thumb|upright=2.0|alt=Map with large green area marked as Lassen Volcanic National Park with a circle is on the lower left corner. Other features, such as Chaos Crags, Brokeoff Mountain, Bumpass Hell and Cinder Cone are also labelled.|Eruptions in the Lassen volcanic area in the last 70,000 years. Circle shows base of [[Mount Tehama. See timeline image.]]
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,
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
right|upright=1.2|alt=Map showing black vertical bars set in a blue field with their ends connected by thin lines. A contoured line with sharp bumps point toward a nearby coastline. In between is the label "Juan de Fuca Plate".|thumb|The major volcanoes of the Cascade Range are fed from heat generated as [[tectonic plates dive below North America.]]
All rock now exposed in the area of the park is volcanic, and unconformably overlies much older sedimentary, metamorphic and igneous rock, which was formed during the hundreds of millions of years when the Lassen region underwent repeated uplifting to form mountains, only to have them worn down and submerged under encroaching seas. During the periods of submersion, sand, mud and limestone were deposited. Occasionally volcanic activity was associated with the mountain building. The rocks that make up the modern Sierra Nevada and the Klamath Mountains were already in existence but deeply buried. Some 70 million years before (140 million years before present), the rocks that now make up the Klamaths broke away from the rocks that now make up the Sierras and moved west, leaving the flooded 'Lassen Strait.' This broad depression was a seaway that connected the marine basin in California with that in east central Oregon. and is of late Pliocene age. An overlying rhyolitic lava flow gives an age of 1.5 million years.
Lassen is the fifth volcanic center to be active in the region. Latour, Yana, Maidu and Dittmar were the four preceding centers; Latour and Yana are only poorly known. One major source of the formation was Mount Yana; centered a few miles (5 km) southwest of Butt Mountain and south of the park. Mount Yana had probably reached its full size of in elevation and in diameter before Mount Maidu, the second source, had acquired half its growth. Mount Maidu, which eventually surpassed Mount Yana in size, was centered over what is now the town of Mineral, California, but has been extinct for hundreds of thousands of years (the grassy plain around the town is Maidu's caldera). A third source situated north of Latour Butte made a lesser contribution to the formation. Minor sources included an area near Hatchet Mountain Pass (northwest of Burney Mountain), dikes south and southwest of Inskip Hill and possibly Campbell Mound (north of Chico, California). of rhyolite magma violently erupted onto the surface, producing massive pyroclastic flows and an ash plume several tens of kilometers high. This plume distributed ash almost entirely over the state of Nevada and sending traces as far as southeastern Idaho. As the eruption progressed the underlying magma chamber was severely drained. This caused the overlying rock that was once supported by the magma to collapse downward. Forming a massive depression known as a caldera. Over 326.7 km<sup>3</sup> of tephra was erupted during this event. eruptions built a large conical stratovolcano called Mount Tehama (also called Brokeoff Volcano) in what is now the southwest corner of the park within the Rockland caldera complex. It was made of roughly alternating layers of andesitic lavas and tephra (volcanic ash, breccia, and pumice) with increasing amounts of tephra with elevation.
thumb|upright=1.0|alt=Map with red labels|Extent of Tehama shown by outline
Tehama eventually reached an elevation of about , was wide at its base, and contained of material.
thumb|upright=1.0|Lassen Peak [[geological map, where an asterisk denotes the location of a vent, HPE al is alluvium/talus/colluvium, HPE t is glacial till, and PE-Lvd is the Brokeoff Volcano Diller Sequence]]
Four shield volcanoes (Raker and Prospect Peaks, Red Mountain and Mount Harkness) grew to elevations of between at the corners of the central plateau. Raker Peak erupted andesite lavas while basalt flowed from the others. Each of these volcanoes developed a cinder cone on its summit during their last stages of eruption. Later, a mass of rhyolite was forced through the north flank of Sifford Mountain and a plug of dacite was pushed up through the west flank of Raker Peak.
thumb|Multiple domes at Jumble Crags
thumb|Mafic inclusion in dacite/rhyodacite of Chaos Crags
Later, but not precisely dated, eruptions from the Lassen volcanic area have formed over 30 smaller steep-sided, mound-shaped accumulations of volcanic rock, called lava domes.
Since then, the volcano has been dormant, although some steam still rises from small vents in its summit and on its flanks. Pumice ejected during the 1915 eruption of Lassen Peak is conspicuously banded with light streaks of dacite and dark andesite, which appears to represent two distinct magmas imperfectly mixed during the eruption. The 1915 eruption of Lassen Peak was the second most recent volcanic outburst in the contiguous 48 U.S. states (after the 1980 eruption of Mount St. Helens in Washington).
Volcanic hazards
Direct eruption hazards
thumb|upright=1.2|alt=Map with circles and blobs of color|Volcano Hazards of the Lassen area<br /> [[image:Volcano Hazards of the Lassen area-explanation.jpg]]
The Lassen area remains volcanically active. The most common volcanic activity over the last 50,000 years in the Lassen volcanic area consists of small to moderate-sized eruptions that produce basaltic lava flows and localized ash falls. A thin crust of material often covers these boiling hot features, making them a serious burn hazard to anyone walking off trail. The waters of the features are typically acidic and, even if cool enough, are not safe for bathing.