The source of heat for volcanism in the Lassen Volcanic National Park area was, and still is, subduction off the Northern California coast of the Gorda Plate diving below the North American Plate.
Formation of basement rocks
In the Cenozoic, uplifting and westward tilting of the Sierra Nevada along with extensive volcanism generated huge lahars (volcanic-derived mud flows) in the Pliocene which became the Tuscan Formation. This formation is not exposed anywhere in the national park but it is just below the surface in many areas.
Also in the Pliocene, basaltic flows erupted from vents and fissures in the southern part of the park. These and later flows covered increasingly large areas and built a lava plateau. In the later Pliocene and into the Pleistocene, these basaltic flows were covered by successive thick and fluid flows of andesite lava, which geologists call the Jupiter lavas and the Twin Lakes lavas. The Twin Lakes lava is black, porphyritic and has abundant xenocrysts of quartz (see Cinder Cone).
Another group of andesite lava flows called the Flatiron, erupted during this time and covered the southwestern part of the park's area. The park by this time was a relatively featureless and large lava plain. Subsequently, the Eastern basalt flows erupted along the eastern boundary of what is now the park, forming low hills that were later eroded into rugged terrain.
Volcanoes rise
Pyroclastic eruptions then started to pile tephra into cones in the northern area of the park.
Mount Tehama (also known as Brokeoff Volcano) rose as a stratovolcano in the southeastern corner of the park during the Pleistocene. It was made of roughly alternating layers of andesitic lavas and tephra (volcanic ash, breccia, and pumice) with increasing amounts of tephra with elevation. At its height, Tehama was about 11,000 feet above the surrounding lava plain.
Approximately 350,000 years ago it's cone collapsed into itself and formed a two-mile wide caldera after it emptied its throat and partially did the same to its magma chamber in a series of eruptions. One of these eruptions occurred where Lassen Peak now stands, and consisted of fluid, black, glassy dacite, which formed a layer 1500 feet thick (outcroppings of which can be seen as columnar rock at Lassen's base).
During glacial periods (ice ages) of the present Wisconsinan glaciation, glaciers have modified and helped to erode the older volcanoes in the park, including the remains of Tehama. Many of these glacial features, deposits and scars, however, have been covered up by tephra and avalanches, or were destroyed by eruptions.
Following a post-Tehama pause in activity around 18,000 years ago, Lassen Peak started to form as a dacite plug quickly pushed its way through Tehama's destroyed north-eastern flank. As the plug pushed its way up, it shattered overlaying rock, including the 1500 foot thick Tehama black dacite, which formed a blanket of talus around the emerging plug dome volcano. Lassen rose and reached its present height in a relatively short time.
Since then, smaller dacite plug domes formed around Lassen. The largest of these, Chaos Crags, is just north of Lassen Peak. Phreatic (steam explosion) eruptions, dacite and basalt lava flows and cinder cone formation have persisted into modern times (see Lassen Peak, Cinder Cone, and Chaos Crags for specifics).