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.
In the 19th-
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.
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.
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.
- 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