What if Shasta reawakened into a state of volcanic activity? A myriad of volcanic hazards are possible at the volcano, some of which could have devastating consequences to the surrounding countryside. California’s Mt. Shasta Volcano | Unofficial Networks

California's Mt. Shasta Volcano

California's Mt. Shasta Volcano


California's Mt. Shasta Volcano


Aaron Pope and James Conley on the Wintun-Hotlam route in June 2009.

Mount Shasta is the classic go-to summer ski for those of us lucky enough to call Lake Tahoe home. Topping out at 14,179 feet, the mountain offers five to seven thousand vertical feet of fantastic corn skiing in the late spring to early summer. When conditions are just right, even longer descents are possible, as the volcano towers 10,000 feet over the surrounding landsacape. Thanks to La Nina and the 50-year winter that we just enjoyed here in Northern California, Shasta will probably have good skiing well on into August.

Shasta is not only the second tallest mountain in the Cascades, and the fifth tallest in California, it’s also an active volcano. Anyone who has made the potentially epic slog to the summit in the summer will tell you that the top smells distinctly of sulfur – an odor reminiscent of rotten eggs – caused by hydrogen sulfide gas that is percolating up through the rocks of the volcano from magma deep within it.

Sunset from high camp on Mt. Shasta, photo by the author

Shasta is by far not the largest of the Cascade volcanoes, but it has produced the most lava of any of them. It is made of at least four different volcanic edifices that have been built up upon each other over the past 590,000 years, and so what we see today is a composite cone that is more than just the result of one single episode of activity. The last eruption of lava from the volcano took place in 1786, and was observed by a ship off the coast of California, the La Perouse. It’s not known whether there were any human casualties from this eruption.

The largest single-source debris avalanche (a.k.a. landslide) in the last 3 million years (the Quaternary) on the planet Earth took place when ancestral Mt. Shasta crumbled into pieces and tumbled to the northwest into the Shasta River Valley. The deposit is thought to be about 11 cubic miles in volme, and covers an incredible 260 square miles (Crandall et al., 1984). That’s the equivalent of more than 150,000 football fields.

Map of the ancient debris flow that devastated ancestral Mt. Shasta (USGS)

The USGS Volcano Hazards Program classifies Shasta as exhibiting a “background” level of volcanic activity. This means that it is exhibiting low levels of activity such as fumaroles degassing and isn’t heading towards an eruption at the present time, but the possibly of one can’t be ruled out. For now, it’s safe to recreate on and around the mountain, as it’s not exhibiting any behaviors that are dangerous besides the typical weather or glacial hazards.

What if Shasta reawakened into a state of volcanic activity? A myriad of volcanic hazards are possible at the volcano, some of which could have devastating consequences to the surrounding countryside. The growth of a new lava dome under the perennial snowpack would undoubtedly melt the snow, causing lahars to cascade down the volcano’s slopes. Lahars are a destructive liquid mix of ash, rocks, water, and mud that flow like a river, but have the consistency of concrete.  Check out this incredible video of an Indonesian Lahar on Semeru volcano in 2003. I met the French volcanologist that filmed this video at a volcanic hazards conference in New Zealand. He is the only person I have ever met that is crazier than Mike Wilson.:


If a new lava dome grew on top of Shasta, it would eventually collapse, shedding pyroclastic flows. Pyroclastic flows are burning hot clouds of ash and rocks, travelling at speeds of up to 450 miles per hour. These are not something you would want to be anywhere near, but they are a breathtaking site when viewed in action.  Take a look at one barreling down the slopes of Mt. Unzen in Japan in 1991 (gets good at 0:49):

Other potential hazards from a reawakened Mt. Shasta include lava flows, or ash or tephra fall from episodes of explosive eruption (Miller, 1980). Recent eruptions have blanketed the volcano’s flanks with deposits ranging in depth from a few centimeters up to a meter in thickness, such as the Red Banks eruption 9,600 years ago, or the much more recent Hotlum dome eruption 200 years ago (Christiansen et al., 1977). Winds around Mt. Shasta are almost always oriented N or NE, and are likely to carry any ash or tephra off the volcano in those directions away from populated areas of Weed and Shasta City, or the very busy I-5 corridor. A bigger eruption, however, would be likely to deposit at least some ash on the Western flanks of the volcano and cause disruptions in those places.

Five hours to the south, here in Lake Tahoe, we’re unlikely to ever feel the effects of an eruption from Shasta, or even the much larger volcanic complex in Long Valley that I wrote about a few weeks ago. We’re surrounded by volcanoes, but they are all too far away to have any significant consequences to us. Tahoe really is utopia!

Have an earth-science question that’s burning a hole in your curious mind? Ask it in the comments of this post and I’ll pick one to answer next week.


Christiansen, R.L., Kleinhampl, F.J., Blakely, R.J., Tuchek, E.T., Johnson, F.L., and Conyak, M.D., 1977, Resource appraisal of the Mount Shasta Wilderness study area, Siskiyou County, California: U.S. Geological Survey Open-File Report 77-250, 53 p.

Crandell, D.R., Miller, C.D., Glicken, H.X., Christiansen, R.L., and Newhall, C.G., 1984, Catastrophic debris avalanche from ancestral Mount Shasta volcano, California: Geology, v. 12, p. 143-146.

Miller, C.D., 1980, Potential hazards from future eruptions in the vicinity of Mount Shasta volcano, Northern California: U.S. Geological Survey Bulletin 1501, 43 p.

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