Red Cinder area, the east side of the Hat Creek Valley and Potato Buttes-Sugarloaf area, and the Red Lake Mountain area. Prolonged basaltic volcanism at a single site can produce a sizeable edifice, like the broad, relatively flat shield volcanoes of Prospect Peak and Sifford Mountain. Unlike other Cascade volcanoes, Lassen's large plug dome and composite volcanoes are in close proximity to the smaller cinder cone volcanoes that surround the volcanic center. Northwest of the park lies the Klamath Mountains (a collective term for the Siskiyou, Trinity, Salmon and Marble mountain ranges). To the west lies the Sacramento Valley. Just south of the park begins the Sierra Nevada mountain range, and to the east lie the Modoc Plateau and then the Great Basin.
Geologic history of the region
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