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The landslide of Köfels (named after a small village in Tyrol) is one of the largest recognized landslides in the Alps - large enough to dam up a 92 meters (300 feet) deep prehistoric lake and divide in two the valley of Ötz. Wood fragments discovered during the construction of a gallery in the landslide deposits were dated to an age of 8.710+/-150 years BP (ca. 9.800 years cal BP).
The landslide of Köfels was recognized already by early geologists mapping the Alps, like Escher von der Linth in 1845 and the two great quaternary geologists Penck & Brückner in 1909. The Tyrolean geologist Klebelsberger describes the morphology of the landslide in his book "Geologie von Tirol" (1935):
"…the landslide slide down apparently from the … [] … mountain ridge above Köfels… great gliding surfaces show the movement of large masses of rocks. It seems that the ridge was considerably lowered by the collapse…"
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Fig.1. View into the valley of Ötz and on the landslide of Köfels (and yes, that is an analog photo). On the left the Tauferberg, with the concave scar left by the prehistoric landslide (the mountain was lowered by 200 meters, more than 600 feet, by the collapse). In the foreground the village of Umhausen. Strippled line = landslide deposits.
Klebelsberger mentions also the occurrence of a strange rock:
"… a particular kind of formation, for which a volcanic origin or at least influence must be considered, like nowhere else [found] in the Alps, are the deposits of pumice in the area surrounding Köfels in the valley of Ötz… a local peculiarity… a postglacial liparitic pumice. It is an in part yellow-grey to yellow-brown, sometimes light grey to dark grey, extraordinary foamy, mostly vesicular to, in rare occasions, a fibrous rock, with common inclusions of mylonized gneiss."
Some authors interpreted the shattered rocks and the pumice as a diatrem - a volcanic dike or the remains of a collapsed magma chamber - evidence for the youngest volcanic activity in the Alps. In 1936 other authors suggested that the rocks were formed by the recent impact of an extraterrestrial body.
Both explanations however couldn't explain the mapped distribution of the strange rocks, not scattered around a supposed impact- or volcanic-crater, but found just on one side of the valley.
Only after 1970, with the discovery of other very large landslides in the Alps and in the Himalaya, geologists recognized the pumice as a Frictionite (also referred as Pseudotachylite or Hyalomylonite). The prevailing rock of the landslide-mass - gneiss - was partially molten by the intense heat generated by the friction along the sliding planes of the gigantic landslide. The heating and cooling occurred so quickly that no new crystals formed from the melt, but a glassy matrix with preserved gas bubbles, resembling volcanic pumice.
The landslide of Köfels is famous also for another strange geological anomaly, also connected to the intense deformation and shattering of the rocks. In 1989 it was discovered that the rate of lung diseases and cancer in the village of Umhausen, build in part on the landslide deposits, was five times higher than in the surrounding communities.
The increase of lung cancer was caused by unusual high concentrations of the radioactive gas Radon found in the buildings. Radon forms naturally by the decay of radioactive elements, like Uranium, found in the accessory minerals, Apatiteand Zircon, of the Ötz-valley gneiss. In an intact rock mass the gas released is, as is the concentration of uranium in the source minerals, very low. However the rocks of the landslide deposits are so intensely shattered, almost to the single mineral grains, that there is a large contact zone between the radioactive minerals and the air. The radioactive gas can so easily escape from the rocks and becomes concentrated in the basements of the buildings, standing on the permeable rocks.
Recognized this geological threat, authorities suggested to residents to isolate their basements or to provide effective ventilation, so to disperse the radioactive radon and lower the concentration inside the house to safe levels.
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