Formed in only a few years , there 27,000 years ; the lava dome of Lassen Peak was then eroded during the last glacial period ( 25,000 to 18,000 years), with the formation of a large circus or bowl-shaped depression extending up to 11 km. the dome. This morphology will influence the course of future eruptions.
After thousands of years of repose , a phreatic eruption shakes the dome , May 30, 1914 . Hundred eighty vapor explosions will succeed until mid- May 1915 , opening a crater 300 meters near the top .
Then the character of the eruption changes : a dacite dome began to grow in the crater ; it will gradually fill and déborder. On May 14, 1915 , you can see glowing blocks bouncing off the sides of the volcano .
On19, in the morning , a steam explosion fragments the dacitic dome , creating a new crater.
The fall of incandescent blocks on snow will generate an avalanche and lahar that will flood the valley of Hat Creek and destroy six ranches, fortunately not yet occupied . On the night of May 19 to 20 , the dacitic lava, more fluid than that emitted 14-15 , will fill the new summit crater , overflow and drip on the western flank and northeast.
The climax of May 22, 1915 :
After two days of calm, Lassen Peak exploded May 22, 1915 in the late afternoon. A plume of gas and ash grows and rises to more than 9,000 meters; it will be visible to Eureka (this can not be invented !) , located 240 km. to the west. The impact of hot pumice pyroclastic surge create a devastating 5 km ² area called Devastated area .
Mixing with snow creates a lahar , which will flood again the Hat Creek Valley. A layer of pumice and ash is traceable for 40 km . to the NE, while the rain of ash reached Nevada, more than 300 km to the east.
Map of deposits from the eruptive climax in 1915 Lassen Peak - scope of " Devastated area" - USGS map
For several years after the eruption , the waters of melting snow percolate in the spring and cause steam explosions , signs that the rocks are still hot . Particularly strong explosion will spray one of two craters in May 1917.
The eruption is considered completed in June 1917 ... but the volcano is only sleeping, as evidenced by hot springs, hot mud pools and fumaroles vents .
NB : according to an article in the NPS - 2004 : Different figures are reported in this article, the plume height of 22 May 1915 : 12,000 meters - end of the eruption in 1921.
The eruption of May 1915 produced four types of rocks vocaniques three days of the black dacite dome forming growth between 14 and 19 May and 19-20 May casting (a ) of dacitic pumice clear non- stratified ( b) , or interbedded with black andesite ( c) , both issued on May 22 , and finally andesitic inclusions hybrid magma supercooled incorporated into the dacite blocks.
These rocks reflect a complex mixing process between andesite and dacite interrupted by the eruption.
Lassen Peak eruption of 1914-1917 - Types of volcanic rocks (see above) - Doc. Michael Clynne / ref. in sources
- Global Volcanism Program - Lassen Volcanic Center
- USGS - Eruptions of Lassen Peak, California, 1914 to 1917 - Link
- USGS - Lassen Peak eruptive activity 1914-1921 - link
- USGS - Volcano Hazards of the Lassen Volcanic National Park Area, California - link
- Journal of Petrology - A complex magma mixing origin for rocks erupted in 1915, Lassen Peak, California - by M.A. Clinne - link