© 2016 Peter Free
Citation — to study
Jessica J. Castleton, Jeffrey R. Moore, Jordan Aaron, Marcus Christl, and Susan Ivy-Ochs, Dynamics and legacy of 4.8 ka rock avalanche that dammed Zion Canyon, Utah, USA, Geological Society of America 26(6): 4-9, DOI: 10.1130/GSATG269A.1 (June 2016)
Citation — to press release
Lee Siegel, How a Huge Landslide Shaped Zion National Park, University of Utah (26 May 2016) (with photographs and simulation graphics)
What follows will mean more to people who have been to Zion National Park in the United States.
From the University of Utah:
A Utah mountainside collapsed 4,800 years ago in a gargantuan landslide known as a “rock avalanche,” creating the flat floor of what is now Zion National Park by damming the Virgin River to create a lake that existed for 700 years.
Computer simulations matched known landslide deposits and show the huge slide rushed southeast across Zion Canyon in about 20 seconds, with an average speed of 112 mph and a peak speed of 180 to 200 mph.
“It was certainly moving more than 150 mph when the huge wall and peak crashed down,” [Jeffrey] Moore says. Then, for 30 more seconds, the slide debris spread up and down Zion Canyon. “By a minute it was pretty much done.”
“The original deposit was 2 miles long and just under a mile wide,” with a maximum thickness of 650 feet and average thickness of 310 feet, he says, adding the landslide’s lower end is at the road junction “right at the mouth of Zion Canyon.”
“We have conducted a rigorous and complete analysis of this landslide for the first time,” Moore says. The study concluded the landslide most likely happened 4,800 years ago as single event, with a range of uncertainty so that it could have happened as early as 5,200 years ago or as recently as 4,400 years ago.
The method exploits the fact that after a landslide, boulders atop the slide have surfaces exposed to the sky for the first time. Particles from incoming cosmic rays begin to hit the boulder surfaces, creating beryllium-10. The longer a boulder is exposed, the greater the amount of beryllium-10, allowing scientists to determine when the boulder’s surface first was exposed by the landslide.
With permission from the National Park Service, Moore and colleagues sampled 12 boulders from the landslide’s surface, crushed the notebook-sized rock samples and analyzed their beryllium-10 content.
© 2016 Lee Siegel, How a Huge Landslide Shaped Zion National Park, University of Utah (26 May 2016) (extracts)
Lee Siegel’s excellent press release contains photographs and graphics.
The same message in scientific language
From the abstract:
The Sentinel rock avalanche blocked the mouth of Zion Canyon, Utah, USA, over a distance of 3.3 km and created a large lake that filled the canyon floor with sediment, transforming this iconic desert landscape.
Reconstructing topography before and after the failure, we calculate an original deposit volume of 286 million m3 with maximum thickness of 200 m. New cosmogenic nuclide surface exposure ages of 12 boulders from across the deposit reveal a mean age of 4.8 ± 0.4 ka and are consistent with single-event emplacement.
Results of 3D numerical runout simulations agree well with mapped deposit boundaries and thickness, affirming our hypothesized failure scenario and indicating an average runout velocity of 50 m/s.
Following partial breach of the landslide dam, we estimate that water levels stabilized for ~700 yr until the lake filled with sediment. Deposited lacustrine clays reveal a period when Zion Canyon was filled by the 3 km2 Sentinel Lake extending more than 7 km upstream.
© 2016 Jessica J. Castleton, Jeffrey R. Moore, Jordan Aaron, Marcus Christl, and Susan Ivy-Ochs, Dynamics and legacy of 4.8 ka rock avalanche that dammed Zion Canyon, Utah, USA, Geological Society of America 26(6): 4-9, DOI: 10.1130/GSATG269A.1 (June 2016) (at Abstract) (extracts)
The moral? — Understanding pertinent geology makes travel more interesting
Appreciation to Lee Siegel and the University of Utah for their visually informative overview.