An Evaluation of the Risks of Lassen Peak - NAM
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An Evaluation of the Risks of Lassen Peak
Lassen Peak is a lava dome volcano located in Northern California as part of the Lassen
Volcanic National Park. It is part of the oceanic Gorda Plate, which is currently subducting
beneath the continental North American plate at around 2.5-3 cm/year. The primary hazards
associated with this volcano include avalanches, pyroclastic flows, mudflows, and flows of hot
ash and gas. This volcano is considered active, with its most recent explosive eruption occurring
in May 1915. Mineral, a nearby town, is at the greatest risk of hazard in the event of volcanic
activity, with Chester, Westwood, and Burney also facing some risks of potentially far-traveled
lava flows and/or lahars. Since the pyroclastic flow produced by Lassen Peak is primarily basalt
(and therefore mafic), it has a low viscosity and can move fast. So, it is important to have a
system of early detection in the event of a potential eruption to evacuate people in the
aforementioned towns to ensure that people can get evacuated in time. This can be fairly easily
achieved by monitoring seismic activity, and issuing orders for evacuation in the case of unusual
seismic activity that may cause an eruption. To deal with any ash fall, buildings should be
airtight to prevent ash from entering ventilation systems. Additionally, nearby towns may have
stations set up to provide N95 masks to residents to ensure they do not inhale any ash, and issue
an order to minimize time spent outdoors during ash fall.
Lassen Peak is located in Northern California within Shasta County as part of the Lassen
Volcanic National Park. This volcano is part of the Cascade Range, and is the southernmost of
the active cascade volcanoes. Created 27,000 years ago, Lassen Peak currently stands at a
maximum elevation of 10,457 feet (3,187 meters). It has a volume of 0.6 cubic miles (2.5 km
3
),
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placing it as the largest known lava dome on the planet. The volcano originated from a portion of
Mount Tehama as a result of a series of eruptions. It has since been significantly eroded by
glaciers, and is now covered with talus deposits.
View of Lassen Peak, Britannica
Map of Cascade Volcanoes, USGS
A 2018 Update to the U.S. Geological Survey National Volcanic Threat Assessment by
multiple geologists for the USGS placed Lassen Peak in the “very high threat” category, ranking
it as the 11th highest threat volcano in the United States, making it the 2nd highest threat volcano
in California, second only to Mount Shasta (Ewert, 2018). These rankings take into account 24
different factors, including likelihood of eruption and potential consequences of eruption. While
it cannot be predicted when Lassen Peak may next erupt, it is considered active due to its most
recent eruption being just over 100 years ago on May 22, 1915. Its main reason for being ranked
so high on the list is because of the potential consequences of an eruption. Since it is the largest
lava dome volcano in the world, it would have a devastating eruption that could completely
destroy nearby communities, and would lay waste to the Lassen Volcanic National Park.
The Lassen Volcanic National Park takes in roughly 300 to 500 thousand visitors every
year, which means that if Lassen Peak were to exhibit volcanic activity, it would put a large
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number of both workers and visitors in severe and immediate danger (
Lassen Volcanic Visitation
by Year
, 2022). The nearby communities that are at greatest risk of an eruption include the towns
of Mineral, Burney, and Chester, with several others also in potential danger. An eruption would
also pose a threat to the surrounding ecosystem, including freshwater sources and a forest full of
diverse wildlife, including endangered species such as the Sierra Nevada red fox (
Mammals -
Lassen Volcanic National Park
, 2022).
As with the rest of the Cascade Range, Lassen Peak formed on a convergent tectonic
boundary. As mentioned previously, Lassen Peak is a fairly new stratovolcano in terms of
geologic time, forming from the much older Mount Tehama. Its volcanic properties are a result
of the Gorda plate (the southernmost fragment of the Juan De Fuca plate) being subducted
underneath the western boundary of the North American plate. Eventually, once the Juan De
Fuca plate has been completely subducted under the North American plate, volcanic activity in
Lassen Peak and the rest of the Cascade Range will cease, however this will take millions of
years to happen, so it is still important and relevant to monitor Lassen Peak’s activity. Due to
having a more recent eruption, much of the rock currently found near Lassen Peak is volcanic,
covering up the sedimentary, igneous, and metamorphic rocks below.
Lassen Peak is classified as a lava dome volcano, and as such it has steep sides and a
rough, rocky surface. The composition of the magma in Lassen Peak is primarily basaltic. This
results in less explosive eruptions, although subsequent pyroclastic flow is less viscous, and can
flow much faster. Since it is still quite young, Lassen peak has only exhibited large-scale
volcanic activity once in its lifetime, with eruptions occurring from 1914-1917. The eruption
began with steam explosions caused by rising magma evaporating ground water. One particularly
large steam explosion fragmented the peak’s dome, creating a new crater. Falling blocks of hot
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lava fell from the dome, resulting in an avalanche that proceeded for 4 miles. The lava and
melting snow formed a lahar which flowed for 7 miles down Lost Creek. A large column of ash
and volcanic gas visible from 150 miles away rose up 30,000 feet, spreading ash as far as
Winnemunca, Nevada (roughly 200 miles east). Flooding occurred multiple times in the lower
Hat Creek Valley. In the following years, snowmelt caused lesser scale steam explosions which
gradually waned off. Steam vents, hot springs, and bubbling hot mud pools indicate that magma
is still near the surface. As such, this volcano remains active, with expected eruptions to occur in
the future (Clynne et al., 2001).
1915 Eruption of Lassen Peak as seen from Red Bluff (44 miles out), National Park Service
Much like several other of the volcanoes in the Cascade Range, the main volcanic hazard
associated with Lassen Peak is the possibility of lahar flows. Lahars form from a mix of
primarily pyroclastic materials, rock debris, and water. Eruptions result in pyroclastic flow being
ejected from volcanoes, and at Lassen Peak there are plenty of loose rocks and talus deposits that
could combine with this material and melted snow or lake water to form a fast-moving, hot, and
deadly lahar. It is for these reasons that lahars are particularly dangerous, as they can travel
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farther and faster due to the water content making them less viscous, especially when combined
with the steep slopes of Lassen Peak which give the lahar more momentum to cause greater
destruction and danger. For instance, in the 1915 eruption of Lassen Peak, a lahar was tracked to
have flowed 7 miles from its source, destroying 6 homes in its path.
Another of the major hazards of Lassen Peak involves the possibility of lava flows.
Lassen Peak actually has two major sources of lava flow, each with different properties. Volcanic
activity in this area could result in “small basaltic lava flows” from vents along the side of the
volcano, however “silica lava flows (like dacite) have formed lava domes in the park, which can
collapse,” resulting in the release of silica-rich lava down the volcano (
Seismic Activity - Lassen
Volcanic National Park
, 2018). The basaltic lava flow results in a less viscous lava flow that is
able to move faster, while silica-rich lava is typically more viscous, and therefore thicker and
slower-moving. Basaltic lava is also typically hotter, however these factors do not mean that
dacitic lava flows are not dangerous, as they typically are accompanied by more explosive
eruptions. The figure below shows the potential range of these lava flows (as well as the
aforementioned lahars).
Hazard range map of Lassen Peak, USGS
5
A third main hazard of Lassen Peak is the launching of volcanic materials into the
atmosphere, resulting in rock fragments raining down and a blanket of volcanic gas and ash
covering the nearby areas. Often overlooked as a hazard, ash fall is the least immediately deadly
hazard of the three, however it is the most far-reaching, meaning it has a greater chance to impact
a wider range of communities. With the past eruption of Lassen Peak, fall of ash and pumice
primarily impacted a 25-mile radius, but some fine ash reached distances of up to 200 miles
away. In addition, “pent-up gases within the volcano blasted and shattered the lava cap” resulting
in “hot lava rocks … careening down the mountain” (Swackhamer, 2012). While it is difficult to
predict the severity and how far the rocks may travel, all it takes is one unfortunate steam
explosion to send massive boulders towards residential areas.
When it comes to assessing the risks of any of these hazards occuring in the near future, it
is very difficult to determine if and when they may occur. However, if an eruption like the one in
1915 were to occur again, it is highly likely that all these hazards (and more) will occur. It would
have devastating effects for the landscape, wildlife, and nearby populations. If proper precautions
are not taken, loss of human life would be likely (mitigation and evacuation plans will be
discussed later in this paper). The town of Mineral would be at the highest risk, being in such
close proximity (roughly 8 miles). However, the Lassen Volcanic National Park that was
established shortly after the eruption of Lassen Peak would be in the greatest danger, as it could
be damaged even without a major eruption.
Lahar flows would be the most dangerous of the 3 major hazards. Since a lahar flow can
form without any major volcanic activity, it is difficult to predict when they may happen. All it
could take is for a heavy rain to expose volcanic ash and cause a landslide, both of which come
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together to form a lahar which ends up taking out the town of Chester. Again, while this would
be incredibly unlikely, it is a nonzero chance that must be prepared for. Due to the fast-moving
nature of lahars, they can appear quickly, and will destroy everything in their path. While nearby
towns like Mineral and Burney aren’t currently shown to be in the path of a potential lahar, they
are still in close enough proximity that they should be prepared for any possibility.
While lahar flows typically follow river streams and have more guided paths, lava flows
can be a bit more variable. As the figure on page 5 of this report shows, the lava flows can
originate from several sources, and can reach a wider range of terrain, placing Burney and
Mineral in the direct path of potential lava flow. The lava from the dome is composed of mostly
dacite, meaning it is felsic and silica-rich, and therefore is more viscous and flows more slowly.
While lava flows from the dome would likely contain a greater total volume of lava, it won’t
flow as far nor fast, so while infrastructure (primarily in the Lassen Volcanic National Park) may
be destroyed, all people should be able to be evacuated safely. Vents throughout the Lassen
Volcanic Center could also result in smaller lava flows that are more basaltic. While the volume
of these flows would be less than from the dome, the basaltic lava is mafic and contains a lower
percent composition of silica, meaning it is less viscous and can travel farther and faster.
Combining this with the fact that these vents are also closer to population centers than Lassen
Peak’s dome, the response time for lava flows must be very rapid to quickly evacuate all
residents in an organized and efficient manner. While these basaltic lava flows are the most
common form of volcanic activity in the Lassen Volcanic Center, they are typically small and not
near population areas, although this does not mean they should not be considered dangerous, as
the flows still affect the natural life and could still impact human life.
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Lahar flows and lava flows can result from Lassen Peak without a major eruption,
however an explosion that would result in ash fall and rock fragments being blasted into the air
would be even less probable. If such an event were to occur, the ash cloud resulting from the
eruption would cause severe air pollution. This could result in a wide range of consequences,
including respiratory issues for people in nearby towns. The effect on wildlife, both on the
ground and in the air, would also be severe. Birds would be impacted first, and the resulting lack
of sunlight would kill off many plants in the area. The particulate pollution and lack of visibility
would mean any aircrafts would have to make a detour around the area, and the lack of visibility
on the ground would make the roads dangerous. This becomes increasingly dangerous
considering that the explosion resulting in the ash fall would likely be accompanied by other
hazards, such as lahars and lava flow. In this case, evacuation of residents must be rapid and
orderly, and the added difficulty of impaired visibility would only increase the chaos of the
situation and delay evacuation protocols.
Since the eruption of Lassen Peak back in 1915, infrastructure in the area has increased.
The Lassen Volcanic National Park was built shortly after, and takes in over 350,000 people each
year. Since the volcano is still active, this is the area that needs the most detailed and responsive
evacuation plan. The most likely event that must be prepared for is lahar flows and lava flows.
Because the national park is so close to the dome of Lassen Peak, not much can be done to
redirect or guide lahar flow away from the park, so evacuation is the best bet for any visitors and
workers. Plenty of vehicles with off-road capability should be available to evacuate people in
case roads are blocked. All visitors should be given a debrief of where to meet and what to do in
the case of an imminent lahar flow. When it comes to actually detecting the lahar flows before
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they happen, research has revealed that “infrasound may be used to detect the approach of
hazardous volcanic mudflows, known as lahars, tens of minutes before their flow fronts arrive”
(Johnson et al., 2023). While this might not sound like a lot of time, it would actually be
incredibly helpful in making sure an evacuation could occur before the situation starts to go
south and people begin to panic more.
In addition to guiding people away from the path of the lahar, the path of the lahar may
also be guided away from people. Two main methods have shown success in both preventing the
formation of a lahar flow and diverting it away from inhabited areas. Slope stabilization and
erosion control techniques can be implemented to “limit shallow landsliding or surface erosion in
disturbed landscapes that could produce extreme sediment inputs to rivers” (Pierson et al., 2014).
By supporting the landscape, the frequency and severity of landslides and general erosion are
greatly reduced, meaning it is less likely that landslide debris can mix with pyroclastic materials
to form a lahar.
Slope stabilization structures to prevent landslides, Unlimited Drilling & Foundations Inc.
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Unfortunately, slope stabilization structures only reduce the likelihood of a landslide
occurring and forming a lahar; they do not completely prevent a lahar from forming. In the case
that a lahar still forms, it is important to have structures in place to divert lahar flow away from
nearby towns and infrastructure. These structures “are designed to redirect/reroute [lahars] away
and around important infrastructure or communities” (Makarenko, 2021). The primary methods
include building walls to direct the flow or constructing steep, narrow, and smooth channels for
the lahar to follow that takes it well away from the path of nearby communities, ensuring that it
does not harm people or their livelihoods.
2 types of lahar diversion structures, Makarenko
To sum it all up, while it is uncertain when the next major volcanic activity in the area
may take place, it is almost certain that it will eventually happen, given its most recent eruption
just over a hundred years ago. The main hazards associated with an eruption of Lassen Peak are
lahar flows, lava flows, and potentially explosive eruptions shooting out hot gas, ash, and debris.
Because lahars can form suddenly, even without a major eruption, and can move very quickly,
they are the main point of concern when it comes to Lassen Peak’s hazards. So, rather than
waiting around for the next event to happen, it is important to make sure there are plenty of
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safety protocols in action to control and mitigate the effects of lahars and ensure that everyone is
evacuated safely. Such protocols include infrasound technology to detect imminent lahar flows,
detailed routes and reliable off-road vehicles for evacuation, slope stabilization and erosion
control to prevent the formation of lahars, and lahar diversion structures such as walls and canals
that can guide lahars away from the nearby communities.
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References
Clynne, Michael A., et al. “Eruptions of Lassen Peak, California, 1914 to 1917.”
U.S. Geological
Survey
, 20 Jan. 2001, pubs.usgs.gov/fs/old.1998/fs173-98/.
“The Eruption of Lassen Peak.”
National Parks Service
, U.S. Department of the Interior, 28 Feb.
2015, www.nps.gov/lavo/learn/nature/eruption_lassen_peak.htm.
Ewert, John W., et al.
2018 Update to the U.S. Geological Survey National Volcanic Threat
Assessment
, 2018, pubs.usgs.gov/sir/2018/5140/sir20185140.pdf.
“How a Retaining Wall Helps with Landslide Prevention.”
Unlimited Drilling & Foundations
Inc.
, unlimiteddrilling.com/how-a-retaining-wall-helps-with-landslide-prevention/.
Johnson, J. B., et al. “Infrasound Detection of Approaching Lahars.”
Nature News
, Nature
Publishing Group, 20 Apr. 2023, www.nature.com/articles/s41598-023-32109-2.
“Lassen Volcanic National Park Visitation Stats.”
National Parked
, 4 Dec. 2022,
www.nationalparked.com/lassen-volcanic/visitation-statistics.
Makarenko, Alexey. “Lahars.”
ArcGIS StoryMaps
, Esri, 12 Jan. 2021,
storymaps.arcgis.com/stories/92c09b82ad6a4251860dc23aa415811e.
“Mammals - Lassen Volcanic National Park.”
National Parks Service
, U.S. Department of the
Interior, 15 July 2022, www.nps.gov/lavo/learn/nature/mammals.htm.
Pierson, Thomas C, et al. “Reducing Risk from Lahar Hazards: Concepts, Case Studies, and
Roles for Scientists.”
BioMed Central
, Springer Berlin Heidelberg, 6 Nov. 2014,
appliedvolc.biomedcentral.com/articles/10.1186/s13617-014-0016-4.
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“Seismic Activity - Lassen Volcanic.”
National Parks Service
, U.S. Department of the Interior,
31 Oct. 2018, www.nps.gov/lavo/learn/nature/seismic-activity.htm.
Swackhamer, Barry. “Hot Rock Historical Marker.”
The Historical Marker Database
, 6 Aug.
2012, www.hmdb.org/m.asp?m=58114.
“Why Study Cascade Volcanoes?”
U.S. Geological Survey
,
www.usgs.gov/observatories/cascades-volcano-observatory/why-study-cascade-volcanoe
s.
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