NSS Convention ~ July 25-30, 2021

Lassen Volcanic National Park

Lassen Volcanic National Park

  • Excerpted from USGS, NPS and Wikipedia Publications

Lassen Volcanic National Park is home to steaming fumaroles, meadows freckled with wildflowers, clear mountain lakes, and numerous volcanoes. Jagged peaks tell the story of its eruptive past while hot water continues to shape the land. At over 100 years old, Lassen Volcanic National Park is one of America's oldest national parks.

Lassen Peak, commonly referred to as Mount Lassen, is the southernmost active volcano in the Cascade Range of the Western United States. Located in the Shasta Cascade region of Northern California, it is part of the Cascade Volcanic Arc, which stretches from southwestern British Columbia to northern California. Lassen Peak reaches an elevation of 10,457 feet, standing above the northern Sacramento Valley. It supports many flora and fauna among its diverse habitats, which are subject to frequent snowfall and reach high elevations.

Lassen Hydro-Thermal Areas

Hydrothermal (hot water) features at Lassen Volcanic National Park fascinate visitors to this region of northeastern California. Boiling mudpots, steaming ground, roaring fumaroles, and sulfurous gases are linked to active volcanism and are all reminders of the ongoing potential for eruptions in the Lassen area. Nowhere else in the Cascade Range of volcanoes can such an array of hydrothermal features be seen. Video courtesy of U.S. National Park Service.

A lava dome, Lassen Peak has a volume of 0.6 cubic miles making it the largest lava dome on Earth. The volcano arose from the destroyed northern flank of now-collapsed Brokeoff Volcano about 27,000 years ago, from a series of eruptions over the course of a few years. The mountain has been significantly eroded by glaciers over the last 25,000 years, and is now covered in talus deposits.

On May 22, 1915, a powerful explosive eruption at Lassen Peak devastated nearby areas, and spread volcanic ash as far as 280 miles to the east. This explosion was the most powerful in a series of eruptions from 1914 through 1917. Lassen Peak and Mount St. Helens were the only two volcanoes in the contiguous United States to erupt during the 20th century.

Lassen Volcanic National Park, which encompasses an area of 106,372 acres, was created to preserve the areas affected by the eruption, for future observation and study, to protect the nearby volcanic features, and to keep away anyone from settling too close to the volcano. The park, along with the nearby Lassen National Forest and Lassen Peak, have become popular destinations for recreational activities. Lassen Peak is still considered alive, meaning the volcano is merely asleep, and it has a functioning magma chamber under the ground still capable of eruptions. Thus it poses a threat to the nearby area through lava flows, pyroclastic flows, lahars, ash, avalanches, and floods. To monitor this threat, Lassen Peak and the surrounding vicinity are closely observed with sensors by the California Volcano Observatory.

Lassen Volcanic Center is located at the southern edge of the Cascade Range, which is bounded on the west by the Sacramento Valley and the Klamath Mountains, on the south by the Sierra Nevada, and on the east by the Basin and Range geologic provinces.

Volcanism in the Lassen segment is a result of subduction of the Juan de Fuca oceanic plate eastward beneath the North American continental plate.

Mount Shasta glaciers

A researcher from the US Geological Service uses a long pole to gather a reading from a mudpot at Sulphur Works hydrothermal area.

Click here to download a USGS geology field-trip guide to Lassen Volcanic National Park.

The southern end of active volcanism in the Cascade Range has moved northward through time in conjunction with the northern progression of the San Andreas Fault system and the migration of the Mendocino Triple Junction off the coast of northern California. At 12 Ma, the south limit of Cascade volcanism was in the Sierran block approximately at the latitude of Lake Tahoe, 111 miles southeast of the Lassen area. At 3 Ma, the south limit of active volcanism was in the area of the Yana Volcanic Center, 30 km south of Lassen Peak. At present, the south limit of active Cascade-related volcanism approximately corresponds to the south boundary of Lassen Volcanic National Park, and the youngest dated regional volcano in the area is Sifford Mountain at about 160 ka. These relations suggest a long-term migration rate for the southern end of Cascade Arc volcanism of 15 to 20 km per m.y. (1.5 to 2 cm per yr, about 1 in per yr) and a rate for at least the last 3 m.y. of about 10 km per m.y. (1 cm per yr, about 0.4 in per yr).

Regional volcanism in the Lassen area has been linked to two distinct originating magmas - one is the same type linked to the creation of the main Cascade Arc, and the other is magma associated with the Basin and Range geologic province to the east of Lassen. The Cascade Arc type magma, called calc-alkaline, dominates the area and erupts to build cinder cones and associated fields of lava flows as well as steep-sided cones or shield-shaped mounds with gentle slopes. There is less lava, by volume, associated with the Basin and Range type magmas, called low-potassium olivine tholeiitic basalt, but it has erupted from fissures between the higher-topography volcanoes and has filled in the valleys surrounding them. This latter eruption style is similar to the the eruptions that occur in Hawai`i forming sheets and tube-fed flows.

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