Ancestral Mount Shasta volcano, California, Destroyed by a Massive Landslide

Strange topography of Shasta Valley
Photograph by C.D. Miller on 22 September 1982	Shasta Valley is a flat, arid plain north of Mount Shasta, punctuated by hundreds of hills, ridges, and small closed depressions (not connected by surface streams). The Shasta River drains northward through the valley to join the Klamath River near the Oregon border. Like many geologists who had pondered this strange topography for decades, we were perplexed and could only speculate about its origin.

Strange topography of Shasta Valley

Aerial view of landslide deposit north of Mount Shasta, California

Photograph by C.D. Miller on 22 September 1982

Shasta Valley is a flat, arid plain north of Mount Shasta, punctuated by hundreds of hills, ridges, and small closed depressions (not connected by surface streams). The Shasta River drains northward through the valley to join the Klamath River near the Oregon border. Like many geologists who had pondered this strange topography for decades, we were perplexed and could only speculate about its origin.

A clue from another volcano
North Fork Toutle River valley, Mount St. Helens Photograph by L. Topinka in 1981	Only after we witnessed the great landslide from the north side of Mount St. Helens on 18 May 1980 and the chaotic topography it created on the floor of the Toutle River valley did we come to understand the significance of the confusing topography of Shasta Valley. More about this image.

A new look at the strange deposit in Shasta Valley
Landslide deposit from Mt. Shasta, California Photograph by R. Christiansen in August 1982	With the example of the 1980 Mount St. Helens landslide in mind, we looked carefully at the deposits that underlie the hummocky topography of Shasta Valley. We found them to be intensely fractured, but they also preserve in a general sort of way the sequence of lavas and surficial deposits from an older volcano much like modern Mount Shasta. Lava fragments in these deposits are at least as young as about 360,000 years. Even younger lava flows covered the eastern margin of these deposits after about 300,000 years ago.

A new look at the strange deposit in Shasta Valley

Geologists examines internal characteristics of landslide deposit at Mount Shasta

Landslide deposit from Mt. Shasta, California
Photograph by R. Christiansen in August 1982

With the example of the 1980 Mount St. Helens landslide in mind, we looked carefully at the deposits that underlie the hummocky topography of Shasta Valley. We found them to be intensely fractured, but they also preserve in a general sort of way the sequence of lavas and surficial deposits from an older volcano much like modern Mount Shasta. Lava fragments in these deposits are at least as young as about 360,000 years. Even younger lava flows covered the eastern margin of these deposits after about 300,000 years ago.

Ancestral Mount Shasta destroyed by landslide
Pathway of landslide	Our work showed that a gigantic sector of a volcano, located at the current site of Mount Shasta and probably just as high, collapsed catastrophically and avalanched into Shasta Valley about 300,000 years ago. This great avalanching landslide was 20 times more voluminous than that of Mount St. Helens! The brown area on the map was covered by the huge avalanche. The green area shows the upper part of the modern Mount Shasta Volcano, above about 6,000 feet elevation. Arrows indicate direction of flow of the avalanche. Bar scale is approximately 10 miles.

Ancestral Mount Shasta destroyed by landslide

Map showing pathway of landslide from Mount Shasta volcano

Pathway of landslide

Our work showed that a gigantic sector of a volcano, located at the current site of Mount Shasta and probably just as high, collapsed catastrophically and avalanched into Shasta Valley about 300,000 years ago. This great avalanching landslide was 20 times more voluminous than that of Mount St. Helens! The brown area on the map was covered by the huge avalanche. The green area shows the upper part of the modern Mount Shasta Volcano, above about 6,000 feet elevation. Arrows indicate direction of flow of the avalanche. Bar scale is approximately 10 miles.

Landslides and volcanic explosions
	We also learned from Mount St. Helens that massive collapse and avalanching of a large sector of a volcano can trigger a violent blast that ejects volcanic ash, rocks, and other debris across the landscape with the power to destroy forests. We haven't yet recognized evidence for such a ground-level blast associated with the Shasta Valley avalanche, but any deposit there could have eroded away in the past 300,000 years. The landslide and explosion sequence at left is based on photographs taken by Gary Rosenquist during the first few minutes of the 18 May 1980 eruption of Mount St. Helens. See explanation of the illustrations.

Future landslides?
Could another sector collapse again destroy Mount Shasta, perhaps accompanied by a violently explosive eruption? Perhaps but past history suggests that such events at any stratovolcano occur only rarely. Thus, though possible, we consider the likelihood of such a repetition at Mount Shasta to be small. However, if future activity should begin to bulge out the side of the volcano, a large landslide would become more likely. Learn about the infamous bulge that developed on the north flank of Mount St. Helens before it collapsed on 18 May 1980, see this presentation from the USGS Cascades Volcano Observatory.

Future landslides?

Could another sector collapse again destroy Mount Shasta, perhaps accompanied by a violently explosive eruption? Perhaps but past history suggests that such events at any stratovolcano occur only rarely. Thus, though possible, we consider the likelihood of such a repetition at Mount Shasta to be small. However, if future activity should begin to bulge out the side of the volcano, a large landslide would become more likely. Learn about the infamous bulge that developed on the north flank of Mount St. Helens before it collapsed on 18 May 1980, see this presentation from the USGS Cascades Volcano Observatory.

Ancestral Mount Shasta destroyed by a massive landslide

Strange topography of Shasta Valley

A clue from another volcano

A new look at the strange deposit in Shasta Valley

Ancestral Mount Shasta destroyed by landslide

Landslides and volcanic explosions

Future landslides?

References

			On to Sargents Ridge cone
Back to Cones of Mount Shasta volcano