The Köfels landslide is known to be the largest landslide in the crystalline Alps. It covers an area about 13 km2 and has the form of a semi-circular basin of about 4 km width. The original volume of displaced rocks is estimated to have been 2,1-2,5 km3, sliding a distance of several hundred metres in less than a minute reaching velocities of 50 m/sec.

The event happened when a large part of the Fundus crest (the western, left side mountain ridge of the Oetz valley) collapsed.

This is easily recognisable geomorphologically but also petrographically because the landslide deposits consist mainly of Augengneiss which builds up the Fundus crest and vanishes to the east and west. The broken crest must have been about 400 m higher than today and situated farther to the east (closer to the Oetz valley). The primary (basal) sliding surface is easily recognisable at various locations along the headwall of the Köfels niche (Preuß 1986).

There are slabs with slope angles of 25^o - 30^o to be seen along trails from Köfels to Fundus valley. At the foot of the headwall the slide surface disappears below the landslide deposits. Pumice outcrops have not been found at the primary sliding surface but close to the secondary sliding surface north of Köfels.

Anaglyph image (oriented towards south) showing in the centre the landslide mass of Köfels landslide which is deeply cut by the river Ötztaler Ache. On the very upper right side part of the sliding plane is visible. The village on the right side ist Niederthai at the lower end of the Hörlachtal, the village in the lower middle is Umhausen and in the upper middle of the image a small part of Längenfeld is visible.

The landslide moved down like a huge sled crashing against the opposite slope of the Oetz valley, splitting into two parts: The lower part remained in the main valley and was heavily shattered and pulverised, as is easily seen along the main road into the Oetz valley.

The upper part continued to move above it and filled the Horlach valley. As a result a secondary slip surface occurred and it is this surface which is visible around Köfels. Close to this the famous pumice have been found. The upper part rests on a high rock terrace. A blockade of the Oetz and Horlach valleys dammed up both rivers ( see Oetz valley). The resulting lake deposits are known by coring to be more than 9o m in the Längenfeld area.

View from Niederthai in the direction of upper Hörlach valley. On the right side there are still very obvious rests of the lake sediments (looking like tongue shaped lobes).

View from upper Hörlach valley in direction Funduskamm. In the foreground Niederthai can be seen. The hill directly behind (in the image about in the middle part) Niederthai is the Taufererberg, which consists of landslide mass. In the background, right from the middle a lowered crest can be seen, which is the Fundus crest - the source of the Köfels landslide.

Due to the rather low inclination of the sliding angle, large parts of the landslide deposits remained more or less coherent and look like bedrock. Subsequently they have been mapped as bedrock in the geological literature of the past.

Jigsaw puzzle effect in Köfels landslide - the landslide deposits are of course fractured but remained coherent.

A tunnel was built in 1951 in 1200 m a.s.l. on the western slope of the Oetz valley for hydrogeological purposes which revealed the contrast between "real" bedrock (Paragneiss) and the in part fractured landslide deposits (Augengneiss). In addition this tunnel showed the pre-existing now buried gorge of the Horlach valley. The Horlach river was deflected to the north by the landslide and plunges now 150 m over a rock cliff. This waterfall is called Stuiben fall and has eroded 6-7 m into bedrock which is a good mark for erosion when one knows the time span.

In the tunnel also a strongly deformed wood was found in the shattered landslide sediment. Dating by 14 C was 8710 + 150 BP

By this the landslide could be dated and also the erosive force in bedrock which seems to be in this case (Horlach river at Stuiben fall) about 1 m in 1000 years.

Before the tree was found in the tunnel the dating of the slide was complicated by the fact that glacial deposits were lacking on the western side of the Maurach gorge (Köfels area) but are ample on the eastern side (Niederthai-Tauferberg with glacial moraines, erratics , tilted and turned around roches moutonnees with crescentic gouges on the surface pointing in the "wrong"-for the Oetz valley glacier-direction!).If one assumes a landslide before the last glacial advance where are the glacial features on the other side ?

Heuberger (1966,1975,1994) explains this:

The glacial features of Tauferberg do not continue beyond the landslide deposits. The idea is that all the glaciated surface from the other side (west) was carried with the landslide to the east and remained at the surface to form the area of Tauferberg today. A comparable event was the landslide Vaiont 1963; also there trees etc. remained on the surface throughout the slide. There are also no signs of periglacial action (solifluction etc.) subsequent to the sliding. The knick point between slope and basin of Niederthai is pretty sharp. This would have been impossible had the slide occurred in late glacial time.

So whereas many other slides in the Alps took place in late glacial time this slide as well as the Tschirgant slide occurred in the Holocene.

The Köfels event became well known when in 1863 pumice or fused rock was found at the site (Pichler,1863). This raised the question of the source of the energy that would be required to produce such a rock structure.

At first the prevalent hypothesis attributed these energies to a volcanic event. The volcano hypothesis lost support when it was seen by drillings that neither the shattered Maurach barrier rock nor the pumice carried on downwards.

A second idea was a meteorite impact because a rather rapid cooling necessary for the pumice production made a volcano hypothesis untenable (Suess,1937). In both theories the formation of the pumice relates to the process which triggered the landslide.

Preuss(1971,1974) one of the investigators of the meteorite impact event at the Nördlinger Ries,Bavaria was the first to refute both above theories by comparing the mineralogical data of both events. He could rule out that the two situations resulted from the same kind of event. His idea was that the required energy to produce rock fusion had been generated by the action of the landslide itself.

Studies of deformation of quartz grains (microstructure density and type of dislocation) and the total lack of glassy planar deformation features clearly indicate that deformation at Köfels occurred in a regime characterised by a lower stress than an impact. No evidence of shock metamorphism can be detected. These defect microstructures in the quartz grains of the gneiss wall rock are typical of plastic deformation at high temperature with very efficient recovery. At relatively short distances from this surface (dimensions of cm) the dislocation microstructure is markedly different. It is typical of a deformation regime controlled by lattice friction with no efficient climb processes. These dislocations microstructures demonstrate the very strong temperature gradient which must have occurred during the landsliding. The glassy veins detected in the crystal relicts have nothing to do with the planar deformation features found in naturally shocked quartz. The silica glass particles found in the pumice is mostly irregular and is not identical at all with the straight narrow amorphous lamellae with controlled orientation found in shocked quartz.

Landslide processes release their energy much more slowly than impacts in such a way that this energy is dissipated in the form of heat without radiation or shock wave effects. The temperature along the glide surface could have been in excess of 1700 C during about half a minute. Under these conditions the base of the rock slide must melt and most of the mechanical energy generated along the friction zone must be transferred into heat in the surrounding wall rock. Other processes for dissipating the energy (cracking, wave radiation etc. must have been negligible.

The fractured gneiss rock involved in the landslide was degassed and amphiboles and biotite where preferentially consumed to form the melt which still contain about 30 % glassy quartz and feldspar. The gneiss nearby the shear zone is intruded by the melt along many fractures. Erismann (1977) did experimental work to confirm the calculations and the term "frictionite" was coined for rock modified by landslides for all stages of deformation ranging from fraction to fusion.

GIS analysis

For investigating the Köfels landslide area a DEM has been created using the cartographic model 1: 50.000 and a set of structure lines derived from areal photography (all of the following examples are oriented towards north).

Exposition in landslide area

Slope classes show the differences between nearly flat valley floors and steep hillslopes.

A hillshaded relief gives an idea of the landscape around Köfels landslide.