Geology of the Lassen area
 | Cinder Cone from the Fantastic Lava Beds. |
|  | Lassen Peak from Devastated Area. |
|
 | Lassen Peak from the summit of Brokeoff Mountain. Photo shows 1915 tongue of lava and Volcan's Eye |
|
| Lassen Peak as seen from lake helen |
|
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.
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.
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 probably about 11,000 feet above the surrounding lava plain.
Approximately 350,000 years ago its 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.
Roughly 27,000 years ago, Lassen Peak started to form as a
dacite lava dome quickly pushed its way through Tehama's destroyed 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 7 miles (11 km) 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.
*
Lassen Peak*
Chaos Crags*
Cinder Cone and the Fantastic Lava BedsGeology of National Parks: Fifth Edition, Ann G. Harris, Esther Tuttle, Sherwood D., Tuttle (Iowa, Kendall/Hunt Publishing; 1997) ISBN 0-7872-5353-7
