JACKSON, Wyo. — On June 23, 1925, a mountainside in the Gros Ventre Range collapsed, causing a landslide of an estimated 38 million cubic meters of rock and debris.
This week’s Caldera Chronicles, written by the scientists at the Yellowstone Volcano Observatory (YVO), delves into the geological reasons for the massive Gros Ventre Slide that happened 100 years ago. The column was written by James Mauch, geologist with the Wyoming State Geological Survey.
According to the article, the Gros Ventre Slide occurred around 4 p.m. on the north side of Sheep Mountain, where 38 million cubic meters of rock and debris tumbled down the mountainside and into the Gros Ventre River valley.
“Within minutes, the valley floor was buried beneath more than 200 feet of rocky debris and the river was dammed, creating Lower Slide Lake,” Mauch wrote.
Many factors worked together to create the perfect storm. According to Mauch, sedimentary rocks underlie the Gros Ventre River valley. The base of the hillslope where the slide occurred is undercut by long-term erosion from the river. The type of rock exposed at the surface of the slope is Tensleep Sandstone, “a layer that groundwater can easily penetrate due to the space between sand grains.” Below that, the Amsden Formation of shale beds forms a barrier which causes groundwater to collect there between the Amsden Formation and the Tensleep Sandstone. This layer is also where weak, heavily weathered siltstone layers are present.
When the weak layers become saturated with groundwater, they become more likely to fail. In the spring of 1925, those conditions were present from abundant rainfall and unusually warm weather causing snowmelt to permeate the surface layers. Although there were no seismic instruments in the area, local residents reported feeling several earthquakes in the weeks leading up to June 23, including one of estimated magnitude 3 to 4 that occurred the night before the slide.
A likely explanation is that ground shaking from the earthquakes kicked off a chain reaction that began with liquefaction of the saturated, weak layers. According to the U.S. Geologic Survey, “liquefaction takes place when loosely-packed, water-logged sediments at or near the ground surface lose their strength in response to strong ground shaking.” This most likely culminated hours later with the massive collapse of the hillside.
“One hundred years later, the Gros Ventre Slide stands as an important milestone in the human and natural history of the Greater Yellowstone region, reminding us of the power and destructive potential of unstable slopes in this dynamic landscape,” wrote Mauch.
