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Largest landslide in the Alps From Wikipedia, the free encyclopedia
The Flims rockslide happened about 10,000 years ago (8000 BC) in eastern Switzerland. It is the largest known landslide in the Alps,[1][2] and the biggest worldwide whose effects are still visible, moving some 12 km3 (2.9 cu mi) of rock, about 300 times that of the historic Swiss Goldau landslide. The town of Flims can be found at the line where the slip surface disappears under the debris. North of Flims the rock face of Flimserstein stands 350 metres (1,148 ft) high whereas more westerly the slide surface is clearly visible. South of Flims is a huge hilly debris area that has been forest ever since, as this area is not suitable for farming, firstly for its shape and, even more for its lack of water. The river Rhine crosses this debris in a gorge called Ruinaulta. The Rhine still runs in debris, which shows that it has not yet reached the level it had before the incident. The highest hill in the debris area is almost 200 metres (656 ft) higher than Flims at the end of the sliding surface.
The top of the slide surface can be found at 2,700 metres (8,858 ft) above sea north of Flims at Fil de Cassons; the end of the slide surface is at about 1100. The pre-slide valley would have been at 600 metres (1,969 ft). The rock is Mesozoic limestone, including Mergel; the angle of slide is (only) 20° to 25°. Debris covers 40 km2 (15 sq mi). Pressure baked the debris together to a somewhat stable rock. Given the river Rhine as a base of the valley, the debris is as high as 600 metres (1,969 ft). This debris dammed the Vorderrhein and created a lake in the Ilanz area. The level of the lake was found to have been at a maximum of 840 metres (2,756 ft) above sea level, which results in a lake some 10 miles (16 km) long.[3]
Clemens Augenstein from the Geologisches Institut der ETH Zürich explored with Flavio Anselmetti the sediments at a small lake called Dachlisee at 1,137 metres (3,730 ft) near Obersaxen. The lake with no river flowing into it lies opposite Flims, some 6 miles (9.7 km) away. They were looking for dust, as an incident of this size would have produced a large dust cloud. Drilling five times into the sediment, they found embedded limestone dust. Using carbon dating the limestone dust was found to be 10,055 years old (±195 years).
A second source is wood that was found inside the debris in the region some 2 miles (3.2 km) upstream of the mouth of the river Rabiusa, which was covered by massive rock identified as being from the Fil de Cassons area, hence reaching this point during the event. It was too old for the dendrochronology line but confirmed the above carbon dating.[4]
Isotopic surface exposure dating of boulders exposed by the rockslide has yielded an age of 8900 ± 700 years.[5]
After the slide most of the water escaped through the upper section of the debris. There are several examples of rivers disappearing in the Alps and also in Scotland. The lakes in the debris area, such as Caumasee, still behave in this way. The Ilanz lake existed some 1000 to 2000 years before the water cut through the debris, creating the gorge.
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