Rockfall in Zion National Park

Zion maintenance worker drilling rockfall at Emerald Pools

NPS / Ally O'Rullian

Zion National Park is defined by its rock canyons and towering cliffs. Although the impressive pinnacles of rock found throughout the park might seem to be monuments to permanence, the opposite is true. The park’s towering red, orange, and white walls sometimes shift, slide, or fall.

Over the course of millions of years, the rock layers in Zion have uplifted, tilted, and eroded, to create the sandstone peaks and canyons we see today. The forces that have shaped the canyon are still in play today.

 

Forces that can change rock

The rocks that make up the canyon’s walls frequently break apart and fall. In fact, every rock at the bottom the canyon started somewhere higher in the canyon’s walls. Some of the natural factors than can change rocks and contribute to rockfall include:

Water

• Floods from heavy rain and snow runoff can erode and weaken rocks.

• Some rocks may absorb water, which can cause them to expand, while others may be unaffected.

• Zion’s rocks have many cracks and crevices. Called "frost wedging" or "freeze-thaw," water can seep into cracks in the rock freeze and expand, causing those cracks to further deform.

• Zion has hundreds of springs in the park that emerge from bedrock aquifers. Pressure from groundwater near the surface can cause deformation in the rocks causing failure.

Temperature

• Thermal cycling, or long- and short-term changes in temperature, can change rocks. Rocks expand when heated and contract when cooled – on daily and season cycles. These near constant changes can make the rocks fracture.

Gravity

• Through the processes mentioned above, rocks can become less stable.

• An initial rockfall can destabilize a larger area, leading to more rockfall.

• Sometimes, rocks just fall!

 

Cable Mountain Rockfall

Left image
Cable Mountain in 2015 prior to rockfall
Credit: NPS

Right image
Dust and debris on Cable Mountain following the 2023 rockfall. The rockfall site can be seen as a white scar on the top right-hand side of the cliffside
Credit: NPS / Ally O'Rullian

In 2019, a slab of sandstone broke off of Cable Mountain and fell down onto the Weeping Rock area and the trails there. Another slab of sandstone fell in November 2023. Though years passed, this area and all of Zion Canyon remain geologically active.

 

Assessing Rockfall

Rockfall can block roads and trails, especially those that are carved into and near canyon walls. When that happens, park geologists and maintenance workers consider how to conserve landscapes, plants, and animals as part of the process to safely and efficiently repair damaged infrastructure.

Rockfall sites can be active for days, weeks or years. In some cases, rockfalls are progressive, meaning it is more likely other rocks will fall because of the initial event. If Zion staff are concerned a site is unstable, they may keep it closed so they can monitor it.

A recent example of progressive rockfall site are the cable mountain rockfalls of 2019 and 2023. In 2019, a large rockfall, classified as a small rock avalanche, destroyed part of the East Rim Trail. After carefully analyzing the damage along with academic and other public service scientists, Zion staff determined the rockfall covering the East Rim Trail was not safe to clear. In 2023, a similar rockfall occurred near the 2019 rockfall site. Though years had passed, the area remains active.

 

Workers drilling to clear rockfall

NPS / Colton Johnston

Rockfall Cleanup

Rockfall can look differently depending on where and how it happened, and different situations require different cleanup. When national park service staff respond after a rockfall, safety is the main priority both for employees and visitors.

Handtools

• Maintenance staff most often use hand tools, rock bars, picks, and shovels working as a team to move debris.

• Staff often move 500-1000 pounds of rock by hand.

Heavy machinery

• Heavy machinery is used to break up large rocks on roads and on trails when possible.

• Smaller rocks are moved off roads and trails to clear the path.

Blasting devices

• In some cases, workers use specialized blasting devices to break rock into smaller pieces.

• These devices focus their force where they are placed, which limits shaking to prevent instability in rocks nearby.

• National park staff only use blasting devices when they are confident that it is not likely to cause additional rockfall.

 

Rockfall has always been a part of Zion National Park.

NPS

Managing rockfall over the long-term

Rocks in Zion National Park fall, frequently. The active landscape is part of what makes Zion special. These geologic forces created and continue to shape the canyons we experience today.

It is impossible to predict when or where the next rockfall will happen. But we can predict that our geologists’ and trail workers’ experience and expertise will ensure we continue to safely and efficiently address rockfall when it happens.

 

Duration:

5 minutes, 3 seconds

Rockfall in Zion is frequent and unpredictable. In this video we explain some causes of rockfall and how our staff respond when it happens. Music from Uppbeat.