Natural Fracture Ventilation Altering the Ground Thermal Regime in Unstable Rock Slopes—A Case Study From Western Norway

Abstract

Open bedrock fractures are common features in unstable rock slopes and alter subsurface thermal conditions of fractured rock masses through convective air ventilation and atmospheric exchange. These processes are particularly relevant in cold mountain environments, where variations in ground temperature affect rock mass stability and may allow for the occurrence of localized permafrost. Here, we investigate fracture‐atmosphere interaction systems in the Stampa unstable rock slope (Norway) by combining remote sensing, field mapping, continuous temperature and airflow monitoring, and radon and thoron surveys. In total, 79 temperature anomalies were identified and classified as: (a) cold vents and fractures during summer, with mean annual air temperatures (MAATs) generally below 0°C (down to −1.65°C), and (b) warm vents during winter, characterized by MAATs ranging from 2.62 to 5.27°C. Radon and thoron concentrations within fracture openings vary widely (8 to about 15,000 Bq m ⁻³ ), reflecting differences in ventilation rates, fracture connectivity and volumes. Monitoring at vents shows that airflow direction and intensity are primarily controlled by temperature gradients between the subsurface and atmosphere, while strong winds can significantly affect ventilation and gas concentrations. We identified four types of fracture‐atmosphere interaction systems: reversible and seasonal chimney ventilation, local convection in deep fractures, and cold‐air trapping in shallow fractures. These processes generate spatial heterogeneity in the rock thermal regime and can maintain frozen conditions far below regional permafrost limits. Our results demonstrate that fracture‐controlled ventilation plays a key role in shaping the thermal regime of unstable rock slopes and influences rock mass stability in mountain environments.