Answer:
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.
