What Triggers Rockfalls? Yosemite Study Shows It's About the Heat
Rockfalls in Yosemite Valley have long puzzled geologists for a simple reason: many of them happen on warm, clear, perfectly calm days — with no earthquake, no rainstorm, no freezing to blame. Research led by the U.S. Geological Survey and the National Park Service found the missing trigger in an unexpected place: ordinary daily sunshine.
Measuring a breathing cliff
Researchers Brian Collins (USGS) and Greg Stock (Yosemite's park geologist) installed instruments behind a large, partially detached granite slab — an exfoliation sheet — on a Yosemite cliff face and monitored it continuously for more than three years. What they recorded was remarkable: the slab moved every single day, bowing outward by as much as a centimeter as afternoon heat expanded the rock, then contracting back at night as it cooled.
Day after day, this thermal flexing works like bending a paperclip back and forth. The repeated cycles gradually extend the fracture behind the sheet, weakening its attachment until, one afternoon — typically at the hottest time of day, in the hottest months — the slab lets go.
Why it matters
The findings, published in Nature Geoscience in 2016, explained a long-standing statistical anomaly: an outsized share of Yosemite's unexplained rockfalls cluster in summer afternoons. They also carry practical weight for a park visited by millions, informing hazard assessment beneath the valley's iconic granite walls, and for infrastructure anywhere built on or beneath exfoliating rock faces.
The deeper lesson is elegant: geological change doesn't always need a dramatic trigger. Sunlight, patiently applied every day for decades, is enough to bring down a cliff.
The study: Collins, B. & Stock, G., "Rockfall triggering by cyclic thermal stressing of exfoliation fractures," Nature Geoscience (2016). See also the USGS summary at usgs.gov.