Unit 2 – Making Mountains – Earthquakes Geosc. 10 Earthquakes! Take a Quick Virtual Tour with Dr. Alley through the U.S. Forest Service Madison River Canyon Earthquake Area (last picture is in Yellowstone National Park) All Photos by Richard Alley

Unit 2 – Making Mountains – Earthquakes Geosc. 10 On the night of August 17, 1959, at 11:37 PM, a large (magnitude 7.5) earthquake struck just northwest of Yellowstone, near Hebgen Lake in the Madison River Canyon of Montana. Land on the west side of the Cabin Creek Fault moved down about 21 feet (almost 7 m) relative to land on the east in a sudden drop that shook the surroundings, forming the Cabin Creek Scarp, shown here. (A scarp or escarpment is a steep ramp connecting places that are more nearly horizontal; think of a wheelchair ramp between sidewalks down here and up there.)

Unit 2 – Making Mountains – Earthquakes Geosc. 10 The shaking caused a massive landslide from the south side of the Madison River Canyon. An estimated 80 to 90 million tons of rock and debris thundered into the canyon, forming a layer estimated at 400 feet (120 m) thick in the bottom of the canyon and bouncing another 400 feet up the north wall. The scar left when the rocks fell is shown here. 26 people died in this vicinity, most in a campground that was buried under the landslide debris. (Two more were killed by a falling boulder elsewhere.)

Unit 2 – Making Mountains – Earthquakes Geosc. 10 This closer view of the scar from the landslide shows large trees next to the slide and growing in the scar, emphasizing the immense size of the slide.

Unit 2 – Making Mountains – Earthquakes Geosc. 10 Huge boulders were carried across the valley and up the other side. The rapidly moving mass of rock blasted air and river water in front of it; some survivors reported having their clothes ripped off by the force of the blast.

Unit 2 – Making Mountains – Earthquakes Geosc. 10 Another shot of the immense boulders carried across the valley by the rapidly moving mass of rock.

Unit 2 – Making Mountains – Earthquakes Geosc. 10 The landslide dammed the Madison River, and the trapped water rose to form Earthquake Lake, drowning many trees that still are visible, as seen here. Quick work by the US Army Corps of Engineers stabilized and lowered the new dam. Landslide-dammed lakes often overtop their dams, cut down through the loose debris, and release a devastating flood; this was avoided by the quick government response.

Unit 2 – Making Mountains – Earthquakes Geosc. 10 Hebgen Lake was a reservoir a little upstream of the new Earthquake Lake. On the night of the earthquake, Hebgen Lake’s dam cracked and water washed over, but the dam did not fail. The land under the lake tilted, raising one side and lowering the other. One person ran safely from her house as the lake rose and the house and land slipped into the waters. Huge waves sloshed back and forth in the lake after the quake. Houses knocked down and flooded by the tilting and waves are still visible, as seen here.

Unit 2 – Making Mountains – Earthquakes Geosc. 10 The quake caused many changes in Yellowstone, stopping some geysers, starting others, and changing the patterns of still others. Many tourists fled in panic, small landslides and rockfalls were triggered, dishes fell off walls in houses, etc., but no one was seriously injured in the park. Earthquakes such as this probably have occurred many times over the geologic history of Yellowstone, although this was the largest experienced in Yellowstone since modern instruments were installed to provide measurements. Monument Geyser, in the Upper Geyser Basin, is shown here; this one was not affected more than most, but it's pretty.