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Geology of the Grimsel area

Some 300 million years ago, magma penetrated from the earth's interior and solidified at a depth of around 13 kilometres to form crystalline rocks. The volume of the rock was reduced during cooling and deep fracture systems were formed. Rocks in liquid form rose through these fractures to form so-called dyke rocks (lamprophyres and aplites).

 

During the next 200 million years, the rocks of the Aar Massif remained virtually unchanged. During the alpine mountain-building phase around 40 million years ago, the Massif was subjected to strong loading. As a result of transport of the alpine nappes towards the north, the Aar Massif subsided and was overlain. At the time of the maximum overburden (around 12 km), the rocks were exposed to high temperatures (around 450°C) and pressures (around 300 MPa). This phase saw the formation of schistosity and shear zones. In the subsequent phase of uplift around 16 million years ago, tension joints or fissures formed in the rock; these are partly filled with beautiful crystals. Uplift is still ongoing today at a rate of around 0.5 to 0.8 millimetres per year.

profil_grimselstr

Geological-tectonic profile along the Grimsel pass road

  1. Sediments of the Wildhorn nappe (Jurassic, Cretaceous)
  2. Autochthonous and parautochthonous sediments (Triassic to Lower Tertiary)
  3. Innertkirchen Crystalline
  4. Altkristallin
  5. Volcanoclastites
  6. Mittagfluh Granite
  7. Central and Southern Aare Granite
  8. Grimsel Granodiorite