Lassen Peak and Devastated Area from Cinder Cone
|Elevation||10,462 ft (3,189 m) NAVD 88|
|Prominence||5,229 ft (1,594 m)|
|Location||Shasta County, California, U.S.|
|Topo map||USGS Lassen Peak|
|Age of rock||Less than 27,000 years|
|Volcanic arc||Cascade Volcanic Arc|
Lassen Peak, also known as Mount Lassen, is the southernmost active volcano in the Cascade Range. It is part of the Cascade Volcanic Arc which is an arc that stretches from southwestern British Columbia to northern California. Located in the Shasta Cascade region of Northern California, Lassen rises 2,000 ft (600 m) above the surrounding terrain and has a volume of 0.5 cu mi (2 km3), making it one of the largest lava domes on Earth. It was created on the destroyed northeastern flank of now gone Mount Tehama, a stratovolcano that was at least 1,000 ft (300 m) higher than Lassen Peak.
On May 22, 1915, a powerful explosive eruption at Lassen Peak devastated nearby areas, and spread volcanic ash as far as 200 mi (300 km) to the east. This explosion was the most powerful in a series of eruptions from 1914 through 1917. Lassen Peak and Mount St. Helens are the only two volcanoes in the contiguous United States to erupt during the 20th century. Lassen Volcanic National Park was created in Shasta County, Calif., to preserve the devastated areas as they were, for future observation and study, and to preserve the nearby volcanic features.
Unlike most lava domes, Lassen Peak is topped by craters. A set of these craters exists around the summit of Lassen Peak, although two of them have been covered up by solidified lava and sulfur deposits. Lassen Peak is the largest of a group of more than 30 volcanoes that have erupted over the past 300,000 years in the Lassen Peak volcanic area.
Lassen Peak has the highest known winter snowfall amounts in California. There is an average annual snowfall of 660 in (1,676 cm), and in some years, more than 1,000 in (2,500 cm) of snow falls at its base altitude of 8,250 ft (2,515 m) at Lake Helen. The Mount Lassen area receives more precipitation (rain, sleet, hail, snow, etc.) than anywhere in the Cascade Range south of the Three Sisters volcanoes in Oregon. The heavy annual snowfall on Lassen Peak creates fourteen permanent patches of snow on and around the mountain top, despite Lassen's rather modest elevation, but no glaciers. Lightning has been known to strike the summit of the volcano frequently during summer thunderstorms.
Lassen Peak is the southernmost in the chain of eighteen large volcanic peaks that stretch from southwestern British Columbia to northern California. These peaks were formed during the past 35 million years while the large Juan de Fuca tectonic plate and the much smaller Gorda plate to its south have been pushed underneath the overriding North American plate. While the oceanic crust in the Juan de Fuca plate melts under the subterranean pressure, this creates pools of molten lava that have uplifted the Cascade Range and created volcanic eruptions from time to time.
Roughly 27,000 years ago, Lassen Peak started to form as a mound-shaped dacite lava dome pushed its way through Tehama's destroyed north-eastern flank. As the lava dome grew it shattered overlaying rock, which formed a blanket of angular talus around the emerging steep-sided volcano. It probably looked much like the nearby 1,100-year-old Chaos Crags. Lassen Peak reached its present height in a relatively short time, probably in just a few years.
From 25,000 to 18,000 years ago, during the last glacial period of the current Ice Age, the shape of Lassen Peak was significantly modified by glacial erosion. For example, the bowl-shaped depression on the volcano’s northeastern flank, called a cirque, was eroded by a glacier that extended out 7 mi (11 km) from the dome.
The most recent eruptive period of Lassen Peak began in 1914, and lasted for about seven years (see below). The most powerful of these eruptions was an explosion in 1915 that sent ash and steam in a 6 mi (10 km) tall mushroom cloud, making it the largest recent eruption in the contiguous United States, until the explosion of Mount St. Helens in 1980. The region around Lassen Peak remains a mildly-volcanic one, with mud pots, active fumaroles, and boiling water features demonstrating their activity. The area around Mount Lassen and the nearby Mount Shasta are considered to be the most likely volcanoes in the Cascade Range to erupt during the coming decades and centuries.
Human history 
Lassen Peak was named in honor of the Danish blacksmith Peter Lassen, who guided immigrants past this peak to the Sacramento Valley during the 1830s. The trail that Lassen blazed never found generalized long-term use because it was considered unsafe. This trail was replaced by the Nobles Emigrant Trail, named for the guide, William Nobles, which linked the Applegate Trail in northwestern Nevada to the northern part of the Sacramento Valley.
Beginning in 1914 and lasting until 1921, Lassen Peak emerged from dormancy with a series of phreatic eruptions (steam explosions), dacite lava flows, and lahars (volcanic mud flows). There were 200 to 400 volcanic eruptions during this period of activity. Because of the eruptive activity and the area's stark volcanic beauty, Lassen Peak, Cinder Cone and the area surrounding were designated as the Lassen Volcanic National Park on August 9, 1916.
Early 20th century eruptions 
Initial rumblings (May 1914 through May 1915) 
On May 30, 1914, Lassen became volcanically active again after 27,000 years dormancy, when it was shaken by a steam explosion. Such steam blasts occur when molten magma rises toward the surface of a volcano and heats shallow-lying ground water. The superheated water rises under pressure through cracks, and upon nearing the surface, it flashes into steam, which then vents explosively through the paths of least resistance. By mid-May 1915, more than 180 steam explosions had blown out a 1,000 ft (300 m) wide crater near the summit of Lassen Peak.
Then the character of the eruption changed dramatically. On the evening of May 14, 1915, incandescent blocks of lava could be seen bouncing down the flanks of Lassen Peak from as far away as the town of Manton, 20 mi (30 km) to the west. By the next morning, a growing dome of dacite lava (lava consisting of 63% to 68% silica) had filled the volcano’s crater.
The volcanic eruption of May 19–20, 1915 
Late on the evening of May 19, a large steam explosion fragmented the dacite dome, creating a new crater at the summit of Lassen Peak. No new magma was ejected in this explosion, but glowing blocks of hot lava from the dome fell on the summit and snow-covered upper flanks of Lassen Peak. These falling blocks launched a half mile (800 m) wide avalanche of snow and volcanic rock that roared 4 mi (10 km) down the volcano’s steep northeast flank and over a low ridge at Emigrant Pass into Hat Creek.
As the hot lava blocks broke into smaller fragments, the snow melted, generating a mudflow of volcanic materials, called a lahar. The bulk of this lahar was deflected northwestward at Emigrant Pass and flowed 7 mi (11.3 km) down Lost Creek. Even after coming to rest, both the avalanche and lahar released huge volumes of water, flooding the lower Hat Creek Valley during the early morning hours of May 20. The lahar and flood destroyed six mostly not-yet-occupied summer ranch houses. The few people in these houses escaped with minor injuries.
Also during the night of May 19–20, dacite lava somewhat more fluid than that which erupted on the night of May 14–15 welled up into and filled the new crater at Lassen’s summit, spilled over low spots on its rim, and flowed 1,000 ft (300 m) down the steep west and northeast flanks of the volcano.
Climactic eruption of May 22, 1915 
Then at 4:30 p.m. on May 22, after two quiet days, Lassen exploded in a powerful eruption (referred to as "the Great Explosion") that blasted volcanic ash, rock fragments, and pumice high into the air. This created the larger and deeper of the two craters seen near the summit of the volcano today. A huge column of volcanic ash and gas rose more than 30,000 ft (10,000 m) into the air, and was visible from as far as Eureka, 150 mi (240 km) to the west, on the Pacific Coast.
Pumice falling onto the northeastern slope of Lassen Peak generated a high-speed avalanche of hot ash, pumice, rock fragments, and gas, called a pyroclastic flow, that swept down the side of the volcano, devastating a 3 sq mi (8 km2) area. The pyroclastic flow rapidly incorporated and melted snow in its path. The water from the melted snow transformed the flow into a highly fluid lahar that followed the path of the lahar of May 19–20, and it rushed nearly 10 miles (16 km) down "Lost Creek" to the Old Station. This new lahar released a large volume of water that flooded the lower Hat Creek Valley a second time.
The powerful climactic eruption of May 22 also swept away the northeast lobe of the lava flow extruded two days earlier. The eruption produced smaller mudflows on all flanks of Lassen Peak, deposited a layer of volcanic ash and pumice traceable for 25 miles (40 km) to the northeast, and rained fine ash at least as far away as Winnemucca, Nevada, 200 mi (320 km) to the east. Together these events created the Devastated Area which is still sparsely populated by trees due to the low nutrient level and high porosity of the soil.
Later volcanic activity 
For several years after the large eruption in 1915, spring snowmelt water percolating down into Mount Lassen triggered steam explosions, an indication that magma beneath the surface of the volcano remained quite hot. Vigorous steam explosions in May 1917 blasted out the second of the two craters that are now seen near the northwest corner of the volcano’s summit. The two older craters were buried.
Steam vents could be found in the area of these craters into the 1950s, but their activity gradually waned, and they are difficult to locate today. Since then, the United States Geological Survey, in cooperation with the United States Park Service, has been monitoring Lassen Peak and other volcanic and geothermal areas in the park.
For more details, see a map of the deposits from the 1915 eruption.
See also 
|Wikimedia Commons has media related to: Lassen Peak|
- Geology of the Pacific Northwest
- High Cascades
- Lassen Volcanic National Park
- List of Ultras of North America
- List of Ultras of the United States
- List of highest points in California by county
- Volcanic Legacy Scenic Byway
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