Humboldt´s Cosmos

"Inside the globe there live mysterious forces, whose effects become apparent on the surface. Outbreaks of vapours, hot slag and new volcanic rocks, as uplifts of islands and mountains"
Alexander von Humboldt

Fig.1. Geologic map and profile of the Pyrenees, after the "Berghaus-Atlas", a supplement to Humboldt's masterpiece "Kosmos" (1845-1862).
In the profile from inside to the outside of the mountains the layers are described as follows "Granitic rocks and general basement" - "transition mountains/rocks" - "secondary mountains". In the map Granite=pink, Basaltic rocks=green, Schist= grey, Clastic rocks/ Limestone= blue, Sandstone= red, Secondary Limestone= yellow, Tertiary rocks= dark-green

The German Alexander von Humboldt (1769-1859) studied finance and also mining engineering and became later a great self-educated explorer and naturalist. Following the tradition prevalent in Germany at the time he was educated to interpret geology with Werner's Neptunism - rocks were formed from liquids and magmatic phenomena are only of local significance.
During his travels in Europe and America he observed also active volcanoes and soon converted to Plutonism - rocks are formed from cooling lava and magmatic forces play a mayor role in shaping earth's surface.
In accordance to the theory of German geologist Leopold von Buch (1774-1835) of "volcanic bubbles" the profile shows the Pyrenees as a result of uplift by a core of magmatic rocks, bending the sedimentary layers of sandstone and limestone formations. According to this hypothesis magmatic rocks are found always as core in mountain ranges. To explain the apparent lack of Grantic rocks (coloured in pink) in the northern areas of the Pyreenes Humboldt apparently suggested selective erosion - explained by the schematic profile at the bottom of the page.

The Pyrenees are a mountain range 1500km long with an average width of 200km, the most western foothills of the Alpine-Himalayan orogenic belt. According to the modern theory of plate tectonics these mountains formed when the Iberian plate was partly subducted under the European plate, in a time period from the late Cretaceous to the Miocene (55-25 Ma).
The profile of the Pyrenees display a fan-shaped structure, with north vergent thrusts in the northern area and south vergent thrusts in the southern part.
In northwest to southeast direction the mountain range can be divided into five structural zones

- The Aquitaine foreland basin with the deformed Cretaceous sediments of the European Plate.
- The North Pyrenean thrust zone with thrust faults developed in the crystalline basement and the Mesozoic to Eocene sediments.
- The Axial zone with three mayor nappes of the crystalline basement
- The South Pyrenean thrust zone with deformed early Eocene to Miocene sediments.
- The Ebro foreland basin molassic, filled with relatively undisturbed sediments.

Fig.2. Axial Zone: Variscan granitoid rocks and Palaeozoic sediments (Humboldt´s granitic rocks); Thrust Zone: Mesozoic to Cainozoic deformed sediments; Foreland basin: Cainozoic undeformed sediments (after SCHELLART 2002).

The asymmetry of the Pyrenees (the estimated shortening is 70km in the south and 32km in the north), recognized already at Humboldt's times, is today explained by rotation due asymmetric mountain roots, where the Iberian plate was partially subducted under the European plate.

Bibliography:

SCHELLART, W.P. 2002: Alpine deformation at the western termination of the Axial Zone, Southern Pyrenees. In: Rosenbaum, G. and Lister, G.S.2002. Reconstruction of the evolution of the Alpine-Himalayan Orogen. Journal of the Virtual Explorer, 8, 35-55