Learn to recognize the potential dangers of rockfall, both natural and human-caused

Rockfall is just one of many accepted risks of venturing into the mountains. Rockfall can be a single boulder careening down a cliff or a mass movement of rock falling down a steep slope. And rockfall can have either a natural or engineered cause.

Cliff or hillside erosion is the means by which Mother Nature exfoliates. This natural process is potentially hazardous should a person stand in the way of a tumbling boulder or sliding cascade of rock. Rockfall is also possible under a block cleaned by a lead climber, or from a rock kicked loose from a mountaineer on a ledge above. Understanding why and how rocks fall can help the aspiring adventurer avoid either triggering or encountering one of these dangerous rock movements.

Heat and Rain

Heat and rain are major contributors to rockfall, causing or facilitating the natural physical and chemical weather of rock layers. Yosemite National Park geologists indicated that rain accounts for about 29% of rockfall.

Temperature fluctuation, especially heating, is the main culprit. The freeze-thaw cycles of seasonal changes, the hot sun-kissed days to chilly nights and the monsoon storms of the Southwest summer, contribute to physical weathering of mountain geology. Temperature fluctuations expand and compress rock, resulting in the loosening and fracturing of the outer layers.

Rock itself is a poor heat conductor. When heated, only the surface of the rock is affected while the core remains cool. This cycle of heating and cooling causes the outermost layers to expand with the heat and contract when cooled, amassing stress known as thermal fatigue. Thermal fatigue causes stress between constituting mineral crystals resulting in fragmentation known as “granular disaggregation.” The resulting stress fractures break and weaken the rock from its connection point at the cliff.

rockfall rock slide area hazard sign
Most areas with danger of rockfall are lucky enough to have a sign.
Chemical Weathering

Weather and seasonal changes also contribute to chemical weathering. Rocks are comprised of minerals. The constituent minerals can have a chemical reaction with oxygen — whether from the air, water from summer storms or snowmelt. A change in the chemical composition of the rock can cause stress between the mineral crystal. Temperature fluctuation again comes into play and promotes fragmentation, fracturing the outermost rock layers.

Other Factors

With the outermost rock layers vulnerable to weathering, mass movements can be triggered by seismic activity, lightning, erosion of surrounding materials during a heavy rainstorm, root growth, extreme wind and more.

The Human Factor

Accepting risk is part of the necessary exchange for enjoying our wondrous planet. Rockfall will happen. With a bit of knowledge, however, one can help minimize the risk of a great day turned calamitous.

Wherever you may venture, avoid or be extremely cautious in areas where other rockfall has obviously occurred. When rockfall occurs, the rock accumulates at the base of a cliff, chute or slope. A talus pile, especially one at the base of a steep slope, indicates that the area below the slope may not be safe for recreation.

Watch the weather. Rain makes rock material less cohesive by reducing friction between cracks. It also contributes weight and volume to the overall mass, thus increasing the gravitational force of the slope itself. Stay alert while adventuring during the summer monsoon seasons.

If you are new to adventuring, it is prudent to go with someone who can assess the safety the situation. Some cliffs are safer than others. Shorter cliffs have less rock material and therefore less rockfall potential, while longer cliffs have more rockfall potential. In keeping to that logic, a cliff with more overhang has more unstable rock material — more pull of gravity upon unsupported rock — than a cliff with less overhang.

For climbers and mountaineers, wearing a helmet could save your life. Always be on the lookout for weathered rock. Assess all questionable blocks, whether rock climbing or ascending a ridge. Pull down rather than out on blocks. Should you happen to pull a block loose, give a warning cry to anyone who may be below.

rockfall rock climbing safety helmet
Rockfall is common in the climbing world. Protect yourself from the dangers of rockfall by wearing a helmet.Terrance Siemon
Engineered Causes

Trundling is the practice of throwing loose rock from a cliff or loosening a boulder to watch it cascade down a slope. Climbers are in the habit of removing bits of bad rock to lessen the risk of a larger accident occurring later. With a good view, the climber can generally control where to safely toss the loose rock to a place where it will not threaten the safety of anyone below.

The blatantly irresponsible trundle is to loosen a boulder to watch it bounce down a slope. There’s no controlling that.

Either way, the practice could result in injury or fatality should someone stand in the path of the falling rock.

Stay safe and enjoy the next adventure with perhaps a little more knowledge than before. 

LIS MCLAUGHLIN is an outdoor enthusiast and freelance writer based in Durango, Colorado. A Fort Lewis College graduate and humble student of philosophy, she balances her free time with writing, reading, fitness and appreciating the beauty of nature.