Selected Examples of Mass Wasting
on Mount Shasta

A Term Paper
by Bob Musgrove
Geography 581 Geomorphology
Professor: Dr. Mairs
Southern Oregon University 
 Introduction | Snow Avalanches | Jokulhlaups | A Unique Debris Flow | Conclusion | Sources


Jokulhlaups in Iceland

The term "jokulhlaup" is derived from Icelandic and refers to an outburst flood event of glacial origin (Hill and Engendoff 1976). Jokulhlaups can start when cavities in or under the glacier rapidly release quantities of water at unpredictable times (Driedger 1986). Some jokulhlaups are relatively small and others such as the November 1996 event in Iceland are cataclysmic events and truly worthy examples of mass wasting.

Figure 5. The great jokulhlaup of 1996, Iceland. (Gumundsson and Palsson.)

   On November 5 though 8, 1996, two scientists from the Science Institute at the University of Iceland, M. T. Gumundsson and F. Palsson, made a series of observational flights over a jokulhlaup that burst from the Skeiararjokull Glacier following a sub-glacial volcanic eruption. Just hours after emerging from the ice sheet, the jokulhlaup had a discharge of 5,000 cubic meters per second, which increased to 15,000 cubic meters per second within 90 minutes. Two large bridges, 380 and 900 meters long, were destroyed, and a large, circular plume of suspended sediment load formed in the ocean waters beyond the outlet of the jokulhlaup. Up on the glacier itself, the the collapse and subsidence associated with the jokulhlaup left an ice canyon 6 kilometers long with an average depth of 100 meters.

A Mount Shasta Jokulhlaup

While California's mountains are home to many remnant glaciers, few fossil records exist to document past occurrences of jokulhlaups (Hill and Engenhoff 1976). The most notable exception to the above are the events associated with the mass wasting that originated on the Konwakiton Glacier on Mount Shasta between 1881 and 1931 (Hill and Engenhoff 1976). The Konwakiton is a relatively small glacier, even for Mount Shasta, and is currently less than 0.7 kilometer long and less than 0.4 kilometer wide. It is located adjacent to the standard Avalanche Gulch climbing route and seemed relatively stable when viewed by myself from Thumb Rock in 1998. Late in the summer climbing season a large bergshrund and several crevasses open up and present potential hazards to climbers who venture off route. Despite its current benign appearance, the Konwakiton has, in the geologically recent past, presented a hazard to area residents and to nearby rivers.

Figure 6. The Konwakiton Glacier. USFS aerial photograph.

   The Konwakiton Glacier drains into Mud Creek canyon, which discharges into the McCloud River, which joins the Pitt River before feeding into the Sacramento River and eventually, San Francisco Bay. In August 1924 a camper in Mud Creek canyon was alarmed by a loud roaring sound accompanied by a trembling of the ground. He soon saw, and quickly escaped from a liquid torrent of earth and debris which enveloped his campsite. The water supply for the town of McCloud was disrupted by the flow and lumber company employees sent to repair the damage reported difficultly with pieces of ice as large as houses. The flow soon blocked the road to Sisson (present day Mt. Shasta City) and was up to a half mile wide and twenty feet deep (Hill and Egenhoff 1976).

   That the mud flow was the result of a glacial outburst, a jokulhlaup, was suspected, and on August 21 locals climbed up to the Konwakiton to investigate. "McCloud people are confident that what is left of the glacier near the top is frozen tight it its moorings," the local newspaper reported (Hill and Egenhoff 1976, p. 155). The site became a popular hiking destination, and people expressed confidence that, unless the weather warmed up, the danger was past. However temperatures did rise, the jokulhlaup resumed its destructive flow, and soon it was 5 miles long, up to 1 mile wide, and 10 feet deep. The alluvial fan created by the flow was reported to encompass 1,500 acres and to hold a volume of 7 million cubic yards of debris. Plants were unable to reestablish themselves in the wake of the devastation. The jokulhlaup polluted the rivers downstream and carried ash into the Sacramento River. The event continued, and up high on Mount Shasta, the torrent peaked at 2 or 3 o'clock each afternoon, and local observers felt that melting of glacial ice by the afternoon sun was the cause of increased flow (Hill and Engenhoff 1976).

Figure 7. This 1997 mudlfow on Mount Shasta produced devastation similar to the events of 1924.

   In 1925 the California Debris Commission conducted an investigation into the causes and mechanisms of the jokulhlaup. Engineer R. L. Egenhoff reported that the "main part of the glacier was about 800 feet wide and 100 feet thick....the material underneath the end of the glacier consisted of unconsolidated volcanic debris of all kinds." He went on to describe the scene: "two large caverns, each about 300 feet wide....extending two or three hundred feet back under the glacier.... a waterfall from under the ice was pouring out of each cavern over the caving material." (Hill and Engenhoff 1976, p. 157).

Causes of the Jokulhlaup on Mount Shasta

   On slopes shear strength holds materials, including glacial ice and volcanic debris, in place. Shear failure can occur if the stress is strong enough to overwhelm the forces of cohesion and resistance in the layers of material (Easterbrook 1999). It may be the case that the melting of the Konwakiton due to increased temperature led to the saturation of the debris which supported that glacier, which in turn led to an increase in stress as the debris reached its plastic or liquid limit (Van Susteren). Thus the increase in atmospheric temperature may have led to a sequence of events resulting in the failure of the materials which supported the glacier.

Figure 8. USFS mud flow hazard map. The dark orange areas are of the greatest hazard, followed by the lighter orange areas of intermediate hazard, and the pink areas are of minimum hazard. (Crandell and Nicols, 1987)



 Introduction | Snow Avalanches | Jokulhlaups | A Unique Debris Flow | Conclusion | Sources

web page authored by Bob Musgrove