The year 2015 marks the centenary of the explosive eruption of Lassen Peak, located in the south of the Cascades volcanic Range. From a volume of 2 cubic kilometers and dominant the environmeny of around 600 meters, the Lassen Peak is the largest dacite lava dome on earth. It is established, there are about 27,000 years, on the northeast side of a now extinct stratovolcano, Mount Tehama.
On 22 May 1915 the Lassen Peak explodes, devastating the nearby areas and dropped a rain of ashes up to 300 km east of the volcano. This eruption has changed forever the landscape and led to the creation of the National Park of the same name.
The eruption of Lassen Peak 1914-1917:
The explosion of May 22 is the strongest of a series of eruptions that characterize the episode of 1914-1947; this series is the latest to have rocked the Cascades Range before the eruption of St. Helens in 1980.
The eruptive episode begins on May 30, 1914, with a phreatic explosion from the summit of Lassen Peak. During the following years, 180 steam explosions dug a 300 meters wide crater at the top.
Lassen Peak in 1914, before the explosive eruption - photo of the Devastated area by BFLoomis / USGS
Lassen Peak - Steam Explosion and blast of June 14, 1914 at 9:45 am - photo BFLoomis of Manzanita Lake, 10 km from Lassen Peak / via Shasta County
Lassen Peak - the summit crater, October 12, 1914, blown by the steam explosions in May - photo BFLoomis / via USGS
From mid-May 1915, the eruption changes of character, with the extrusion of a lava dome in the summit crater. This dome will expand to the west and the east extending beyond the walls of the crater. On May 19, a large explosion sprayed the dacitic dome, creating a new summit crater. Incandescent blocks fall on the top and the upper slopes of Lassen Peak, covered with a thick layer of snow. An avalanche of snow and lava blocks will travel 6,500 meters and spend Emigrant Pass, a pass used by the pioneers. The large rock, dubbed "Hot rock" by Loomis, is a piece of the lava dome carted away by the avalanche (see map below).
On May 22, an explosive eruption is accompanied by a pyroclastic flow that devastates an area which extends up to 6 km northeast of the crater. It is called from “the Devastated Area of Lassen Volcanic Park” (8 km²); this area have still little tree, following the low in nutrients and an high soil porosity.
Lassen Peak - Devastated area created by the explosion of 19.05 - the "Hot Rock", still a hot boulder at the photo taken by BF Loomis between 19 and 05.22.1915 - Archive VI-PH-C1. 63 / Lassen NPS / Flickr
The eruptive column rises vertically to more than 9,500 meters above the crater and deposited a lobe of pumice tephra over 30 km to ENE. Fine ash is found to Winnemucca, Nevada, more than 325 km. east of Lassen Peak.
These events are causing more snow melt, that feeds mudflows / lahars and flooding of Lost Creek and Hat Creek valley on more than 20 kilometers.
Lassen peak - the eruptive plume 05/22/1915 - photo RE Stinson - Archive VI-PH-C1.192 - Lassen NPS / Flickr
Lassen Peak from the west, shortly after the eruptive climax of 22 May 1915. The hot pumice fallout on the snow covered flanks of the volcano generated low-volume lahars more viscous and not reaching the base of Lassen Peak, unlike the lahars pf 19-20.05.15 - photo BF Loomis - Archive VI-PH-C1.27 Lassen NPS / Flickr
Intermittent eruptions will succeed until mid 1917 ... the percolation of snow meltwater engendered steam explosions, indicating the heat below the surface. In May 1917, a very strong steam explosion blew the northernmost crater. Steam vents could still be at the top into the 50s, but are difficult to detect today.
The total volume of 1915 eruptions is relatively low, about 0.03 cubic kilometers, compared to the well-known St Helens eruption in 1980, of the order of 1 cubic km.
Today, the Lassen Peak is dormant, but steam vents, hot springs and bubbling mud pools still exist in the Lassen Volcanic National Park.
A collector : the poster of Lassen Volcanic National Park - published in 1938 by the National Park Service
- USGS - A Sight “Fearfully Grand”—Eruptions of Lassen Peak, California, 1914 to 1917 - By Michael A. Clynne, Robert L. Christiansen, Peter H. Stauffer, James W. Hendley II, and Heather Bleick - link
- Photos d'archives : Lassen NPS on Flickr // creative /
- Flickr Lassen peak eruption - link