This article began as a regular piece on Lassen Peak but quickly expanded as I discovered how incredibly complex the geology and history of the Lassen Volcanic Complex was. Unlike nearby Shasta, Lassen Peak is but a dacite lava dome, one of the largest lava domes on Earth but only one of the dozens of lava domes and peaks to be found within the Lassen Volcanic National Park. Before we begin to unravel the very complex history of eruptions, collapses and glacial erosion, it is a good idea to acquaint ourselves with the area and its main features:
Volcanic activity in the area goes back at least three million years. About 600,000 years ago, contemporary with the formation of nearby Mount Shasta, a series of large eruptions formed a huge volcano, Mount Tehama. Just to put it into perspective, the largest of these eruptions was about 50 times as powerful as the 1980 eruption of Mount St Helens or five times that of Mount Pinatubo. Mount Tehama was immense. At its base, it measured 18 km (11 miles) by 24 km (15 miles) wide and it stood more than 3,353 m (11,000 feet) tall. Its total volume must have been well in excess of 400 kubic kilometres, a rough calculation yields a volume of 425 cu km, comfortably larger than either present-day Mount Shasta or Mount Hood, both of which have volumes of about 350 cubic kilometres.
Activity at Mount Tehama continued for about 200,000 years and consisted mostly of pyroclastics and andesitic lava flows containing olivine. During the same period of time, other volcanic vents began to appear such as the shield volcanoes Raker Peak, Red Mountain, Prospect Peak and Mount Harkness. The distances from the central volcano, never more than eight miles, argues that they were satellite cones of Mount Tehama. As for the central volcano itself, there was a final, large lava flow on the north-eastern flank consisting of glassy dacite and further dacite flows formed smaller cones such as Mount Conard and Diamond Peak on the flanks of Mount Tehama.
Subsequent glaciation combined with extensive hydrothermal activity led to major erosion and to a subsequent collapse of Mount Tehama of which today the only visible, large remnant is Brokeoff Volcano, also known as Brokeoff Mountain. The collapse structure, a caldera about 3 km (2 miles) in diameter, was quickly infilled by debris and material from later eruptions. Further activity built more than thirty other cones which are sometimes referred to as the Lassen Domes.
As we have already seen, Mount Tehama began life as a rather typical andesitic stratovolcano, but subsequent activity became more and more silicic. After its collapse and disappearance, local volcanism shifted to what had been its northern flank. The last 400,000 years have seen at least three known lava flows, parts of which can be seen on or near Raker Peak and Mount Conard. There is also evidence of a very large explosive eruption which ejected an estimated 50 km3 (12 cubic miles) of material and created a large crater which was soon obliterated by subsequent lava flows and emplacements. During this period, a further twelve lava domes were emplaced throughout the Park area. The next period of eruptive activity spanned some 50,000 years and ejected between 15 to 25 km3 (3.5 to 6 cubic miles) in the form of pyroclastic flows, viscous lava flows and mainly dacitic lava domes.
About 27,000 years ago,Lassen Peak started to form as a mound-shaped dacite lava dome pushed its way through the shattered north-eastern flank of Mount Tehama. As the lava dome grew, it shattered overlaying rock, which formed a collar of angular talus around the steep-sided volcanic dome. Lassen Peak reached its present dimensions of nearly two km across at the base and prominence of 606 metres above the surrounding landscape in a remarkably short time, probably in just a few years or at most tens of 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. The bowl-shaped depression – cirque – on the volcano’s north-eastern flank is the result of erosion by a glacier that extended some 11 km (7 miles) from the dome.
To inject a piece of human history, Lassen Peak was named after the Danish blacksmith Peter Lassen (most likely an Americanisation of Larsen), who guided immigrants in covered wagons past this peak into the Sacramento Valley during the 1830s. The trail that Lassen blazed never found general long-term use because it was considered to be unsafe.
Just over 1,000 years ago, a major period of activity resulted in the extrusion of five dacite lava domes that form the 177 metres high Chaos Crags. Initially, they must have been considerably higher as a major rockslide occurred just 300 years ago. Riding on a cushion of compressed air, the sturzstrom landslide travelled 20 to 30 times the vertical fall height and resulted in the formation of beautiful Lake Manzanita, 3 km (2 miles) from the Chaos Crags.
The most recent period of activity at Lassen Peak took place between 1914 and 1921. After 27,000 years of dormancy, Lassen Peak was shaken by a steam explosion on May 30th, 1914. By mid-May 1915, more than 180 steam explosions had gouged out a 300 metre-wide (1,000 ft) crater near the summit. On the evening of May 14th, 1915 the character of the eruption changed dramatically as incandescent blocks of lava could be seen bouncing down the flanks of Lassen Peak from as far away as the town of Manton, 30 km to the west. Next morning, the crater had been filled by a rapidly growing dacite lava dome.
Late in the evening of May 19th, the lava dome was fragmented by a large hydrothermal explosion. Glowing blocks of lava fell on the summit and snow-covered upper flanks of Lassen Peak and launched an 800 metre (half mile) wide avalanche of snow and volcanic rocks that roared 10 km (6 miles) down the volcano’s steep northeast flank, over a low ridge at Emigrant Pass and into Hat Creek. The lahar was deflected north-westward at Emigrant Pass and flowed more than 11 kilometres (7 miles) down Lost Creek. During the night of May 19–20th, dacite lava slightly less viscous than that which erupted on the night of May 14–15th, welled up into and filled the new crater at Lassen’s summit, spilled over low spots on its rim, and flowed 300 metres (1,000 ft) down the steep west and northeast flanks of the volcano.
At 4:30 p.m. on May 22nd 1915, Lassen Peak exploded after two quiet days in what is referred to as “the Great Explosion”. The eruption column rose more than 10,000 metres (30,000 ft) and was visible from as far away as the Pacific coast 240 km (150 mi) to the west.
The blast created the larger and deeper of the two craters seen near the summit of the volcano today. Pyroclastic flows devastated an area of 8 km2 (3 sq miles) below the volcano, and melting snow created a series of lahars that rushed nearly 16 km (10 miles) down “Lost Creek” to the Old Station. The lahars released large masses of water that flooded the lower Hat Creek Valley a second time. A layer of volcanic ash and pumice was traceable for 25 miles (40 km) to the northeast and fine particles of ash rained down as far away as 320 km (200 mi) to the east.
For several years after the main eruption, rainfall triggered steam explosions indicative of fresh lava just below the surface, and in May 1917, a series of vigorous hydrothermal explosions obliterated the two older craters and dug out the second of the two summit craters visible today. It was not until 1921 that the eruptive period was officially declared over.
Mount Tehama today. Activity at the Lassen Volcanic complex is far from over. There are several areas of hot springs and fumaroles of which the Sulphur Works and the hot mud springs at Bumpass Hell on the Lassen trail are the most well known. With major eruptive activities during the Holocene, the area around Mount Lassen, which more accurately ought to be referred to as Mount Tehama, 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. Should they do so during our lifetimes, it is not inconceivable that we might see a repeat of the rapid growth of Mount Shastina, Lassen Peak or the Chaos Crags. I for one would much rather see this volcanic creation process than a truly huge eruption that has a “human impact”, to put it euphemistically…
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