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Center of Mongolia, the Orkhon Valley

Center of Mongolia, Orkhon Valley

The center of Mongolia is an area mountainous witness of the orogen from the center of Asia (Central Asian Orogenic Belt, CAOB). The Orkhon Valley is 320 km from Ulan Bator in a volcanic area and sees past the Selenga River. There are many volcanic rocks as well as hot springs. Classified by UNESCO, the centre of Mongolia has been the witness of many events of the Mongolian empire. Indeed, she welcomed the capital of the Mongol empire, Karakorum, during the reign of Genghis Khan and retains many archaeological remains. The region is also known for its mineral wealth.

Mongolia hot spring
Hot spring in the Orkhon Valley

Geologic setting

Central Asian Orogenic Belt CAOB

The orogen of Central Asia and the formation of the mountains such as the Urals, Altai took place during the period of the Mesoproterozoic (-1600Ma to-1000Ma) Triassic (-252 my to-201 my). This training represents an area giant and emblematic of a long evolution of an orogen with duration between more than 800Ma.

Central Asian Orogenic Belt, orogen from the Urals to the Pacific ocean.


The CAOB is due to reconciliation of the supercontinent Gondwana with the Laurusia and more precisely with the Siberia craton and the Tarim – North China craton. This represents a huge area of orogenes, collisions, rotations and training of micro-continents from the Urals to the Pacific ocean.

training CAOB
Reconciliation of Gondwana with the Laurasia during the CAOB
Geological composition of the rocks

The Center of Mongolia consists of magmatic rocks such as basalt and granite.

Granite from the cooling of the magma. It is composed of quartz, feldspar, potassium (orthoses) and plagioclases, micas (biotite or muscovite)
Basalt from a magma to the surface. It is composed of plagioclases (50%), pyroxene (25 to 40%), olivine (10-25%), and 2-3% of magnetite


These rocks are from two types of volcanic activity have allowed the formation of the Mongolia.

Indeed a magma can evolve in different ways depending on its geographical area of creation. Whether in a ridge, a subduction or a hot spot, magma will have a different chemical composition. It is through this chemical composition in alkaline (K potassium and sodium Na) and silica (SiO2) that we will be able to identify the place of formation of magma. Thus, the center of the Mongolia, we have:

The first magmatic activity took place at the beginning of the Paléozoïque(542 Ma à 480 Ma) having trained calco-alkaline magma. Its report (KO2 + NaO2) / SiO2 is low and its NaO2 concentration is higher than KO2. This magma comes from the subduction of one plate under another plate creating a magmatic activity and forming a chain of mountains (Rockies, Andes, etc…).

Training of the calco-alkaline magma in a subduction


The second is alkaline magma was brought late Carboniferous (~ 300 my) early Mesozoic (251 my). Its report (KO2 + NaO2) / SiO2 is high, which means that its concentration in KO2 + NaO2 is greater than the concentration of SiO2. This one comes from a hot spot within a plate such as the island of Réunion, or Mauritius.

hot spot
Formation of an alkaline magma on a hot spot


The Orkhon Valley has for many magmatic reliefs emphasizing intense activity during his training. Ulaan Tsutgalan waterfall, within the Orkhon Valley, measure more than 20 meters high and has a very good example of volcanic organ.

Volcanic organ

Volcanic organs are a volcanic formation composed of regular hexagonal columns. They are relics of the release of gases in a magmatic flow. During cooling, energy will break free by the shortest and fastest. This path is perpendicular to the direction of the casting and takes the form of hexagon. Why a hexagon? Because it is the geometric form that best expressed a quick and release little heat during cooling energy. In this context we get organs like the road of the Giants in North Ireland.

volcanic organ - basaltic organ
Volcanic organ. The evacuation of energy (blue) is perpendicular to the direction of flow (red).
For all definitions: go to this page.
For the time scale is here!


“Holocene geomorphological processes and soil development have indicator for environmental change around Karakorum, Upper Orkhon Valley (Central Mongolia)”, Frank Lehmkuhl, Alexandra Hilgers; 2011.

“Foreword to the special volume ‘ Magmatic evolution of Mongolian part of the Central Asian Orogenic Belt and its geodynamic significance.” Karel Schulmann1, 2 *, Ochir Gerel3, Wen Jiao Xiao; 2016

“The Altaids of Central Asia: A tectonic evolutionary and innovative review.” WHILEM C., WINDLEY B., SCOTT G., 2012

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