Field work in lonely and spectacular places is a privilege. Though it can be great, boredom sometimes sets in, which is hard for the lone geologist. Today, I guess a cell phone would help, especially in high places where the signal is good. That means of communication and entertainment only emerged in the 1980s and did not reach wild places until well into the 90s. Pre-cellnet boredom could be relieved by what remains a dark secret: lone geologists once rolled large boulders down mountains and valley sides, shouting ‘Below!’ as a warning to others. Their excuse to themselves for this unique thrill (bounding boulders reach speeds of up to 40 m s-1) was vaguely scientific: sooner or later a precarious rock would fall anyway. This week it emerged that Andrin Caviezel of the Institute for Snow and Avalanche Research in Davos, Switzerland, an Alpine geoscientist, rolls boulders for a living (Caviezel, A. 2022. The gravity of rockfalls. Where I work, Nature, v. 607, p. 838; DOI: 10.1038/d41586-022-02044-9). He finds that ‘…flinging giant objects down a mountain is still super fun’. The serious part of his job attempts to model how rockfalls actually move downslope, as an aid to risk assessment (Caviezel, A. and 23 others 2021. The relevance of rock shape over mass – implications for rockfall hazard assessments. Nature Communications, v. 12, article 5546; DOI: 10.1038/s41467-021-25794-y)
Caviezel’s team (@teamcaviezel) don’t use actual rocks but garishly painted, symmetrical blocks of reinforced concrete weighing up to 3 tonnes, which are more durable than most outcropping rock and can be re-used. A Super Puma helicopter shifts a block to the top of a slope, from which it is levered over the edge (watch video). The team deploys two types of block, one equant and resembling a giant garnet crystal, the other wheel-shaped with facets. The first represents boulders of rock types with uniform properties throughout, such as granite. The wheel type mimics boulders formed from rocks that are bedded or foliated, which are usually plate-like or spindly.
Unlike other gravity-driven hazards, such as avalanches and mudflows, the directions that rockfalls may follow by are impossible to predict. Rather than hugging the surface, boulders interact with it, bouncing and being deflected, and they spin rapidly. To follow each experiment’s trajectory a block contains a motion sensor, measuring speed and acceleration, and a gyroscope that shows rotation, wobbling and motion direction, while filming records jump heights – up to 11 m in the experiments. Despite the similarity of the blocks, the same release point for each roll and a uniform mountainside slope, with one cliff line, the final resting places are widely spread. That hazard zone of rockfalls is distinctly wider than that of snow avalanches; observing a boulder once it starts to move gives a potential victim little means of knowing a safe place to shelter.
The most important conclusion from the experiments is that the widest spread of tumbling ‘boulders’ is shown by the wheel-shaped ones. So, slopes made from bedded or foliated sedimentary and metamorphic rocks may pose wider hazards from rockfalls than do those underpinned by uniform rocks. However, plate-like or spindly boulders are more stable at rest than are equant ones. Yet boulders rarely fall as a result of being pushed (except in avalanches). On moderate slopes they are undermined by erosion, and on steep slopes or cliffs winter ice wedges open joints allowing blocks to fall during a thaw.