DESCRIPTION:
Lassen Peak Eruptive Activity 1914-1921

[Map,25K,InlineGIF]
Lassen Peak, 1915 Eruptions
-- Modified from: Clynne, et.al., 1999

Long before the recent activity at Mount St. Helens, a series of spectacular eruptions from Lassen Peak between 1914 and 1917 demonstrated the explosive potential of Cascade volcanoes. Small phreatic explosions began on May 30, 1914, and were followed during the next 12 months by more than 150 explosions that sent clouds of ash as high as 3 kilometers above the peak. The activity changed character in May 1915, when a lava flow was observed in the summit crater. A deep red glow from the hot lava was visible at night 34 kilometers away. On May 19, an avalanche of hot rocks from the lava spilled onto snow and triggered a lahar that extended more than 15 kilometers from the volcano.

The most destructive explosion occurred on May 21, when a pyroclastic flow devastated forests as far as 6.5 kilometers northeast of the summit and lahars swept down several valleys radiating form the volcano. An enormous ash plume rose more than 9 kilometers above the peak, and the prevailing winds scattered the ash across Nevada as far as 500 kilometers to the east. Lassen Peak continued to produce smaller eruptions until about the middle of 1917.

The most recent eruptive activity occurred at Lassen Peak in 1914-1917 A.D.. This eruptive episode began on May 30, 1914, when a small phreatic eruption occurred at a new vent near the summit of the peak. More than 150 explosions of various sizes occurred during the following year. By mid-May 1915, the eruption changed in character; lava appeared in the summit crater and subsequently flowed about 100 meters over the west and probably over the east crater walls. Disruption of the sticky lava on the upper east side of Lassen Peak on May 19 resulted in an avalanche of hot rock onto a snowfield. A lahar was generated that reached more than 18 kilometers down Lost Creek. On May 22, an explosive eruption produced a pyroclastic flow that devastated an area as far as 6 kilometers northeast of the summit. The eruption also generated lahars that traveled more than 20 kilometers down Lost Creek and floods that went down Hat Creek. A vertical eruption column resulting from the pyroclastic eruption rose to an altitude of more than 9 kilometers above the vent and deposited a lobe of pumiceous tephra that can be traced as far as 30 kilometers to the east-northeast The fall of fine ash was reported as far away as Elko Nevada, more than 500 kilometers east of Lassen Peak. Intermittent eruptions of variable intensity continued until about the middle of 1917.

Dramatic eruptive activity in the Cascades has been rare so far in the 20th century. Until the recent eruptions at Mount St. Helens, the only Cascade volcano that had a major eruption during this century was Lassen Peak in California. A series of intermittent eruptions of steam and volcanic ash beginning in May 1914 and lasting until 1921 climaxed, during the 4 days from May 19 to 22, 1915, in a series of violent events comprising small lava flows, massive lava-triggered mudflows, and explosive eruptions of ash. The most destructive of these eruptions included a nearly horizontal (lateral) blast that reached only about one-fifth as far as the recent Mount St. Helens lateral blast.

Around the mid 1700s a series of eruptions produced the Cinder Cone in the northeast corner of the park, mantling an area of 30 square miles with ejecta in the process. In the meantime, ashes falling on the streams of lava pouring from the cone's east flank formed the Painted Dunes. At the same time another lava flow poured from the Cinder Cone and entered Butte Lake and damned the drainage into Butte Lake to form a new lake--Snag Lake. In the late 1700s Cinder Cone had its most recent eruption and lava flow. Steam rose from the domes of Chaos Crags until 1857, but no important eruptions occurred again until Lassen Peak burst into activity in 1914.

For one year, explosions recurred at irregular intervals. Then, on May 19, 1915, a mass of lava rose in the summit crater and spilled over the southwestern and northeastern sides. On the southwestern slope glowing lava descended 1,000 feet toward the Sacramento Valley, then cooled and hardened. Extensive mudflows were created on the northeastern side as snowbanks were melted. The resulting debris swept down the slope. Divided by Raker Peak, part of this mudflow raced down Lost Creek; the remaining flow passed over the 100 foot rise east of the park road and rushed down Hat Creek. A wide barren swath was brutally torn through the forest.

Three days later, May 22, 1915, a great explosion blasted out a new crater. A volcanic cloud rose 40,000 feet, but a portion of the explosive force was deflected downward. The resulting Hot Blast (nuee ardente) roared down the same path taken by the mudflow, resulting in further damage along the headwaters of Hat and Lost Creeks. Thereafter, activity declined, finally ending in 1921. Since then, the volcano has lain dormant, although a little steam still rises from small vents in its summit and on its flanks.

Pumice ejected during the 1915 eruption of Lassen Peak is conspicuously banded with light streaks of dacite and dark ones of andesite, which appears to represent two distinct magmas imperfectly mixed during the eruption.

[Image, 47K,JPG]
"Hotrock" and Mount Lassen, 1926 and 1984
-- 1926 photo courtesy Lassen Volcanic National Park, by B.F. Loomis, and 1984 Photo by Lyn Topinka, USGS

Eruptions of Lassen Peak, California, 1914 to 1917 -- Clynne, 1999

Return to:
[Lassen Peak Menu] ...
[Lassen Peak Eruptive History Menu] ...