Orogeny

The mountain formation or orogenesis (composed of the Greek words ὄρος óros 'mountain' and γένεσις génesis 'emergence, procreation, birth') is caused by tectonic processes , which in many cases are directly generated by the displacement of continental plates. Special cases of orogeny concern the formation of rupture clod mountains and rupture fold mountains , which are not directly created by the displacement of continental plates.

Orogenesis used to be understood as a time-limited process, as only the processes that determine the structure of the affected rocks were considered. However, the investigation of active orogenes such as the mountains on the Pacific coast of America shows that these are often prolonged and continuous processes.

Ore formation processes

According to today's understanding, mountain formation can be traced back to plate tectonic processes with a few exceptions . The type of rock , its internal structure and the water content of the sediments involved influence the process, as do external influences such as climatic factors and erosion processes . They determine the shape of a mountain range ( orogen ), how high it will be at a certain rate of uplift, and how long it will take to build up or gradually level it.

According to earlier beliefs, orogenesis was preceded by the formation of a so-called geosyncline : a large depression in which thick layers of deep sea and other sediments were deposited before the subsidence became an uplift area due to large tectonic processes . These rocks were later transformed and sometimes shifted over hundreds of kilometers, so that they can be found in the mountains today. According to today's ideas, the "geosyncline" corresponds to the ocean or marginal sea located between the continental plates involved in plate tectonic processes.

Collision of continental blocks

Scheme of the Alpid mountain belt

According to the theory of plate tectonics, the earth's surface is made up of larger and smaller lithospheric plates ("continental plates "), which move against each other at a rate of a few centimeters per year. In some places these plates move towards each other, or rather one plate slides over the other. This is known as convergence . From a geotectonic point of view, continents are crustal areas with relatively strong upwelling . As a rule, lithospheric plates not only bear continental crust , but continental blocks regularly collide over the course of many millions of years in the course of plate convergence. This leads to intensive compression processes at the edges of the colliding continental blocks. As a result, a mountain range forms along the collision zone . So the Himalayas came into being when the Indian continental block met that of the Eurasian plate . Around the same period, around 50–30 million years ago ( Eocene ), the folding of the Alps also took place through the collision of the continental block of the African plate with that of the Eurasian plate. Such mountains are called collision mountains .

Subduction of oceanic plates

If a continental plate meets an oceanic plate, the oceanic plate mostly plunges into the earth's mantle due to its higher density and magmas are created in the course of the drainage of the plate. This is known as subduction . Earthquakes often occur in the subduction zones . Above the subduction zones there are volcanoes of the explosive type. Mountains are created by the tectonic upheavals / thrusts, but also by the rising of magmas that arose during subduction in the earth's interior. The Andes are a result of the collision of the Nazca plate with the South American plate . Other examples are the North American Cascade Mountains and the Japanese Islands .

Autopsy of oceanic plates

When oceanic plates collide with others, there is sometimes no complete subduction of the oceanic crust. Parts of the oceanic plate are scraped off from their base and pushed onto the upper plate ( autopsy ). Such rock bodies can be found in many mountains, mostly as lens-shaped and not very extensive occurrences. The rocks exposed there are called ophiolites and have a very characteristic formation that differs significantly from the surrounding rocks. In rare cases, larger parts of the ocean crust are autopsied, for example in the ophiolite complex of Oman .

The role of erosion in mountain formation

Even during the formation of a mountain, erosive forces become effective as soon as the mountain body rises above its surroundings. Physical factors (solar radiation, thawing and re-freezing of the rock), chemical processes and mechanical factors (mainly due to glaciers and flowing water) play a decisive role. Depending on the rate of uplift of a mountain range in relation to the rate of erosion, the result is whether a mountain is gaining in height or is being eroded faster. In principle, higher mountains have higher erosion rates, since the relief energy of high mountains is higher than that of low mountains and this in turn is higher than in flat areas. The higher erosive effect of a raging mountain stream compared to a meadow meandering on the plain is easy to imagine. In general, denudation , the extensive erosion caused by frost, for example, has a stronger effect than the linear erosion of a river.

Simulations of ore formation processes showed that the erosion may have a controlling influence on the ore formation process.

Isostasis

Arathem	system	Beginning ( mya )	Orogenesis
Cenozoic Modern Earth Duration: 66 Ma	quaternary	2,588	Alpid orogeny
	Neogene	23.03
	Paleogene	66
Mesozoic Middle Ages Duration: 186.2 Ma	chalk	145
	law	201.3
	Triad	251.9	Variscan orogeny
Paleozoic Earthen Period Duration: 288.8 Ma	Perm	298.9
	Carbon	358.9
	Devon	419.2
	Silurian	443.4	Caledonian orogeny
	Ordovician	485.4
	Cambrian	541	Cadomic orogeny
Neoproterozoic Jungproterozoikum Duration: 459 Ma	Ediacarium	635
	Cryogenium	720	various Precambrian rock formations
	Tonium	1000
Mesoproterozoic Mittelproterozoikum Duration: 600 Ma	Stenium	1200
	Ectasium	1400
	Calymmium	1600
Paleoproterozoic Altproterozoikum Duration: 900 Ma	Statherium	1800
	Orosirium	2050
	Rhyacium	2300
	Siderium	2500
Neo-Archaic Duration: 300 Ma		2800
Mesoarchean Duration: 400 Ma		3200
Paleoarchean Duration: 400 Ma		3600
Eoarchic Duration: 400 Ma		4000
Hadaikum Duration: 600 Ma		4600
Please note that this table is only intended to provide a rough overview. Information in the specialist literature at the beginning and end of a particular orogenesis may differ from the information in the table. a. because there are different concepts and definitions depending on the region and author .

Mountains are in a kind of swimming equilibrium ( isostasy ) with the tough plastic mantle below . The mountain block dips so deep into the mantle that the mass of the displaced mantle rock corresponds to its own mass. This is comparable to ships which, according to Archimedes' principle, dive deeper into the water the heavier they are.

A mountain massif protrudes about 5 to 6 times its height above sea level into the earth's mantle. If rock is removed by erosion on the surface, the entire mountain range rises until around 80% of the removed rock mass is replaced. Even if the tectonic upward movement has long since come to a standstill, mountain regions can remain at their altitude for many millions of years before erosion gains the upper hand.

Phases of ore formation in the history of the earth

The subdivision of the geological development of Europe into four main phases of mountain formation goes back to Hans Stille : the Fennosarmatic , Caledonian , Variscan and Alpidic phases.

Almost all of the “young” fold mountains on earth were created in the last 20 to 40 million years during the alpine mountain formation, the last of these phases. During this phase, the High Atlas , the Pyrenees , the Alps and the Carpathians , the Dinarides , the mountain ranges of Turkey , the Zagros Mountains in Persia, the Himalayas and the western mountains of Burma , Thailand and Indonesia were created . In today's Alpine region, the parent rocks were deposited in several sea areas; The sea sediments formed at that time were up to a few kilometers thick and were unfolded into a mountain range in a complicated process, the main phase of which began around 70 million years ago. After the strongest uplift phase (around 25 million years ago), the erosion furrowed the extensive uplifted area in tectonic weak zones through long and shorter valleys . The uplift of the Alps continues to this day (at 1–3 mm per year), it is offset by roughly the same amount of erosion.

For historical reasons, the names of Hans Stille are still used today, partly not only in Europe, but also on other continents. In addition, there is a large number of names for orogenesis, which are derived from the mountains of the respective region. Characteristic designations mostly more or less extensive or classified as significant orogenesis of the earth's history are listed in the following table.

Age of orogenesis	Initial phase Information in mya	Climax or end	Current phase	Where? Continent?
alpidic - Cretaceous , Cenozoic	100	50	recently growing	Alps , Himalayas , Carpathians , Rocky Mountains - Continent Eurasia and subcontinent India
variskisch - alleghenisch middle Paleozoic	400	300	erosion	Southern Appalachian Mountains , the pre- Rocky Mountains and Andes , Urals , Black Forest , Harz , Rhenish Slate Mountains - Supercontinent Pangea
Caledonian - early Paleozoic	510	410	erosion	Northern Appalachian Mountains, Scotland , Norway - Old major continents Laurussia , Laurasia
Cadomic - (or Assyntic Orogeny) Neoproterozoic	650	545	Overlaid by plate tectonics, sedimentation , volcanism .	Dobra Gneiss (1377 mya ) of the Bohemian Mass , in the Waldviertel in Austria - Supercontinent Pannotia or the great continent Gondwana
Pan-African orogeny Neoproterozoic	1,000	530	erosion	Supercontinent Pannotia or Greater Continent Gondwana , Continent Africa
Grenville , svekonorwegian - Mesoproterozoic	1,200	1,100	Formerly overlaid, partially exposed by abrasion during the Ice Age .	In the Eastern Canadian Shield , Southwestern Sweden , Southern Norway , Northern Australia - Supercontinent Rodinia
dano-polonian - Mesoproterozoic	1,500	1,400	overlaid by plate tectonics, sedimentation	In Poland , Ukraine , south of Blekinge and north of Bornholm - Merger of the continents Nena and Atlantica
Wopmay , svekofennisch - Paleoproterozoic	2,000	1,700	Formerly overlaid, partially exposed by abrasion during the Ice Age.	In the Western Canadian Shield , on Greenland , in Northwest Australia , in South Africa and in the Western Baltic Shield - Continents Nena , Atlantica , at the end of the supercontinent Columbia
Example: loopisch - Neo-Archean , Paleoproterozoic	2,700	2,300	Formerly overlaid, partially exposed by abrasion during the Ice Age.	In the Northwest Baltic Shield - Small continents Fennoscandia , Sarmatia , Volgo-Uralia , in the end the supercontinent Kenorland ?
archaic orrogens	approx. 4,000	?	Overlaid and exposed in the smallest areas of the cratons by Ice Age glaciers .	Acasta gneiss in the slave province and Nuvvuagittuq greenstone belt in the superior province in Canada, Isua gneiss in Greenland and other rock units in the shields of the continents - Supercontinent "First Earth" ?

See also

• Wilson cycle

literature

• Wolfgang Frisch , Martin Meschede : Plate tectonics. Continental drift and mountain formation. Primus Verlag, Darmstadt 2005, ISBN 3-89678-525-7 .
• Andreas Heitkamp: Mountain building - When mountains grow into the sky. In: Nadja Podbregar; Dieter Lohmann: In focus: geological knowledge. How does our planet work? Springer Verlag, Berlin, Heidelberg, 2013, e- ISBN 978-3-642-34791-7 , 105-119.
• Florian Neukirchen: Moving mountains. Mountains and how they are formed. Spektrum Akademischer Verlag, Heidelberg 2011, ISBN 978-3-8274-2753-3 .
• Miyashiro, Akiho, Keiiti Aki: Orogeny. Basics of the mountain formation Deuticke / Vienna, 1985. ISBN 3-7005-4552-5 .

Web links

Commons : Orogenesis  - collection of images, videos and audio files

Wiktionary: Bergbildung  - explanations of meanings, word origins, synonyms, translations

• In the shadow of old continents - Austria in the Proterozoic , Geological Federal Institute Vienna
• The formation of the Appalachians and the first approaches of the Rocky Mountains in the late Carboniferous (en.)
• scinexx .de: Mountain formation November 26, 2004

Individual evidence

1. ^ Hans Murawski, Wilhelm Meyer: Geological dictionary . 11th edition. Elsevier / Spektrum, Heidelberg 2004, ISBN 3-8274-1445-8 , pp. 262 .
2. ^ Hans Stille: The leitmotif of the geotectonic earth development , lectures and writings of the German Academy of Sciences in Berlin; H. 32, 27 p., 1949.
3. ^ Oman: an autopsy orogen , lectures on structural geology, Jean-Pierre Burg, ETH Zurich (PDF, 2.4 MB).
4. ^ Mark T. Brandon; Nicholas Pinter: The Contribution of Erosion to Mountain Formation , Spectrum of Science, September 1997. Executive summary .
5. ↑ Djordje Grujic, Christof Lindenbeck: Geological Expedition in the Kingdom of Bhutan. Field observations and model presentations for the formation of the Himalayas - the interplay of erosion and climate , Geological Institute of the Albert Ludwig University, Freiburg im Breisgau .