Over the last 3 million years, regional volcanism in the Lassen segment of the Cascade arc constructed a broad platform of volcanic rocks. In periods of about 50,000 - 200,000 years, concentrated volcanism occurred in localized areas. These "volcanic centers" are defined as large, long-lived edifices erupting the full range of lava compositions from basalt to rhyolite.
Yana and Latour Volcanic Centers and the Tuscan Formation
The oldest volcanic rocks in the Lassen vicinity comprise the Latour and Yana volcanic centers. Rocks of the Latour Volcanic Center (>3 Ma) are present northwest of Lassen Volcanic National Park, whereas rocks of the Yana Volcanic Center (~3.4–2.4 Ma) dominate the area southwest of Lake Almanor. These two volcanic centers are important as the primary sources of the Tuscan Formation (~3.5 to 2.5 My old), a broad volcanic and volcaniclastic wedge of fragmental material emplaced on west slope of the southernmost Cascade Range. The Tuscan Formation consists primarily of volcanic debris flows (lahars), conglomerate, sandstone, and siltstone, with minor silicic ash-flow and airfall tuffs and several basaltic to andesitic lava flows.
Dittmar Volcanic Center
The Dittmar Volcanic Center (2.4–1.4 Ma) lies at the NW end of the Lake Almanor Graben. Deposits of the Dittmar Volcanic Center are grouped into 3 stratigraphic stages: Stage 1, early growth of the composite cone (1.6 to 2.3 Ma) with thin lava flows and intermixed layers (interbedded) of air-fall and volcanoclastic deposits; Stage 2, later growth of the composite cone (1.4 Ma) with thick lava flows and some intermixed explosive or sedimented deposits; and Stage 3, late silicic volcanism (1.3 Ma) with a few lava flows and domes of rhyodacite and rhyolite.
Maidu Volcanic Center
The Maidu Volcanic Center (2.4–1.2 Ma) formed at a similar time as the Dittmar Volcanic Center and is located southwest of Lassen Volcanic National Park where it overlies the Tuscan Formation and rocks of the Yana Volcanic Center. Like the Dittmar, it consists of three stratigraphic stages: Stage 1, early composite cone growth with thin lava flows and fragmental deposits erupted from a central vent; Stage 2, later composite cone growth with thin dacite lava flows erupted from flank vents; and stage 3, late silicic volcanism comprised of five, extensive and thick rhyodacite and rhyolite lava flows. Stage 3 flows are some of the largest silicic lava flows in the Cascade Range with an aggregate volume of ~35 km3 (8.4 mi3).
Lassen Volcanic Center
The major stratigraphic divisions of the Lassen Volcanic Center are designated as the Rockland caldera complex, Brokeoff Volcano, and the Lassen dome field. These eruptive stages are connected periods volcanism distinctly different from one another but linked by a common magmatic system.
Rockland caldera complex
The Rockland caldera complex consists of the Rockland tephra and a group of dacite to rhyolite domes and flows ranging in age from ~825 to 609 ka. Effusive eruption (extrusion) of these domes and flows culminated in explosive eruption of the Rockland tephra about 610 ka.
Brokeoff Volcano, also designated “Mount Tehama” by the National Park Service, consists of a large, 80-km3(19.2 mi3) composite volcano. Almost immediately after eruption of the Rockland tephra, its caldera began to fill as renewed activity formed Brokeoff Volcano. The stratigraphy of Brokeoff Volcano is described by Clynne and Muffler (USGS Scientific Investigations Map 2899) as two sequences of deposits: the Mill Canyon sequence and the Diller sequence. The Mill Canyon sequence consists of dozens of small-volume basaltic andesite to dacite lavaflows and interlaid layers of explosive deposits erupted from a central vent between about 590 and 470 ka. The Diller sequence consists primarily of 6 thick, large-volume, lithologically similar, lava flows that erupted from flank vents between 470 and 385 ka.
Between about 385 and 315 ka the character and locus of volcanism in the Lassen Volcanic Center changed dramatically from the andesitic stratocone to the Lassen domefield, which consists of a core of dacite domessurrounded 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 (~300–190 ka) and Eagle Peak (~70–0 ka) sequences. The hybrid andesite units erupted in two groups called the older (~315–~240 ka) and younger (~90–0 ka) Twin Lakes sequence and are contemporaneous with the Bumpass and Eagle Peak sequences, respectively. No volcanism is known in the LVC during the period 190–90 ka.
The Bumpass, Eagle Peak, and Twin Lakes sequences belong to a suite of lavas formed by a continuum of magma-mixing processes that cause many rock units to be variable in appearance and composition. The Bumpass sequence is a group of 15 dacite to rhyodacite lava domes and flows, emplaced in the south part of the Lassen domefield. The Eagle Peak sequence is made up of seven dacite and rhyodacite lava domes and flows and their associated pyroclastic deposits erupted in the north part of Lassen domefield. These include the most prominent young volcanic features in in Lassen Volcanic National Park: Lassen Peak (27 ka) and Chaos Crags (1.1 ka), as well as the products of 5 older eruptions. The Twin Lakes sequence consists of two groups of mostly andesite andbasaltic andesite lava flows and agglutinate cones erupted around the periphery of the Lassen domefield, but primarily on the Central Plateau of Lassen Volcanic National Park. The two youngest eruptions in the park, the basaltic andesites of Cinder Cone and deposits of the 1914–17 eruption of Lassen Peak, are parts of the younger Twin Lakes sequence.
The Lassen Volcanic Center is still active, and three eruptions of Holocene age have occurred: Chaos Crags, Cinder Cone, and the 1914–1917 eruption at the summit of Lassen Peak. Chaos Crags is part of the Eagle Peak sequence; Cinder Cone and the eruption at the summit of Lassen Peak are part of the younger Twin Lakes sequence. No other eruptions documented to be Holocene have occurred in the Lassen region.