Formation of basement rocks s at Sulphur Works 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 Juniper 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 , 1915
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 southwestern 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 probably about high. Approximately 350,000 years ago its cone collapsed into itself and formed a two-mile (3.2 km) 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 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. Roughly 27,000 years ago (older data gave an age of 18,000 years), Lassen Peak started to form as a
dacite lava dome quickly pushed its way through Tehama's former north-eastern flank. As the lava dome pushed its way up, it shattered overlaying rock, which formed a blanket of talus around the emerging volcano. Lassen rose and reached its present height in a relatively short time, probably in as little as a few years. Lassen Peak has also been partially eroded by Ice Age glaciers, at least one of which extended as much as from the volcano itself. Since then, smaller dacite domes formed around Lassen. The largest of these,
Chaos Crags, is just north of Lassen Peak.
Phreatic (steam explosion) eruptions,
dacite and
andesite lava flows and
cinder cone formation have persisted into modern times. There are active hot springs and mud pots in the Lassen area. Some of these springs are the site of occurrence of certain
extremophile micro-organisms, that are capable of surviving in extremely hot environments. ==See also==