From 1914 to early 1915, Lassen spewed steam and ashes in more than 150 eruptions. On May 19, 1915, the mountaintop exploded. A 20-foot-high wall of mud, ash, and melted snow roared down the mountain, snapping tree trunks. Three days later, a huge mass of ashes and gases shot out of the volcano, devastating a swath a mile wide and three miles long. Since 1921 Lassen Peak has remained quiet, but it is still considered an active volcano.
(National Geougraphic)

National Park Service, first 100 Years
Lassen Volcanic National Park
Northern California
Established August 9, 1916