Geologic Background:
The Lassen volcanic center consists of the
andesitic Brokeoff stratovolcano, a 6-km-wide caldera
largely filled by a dacitic lava dome field, and
peripheral small andesitic shield volcanoes and
pyroclastic cones. Activity spanning 600,000 years began
with construction of Brokeoff stratovolcano. Beginning
400,000 years ago activity shifted to the north flank of
Brokeoff, where episodic, more silicic eruptions produced
a small caldera and field of a dozen dacitic lava domes
along the inferred margin of the caldera, including
Bumpass Mountain, Mount Helen, Ski Heil Peak, and Reading
Peak. At least twelve eruptive episodes took place during
the past 100,000 years, with Lassen Peak being
constructed about 28,000 years ago. The Chaos Crags dome
complex was constructed about 1100-1000 years ago north
of Lassen Peak. The Cinder Cone complex NE of Lassen Peak
was erupted in a single episode several hundred years
before present and is considered part of the Lassen
volcanic center. Wide range of volcanic products
including a andesitic stratovolcano, dacite domes, small
andesitic shield volcanoes, and cinder cones and flows.
Much of the volcanic center is located within Lassen
Volcanic National Park.
Historic Activity:
- A series of eruptions from Lassen Peak from 1914 to
1917 marks the most recent eruptive activity in the
southern Cascade Range.
- The 1914-1917 eruptions of Lassen Peak began with
phreatic eruptions and included emplacement of a small
summit lava dome, subplinian explosions, mudflows, and
pyroclastic flows.
Recent Activity:
- Several prominent active steam-and-sulfur springs
exist on the south flank of Lassen Peak. Of the three
major Holocene volcanoes in the California Cascades,
Lassen (about 800 km NNW of the epicenter) had the
strongest response to the June 28, 1992, M 7.5
earthquake. 32 events occurred within the first 24 hours
following the quake, one was as large as M
2.8.
Data Sources:
- Smithsonian's Global Volcanism Network (V. 17, No.
6).
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