September 28, 2013 | 1
In 1820 the Italian engineer Count Giuseppe Marzari-Pencati (1779-1836) published a short article about the stratigraphic succession found near the small village of Predazzo. At the “Canzoccoli” -outcrop Pencati observed a grayish granitic rock overlying white marbles. What today is described in any geological textbook as an “unconformity” was at the time a geological impossibility.
Fig.1. & 2. The outcrop “Canzoccoli” today and a sketch of the stratigraphy by a geologist in 1849, the limestone-marble (“Kalkstein”), also referred 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 earth was imagined to consist of ordered layers of various rock-types (figure in public domain).
In the 18th and 19th century most European geologists applied the scientific theory of “Neptunism“, named after the Roman god of the sea, to explain the stratigraphic successions as found in most mountain ranges. Many mountains are formed at the base by metamorphic or granitic rocks, with coarse mineral grains, followed by sedimentary rocks, like shales and limestone, with small mineral grains.
Fig.3. An idealized section of earth´s crust, from Emile With “L´Ecorce terrestre” (1874), showing the stratigraphic order of the sediments deposited during various geological epochs. Magmatic and other metamorphic rocks are found mostly at the base, only some rare and localized volcanic vents bring molten rocks near the surface of earth (figure in public domain).
The great German mine inspector and professor of mineralogy Abraham Gottlob Werner (1749-1817) published in 1787 a book with the title “Kurze Klassifikation und Beschreibung der verschiedenen Gesteinsarten” (Short classification and description of the various rock-types) were he explained this difference in composition and texture by crystallization of all the rocks from a primordial ocean.
Early earth, born by the agglutination of cosmic matter, was at first covered by a primordial ocean, where all the elements, necessary to form minerals, were dissolved in the water. In the cooling ocean crystallization and deposition of the rocks started: first the oldest and hardest rocks – granite, gneiss and schist, then basalt and finally limestone and sandstone. After the last rocks crystallized, the sea dropped and erosion formed the last, and more recent, rocks and soils.
Figure 4. from “De omni rerum fossilium genere,…()”, published in 1565 by Swiss naturalist Konrad Gesner, with basalt columns, however shown as large crystals. The idea that crystals – therefore minerals and rocks – were formed by crystallization from water had a long tradition (figure in public domain).
Johann Wolfgang Goethe, philosopher, poet and convinced “Neptunist-geologist” summarizes the argument as follows:
“All observations agree, as much as were carried out recently, that granite is the lowermost kind of mountain on our earth, that all the rest is found on or beside it, it itself in contrast covers nothing else, so it, even if it not composes the entire earth, nevertheless composes the lowermost crust that is accessible to us.“
Werner published in 1788 “Bekanntmachungen einer von ihm am Scheibenberger Hügel über die Entstehung des Basalts gemachte Entdeckung” (Notes about a discovery at the hill of Scheibenberg about the formation of basalt), where he describes an outcrop in Saxonia with a horizontal bedded – and seemingly gradual – transition of clastic sediments to basalt*. For Werner this outcrop was undisputable proof of the veracity of Neptunism (*today we know that the basalt overflow older sediments, vaporizing the water in the wet clay and sand a layer of fragmented lava was formed – blurring the contact between the different rock-types).
However just some years earlier a Scottish self-taught naturalist, James Hutton, had formulated an alternative geological theory – primordial rocks with coarse mineral grains are formed by magmatic intrusions and slow cooling and crystallization of the magma. Also volcanoes can form new rocks, however fine-grained as the erupted lava cools fast and the minerals have not enough time to grow larger. This theory was named after the Roman god of the underworld “Plutonism“.
Marzari-Pencati had visited the active volcanoes in Italy and as “Plutonist-geologist” he was convinced that volcanoes play an important role in the formation of new rocks. He interpreted the white rocks at the Canzoccoli - classified by some geologist as a peculiar sedimentary rock named “Predazzite” – above the granitic rock (a Monzonite-Syenite) as an older limestone, however altered by the intense heat of the later intruded melted magmatic rocks.
Many German geologists and former students of Werner, like Leopold von Buch and Alexander von Humboldt, rejected this explanation and travelled to Predazzo to find other explanations – like a large landslide distorting the natural order of the rocks.
However with time more and more geologists, studying the peculiar geology at the Canzoccoli, accepted that magmatic processes deep inside earth play a major role in the rock-cycle.
Fig.6. Magmatic dikes (referred after an old name for dark volcanic rocks as “Serpentinite”) cutting through marbles (“modified limestone”), the limestone is also in contact with a younger magmatic intrusion of “granite” – outcrop as seen by a “Plutonist-geologist” in 1848 (figure from “Geo-Mineralogische Skizzen über einige Täler Tirols”, figure in public domain).
Fig.7. Magmatic dikes, 228-237 Ma old, cutting into a former reef of the Dolomites, as seen at the outcrop of Mountain “Dos Capel”, near Predazzo. Such outcrops convinced most geologist that volcanic rocks form by crystallization from molten magma, coming from deep inside earth. However the debate between Neptunists and Plutonists continued long after 1890.
However soon after the debate between Neptunists and Plutonists was solved, geologists became aware of another great riddle – the chemical composition of granite differs significantly from what we know of the chemical composition of the earth´s interior – how could therefore granite form on earth?
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
HÖLDER, H. (1989): Kurze Geschichte der Geologie und Paläontologie – Ein Lesebuch. Springer Verlag, Heidelberg: 243
LOOK, E.-R. & FELDMANN, L. (Hrsg.)(2006): Faszination Geologie – Die bedeutendsten Geotope Deutschlands. E. Schweizerbart´sche Verlagsbuchhandlung, Stuttgart: 179
WAGENBRETH, O.(1999): Geschichte der Geologie Deutschland. Georg Thieme Verlag: 264