On September 11, 1881, a massive volume of rock, estimated at 10 million cubic meters, broke away from the Plattenberg mountain face above the village of Elm in the Canton of Glarus, Switzerland. This rapid, long-runout failure, known as a sturzstrom, destroyed a significant portion of the village and surrounding land, resulting in the deaths of 115 people. The disaster was not simply a natural landslide, but the result of a long-term geological predisposition that was fatally exacerbated by decades of human intervention near the slope’s base.
The Underlying Geological Instability
The Plattenberg mountain, situated within the Glarus Alps, was inherently susceptible to failure due to a complex geological structure formed by immense tectonic forces. This region is home to the Glarus Thrust, a famous fault line where older rock layers have been pushed horizontally over much younger rock layers. Specifically, ancient Permo-Triassic rock, part of the Verrucano group, rested upon younger formations, creating a fundamental instability across the entire slope face.
The mountain was composed largely of schist and slate, which formed stratified layers dipping steeply parallel to the slope itself. This orientation meant that the rock layers acted like a stack of cards leaning outward, ready to slide along pre-existing planes of weakness. Water infiltration, intensified by heavy rainfall in the weeks leading up to the collapse, further lubricated these parallel bedding planes and fractures, increasing the hydrostatic pressure within the rock mass. This natural, long-term instability provided the necessary precondition for a major rock failure.
The Destabilizing Role of Quarrying
The natural instability of the mountain was exacerbated by extensive slate quarrying operations that had been underway since the mid-19th century. Local miners had been extracting high-quality slate from the base of the Plattenberg mountain. Over time, this quarrying systematically removed the natural buttress at the toe of the slope, which was the foundational support for the massive rock mass above it.
By the time of the collapse, the quarrying had created a semi-open cavity approximately 180 meters wide and 65 meters deep at the base of the unstable mass. This massive excavation dramatically altered the stress field within the mountain, promoting the toppling and shearing of the steeply dipping rock layers. The removal of this rock buttress was the direct, human-induced trigger that undermined the mountain’s stability.
Numerous warning signs were observed and largely ignored in the years leading up to the disaster. Rock movements, minor falls, and the appearance of fissures near the quarry were noted as early as 1878. In the summer of 1881, a prominent crack at a higher elevation widened to between 2 and 3 meters, with the ground below dropping by several meters. This crack effectively isolated a large section of the slope, signaling that the mountain was separating from its base. Despite the clear evidence of impending failure, the damage was already done, and the mountain was poised for collapse.
Understanding the Sturzstrom Phenomenon
The event is classified as a sturzstrom, or rock avalanche, distinguished by its extreme mobility and long runout distance. The 10 million cubic meters of rock traveled an astonishing distance of approximately 2 kilometers, surging across the valley floor and running up the opposite slope. This behavior is scientifically puzzling because the mass moved much farther than expected based on the angle of the slope, seemingly defying the normal laws of friction.
The extreme speed, estimated to be up to 200 kilometers per hour, is characteristic of the sturzstrom phenomenon. Scientists propose several theories to explain this low-friction movement, often involving a process of fluidization within the mass. One theory suggests basal fluidization, where air or steam is trapped beneath the rapid flow, creating a cushion that reduces friction. Another common explanation is acoustic fluidization, where vibrations within the granular debris reduce the internal friction between the moving blocks.
The intense fragmentation of the rock during the fall also contributes to the mobility. The destruction of large blocks generates a fine-grained mixture. This pulverized material acts as a form of “granular lubricant,” which helps the entire mass flow quickly and efficiently over the valley terrain. The Elm sturzstrom was a three-phase event, with the final, largest phase occurring 21 minutes after the initial smaller slides, generating a dense, fast-moving debris cloud that devastated a square kilometer of land.