The term landslide is often associated with large disasters that result in loss of life and material damages. Catastrophic events, such as snow avalanches in Vassdalen and Odda, rock avalanches and tsunamis in Loen and Taford, and quick clay slides in Rissa and Verdal, have left their marks in history. Within the last 150 years, approximately 2000 people have lost their lives in Norway due to landslides and snow avalanches. Landslides, however, are natural processes, occurring regularly and helping to shape the landscape over time.
An exposed rock surface will weather over time. Fractures can be widened by the actions of freezing and thawing as well as roots from vegetation. This can lead to small blocks of rocks loosening and falling. We call this a rockfall. If the block is between 100 and 10 000 cubic metres, we call it a rockslide. NGU has produced a national susceptibility map for rockfalls, which shows approximately how far a rock can travel from a mountainside or cliff.
Slow movements along discontinuities in bedrock can eventually cause mountainsides along fjords and valleys to become unstable. When large volumes of rock, often millions of cubic metres, collapse from a mountainside, we call it a rock avalanche. The greatest hazard is found when the avalanche can hit a fjord or lake, resulting in the creation of large, destructive waves. It is impossible to stabilize such large slopes, so the hazard has to be dealt with by mapping and monitoring the slopes such that a collapse can hopefully be predicted. NGU has the role of mapping unstable rock slopes throughout Norway.
Landslides in loose soil
During the last glaciation, the bedrock in Norway was covered by moraine material; an unsorted mixture of all fragment sizes, from clay to large boulders. Weathering of bedrock has also created sediments. Lengthy or intense rainfall can reduce the strength of soil cover so much that a debris slide can occur along steep valley sides. The same can happen as a result of rapid melting of snow. NGU’s susceptibility maps for debris floods and avalanches indicate the areas that are potentially affected by such landslides. Human activities that alter the natural drainage patterns, such as logging or the construction of roads, can increase the likelihood of such landslides.
Liquifaction of clay
A number of regions that were under sea level after the last glaciers melted 10 000 years ago have now risen above sea level and are covered in marine clay. This type of clay can be stable as long as it is undisturbed, but can collapse and liquefy if it is disturbed or subjected to extra loading. We call it quick clay. A small collapse along a creek or shoreline, or even underwater, can evolve into a large quick clay slide as each collapse destabilizes the clay behind it. The collapsed clay flows as a liquid. NGU’s surficial geology maps indicate which areas are covered by marine clays.