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History of Geology

History of Geology

What rocks tell and how we came to understand it

Of Mountain Building and Dwarven Treasures

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Alpine-Type Fissures, fissures filled often with large and beautiful crystals of Quartz, Plagioclase, Rutile , Amphibole and even Gold, are - according to Alpine folklore the treasure chambers of dwarves - but how these treasures formed is even more fascinating than legends could figure out...

Soon after the basic principles of the succession of rocks were recognized, geologists started to wonder how mountains - with their folded and disturbed layers - form. The first mapped areas in France and the United Kingdom required only vertical movements to be explained, for example uplift of a stack of sediments by igneous intrusions (idea proposed by Hutton) or movements resulting from the contraction of a slowly cooling earth.

Fig.2. French geologist Elie de Beaumont (1798-1874) explained tilted layers of sediments by periodic "magmatic" pulses. In a first phase the horizontal bedded sediments (c) are tilted (b) by the intrusion of a large magmatic core (a). In a second phase the already tilted layers become even steeper (b) and new layers, start to tilt (c + d). Various pulses could therefore explain different tilted layers as found in mountains (image in public domain).

Geologist De Beaumont's hypothesis of mountain formation associated with vertical uplifts was accepted by most European and American geologists at the time, however in the late 19th century the idea of horizontally displaced tectonic nappes was introduced and alpine tectonics became even more a mess.

Marcel Bertrand (1847-1907), a young French geologist that actually never worked in the Alps, reinterpreted in 1884 the geology of a classic outcrop - the Glarus thrust -located in the Swiss Alps, revolutionizing the understanding of the tectonic history of the Alps.

Fig.3. The Glarus thrust as depicted in a painting by geologist H.C. Escher in 1812. The thrust forms the contact between older rock layers of the dark Verrucano (Permian - Triassic sandstones and conglomerates) and younger white limestone (Jurassic and Cretaceous) and Flysch and Molasse (Paleogene) - image in public domain.

This contradictory rock succession, where older rocks overlay younger rocks, was explained by famous Swiss geologist Albert Heim (1849-1937) with a very complicated "double fold" - a large fold composed of a series of smaller folds.

Fig.4. Geological section with the infamous "double fold" as envisaged by Albert Heim, from Livret- Guide Géologique, 1894 (image in public domain - click to enlarge).

Bertrand replaced the complicated double fold by an elegant single element - a single tectonic nappe, horizontally displaced by 40km - however his paper was widely ignored and even the eminent Austrian geologist Eduard Suess (1831-1914), convinced of Bertrand's hypothesis after a visit to the outcrop in 1892, failed to persuade the geological community.

However between 1893 and 1898 the Geologist Hans Schardt (1859-1931) demonstrated that some isolated mountains of the Swiss Prealps are in fact eroded remains of a previously much larger tectonic nappe. Finally French geologist Pierre-Marie Termier (1859-1930) extended in 1904 the nappe-structure to the entire Eastern Alps.

Fig.5. The Alps explained as a succession of tectonic nappes and folds, geological section by Swiss geologist Émile Argand (1879-1940) - image in public domain.

During the alpine orogenesis, when ancient marine sediments were thrust as tectonic nappes onto magmatic rocks, cracks and fissures opened. Circulating fluids, heated by the magmatic rocks and the pressure of more and more overburden being involved in the mountain building, transported dissolved elements to these fissures, filling nature's treasure chambers with great riches…

Bibliography:

DalPIAZ, G.V. (2001): History of tectonic interpretations of the Alps. Journal of Geodynamics 32: 99-114

FRANKS, S. & TRÜMPY, R. (2005): The Sixth International Geological Congress: Zürich, 1894. Episodes, Vol. 28(3): 187 - 192

LAMMERER, B. (1975): Geologische Wanderungen in den westlichen Zillertaler Alpen. Alpenvereins-Jahrbuch Bd. 100: 13-25

PFIFFNER, O.A. (2009): Geologie der Alpen. Haupt Verlag Bern-Stuttgart-Wien: 359

ROST, H. (1989): Zur Geologie, Petrographie und Tektonik des Pennins, der Matreier Zone und des Altkristallins zwischen Pürschbach und Grossklausenbach (Durreck-Gruppe, Ahrntal, Südtirol). Unveröffentlichte Diplomarbeit am institut für Geologie und Mineralogie Friedrich-Alexander-Universität Erlangen-Nürnberg: 192

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

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