Why Plate Tectonics was not invented in the Alps

“Like Venus, the theory of plate tectonics is very beautiful and born out of the sea.”

R. Trümpy, 2001

For over 200 years the Alps have been visited by geologists. For most of this time, they wondered how mountain ranges like the Alps formed. Folded sediments suggested forces pushing and squeezing the rocks. In the 18th century Swiss naturalist Johann Jakob Scheuchzer depicts and describes folds in the Swiss Alps, explaining them as layers deposited and then folded by the biblical flood.

The mountains around the Urnersee, from Scheuchzer´s “Helvetiae Stoicheiographia”, published in 1716.

German geologist Leopold von Buch (1774-1853) was convinced that mountains form like a bubble in Earth’s crust. Large magma intrusions displace and fold the superficial sedimentary layers. Von Buch believed that his theory could also explain the complex geology of the Alps, with magmatic and metamorphic rocks forming the inner zones and sedimentary rocks (like found in the Dolomites and the Northern Calcareous Alps) forming the outer borders. Based on von Buch’s research, French geologist Elie de Beaumont proposed that tilted and folded layers of different age were formed by periodic “magmatic” pulses. At first, the horizontally deposited sediments are uplifted by the intrusion of a magmatic core. In a second phase, the layers become tilted and then new layers form by the erosion of older layers. The undeformed layers are tilted by a new orogenic cycle and so on. However, British geologists later showed that this theory couldn’t work as proposed. If a mountain formed around a magma intrusion, all the sedimentary layers should show similar strike and dip, but the strata in the Alps were tilted chaotically.

Central Gneiss of the Tauern Window (covered by snow) surrounded by former sedimentary rocks, now thick-banked marble (mountain in the middle of the photo) and schist (on the left) of the Penninic Ocean. Seen at first as evidence of von Buch’s theory of magmatic rocks uplifting sedimentary layers, nowadays it is seen as a example of the Alpine nappe structure. Here partial erosion removed part of the nappe, forming a tectonic window, where the oldest rocks found in the Alps remerge to the surface.
Elie De Beaumont’s mountain-building theory: (1) previously horizontal beds (b), tilted up and contorted on flanks of rising core (a), and younger flat beds (c) extending up to the foot of the chain;(2) in this case, also beds (c) are disturbed and flanked by new horizontal deposits (d).

A new theory – the Contracting Earth theory – was later formulated by American geologist James Dwigth Dana. This theory explained mountains and continents as products of a cooling and shrinking Earth. Like the surface of an old and dry apple, the shrinking Earth would develop fissures (basins) and wrinkles (mountains).

Austrian geologist Eduard Suess suggested in his book Die Entstehung der Alpen (1875; The Origin of the Alps) and multi-volume work Das Antlitz der Erde (1883-1909; The Face of the Earth, English edition 1904-1924) that deep-sea trenches found along the borders of the Pacific Ocean are zones where the seafloor is pushed beneath the continents. However, also Suess imagined that “the horizontal and uniform movements” of rock layers could be explained by variations in Earth’s circumference. In 1906, Austrian geologist Otto Ampferer imagined with his “Unterströmungstheorie“ large currents in Earth’s mantle, pulling the upper crust, creating mountains like folds in a carpet. However, Ampferer and many other geologists working in the Alps used such theories only to explain very localized tectonic movements, like the thrust belt found in the Northern Calcareous Alps, mapped by Ampferer.

Thrusts had been noted in the Alps since the middle of the century, for example by Bernhard Studer (1853) and Arnold Escher (1841). In the Glarus Alps a spectacular thrust – here older Permian red beds and Mesozoic limestone cover younger Eocene to Oligocene Flysch – was explained by Escher and later by Albert Heim as a large “double fold”, a recumbent fold with inverted layers. In 1884, Marcel Bertrand proposed that a single, north-facing tectonic nappe could explain the inverted stratigraphy. The nappe was thrusted on older layers by the gravitational collapse of the mountains, when single sheets of sedimentary rocks sliding downwards get stacked atop each other.

Section with the “Glarus double fold” by Albert Heim, from Livret- Guide Géologique, 1894.

The Contracting Earth theory could explain the immense forces needed to crack and fold rocks on a global scale. However, it failed to explain the irregular distribution of mountains on Earth. According to the Contracting Earth theory,  features like mountain ranges should be distributed randomly on the uniformly shrinking planet. However, even a short glimpse on a map or globe shows that mountain ranges are not randomly distributed, but rather form long chains, like the Alps, the Caucasus and the Himalayas; or are instead found along one side of a continent, like the Rocky Mountains or the Andes, but not on the other side.

Tectonic map of Europe published by Eduard Suess in 1893. Suess was among the first to describe the tectonic structure of the Alps and together with Franz von Hauer he worked on a geological section. He recognized that European mountain-ranges were the product of at least three distinct orogenic cycles – the Alpine System (Alps, Pyrenees, Dinarides), the Variscian System (Bohemian Mass and truncated uplands in Spain and France) and the Caledonian System (truncated uplands in England and Scandinavia).

In January 1912 the German meteorologist Alfred Wegener presented in his public lecture Die Heraushebung der Großformen der Erdrinde (Kontinente und Ozeane) auf geophysikalischer Grundlage (The formation of large features of Earth’s crust (Continents and Oceans) explained on a geophysical basis) for the first time his idea of the ancient supercontinent Pangaea, from which all modern continents split apart. Three years later he publishes his book Entstehung der Kontinente und Ozeane, translated in the third edition and published in 1922 as The origin of continents and oceans. According to Wegener, ocean basins form as continents split apart, mountains are formed as continental crust collides with the oceanic crust or other fragments of continental crust. Swiss geologist Émile Argand used in 1916 Wegener’s hypothesis to explain the closure of the Tethys Ocean, once located between Europe and Africa, and subsequent folding and overthrust of marine sediments on the continental crust of Europe.

Swiss geologist Emile Argand’s 1916 diagram of the western-Alpine geosyncline during its initial contraction (embryotectonics) with syn-orogenic emplacement of mafic magma (black, Piedmont ophiolites) along the sheared lower limb of the Dolin-Dent Blanche geoanticline. Simplified legend: (1) rigid foreland, (2) epicontinental basin, (3) Valais foredeep, (4) Gran St. Bernard nappe (5) Piedmont basin, (6) Dolin-Dent Blanche nappe.
Argand adopted between 1909 and 1934 the idea of nappes in the geology of the Alps, here a generalized view of the Europe-vergent Alpine thrust belt. Note that the Eastern Alps (4) override the western Alpine nappe stack (2-3), and its root zone is indented and back-folded by the Southalpine hinterland, in turn, deformed by south-vergent thrust. The Western Alps consist of ophiolite-bearing cover sequences (3) and Penninic nappes (2), squeezed out from the contraction of Alpine geosyncline (I-III: Simplon-Ticino nappes;IV-V-VI: Gran St. Bernard-Monte Rosa-Dent Blanche nappes), and overthrown onto the sliced (a-b: Helvetic basement) and undeformed (c) European foreland (1).

Despite Argand’s nappe theory could explain many mysteries of Alpine geology, like old and young rocks found together or the tectonic structure of the Alps, it would need almost another 50 years until it was widely accepted.

Argand’s 1911 cross-section of the Swiss Alps showing the tectonic nappes of the Adriatic microplate (in red and numbered VI), subducted Penninic Ocean (blue), Briançonnais microcontinent (violet and numbered V), European Plate (pink and numbered IV). The Dent Blanche nappe hosts also the famous Matterhorn, old African continental crust overthrusted onto younger sediments and oceanic crust of the Penninic Ocean.

Wegener’s continental drift theory (a catchy phrase adopted mainly by his critics, as Wegener talks more general of displacement theory) was received with mixed feelings. Most geologists regarded it as cherry-picking of data. Only a few geologists became convinced of his idea. Wegner himself reacted to the critics and tried to respond to them in various editions of his book, however with moderate success. The greatest problem facing Wegener was the lack of direct evidence for the movements of continents. No mechanism was known to be powerful enough to move entire continents. Wegener proposed gravitational pull, tidal and centrifugal forces, but British geophysicist and astronomer Harold Jeffreys (1891-1989) demonstrated that such forces are too weak to explain moving continents. Wegener will die in 1930. His continental drift theory is in many aspects erroneous. Not the single continents move, but fragments of Earth’s crust and the driving forces comes from within the planet, not from the outside. But Wegener’s work introduced the idea of moving continents to the scientific community and the public and decades later this legacy will influence a new kind of theory – modern Plate Tectonics.

Between 1959 and 1977, geologists Marie Tharp and Bruce Charles Heezen, published the first maps of the seafloor, showing what seemed to be large rift zones, where new crust can form as lava pours out from submarine fissures. Canadian geologist John “Jock” T. Wilson introduces in the 1960s with the mid-ocean ridges (where new crust forms), subduction zones (where old crust sinks back into Earth’s mantle) and transform faults (accommodating lateral movements) the modern elements of plate tectonics. Harry Hammond Hess, US Navy commander at Iwo Jima, a prospector in Zambia and later professor at Princeton, in 1962 publishes a paper that will become one of the most widely cited geophysics paper for years. He hypothesized that the seafloor widens along the mid-ocean rifts and crust movements are driven by currents in Earth’s mantle, providing also a mechanism for plate tectonics and so mountain building. (to be continued).

Austrian geologist Albrecht Spitz’s geologic cross sections of the Engadin Dolomites (1914), showing tectonic nappes and faults – a novelty at a time when most structures in the Alps were interpretated as large-scale folds.

References:

  • 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, no. 3: 187-192
  • HEIM, A. (1919-1922): Die Geologie der Schweiz.
  • SEARLE, M. (2013): Colliding Continents: A geological exploration of the Himalaya. Oxford University Press: 438
  • STÜWE, K. & HOMBERGER, R. (2011): Die Geologie der Alpen aus der Luft. Weishaupt Verlag: 296
  • TRÜMPY, R. (2001): Why Plate Tectonics was not invented in the Alps. Int J Earth Sciences Vol. 90: 477-483

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