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The radon problem in Kinsarvik, Ullensvang municipality in Western Norway, is among the most severe in the world. Annual average indoor radon concentrations as high as 56,000 Bq/m3 have been measured. The problem affects >100 houses, where average yearly indoor radon concentrations of 4340 Bq/m3 has been reported. Indoor radon concentrations vary during the year, which has been explained by thermally induced flows of radon-bearing air through a porous ice-margin deposit. Updated geological knowledge and new mapping methods (LIDAR data) have allowed for a re-examination of the problem. There is significant evidence that does not fit with the established geological understanding that classifies the Kinsarvik deposit to be an ice-marginal moraine. Instead, the landforms and internal structures of the deposit are characteristic of a rock-avalanche deposit. The LIDAR data suggests a rockslide deposit of approximately 50 million m3. Airborne and ground gamma-ray spectroscopy show elevated uranium content in the granitic bedrock north and east of Kinsarvik. These uranium-bearing rocks are also found in an open pit in the Kinsarvik deposit. Terrestrial cosmogenic nuclide dating of four surface boulders shows an average age of 10,900 ± 600 years. This suggests the rock avalanche fell immediately after the last deglaciation of Western Norway. Our results support that the radon problem in Kinsarvik is caused by uranium bearing bedrock and a porous rock-avalanche deposit that emanates radon, which in turn exhibits an alternating, temperature driven flow direction. • Extremely high indoor radon concentrations are documented in Kinsarvik Norway. • Granitic rocks have an increased uranium content that produces radon gas. • The actual deposit is proved to be a result of a rock avalanche. • The rock avalanche has crushed the uranium bearing rocks and makes for radon emanate easy. • The combination of uranium bearing rock and a rock avalanche can be a worldwide problem. [ABSTRACT FROM AUTHOR]

On 22 February 1756 the largest historically recorded rockslide in Norway took place at Tjelle in the Langfjord (Western Norway). The rockslide created three displacement waves of up to 50 m in height that caused 32 casualties and destroyed most houses and boats along the shores of the Langfjord. The trigger and contributing factors leading to the Tjellefonna rockslide are largely unknown and even seismic triggering has previously been suggested. This study provides a thorough back-analysis of the Tjellefonna rockslide using detailed geomorphological, engineering geological and tectonic field mapping in combination with topographic reconstructions, bathymetry analysis, volume estimations and numerical slope stability analysis. The back-scarp and eastern flank of the Tjellefonna scar form several tens of meter high rock walls, while the basal failure surface and other parts of the scar are covered by rock avalanche debris that extend from the back-scarp down to the bottom of the Langfjord. The rockslide occurred in granodioritic gneisses with variably developed metamorphic foliation that is folded and strike parallel to the fjord. Two prominent fault zones are present in close proximity to the Tjellefonna scar; one is steeply SE-dipping (Tjelle fault), while the other one is sub-horizontal to shallow SE-dipping (Ritlehamran fault). Both fault zones are linked to the Møre-Trøndelag Fault Complex, with one of its branches forming the Langfjord lineament and probably also the faults at Tjellefonna. Additionally, there are four persistent joint sets that together with the metamorphic foliation and the Tjelle fault define the back-scarp of the rockslide and give a fracturing of the rock mass corresponding to a Geological Strength Index (GSI) of 45–55. The GSI decreases significantly to 10–20 in the fault zones, which form distinct weakness zones in the rock slope. Volume estimates based on a reconstruction of the ante-rockslide topography range from 9.3 to 10.4 million m 3 , which is lower than previous volume estimates (12–15 million m 3 ). Large portions of the failed rock mass remained on land and only approximately 3.9 million m 3 entered the fjord. The observed discontinuities in the rock mass at Tjellefonna do not allow for a simple kinematic failure mechanism due to the lack of moderately SE-dipping structures. The basal failure surface was most likely not composed of a single structure, but of a complex interplay of fault zones, metamorphic foliation, joints and broken rock bridges. Numerical slope stability modelling highlights that weak fault zones are essential for the development of the failure surface over a long time. This progressive failure was likely aided by low- to medium-magnitude earthquakes that are frequent in the region. Numerical slope stability modelling and historical accounts suggest, however, that heavy, long-lasting rainfall was the triggering factor for the 1756 Tjellefonna rockslide rather than an earthquake. [ABSTRACT FROM AUTHOR]

Assessing initiation of rock-slope deformation and paleo-slip rates of rockslides is important to understand the impact of climate variability - in particular permafrost changes - on rockslide activity. Norway, with 6–6.5% permafrost cover today, continues to experience spatial and temporal variations in permafrost. We assess the timing of deformation initiation and potential late Pleistocene and Holocene sliding rates for six active gradually deforming rockslides in Norway using terrestrial cosmogenic nuclide dating. We evaluate the timing and rates of deformation considering a possible climate control by modelling the permafrost evolution since deglaciation for three rockslide locations: Mannen, Revdalsfjellet and Gamanjunni 3. Deformation at these sites started during or at the end of the Holocene Thermal Maximum (HTM), between 8 and 4.5 ka when permafrost in those regions was mostly degraded. At two low elevation sites, Oppstadhornet and Skjeringahaugane in western Norway, where permafrost remained absent during the Holocene, deformation started shortly after deglaciation. The timings suggest that the presence of permafrost in Norwegian rock slopes had a stabilizing effect over several millennia after deglaciation. Vertical transects of exposure ages along outcropping sliding surfaces indicate that pre-historical sliding rates seem to have decreased throughout the Holocene at all analysed rock-slope instabilities. However, modern measured sliding rates at four sites indicate a moderate to strong acceleration, compared to previous deformation rates. Three of these sites are located above or at the lower limit of alpine permafrost, where recent permafrost degradation may enhance slope destabilisation. The implication is that slope failures may become more frequent during accelerated warming in the Arctic and Subarctic. • Paleo-slip rates of gradual rock-slope deformations are resolved with 10Be and 36Cl. • High recent deformation suggests acceleration for at least four rockslides in Norway. • Two of eight rockslides activated during or shortly after deglaciation. • Rockslide activation often coincides with increased temperatures and precipitation. • Degrading permafrost likely drives rock-slope destabilisation in the (Sub-)Arctic. [ABSTRACT FROM AUTHOR]