Increasing temperatures controlled rockfall activity in the Rwenzori Mountains (Uganda) over the past 11,000 years

Audrey Margirier, Konstanze Stübner, Christoph Schmidt, Johannes Lachner, Georg Rugel, Salome Oehler, Pontien Niyonzima, Rosemary Nalwanga, Anne Voigtländer

Rockfalls and other gravitational mass movements are expected to become more frequent under ongoing global warming in temperate and cold mountainous regions. In contrast, although high numbers of rockfalls are observed in humid tropical mountains, the processes controlling their occurrence remain poorly understood. These warmer regions offer valuable natural laboratories for anticipating the impacts of future warming on slope stability in currently colder environments. We used 10Be surface exposure dating to establish a chronology for seven individual rockfall deposits in the Rwenzori Mountains (Uganda) and assess climatic controls on slope destabilisation. The combination of steep valley flanks and pervasive fracturing in the basement rocks makes the Rwenzori Mountains prone to rockfalls. The 10Be ages range from 13.6 ± 0.9 ka to 1.0 ± 0.1 ka and cluster into three distinct periods: 11–9 ka, 7–5 ka and ~2 ka. These periods align with local temperature fluctuations, suggesting that rockfalls occurred episodically in response to Holocene temperature fluctuations. Early Holocene rockfall activity (~11–9 ka) likely reflects enhanced mechanical weathering and fracture propagation due to glacier retreat and freeze–thaw cycles. In contrast, mid-Holocene (7–5 ka) and late Holocene (~2 ka) rockfall activity coincide with warmer conditions that enabled chemical and biological weathering and enhanced subcritical crack growth, promoting fracture progression and slope destabilization. These results highlight the role of climate – especially temperature – driven chemical and biological weathering in preparing rock slope failures and rockfalls in tropical mountainous landscapes. With continued warming, chemical weathering may increasingly control rockfall activity in temperate and cold climates.