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.

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 SiO
₂; 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.

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.

About 70 million years ago, the area where the Cascade Range is now situated was under the most recent encroachment by the Pacific Ocean. 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 60 mi (97 km) 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.

The entire western portion of North America was being deformed from the Laramide orogeny starting around 70 million years ago. Gradually during millions of years, crustal rocks were folded and fractured and the seas driven away. This same bending and breaking of rocks relieved pressure on the hot material beneath the Earth's crust and permitted magma to rise toward the surface. Volcanoes burst into activity starting 30 million years ago from Washington southward along the Cascades and in the area now occupied by the Sierra Nevada. This activity continued until approximately 11 or 12 million years ago. Lava and ash reached a thickness of up to 10,000 ft (3,000 m) in some areas, forming what is now known as the western Cascades. These have been eroded until they are now rolling hills. The northern end of the San Andreas Fault and the Mendocino triple junction have moved northward over time, and with them the southern margin of Cascade volcanism retreats north; it currently is located at the southern end of the Lassen national park. The extensional tectonics of the Basin and Range and the widespread fault system of the Walker Lane are also encroaching on the Lassen region and faults associated with them provide pathways for magma to reach the surface.

Meanwhile, toward the end of this activity, eruptions of a different kind took place on an unprecedented scale in eastern Oregon and Washington. From innumerable cracks, floods of highly fluid basaltic lava spread to cover an area of over 200,000 sq mi (520,000 km). Now known as the Columbia Plateau, this great lava bed of flood basalt covers much of Oregon and Washington and even parts of Idaho. Northern California's Modoc Plateau is a thinner basaltic flow which some geologists associate with the Columbia Plateau, but there are technical objections to this. The High Cascades took shape as a distinct mountain belt as a result of this upheaval and the bending of the thick blanket of volcanic rocks. During the next 10 million years, a series of new basaltic volcanic cones similar to the shield volcanoes now found in Hawaii were built.

Between two and three million years ago, during the Pliocene, the Sierra Nevada was uplifted and tilted westward. A series of volcanic mudflows (lahars) from three major source areas contributed debris that covered almost 2,000 sq mi (5,200 km) to form the oldest distinctive geologic formation in the High Cascades. The resulting Tuscan Formation is not exposed anywhere in the national park but is just below the surface in many places within it. It consists mainly of tuffs but also contains conglomerates and lava sheets. The formation can reach thicknesses exceeding 1,000 ft (300 m) 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 10,000 ft (3,000 m) in elevation and 15 mi (24 km) 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).

Also during the Pliocene, basaltic lavas poured forth in the vicinity of Willow Lake in the southwestern portion of the park. These were followed by a very thick sequence of very fluid andesitic lavas which erupted near Juniper Lake and flowed westward about four miles (6 km). At about the same time, other andesitic lavas poured from several vents on the central plateau to cover an area of at least 30 sq mi (78 km). Included among these flows were the Twin Lake lavas of black porphyritic andesite, which are notable in that they contain xenocrysts of quartz. The Flatiron andesites spread over the southwestern part of the park area around this time.

Somewhat later, andesitic lavas poured out from what is now Reading Peak and mainly flowed to the south and east, reaching the head of Warner Valley. By this time, the park's eastern portion had been transformed into a relatively flat plain. The activity was followed by an eruption of the Eastern basalts from volcanoes east of the park. These thick flows have subsequently eroded to produce rugged hills that limit the park on the east. Taken together, these flows built the lava plateau upon which the Lassen volcanic area is located.

The earliest volcanic activity of the Lassen Volcanic Center commenced 825,000 years ago. The Rockland complex formed between 825,000 and 609,000 years ago. It was the source of lava domes and lava flows of dacitic composition. Around 610,000 years ago over 130 km 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 of tephra was erupted during this event. This could very well make the Rockland tephra the most voluminous eruption to have occurred in the Cascade Volcanic Arc, nearly twice the size of the Mazama ash. From 600,000 to 400,000 years ago, 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.

Tehama eventually reached an elevation of about 11,000 ft (3,400 m), was 11 to 15 miles (18 to 24 km) wide at its base, and contained 80 km (19 cu mi) of material. Its principal vent lay in the neighborhood of what is now Sulphur Works, but a second vent from which no lavas issued lay on the eastern flank of Little Hot Springs Valley. Contrary to popular belief, Bumpass Hell is not one of Tehama's main vents since it is located outside of the caldera. Tehama collapsed during the late Pleistocene, and numerous glacial advances tore down its remnants.

The largest remnants of Tehama include Brokeoff Mountain, Mount Conard, Mount Diller, and Pilot Pinnacle. Between about 385,000 and 315,000 years ago the character and locus of volcanism in the Lassen Volcanic Center changed dramatically from the andesitic (silica content between basalt and dacite) stratocone to the Lassen domefield, which consists of a core of dacite lava domes surrounded by an arc of hybrid andesite flows. The dacite domes erupted along the northern flank of Brokeoff Volcano and are divided on the basis of age into the Bumpass sequence (about 300,000–190,000 years ago) and Eagle Peak sequence (about 70,000 years ago to present day). The hybrid andesite units erupted in two groups called the older Twin Lakes sequence (about 315,000 to 240,000 years ago, contemporaneous with the Bumpass Peak sequence) and younger Twin Lakes sequence (about 90,000 to present day, contemporaneous with the Eagle Peak sequence). No volcanism is known in the Lassen volcanic center during the period from 190,000 to 90,000 years ago.

Four shield volcanoes (Raker and Prospect Peaks, Red Mountain and Mount Harkness) grew to elevations of between 7,000 to 8,400 ft (2,100 to 2,600 m) 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.

In the past 50,000 years, at least seven major episodes of dacitic volcanism produced lava domes and pyroclastic deposits in the Lassen volcanic area, and another five episodes produced basaltic and andesitic lava flows. Eruptions have occurred at sites including Lassen Peak, Chaos Crags, and Sunflower Flat (explosive dacite eruptions followed by dome growth) and Tumble Buttes, Hat Mountain, and Prospect Peak (basalt eruptions). In addition, about 30 smaller volcanoes erupted basaltic lavas in the larger region surrounding the Lassen volcanic center. These volcanoes are geologically related to the Basin and Range volcanic province.

Radiometric dating indicates that around 31,000 years ago a new vent opened up on the northeastern slope of Tehama, probably close to where Lassen Peak now stands. Streams of fluid dacite flowed chiefly toward the north, reaching a thickness of 1,500 ft (460 m) and covering perhaps 20 square miles (52 km). Known as the Loomis Sequence, these pre-Lassen dacites are the black, glassy, columnar lavas that now surround Lassen Peak.

Sometime between 25,000 and 31,000 years ago, Lassen Peak, a Pelean lava dome volcano, was pushed up through the pre-Lassen dacites. Lassen grew past the normal maximum size of plug dome volcanoes, 1,000 ft (300 m), and reached a height of 1,800 feet (550 m) above the surrounding plateau in as little as a few years. The growth of the pile of lava shattered the rocks previously there, forming enormous banks of talus. When Lassen Peak formed, it looked much like the nearby Chaos Crags domes do today, with steep sides covered by angular rock talus. Lassen Peak's shape was significantly altered by glacial erosion from 25,000 to 18,000 years ago during the Wisconsin glaciation. In turn the growth of Lassen Peak intercepted moisture, allowing a glacier on its northern flank to grow to almost 10 kilometers (6.2 mi) long.

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. Crescent Crater, which at first glance appears as a parasite on Lassen's northeast flank, has been more heavily glaciated and thus is older. Other dacite domes which rose on Tehama's flanks are Bumpass Mountain, Helen Ridge, Eagle Peak and Vulcan's Castle. An upper limit of 10,000 years has been set for the domes next to Lost Creek (north domes). All of these domes must have risen with great rapidity.