Very large rockslides are rare but very dangerous events that can have catastrophic effects on entire human settlements. One of the greatest disaster of this kind happened in 1963, when a rockslide felt into the artificial lake of Vajont, causing a flood wave that destroyed various villages and killed 2.000 people. To prevent similar disasters it is important to understand the factors that can cause such a large rockslide. Early explanations involved only earthquakes, but since the mid of the 19th century also climate change is considered an important factor that can increase the occurrence of catastrophic rockslides.
According to this hypothesis temperature oscillations increase the weathering rate of rock surfaces and the rock becomes strongly cracked and fissured. During periods with a more humid climate water will infiltrate the rocks trough these fissures. The water acts like a lubricant and causes huge blocks to slip off - a rockslide occurs.
To test this hypothesis it is necessary to compare the occurrence of rockslides with past climatic variations. In the Alps written records of rainfall or temperature span approximately 250 years and in the same period only few large landslides occurred - like in 1806 when a rockslide destroyed the Swiss village of Goldau and killed 500 people.
To improve this limited database geologists reconstructed the climate of the last 10.000 years and dated as many fossil rockslides as possible.
The climate in the Alps can be reconstructed with various methods: the chemical composition and fossil content of sediments deposited in alpine lakes can be used to estimate the amount of rainfall during the period when these sediments formed. The fluctuations of glaciers, inferred from the preserved moraines, are used to reconstruct the oscillations of temperature in the same time span.
To date fossil rockslides until 50 years ago the only applicable method was radiocarbon dating, where the concentration of the slowly decaying element Carbon-14 is measured in organic remains buried by a landslide. One of the first catastrophic landslides investigated with this method was the rockslide of Köfels in Tyrol, where a piece of wood was dated to more than 9.800 years. However such findings are rare and the age of many fossil rockslides was until now not measurable.
A recently developed dating method has significantly increased the number of datable deposits of old rockslides. Many rockslides in the Alps occurred in regions characterized by carbonate rocks, like limestone (composed of the mineral calcite) or dolostone (composed of the mineral dolomite). Both these minerals, formed by the elements Calcium and Magnesium and with traces of the radioactive element Uranium, are soluble in water. When a rockslide occurs the superficial debris is rapidly dissolved by rainfall. The saturated water then percolates in the underground where it deposits a part of the dissolved elements, forming a new generation of minerals inside the cavities of the rockslide debris. In geology such new formed minerals are referred as "cement", and this new formed cement has almost the same age as the rockslide event.
Fig.1. A large boulder of dolostone at the Tschirgant rockslide (Tyrol). Note the smaller pebbles at the basis of the boulder, which are hold together by the cement that formed after the rockslide occurred.
The radioactive Uranium incorporated into the cement slowly decays forming the daughter element Thorium. Similar to the radiocarbon dating method it is possible by measuring the concentration of these two elements, and knowing the rate in which Uranium is transformed into Thorium, to calculate the age of formation of the cement and therefore the age of the rockslide.
With this method it was possible to date various catastrophic rockslides of unknown age situated in the Austrian region of Tyrol, like the rockslide at the mountain pass of Fernpass or at the Tschirgant Mountain. Both fossil rockslides were dated to an age interval between 4.000 and 3.000 years.
In 2008, Prager and collaborators reviewed the available ages of these and other large fossil landslides and debris flows in the Central Alps. They found out that during the Subboreal - a time period between 4.200 and 3.000 years ago - there seems to be indeed a clustering of events.
From the studied sediments deposited in the Swiss lake of Gerzen and from the reconstructed fluctuations of the alpine glaciers we also know that the Subboreal was a period characterized by a humid climate and with strong oscillations in temperature.
Fig. 2. Temporal distribution of fossil landslides in the Tyrol and its surrounding areas compared to climatic variations (humidity and temperature), note the cluster of events around 4.000 years ago (modified after PRAGER et al. 2008).
However, the authors note also that the available data are still limited and there are significant regional variations in the occurrence of landslides, possibly related to local variations in rainfall or temperature.
With a steadily growing database, using old and new dating methods, it will become clearer how rockslides are triggered by these environmental factors and how the occurrence of catastrophic events is controlled by climate change.
OSTERMANN, M.; SANDERS, D.; PRAGER, C. & KRAMERS, J. (2007): Aragonite and calcite cementation in "boulder-controlled" meteoric environments on the Fern Pass rockslide (Austria): implications for radiometric age dating of catastrophic mass movements. Facies 53(2):189-208
PRAGER, C.; ZANGERL, C.; PATZELT, G. & BRANDNER, R. (2008): Age distribution of fossil landslides in the Tyrol (Austria) and its surrounding areas. Nat. Hazards Earth Syst. Sci. 8: 377-407
SANDERS, D.; OSTERMANN, M.; BRANDNER, R. & PRAGER, C. (2010): Meteoric lithification of catastrophic rockslide deposits: Diagenesis and significance. Sedimentary Geology 223: 150-161