Alexander von Humboldt in the Dolomites

In September 1822 the two German geologists Alexander von Humboldt and Leopold von Buch visited the village of Predazzo in the valley of Fassa, Italian Dolomites.

Geological section of the Alps with the Dolomites in the Berghaus-Atlas (1845-1862), a collection of maps printed as supplement to Humboldt’s Cosmos.

In the 19th-century this site was famous among geologists due to a geological mystery found nearby.

Alexander von Humboldt and other famous geologists visited and studied the Dolomites in the 19th-century.

According to Neptunism, a scientific theory very popular at the time among German geologists, all rocks were formed by sedimentation from a primordial sea. Coarse-grained granite was the first rock to crystallize (as the crystals visible in the rock suggest) and so a primordial formation, always followed by younger layers of limestone and other sedimentary rocks. However, near Predazzo a massive granite covers the layers of limestone and therefore is the younger geological formation. Von Buch explained the puzzling observation as a result of a large landslide disturbing the order of the rocks, but Humboldt was not convinced by this explanation.

The outcrop above the village of Predazzo today and a sketch from 1849. The limestone-marble (“Kalkstein”), also referred to as “predazzite“, surrounds a large intrusive body of “granite” (a monzonite-syenite). This was impossible according to the prevailing geological theories of the 19th-century, as the crust of the Earth was imagined to consist of ordered layers of various rock-types.
Samples of Predazzite – a contact-metamorphic limestone named after the village of Predazzo.
The volcanic rocks of Predazzo are associated with the volcanic system of Monzoni, a large volcano that erupted some 230 million years ago. It’s also the type locality of “Monzonite”.

After his visit to the Dolomites, Humboldt quickly “converted” to Plutonism. Plutonists, a group of geologists named after the Roman god of the underworld, believed that volcanism played a major role in the formtion of rocks. Large chambers of molten magma exist within Earth’s crust. Volcanoes are connected to those magmatic chambers by volcanic conduits and as the magma erupts, it cools quickly and forms the fine-grained lava. If the magma cools slowly still stuck in the subterranean chambers, it will form an igneous rock with large crystals. Later erosion will remove the overlying rocks and expose the crystallized rock as granite. This, so Humboldt, likely happened also near Predazzo.

228-237 million years old magmatic dikes cutting through marbles (former reef limestone), as seen at the locality of Mountain of Dos Capel near Predazzo.
Contact metamorphism between a basaltic dike and former reef limestone.
Basaltic dikes (“serpentinite”) cutting through marbles (“modified limestone”) in contact with a magmatic intrusion of “granite”. Figure from Geo-Mineralogische Skizzen über einige Täler Tirols, 1848.

We nowadays know that 230 million years ago molten magma was injected under great pressure in the older limestone formation, deposited in an ancient sea. The magmatic intrusion and magmatic dikes cut through the limestone, causing the rock succession that baffled the 19th-century geologists. Slowly cooling over the ages, the magma solidified and crystallized to form the monzonite-syenite and the limestone was transformed by the great heat into marble.

Literatur:

  • AVANZINI, M. & WACHTLER, M. (1999): Dolomiti – La storia di una scoperta. Athesia, Bolzano: 150
  • DELLANTONIO, E. (1996): Geologia delle Valli di Fiemme e Fassa. Museo Civico “Geologia e Etnografia” Predazzo: 72

Autor: David Bressan

Bressan-Geoconsult bietet geologische Dienste im Alpenraum an, mit Schwerpunkt auf geologische Kartierung, Betreuung von Bohrungen, Quartärgeologie, Hydrogeologie und Baugeologie. Kontakt: david@bressan-geoconsult.eu

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