There is something about the west coast of the contiguous US. The region combines a wonderful climate with a high standard of living where the land is big, diverse and beautiful. It is a magnet for people, on par with Florida. But it has something which Florida lacks. Whilst Florida is a subtropical Caribbean paradise, the state is entirely volcanically inactive. In contrast, all three west coast states have volcanoes in full working order.
These active volcanoes are surprisingly equally distributed among the coastal states. Washington State has five: Mount Baker, Mount Rainier, Mount Adams, Glacier peak and Mount St Helens (the only holy one, and of course the one that erupted so famously). ‘Active’ here is defined as a volcano with active magma which could well erupt. Indeed, of the five, four have shown activity in the past 250 years!
Moving south, Oregon is a very different state but it has the same number of active volcanic systems: Mount Hood, Mount Jefferson, Newberry, Three Sisters (which I here count as one) and the bad one, Crater Lake. Mount Hood had an eruption less than 250 years ago: the others last erupted 1000 to 5000 years ago.
The unexpected winner is, though only just, California. It has 6 volcanoes which have erupted in the past 5000 years: Medicine Lake Volcano, nearby Mount Shasta, Lassen Volcanic Center, Long Valley Volcanic Region, Ubehebe Craters (did you know that one?), and Salton Buttes. (Clear Lake Volcanic Field is a 7th volcano which could erupt but it has been quiet for the past 10,000 years.)
Winning by numbers is not the same as winning by results. In terms of impact, of the six Californian volcanoes only Mount Shasta ranks among the 10 most dangerous volcanoes in the US. It last erupted around 1250 (not 1786 as some say, which was a grass fire). But number 11 on the list is also Californian: it is unassuming Lassen Peak which erupted as recent as the 20th century. USGS views it with caution for a reason.
At the very end of the Cascades lies this shapely mountain, 3100 meter tall. The mountain looks barren brown – trees grow only at some distance from the peak. This emptiness is evidence of its activity. This is the location of the other Cascade eruption of the 20th century. Before St Helens, there was Lassen Peak.
The area is a national park which describes itself as ‘ home to steaming fumaroles, meadows freckled with wildflowers, clear mountain lakes, and numerous volcanoes. Jagged peaks tell the story of its eruptive past while hot water continues to shape the land.’ The description tells you that this area grew into a volcanic field rather than a single mountain. The land is deeply eroded by the ice age glaciers. (So much of the beauty of the natural world which we take for granted came from the Ice Age. How different our mountains were before the ice.) Hiking is the best way to see the Park: there are many well maintained trails, although not in winter when they can be covered under meters of snow and the park is closed. The fumaroles steam even better in the cold, though. They are fed by water from the snow melt, which percolates down to depth where magma is present. The heated water returns to the surface in hot springs and burping mud. The area is wild and diverse with buttes, cones, domes, lakes and sand. Much of the forest was sadly destroyed in the Dixie fire of 2021. Lassen peak is the largest of the domes, and according to wikipedia is among the largest plug dome volcanoes in the world.
Lassen peak formed in a brief phase where dacitic magma reached the surface and pushed out in a big dome, too viscous to flow far. For some reason to grew much larger than such volcanoes normally do. It happened within the space of a few years, some 27,000 years ago.
It is not the only volcano in the park. There are four extinct shield volcanoes, much older than Lassen Peak, and over 30 other volcanic peaks of varying ages – some very young.
The Park and the mountain are named after Don Pedro (Peter) Lassen, who in spite of his adopted name (the territory was Mexican at the time) was a Danish immigrant. He moved to California in 1840 where he worked as a traveling blacksmith and adventurer and later as a rancher. Lassen has left us a mixed legacy. One of his projects was the Lassen cut-off, an alternative to the final section of the California Trail along which people traveled to the state. He advertised it as shorter and better while in fact it was longer and worse and people who took it regretted it – some needed rescuing. Lassen started his cut-off around 1848 . It quickly became known as the death trail and was abandoned in 1853. The cut-off ended near his ranch – where he was ready to sell new supplies to the exhausted and desperate travelers, having made sure in the advertisement that those migrants would have carried minimal supplies for this ‘shorter’ route. (It was twice as long as he said it was.) A third of the gold rush ’49ers’ took this route. This was the era of the gold rush, and it is perhaps a sign of how the wild west brought out the worst in people. Lassen was murdered in 1859. It is not known who did the deed but there were plenty of potential suspects. This criminal fraudster was awarded with the naming rights for the area. Perhaps a reassessment would be in order? A more worthy name is that of Helen Brodt, the first woman to climb Lassen peak in 1864: Lake Helen is already named after her. But perhaps the native Americans have a name which has primacy and could be used. Tehama Park, perhaps? Any name would be better!
The eastern half of the park is a volcanic plateau. There are some cinder cones here. The youngest is called in typical creative language, Cinder Cone: it formed from eruptions around 1650. Hot springs are found in the south and southwest. They sit at the location of an ancestral volcano, Mount Tehama which has long gone. The eruptions here have been mainly basaltic and andesite, but Lassen peak is made of dacite, a more evolved version. It formed 27,000 years ago as a dacitic dome. Other domes formed around it at various times, including Chaos Crags, 1100 years ago. The whole area remains at risk from eruptions, but also from more pedestrian hazards such as rock falls and mud flows. And of course, the main hazard are the tourists themselves who ignore barriers to explore where no one has gone before (and returned), and get badly burned when breaking through the thin soils into the hot springs below.
The presence of both shield volcanoes and dacitic domes shows that the region is complex. What they have in common is that the eruptions are mostly effusive rather than explosive. The shield volcanoes formed from basaltic eruptions: they are located along tectonic faults which allowed magma to reach the surface. The dacitic domes formed from more viscous magma which slowly pushes its way up and out, but has difficulty flowing far. Both avoid explosions because of a lack of gas. The cinder cones form the third type of eruptions here.
Volcanoes in the Park have come and gone. Tehama was around half a million years ago. It build up a shield a little larger than Lassen Peak is now. The mountain itself is gone, largely through erosion, but a variety of ridges remain. Route 89, providing access to the park, runs through the remnants of Tehama. After it died, the magma became more dacitic and a series of cones formed around the current area of Lassen peak. Now the four shield volcanoes grew. Over the past 50,000 years, there has been a variety of eruptions, some producing basalt, some going for dacite. The basaltic eruptions have filled in a NNW to SSE oriented graben which runs from Hat Creek Valley to Lake Almanor.
One phase of eruptions produced thick dacitic lava flows. These came from the region of the later Lassen Peak. The oldest flows are seen at King’s Creek: they have been dated to 38,000 years ago. Over some 10,000 years, the flows formed a small plateau some 400 meters high. The dacitic dome of Lassen peak formed on the plateau, probably within a very short space of time, perhaps as short as a few years.
Since that time, a series of smaller lava domes have formed around it, on average around 1 per 1000 years. The most recent of these are Chaos Crags: a series of 6 separate domes on the northern flank of Lassen peak. They are rough-looking, steep, and crumbly!
The Lassen Peak Volcanic Field is at the southern end of the Cascades. The Cascades are the main belt of west-coast volcanoes stretching from northern California to Canada, sitting on the west side of the Rocky Mountains. Here are the Big Beasts of the West: Rainier, St Helens, Shasta. It is a long and rugged mountain range, with isolated volcanoes rising far above the other mountains. The range is young, as mountains go: these mountains began to grow around 7 million years ago, long after the Rockies.
The Cascade’s cause lies to the west, in the Pacific ocean where the Juan de Fuca plate is being extinguished. It is subducted – reluctantly – underneath the American continent. Only a small fragment of it is left. It has split up in three parts, separated by transform faults. The southern part, off the coast at Lassen Peak, is called the Gorda plate. But in effect this is still the same, dying plate. Different from most subduction zones, there is no trench along the coast. This is because the plate here is young and remains buoyant: It does not sink much while it is overridden by the continental plate.
Underneath the Cascadian range is a melt zone where the subducted plate releases its water, the water rises into the bit of mantle just above and lowers its melt temperature. Magma forms, begins to move upward, and shapes volcanoes. The magma also pushes up the land around it: it has lower density than the solid rock, and therefore the land above sits a little higher. Erosion now makes the texture of the land. This rising, eroded land becomes topped by the volcanoes. Some of the eruptions build up plateaus rather than mountains. It is all part of the mountain-building game, forever played by the Plates. This game gave us the US’ most impressive volcanoes – and it gave us the wild one, Lassen Peak. In 1914, Lassen was ready to show us its nature.
The 1914 eruption
After a long quiescence, a phreatic explosion on 30 May 1914 heralded the re-awakeing of Lassen Peak. The explosion came from the summit of Lassen peak. More such explosions followed. Over a year, some 180 explosions dug out a summit crater which finally grew to 350 meter across.
14 May 1915 was the day it went for it Lava reached the surface and was thrown out in the day’s explosions. The lava heralded the arrival of dacitic magma. As in an echo of the formation of the Lassen Peak 27,000 years before, a new dome formed on top of the old dome, starting inside the crater but quickly growing beyond it.
The next phreatic explosion occurred on 19 May. It blew the dacitic dome apart and covered the summit and flanks in the debris. Now the real event started. The steep, snow covered slopes were carrying the weight, and the still hot dacite block melted the snow. An avalanche of rocks and mud came down the northeast side and reached Hat Creek and Lost Creek Valley, 7 miles away. This was a lahar before lahars were known. The melt water flooded Hat Creek Valley.
The removal of the dome opened up the conduit for more fluid lava. It came down the flanks in short flows, up to 300 meters long.
For two days nothing happened. Then, on 22 May 1915, the mountain exploded again. What caused this is not known. Did something allow water to enter below the lava? Or had gas collected in the conduit, unable to pass through the cooling lava surface? This explosion was the largest one. It formed a crater which still exists. The eruption column was 10 km high. It was no St Helens, but it certainly attracted attention. A pyroclastic flow formed, following the same path as the lahar of 3 days before. It carried with it the mud and melt of the previous flow, entered Lost Creek and flooded Hat Creek – again. Light ash fell 300 kilometer away, blown with the wind. The area which was devastated by this flow is still called the Devastated Area.
In a way, this was the end. In another way, it wasn’t. The lava was finished but the water was still there. The hot rocks continued to cause phreatic explosions especially during the snowmelt season. In May 1917, the largest of these carved out the second of the two current summit craters. Over time, the explosions lessened and finally ceased. In 1921, the phreatic season did not return. Steam would continue for decades, but the eruption was over.
There is a short film of one of the phreatic explosions of 1915, republished on youtube:
Why was the activity always in May? This shows the importance of water. May is when the thick snow (over 10 meters is possible!) begins to melt and turn the ground to mud. The phreatic explosions reacted to water coming from the melting snow. This volcano is driven by weather.
The 1914 eruption had come unexpectedly. Could we have seen it coming? With modern instrumentation, that is more than likely. But there had been other signs that the Lassen Peak area was not dead, such as the young-looking ejecta in the region. One of these was the Chaos Crags, on the northern flank of Lassen Peak. It was clearly a fairly recent eruption.
The Chaos Crags eruptions have been carbon dated from pyroclastic deposits. The age ranges of 1000 to 1100 years. It started with an explosive phase which deposited about 0.15 km3 of pumice and pyroclastic flows. The lava flows came later. The Crags themselves contain a little over 1 km3 of lava, which was erupted quickly. About 70 years later, one of the domes collapsed and exploded. Nothing more happened until 300 years ago, when a second dome collapsed. The collapse obliterated the area to the north: it is now called the Chaos Jumbles. The oldest trees growing around the Jumbles here are indeed 300 years old. The Chaos eruption was ten times larger than the 1914-1921 eruption. It shows what the area can do and why it is so high on the list of the most dangerous US volcanoes.
This double hill of loose scoria is more recent than Chaos Crag. (The older hill is almost entirely covered by the younger one.) The lava erupted into a lake a small part of the lake survived on either side of the hummocky lava flow: this flow is called, for unclear reasons, the Fantastic Lava Beds. (The naming in the area seems open to improvement!) The eruption was at first thought to date around 1850, but it was quickly realized that trees in the supposedly erupted area were 150-200 years old. This changed the date to around 1650. A tree ring record showed that there was a notably poor year for tree growth in 1666, and this is considered as one possible date for the eruption. Although there are several different lava flows and ash deposits, they were all erupted within a few months, as part of a single event.
There is a story behind the eruption date of Cinder Cone. Some early explorers (around 1870) thought that the lava looked so fresh that it must have been erupted only decades before. In fact there had been reports of shooting lava and hot rocks from people traveling on the gold rush trail. Some reports were from the spring 1851 but were not reported until later. However, there is also an earlier report from the Daily Pacific News in San Fransisco of 21 August 1850, of an erupting volcano with “burning lava still running down the sides.” Other claims were dated to the winter of 1850. For a while this was taken as fact and attributed to Cinder Cone. Later it was realised that it could not be correct: from traveler reports, it was clear that the mountain had not changed before and after this year, and none reported any signs of an eruption. Even a tree growing near the summit was still there and was in good shape. Was Lassen involved in the story, getting in some advertisement for his trail? After all, an other alternative and competitor, the Nobles Trail, ran close to Cinder Cone. It is a guess, but it would fit with the character of the guy.
The original magma of the Chaos eruption was rhyolite. This was mixed with a smaller amount of basaltic magma, with high aluminium content. The basaltic fraction increased during the eruptionL the first ejecta had very little, later ones had about 20%. The eruption came from a rhyolitic magma chamber which suffered a basaltic intrusion. At the time of mixing, the original magma was about 70% liquid and 30% crystalized.
Cinder cone also showed a changeable magma composition, although not as diverse as Chaos Crag. It started out as basaltic andesite, changed to andesite and ended up again as a basaltic andesite bit now with higher titanium.
The Lassen peak eruption showed four different types of lava, all erupted within the three days of the main phase: black dacitic lava, andesitic lava, banded lava with dark andesite and light dacite, and plain light dacite, which were erupted in this order. The andesitic lava formed inclusions in the black dacite: the inclusions ranged in size from a few millimeters to half a meter, and accounted for 5-10% of the lava. As in the other cases, there was a host magma (dacite) and an intrusion (andesite), where the magma was partly mixed and partly unmixed. The first lava formed the dome out of glassy black dacite. Some of it was apparently solidified lava which had been blocking the conduit. The lava that followed the dome explosion was again dacite. The banded and unbanded light dacite was erupted on the 22 May.
The intrusions varied between eruptions. Chaos Crags contained the most mafic of the lavas erupted in this period, the other intrusions were more evolved. The intrusion was always hotter than the magma it intruded. This temperature contrast prevented mixing, and caused the new magma to form separate inclusions. Later the temperatures equalized and the magma types now formed a more homogenous mix. Perhaps the best way to visualize this is with a bread maker. Although I do not recommend pouring magma into one!
It seems clear that there is a significant reservoir of uneruptable old magma in the area, which can activates when an intrusion of hotter new magma mobilises it. How long this process takes we don’t know. Perhaps the next eruption will tell us.
Based on past events, Chaos Crags has long been identified as the most dangerous part of the park. Eruptions give warning, but rock falls come suddenly. The Kazbek disaster has shown how dangerous these can be. Now add the risk of one of the domes collapsing completely, with the resulting rock avalanche reaching the tourist areas at extreme speed and without warning. This hasn’t happened for 300 years – but one in 300 is a significant risk. USGS monitors the Crags closely, looking for the early signs of movement.
Could there be another eruption? We have no idea. Was the 1914 eruption related to the one 1000 years ago? Is there a living magma chamber here, ready to receive an infusion of fresh basalt? The answer to that is probably yes. But that might also be the start of a tourist eruption. The contiguous US has never had one. I do hope they fix the names before it happens.
Albert, December 2022