To put this eruptive process 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 cubic kilometers, a rough calculation yields a volume of 425 cu km, comfortably larger than either present-day Mount Shasta or Mount Adams, both of which have volumes of about 350 cubic kilometers.
Here named Brokeoff Volcano, this shows the size and position of this monstrous mountain as it was 300 thousand years ago.
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 vents 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.
A current view of one flank of the ancient Mount Tehama, now named Brokeoff Mountain.
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.
Soon, perhaps, this might be on the menu for local entertainment here. Who's bringing the popcorn?
Hey, don't laugh, it's happened relatively recently!
This view of the climactic eruption of May 22nd, 1915, was taken from Anderson, California, more than 20 miles from 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 into the sky more than 30,000 feet and was visible from as far away as the Pacific coast 150 miles 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 3 square miles below the volcano, and melting snow created a series of lahars that rushed nearly 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 to the northeast and fine particles of ash rained down as far away as 200 miles 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.
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.
If they erupt, the climate in Redding will change
ReplyDeleteI heard St.Helens blow. And I was 200 miles away. Thought it was someone dynamiting a roadway. Till I got back back to camp and listened to the radio.
ReplyDeleteIf you drive up the Puyallup river toward Enumclaw,( I promise I am not making these names up) toward Mt. Rainer, there will appear a series of hills, oddly shaped in that they seem to rise up out of nothing, like a bowling ball sitting in a mud puddle. In fact, that is exactly what they are- the entire valley has a thick mud flow in it, burying the lower,normal ,gradual slope of the hills.
ain't geology wonderful!
ReplyDeletecentral washington university has a few very good youtube videos that I could recommend you watch if interested.
ReplyDelete...and they keep driving their Prius's (sp) trying to save the climate :)
ReplyDeleteThese volcanoes must have been awe inspiring to see. The super volcanoe at yosemite being the grand daddy of all.
There is a section of I-5, north of Redding, (in the neighborhood of the barn roof that says "State of Jefferson" as I recall) if you look off to the east across the valley you can see several volcano looking mountains.
ReplyDeleteI'm not an artists but I think if those volcanoes looked "not dormant" (smoke rising from them kinda look) in a present day setting it would look neat!
It's been awhile since I was up that way....
Oh, why worry about a few exploding mountains? Global Warmening is going to kill us all in ten years, Robert Francis says.
ReplyDeleteIf you've gotta worry about something... wild fire
ReplyDelete