Mount St Helens(1)(1)
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Dec 6, 2023
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Monitoring Volcanoes: Mount St. Helens Case Study
Modified from Dzurisin et al., 2013, by Rachel Teasdale (California State University, Chico) and Kaatje van der
Hoeven Kraft (Whatcom Community College)
Read the following to help you with answering questions about the eruptions of Mount St.
Helens.
Mount St. Helens Background
Mount St. Helens seized the world’s
attention in 1980 when the largest
historical landslide on Earth and a
powerful explosive eruption
reshaped the volcano (Figure 1&5),
created its distinctive crater, and
dramatically modified the
surrounding landscape in
southwestern Washington State. An
enormous lava dome grew
episodically in the crater until 1986,
when the volcano became relatively
quiet. A new glacier grew in the
crater, wrapping around and partly burying the lava dome.
From 1987 to 2003, sporadic earthquake swarms and small steam explosions indicated that
magma (molten rock) was being replenished deep underground. Scientists with the U.S.
Geological Survey (USGS) and University of Washington’s Pacific Northwest Seismograph
Network (PNSN) monitor activity at Mount St. Helens and other Cascade volcanoes.
The 1980 eruption at Mount St. Helens opened a new episode in the volcano’s history that began
more than 250,000 years ago and will continue into the future. Following the eruption, lava
domes grew in the 1980 crater from 1980–86. A new glacier— Crater Glacier —formed, and
streams began eroding and transporting millions of tons of sediment downstream (Figure 2).
Years later, hot dome rocks are still steaming, Crater Glacier is still advancing, rock falls send
plumes of dust skyward, and streams continue to erode and transport sediment away from the
volcano. Meanwhile, magma (molten rock) is accumulating again beneath the volcano. Scientists
are keeping a close watch on Mount St. Helens and the other Cascade volcanoes to assess
hazards and provide timely warnings of future activity.
1980—Earth’s Inner Fury Uncorked
On May 18, 1980, Mount St. Helens erupted violently, killing 57 people. The picturesque conical
volcano had been rumbling from earthquakes for months, but on that morning intense earthquake
activity was followed by a massive landslide and collapse of its north side, resulting in one of the
largest debris avalanches (landslides) in history. The landslide triggered a lateral blast that
devastated 150 square miles and erupted an ash column that reached the stratosphere and
Questions or comments please contact education AT unavco.org
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Page 1
Version May 30, 2019
Figure 1: Mount St. Helens erupting in 1980. (USGS)
Unit 1: Mount St. Helens volcano background information student pre-reading
Questions or comments please contact education AT unavco.org
or contact your instructor
Page 2
Figure 3: The U.S. Army Corps of Engineers
dredged sediment from the Toutle (shown
here), Cowlitz, and Columbia Rivers. By
1987, the volume of material removed was
equivalent to a highway 12 lanes wide, one
foot thick, stretching from New York to San
Francisco. (USGS)
Figure 4: Steep-walled channel eroded into
hummocky 1980 landslide deposit along
North Fork of Toutle River. Sediment
produced by rapid cutting down is
deposited downstream, increasing threat of
floods and reducing salmon runs. USGS
photo by Jon Major.
Figure 2: Lidar map of Mt. St. Helens, showing a dynamic landscape that is constantly
changing. Question 1:What kinds of eruption products give the volcano its shape? Which
features in this image changed dramatically on May 18, 1980, or since? Almost all the
terrain in the foreground and middle ground of this image (the devastated area) was
affected by the 1980 blast. Can you name some geologic processes that are active now?
This shaded relief image was produced from lidar data (LIght Detection And Ranging), a
remote sensing technique used to map topography more accurately than was possible
with older methods. The crater is 1.2 miles (1.9 km) wide east-west. Elsewhere the
scale varies owing t to the oblique viewing angle. (USGS)
Unit 1: Mount St. Helens volcano background information student pre-reading
dispersed ash areas hundreds of miles downwind. The eruption also initiated lahars (volcanic
mudflows) that choked nearby rivers with large volumes of sediment (Table 1).
The landslide deposit its surface strewn with huge
blocks of shattered rock called hummocks (Figure
2)—buried the North Fork of the Toutle River
valley as much as 600 feet (180 m) deep for a
distance of 13 miles (21 km) Part of the landslide
overtopped a ridge 1,150 feet (350 m) high 6 miles
(10 km) north of the volcano, leaving a deposit
called The Run-Up or The Spillover (Figure 2). The
eruption destroyed nearly all of the volcano’s
glaciers, and lahars choked the Toutle, Cowlitz, and
Columbia River channels with sediment (Figures
3&4) impacting residents and the shipping industry
downstream.
Pyroclastic flows (ground-hugging avalanches of
hot volcanic ash, pumice, rock fragments, and gases
that destroy everything in their path) rushed out of
the crater left by the landslide and formed the
Pumice Plain in the valley below. Five more
explosive eruptions in summer 1980 sent columns
of ash jetting into the stratosphere, disrupting life in
the Pacific Northwest and threatening air travel.
From December 1980 to October 1986, a lava dome
grew episodically on the crater floor, eventually
reaching a height of nearly 1,000 feet (305 m) and a
volume of 120 million cubic yards (92 million cubic
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Page 3
Figure 5: Images of Mount St. Helens before,
during and after the 1980 eruption.
Table 1: Results of the 1980 eruption of Mount St.
Helens.
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Unit 1: Mount St. Helens volcano background information student pre-reading
m). Snowfall and avalanches from the steep crater walls collected on the crater floor, where they
were shaded from sunlight during most of the year. As a result, Crater Glacier formed and began
to flow—the newest and only expanding glacier in the Cascade Range. Newly formed streams
began the monumental task of eroding and transporting millions of tons of sediment downstream.
More than three decades later, only about 8% of the sediment that entered the river system in
1980 has been eroded away. The downstream flood threat from rivers clogged with mobilized
sediment will require attention for decades to come.
Videos to watch
7-minute USGS video on the 1980 eruption
:
https://pubs.usgs.gov/fs/2013/3014/videos/May_18_1980.mp4
4-minute video on Geodetic Monitoring of Volcanoes:
https://serc.carleton.edu/details/files/258187.html
Monitoring Can Provide Advance Warning of Future Eruptions
Volcanoes generally do not produce large earthquakes like those that occur along plate
boundaries, but sometimes swarms of hundreds to thousands of small quakes occur beneath a
volcano and provide clues to processes occurring deep underground. Ground deformation
(swelling, stretching) occurs when the pressure beneath a volcano changes, which can mean that
magma is accumulating or moving toward the surface. A change in the amount or composition of
gases released from magma as it rises toward the surface also can foretell an impending eruption.
By monitoring and analyzing earthquakes, ground deformation, and volcanic gas emissions,
scientists are better able to understand a volcano’s behavior, to assess hazards and potential
impacts, and to provide timely warnings of future events.
Mount St. Helens is a world-class natural laboratory where USGS and other scientists are
learning about processes of rapid landscape change, the magma system that feeds the volcano,
and the volcanic and tectonic forces that shape the Pacific Northwest, as well as the warning
signs and hazards associated with eruptions. Mount St. Helens is the volcano in the Cascades
most likely to erupt again in our lifetimes. The exact timing and magnitude of the next eruption
cannot be forecast long in advance, but our growing knowledge and continued monitoring will
enable the USGS to provide short-term forecasts and warnings—as was done in 1980.
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Page 4
Figure 6. Various methods used to monitor volcanoes.
Unit 1: Mount St. Helens volcano background information student pre-reading
Preparation for Future Eruptions
Mount St. Helens tends to be relatively quiet between eruptions, with occasional earthquake
swarms and minor amounts of ground movement (deformation) and volcanic gas emission.
Scientists continue to monitor the volcano closely for any sign of a change from the normal
background level of activity, but until that happens the timing of the next eruption cannot be
known. On the other hand, we know from the geologic record at Mount St. Helens that additional
explosive eruptions are possible and that dome building can go on episodically for decades to
centuries. If past trends are repeated, renewed dome growth will be preceded by a few days to
weeks of heightened earthquake activity, ground deformation, and gas emissions, with occasional
earthquake swarms and minor amounts of ground movement (deformation).
Volcano hazards in the Mt. St. Helens region
The simplified hazard map by Wolfe and Pierson, 1995, (Figure 7) shows areas that are prone to
ground-based hazards from eruptions at Mount St. Helens. Becoming familiar with the hazard
map is a good start to understanding areas of safety in the event of an eruption.
References & Resources
Dzurisin, D., Driedger, C.L. and Faust, L.M., 2013, Mount St. Helens, 1980 to Now —What’s
Going On? USGS Fact Sheet 2013-3014, U.S. Department of the Interior U.S. Geological
Survey. Retrieved July, 2018 from
http://pubs.usgs.gov/fs/2013/3014
.
Wolfe, EW and Pierson, TC, 1995, Volcanic-Hazard Zonation for Mount St. Helens, Washington,
1995, USGS Open File Report 95-497.
Reading Questions: Answer the following questions based on the reading and
video.
1.
(Refer to Figure 2, on page 2)
What kinds of eruption products give the volcano its
shape? Which features in this image changed dramatically on May 18, 2980, or since?
2.
What are the eruptive products from a volcano like MSH and what are the hazards
associated with each?
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Page 5
Unit 1: Mount St. Helens volcano background information student pre-reading
3.
What are at least three geologic indicators you would look for to determine if Mount St.
Helens were going to erupt in the near future? Your answer should include a description
of the characteristics of geologic indicators and why the characteristics suggest volcanic
activity is increasing.
4.
How would you notify and evacuate the surrounding populace? How would propose
convincing residents to take the threat seriously - without causing panic? Name 1-3
things you would specifically do differently from the protocol utilized in the 1980
Eruption.
Questions or comments please contact education AT unavco.org
or contact your instructor
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Page 7
Zone 1 represents the area vulnerable to passage of high-concentration (high-density) flows,
including pyroclastic flows, lava flows, and proximal parts of lahars.
Zone 2 represents the area that could be overrun by pyroclastic surges, which are low-concentration
(low-density) flows.
Zone 3 includes the intermediate and lower reaches of valleys that could be inundated by lahars.
Town boundaries shown are not official corporate boundaries but are drawn by the authors around
areas of de-facto urban-suburban area as indicated by the highest concentrations of roads depicted
on USGS 1:100,000 quadrangles of late-1980’s vintage.
Water body
Stream