section_path: Geology and orogeny
section_name: Geology and orogeny
paragraph_text: Geology and orogeny
  
  Important geological concepts were established as naturalists began studying the rock formations of the Alps in the 18th century. In the mid-19th century, the now-defunct idea of geosynclines was used to explain the presence of "folded" mountain chains. This theory was replaced in the mid-20th century by the theory of plate tectonics.
  
  The formation of the Alps (the Alpine orogeny) was an episodic process that began about 300 million years ago. In the Paleozoic Era the Pangaean supercontinent consisted of a single tectonic plate; it broke into separate plates during the Mesozoic Era and the Tethys sea developed between Laurasia and Gondwana during the Jurassic Period. The Tethys was later squeezed between colliding plates causing the formation of mountain ranges called the Alpide belt, from Gibraltar through the Himalayas to Indonesia —a process that began at the end of the Mesozoic and continues into the present. The formation of the Alps was a segment of this orogenic process, caused by the collision between the African and the Eurasian plates that began in the late Cretaceous Period.
  
  Under extreme compressive stresses and pressure, marine sedimentary rocks were uplifted, forming characteristic recumbent folds, and thrust faults. As the rising peaks underwent erosion, a layer of marine flysch sediments was deposited in the foreland basin, and the sediments became involved in younger folds as the orogeny progressed. Coarse sediments from the continual uplift and erosion were later deposited in foreland areas north of the Alps. These regions in Switzerland and Bavaria are well-developed, containing classic examples of flysch, which is sedimentary rock formed during mountain building.
  
  The Alpine orogeny occurred in ongoing cycles through to the Paleogene causing differences in folded structures, with a late-stage orogeny causing the development of the Jura Mountains. A series of tectonic events in the Triassic, Jurassic and Cretaceous periods caused different paleogeographic regions. The Alps are subdivided by different lithology (rock composition) and nappe structures according to the orogenic events that affected them. The geological subdivision differentiates the Western, Eastern Alps, and Southern Alps: the Helveticum in the north, the Penninicum and Austroalpine system in the centre and, south of the Periadriatic Seam, the Southern Alpine system.
  
  According to geologist Stefan Schmid, because the Western Alps underwent a metamorphic event in the Cenozoic Era while the Austroalpine peaks underwent an event in the Cretaceous Period, the two areas show distinct differences in nappe formations. Flysch deposits in the Southern Alps of Lombardy probably occurred in the Cretaceous or later.
  
  Peaks in France, Italy and Switzerland lie in the "Houillière zone", which consists of basement with sediments from the Mesozoic Era. High "massifs" with external sedimentary cover are more common in the Western Alps and were affected by Neogene Period thin-skinned thrusting whereas the Eastern Alps have comparatively few high peaked massifs. Similarly the peaks in eastern Switzerland extending to western Austria (Helvetic nappes) consist of thin-skinned sedimentary folding that detached from former basement rock.
  
  In simple terms, the structure of the Alps consists of layers of rock of European, African, and oceanic (Tethyan) origin. The bottom nappe structure is of continental European origin, above which are stacked marine sediment nappes, topped off by nappes derived from the African plate. The Matterhorn is an example of the ongoing orogeny and shows evidence of great folding. The tip of the mountain consists of gneisses from the African plate; the base of the peak, below the glaciated area, consists of European basement rock. The sequence of Tethyan marine sediments and their oceanic basement is sandwiched between rock derived from the African and European plates.
  
  The core regions of the Alpine orogenic belt have been folded and fractured in such a manner that erosion produced the characteristic steep vertical peaks of the Swiss Alps that rise seemingly straight out of the foreland areas. Peaks such as Mont Blanc, the Matterhorn, and high peaks in the Pennine Alps, the Briançonnais, and Hohe Tauern consist of layers of rock from the various orogenies including exposures of basement rock.
  
  Due to the ever-present geologic instability, earthquakes continue in the Alps to this day. Typically, the largest earthquakes in the alps have been between magnitude 6 and 7 on the Richter scale. Geodetic measurements show ongoing topographic uplift at rates of up to about 2.5 mm per year in the North, Western and Central Alps, and at ~1 mm per year in the Eastern and South-Western Alps. The underlying mechanisms that jointly drive the present-day uplift pattern are the isostatic rebound due to the melting of the last glacial maximum ice-cap or long-term erosion, detachment of the Western Alpine subducting slab, mantle convection as well as ongoing horizontal convergence between Africa and Europe, but their relative contributions to the uplift of the Alps are difficult to quantify and likely to vary significantly in space and time.