7.5: Mount Shasta
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American poet and novelist, Joaquin Miller wrote, "As lone as God, and white as a winter moon, Mount Shasta starts up sudden and solitary from the heart of the great black forests of Northern California." At an elevation of 4,317 m (14,163 ft), the composite volcano of Mount Shasta towers over the surrounding landscape (Figure \(\PageIndex{1}\)). With nearly 10,000 ft (3,000 m) of local prominence, Mount Shasta has been a source of fascination and inspiration for as long as humans have lived in the region. Mount Shasta is located at the ancestral territorial boundary of the Shasta, Modoc, Ajumawi/Atsuwegi, and Wintu peoples. It can also be seen from the ancestral lands of the Karuk and Klamath, making the mountain a unique and important traditional landmark. As such an important and prominent landmark, Mount Shasta has had many different names in many languages. Wakanunee-Tuki-wuki is a name that was used by the Shasta people, but neighboring tribes like the Ajumawi, Karuk, and Wintu also have names for it, such as Ako-Yet, Uytaahkoo, and Bohem Puyuik, respectively.
Although Mount Shasta has not had a volcanic eruption in historic times, within the last 4,000 years, it has been one of the more active among the Cascade volcanoes. The U.S. Geologic Survey has ranked Mount Shasta fifth in the list of volcanic threats in the United States, a list that takes into account not only the likelihood of an eruption but also the severity of damage if an eruption were to occur. Within the Cascade volcanoes, it ranks only behind Mount Saint Helens and Mount Rainier, both in Washington. The USGS monitors Mount Shasta (and other volcanoes throughout California) for signs of unrest, and issues an alert level of ‘normal’, ‘advisory’, ‘watch, or ‘warning’. The current alert level is published on the California Volcano Observatory (CalVO) website.
Eruption History
Before the Mount Shasta that we know today began to form, a similar volcano was built in the same place, beginning 590,000 years ago. Around 300,000 years ago, this ancestral composite cone collapsed, destroying the edifice of the volcano and spawning one of the largest landslides known on Earth, covering more than 440 km2 (170 mi2) of Shasta Valley to the northeast. The hummocky terrain of this landslide deposit remained a mystery to geologists for many decades (Figure \(\PageIndex{2}\)). Eruptions that caused edifice collapse had not often been observed in action and so little was known about the processes leading up to and during such an eruption. Then, on May 18, 1980, geologists witnessed an edifice-collapsing eruption at Mount Saint Helens, a volcano in the Washington Cascades. The influx of magma swelled the volcano, oversteepening its northern flank. This resulted in a large landslide, which in turn triggered the explosive eruption. When the dust settled, geologists found hummocky terrain, similar to that of Shasta Valley. True to the mantra, “the present is the key to the past,” this new development allowed geologists to re-interpret the hummocky terrain in Shasta Valley and conclude that it was produced in a similar, though much larger, volcanic eruption that included a landslide.
If you would like to learn more about this hummocky terrain in the Shasta Valley, and to see a variety of images, watch “Mystery Mounds at Mt Shasta; A Landslide that Traveled 30 Miles”.
The modern edifice of Mount Shasta was primarily constructed during four major cone-building episodes that were centered on separate vents, shown in Figure \(\PageIndex{3}\). The eruptions that formed these cones probably lasted for only a few hundred or a few thousand years, during which numerous lavas erupted, mainly from each cone's central vent. The final major eruptions from each of the central craters produced dacitic domes and dense-fragment pyroclastic flows. After each episode of rapid cone building, the volcano underwent significant erosion while less frequent central- and flank-vent eruptions occurred. The Sargents Ridge cone, the oldest of the four, is younger than approximately 250,000 years, has undergone two major glaciations, and is exposed mainly on the south side of Mount Shasta. The next younger Misery Hill cone is younger than approximately 130,000 years, has been sculpted in one major glaciation, and forms much of the upper part of the mountain.
Two of the main eruptive centers at Mount Shasta, Shastina (3,758 m 12,303 ft), and Hotlum cones were constructed during Holocene time, which includes about the last 10,000 years. Holocene eruptions also occurred at Black Butte, a group of overlapping dacite domes about 13 km (8 mi) west of Mount Shasta (Figure \(\PageIndex{2}\)). Evidence of geologically recent eruptions at these two main vents and at flank vents forms the chief basis for assessing the most likely kinds of future eruptive activity and associated potential hazards.
The youngest, well-documented eruption of Mount Shasta was about 3,000 years ago. Small, short-lived blasts of steam and ash may have occurred more recently, perhaps as recently as 1,800 to 200 years ago, but these events need additional field verification. Hot springs and volcanic gasses seep from the summit indicating a relatively young and still-hot system. The record of eruptions over the last 10,000 years suggests that, on average, at least one eruption occurs every 800 to 600 years at Mt Shasta, which is one of the reasons for its fifth place ranking in the USGS National Volcanic Threat Assessment.
Review your rock identification skills (7.2: Rock Types of the Cascades) and apply them in the activity, “Rock Identification: Mount Shasta” to learn more about Mount Shasta.
Volcano Hazard Map
Figure \(\PageIndex{4}\) shows the USGS Volcano Hazards Zones map for Mount Shasta and the surrounding region. The towns of Weed, Mount Shasta and McCloud all lie within the Near-volcano hazard zone, which is susceptible to lava flows, pyroclastic flows, tephra fall and lahars during an eruption. Lahars are also likely to affect river valleys downstream of the volcano and ash fall may affect a very large region downwind (not pictured on the hazard map). The hazard map also shows an area susceptible to regional lava flows. Flows originating from the cones that make up Mount Shasta would not reach this area. Rather, the area is susceptible to lava flows from smaller volcanic vents.
In an explosive eruption of Mount Shasta, ash fall would affect a large region surrounding the volcano. Ash fall is not pictured on the volcano hazard map (Figure \(\PageIndex{4}\)), in part because the area likely to be affected is larger than that of other hazards and in part because the precise patterns of ash fall are difficult to predict. The thickness and area of ash distribution depend not only on the eruption itself but also on the wind conditions at the time of the eruption. In general, the prevailing wind direction at Mount Shasta is from the west, and therefore the region most likely to be affected by ashfall is the region to the east of the volcano.
Although Mount Shasta has less snow and ice than its sister volcanoes further north, there is enough to create the potential for significant lahars during an eruption. Eruptions at the Hotlum and Shastina vents, about 10,000 years ago, produced many lahars, about 20 percent of which reached more than 20 km from the summit of Mount Shasta, and spread out on fans around the base of the volcano. The largest lahars and floods extended beyond the base of the volcano and entered the McCloud and Sacramento Rivers. If they occur in the future, lahars may cover valley floors and other low areas as much as several tens of kilometers from the volcano. They will also impact the systems of rivers that flow from Mount Shasta. A primary concern in this category is the Sacramento River.
There is also a history of non-eruptive lahars at Mount Shasta. These events typically occur after the snowpack has diminished during mid-to-late summer. In the last 1,000 years, more than 70 such mudflows and debris flows have inundated stream channels surrounding Mount Shasta.
Aptly named Mud Creek is the longest and deepest drainage on Mount Shasta and is the site of the most frequent non-eruptive lahars. Between 1924 and 1931, the Konwakiton glacier at the head of the Mud Creek drainage broke apart, causing a series of debris flows that threatened the town of McCloud. Though none of these flows inundated the town itself, roads were closed, railways washed out and the town's main water supply cut off. One of these mudflows, in 1924, sent a pulse of mud into the Sacramento River, which was detected as far away as San Francisco Bay.
The community of McLoud remains vulnerable to lahars. Figure \(\PageIndex{5}\) shows a roadway buried by a mudflow along Mud Creek Canyon in Sept. 2014. As recently as 2021, nearby roads were closed and the water supply was once again threatened by debris flows. As the climate warms, there is concern over the increased potential for non-eruptive mudflows. In a warmer climate, there is greater potential for rapid snowmelt and heavy rain events. Additionally, as glaciers recede, they leave behind loose material that can easily be eroded by meltwater or rainwater.
Acknowledgments
Parts of the text on this page were taken with minimal editing from sources provided by the USGS, which are in the public domain. Links to the original text can be found in the reference section on this page.
References
- 2 roads, bridge remain closed due to Mt. Shasta mudslide triggered by melting glacier | Fox News. (n.d.). Retrieved May 28, 2024, from https://www.foxnews.com/us/2-roads-bridge-remain-closed-due-to-mt-shasta-mudslide-triggered-by-melting-glacier
- Blodgett, J. C., Poeschel, K. R., & Osterkamp, W. R. (1996). Characteristics of debris flows of noneruptive origin on Mount Shasta, northern California (Open-File Report 96–144). US Geological Survey. https://books.google.com/books?hl=en&lr=&id=7WBRAQAAIAAJ&oi=fnd&pg=PA1&dq=Characteristics+of+Debris+Flows+of+Noneruptive+Origin+on+Mount+Shasta,+Northern+California&ots=cJ0gKKTNiK&sig=4tzZYyqrbfnvy204ovJi5kJItNc
- bubbasuess. (n.d.). The 4 Eruption Cones Of Mount Shasta. Hike Mt. Shasta. Retrieved May 24, 2024, from https://hikemtshasta.com/2018/01/12/the-4-eruption-cones-of-mount-shasta/
- Chapman, M. (n.d.). Mudflow near flank of Mt. Shasta covers road northeast of McCloud. Record Searchlight. Retrieved March 17, 2024, from https://www.redding.com/story/news/2021/07/10/mudflow-near-flank-mt-shasta-covers-road-east-mccloud/7926982002/
- College of the Siskiyous Library. (2005). Mount Shasta Fact Sheet. https://www.siskiyous.edu/library/shasta/documents/MS_Fact_Sheet_20050617_4pp.pdf
- Ewert, J. W., Diefenbach, A. K., & Ramsey, D. W. (2018). 2018 update to the US Geological Survey national volcanic threat assessment. US Geological Survey. https://pubs.usgs.gov/publication/sir20185140
- Mangan, M., Ball, J., Wood, N., Jones, J. L., Peters, J., Abdollahian, N., Dinitz, L., Blankenheim, S., Fenton, J., & Pridmore, C. (2019). California’s exposure to volcanic hazards. In Scientific Investigations Report (2018–5159). U.S. Geological Survey. https://doi.org/10.3133/sir20185159
- McClung, S. C. (2005). Lahar hazard mapping of Mount Shasta, California: A GIS-based delineation of potential inundation zones in Mud and Whitney Creek basins [Master’s Thesis, Oregon State University]. https://ir.library.oregonstate.edu/concern/graduate_thesis_or_dissertations/1c18dk67z?locale=en
- Miesse, W. C. (1993). Mount Shasta: An annotated bibliography. College of the Siskiyous.
- Sahagún, L. (2021, September 8). First the snow vanished, then the mudslides began: Mt. Shasta’s summer of pain. Los Angeles Times. https://www.latimes.com/environment/story/2021-09-08/mt-shasta-snow-vanished-replaced-by-mudflows
- U.S. Geological Survey. (n.d.-a). Geology and History of Mount Shasta. Retrieved March 13, 2024, from https://www.usgs.gov/volcanoes/mount-shasta/science/geology-and-history-mount-shasta
- U.S. Geological Survey. (n.d.-b). Lahars at Mount Shasta. Retrieved March 17, 2024, from https://www.usgs.gov/volcanoes/mount-shasta/science/lahars-mount-shasta