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 Lassen Peak

Lassen Peak, California. Long before the recent activity of 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 from 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.

Giant mushroom-shaped ash cloud of May 22, 1915, viewed from 80 kilometers west of Lassen Peak. (Photograph provided by National Park Service.)

Maintained by John Watson
Updated 06.24.97

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