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Granite Wars – Episode I: Fire & Water


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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 theCanzoccoli” -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.

Fig.5. Rock samples of “Predazzite”, believed to be a peculiar sedimentary rock deposited from the primordial ocean after the granite, it was later identified as common contact-metamorphic limestone.

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?

Bibliography:

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

David Bressan About the Author: Freelance geologist dealing with quaternary outcrops interested in the history and the development of geological concepts through time. Follow on Twitter @David_Bressan.

The views expressed are those of the author and are not necessarily those of Scientific American.





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  1. 1. Snowballsolarsystem 3:14 pm 09/28/2013

    An alternative hypothesis attempts to revive the Neptunism vs. Plutonism debate by suggesting that granite plutons formed through Neptunian aqueous differentiation of comet cores, forming authigenic sedimentary comet cores that went on to melt and become plutonic rock due to violent chemical reactions of the highly-chemically-reduced solar-plasma comet precursors.

    But the comet cores predominantly arrived on Earth and Venus in the form of long-period dwarf-planets whose extremely-eccentric, circular-perihelia orbital sections predominantly matched up with two planets with the most circular orbits, Venus and Earth.

    Dwarf planets, in turn, are primarily composed of accretions of 100 km and larger trans-Neptunian object (TNO) cores. TNOs may have formed by gravitational instability (GI) at the inner edge of our former protoplanetary disk at the pressure dam of the corotation zone and were carried to the Kuiper belt by Neptune’s outer resonances as Neptune spiraled out from its spin-off fission from the circumsecondary orbit around our former binary Sun’s smaller companion ‘B’ star.

    Proto-TNOs formed by GI from the highly-oxidized presolar dust and ice of the protoplanetary disk similarly typically bifurcated due to excess angular momentum, and the binary TNOs that spiraled in to merge and initiate aqueous differentiation to form authigenic, sedimentary gneiss-dome cores with hydrothermal schist and carbonate-rock mantles.

    The protosun bifurcated due to excess angular momentum during initial gravitational collapse. Then the two binary solar components spun off pairs of proto-planets during, perhaps, the second phase of gravitational collapse as the solar cores were endothermically converted from molecular hydrogen to atomic hydrogen at a clamped core temperature of about 2000 K. Jupiter and Saturn spun off from the larger ‘A’ component and Uranus and Neptune from the smaller ‘B’ component, again due to excess angular momentum.

    The proto ‘spin-off’ planets Jupiter, Saturn, Uranus and Neptune also bifurcated due to excess angular momentum and spiraled out to their current orbits, fueled by the energy and angular momentum of their former binary components and from the binary-binary, resonant secular perturbation between binary Sun and binary planets. Binary Sun merged at 4,567 Ma in a luminous red nova (LRN), forming the short-lived r-process radionuclides of our early solar system, along with enriching the Sun and LRN debris (chondrules, chondrites and highly-chemically reduced comets) in alpha-process isotopes, 12C, 16O, 20Ne, 24Mg and 28Si.

    Highly-chemically-reduced, solar-plasma LRN debris condensed in dammed outer resonances to form comets by GI. Proto-comets typically bifurcated due to excess angular momentum, forming binary comets which may spiral in to merge and initiate aqueous differentiation, forming peanut-shaped ‘contact-binary’ comets. And the violent chemical reactions of chemically-reduced solar plasma condensates typically cause the authigenic sedimentary cores to melt, forming I-type plutonic granite or S-type layered granite in comet cores that fail to melt but merely lithify, while A-type granite may be terrestrial melts over ‘hot spots’ as theorized.

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