Orogenesis refers to all the processes leading to the formation of mountains.
It is the movement of tectonic plates that allows us to understand and explain the formation of mountain ranges. These lithospheric plates move a few centimetres per year as a result of the convection movements of magma inside the mantle. Mountain formation is an extremely slow process that takes place over millions of years. They appear, grow, flatten out through erosion and glaciation, and finally disappear.
There are different reasons to explain the formation of a mountain range: subduction, collision, or compression. Moreover, hot spots and extinct volcanoes can also lead to the formation of mountains.
A subduction zone forms when a dense tectonic plate plunges under a less dense tectonic plate as they collide. The rim of the top plate folds and lifts to form mountains.
Subduction zones are created when an oceanic plate collides with a continental plate. Their difference in density causes one plate to go over the other. When marine sediments are deposited on the crust of the oceanic plate and cling onto the edge of the continental plate, the formation of a mountain range is likely to occur. This is called an accretion prism. Very often it is a volcanic mountain range.
The Andes mountain range is an example of oceanic-continental subduction orogenesis. It formed due to the subduction of the Nazca oceanic plate under the South American continental plate.
Furthermore, the subduction zones exhibit strong volcanic activity. The Pacific Ring of Fire is a very good example of subduction orogenesis.
Formation by collision occurs when two tectonic plates of the same type meet, either continental-continental or oceanic-oceanic.
When two continental plates come together, the impact is powerful. Since there is no difference in the density of similar types of plates, a collision occurs. The two plates collapse against each other and crumple. The Earth's crust rises and thickens to form very high mountains. It is worth noting that ranges formed by collision never result in the formation of volcanoes.
The collision between the Indian continental plate and the Eurasian continental plate formed the Himalayan range. Since the tectonic plates are always in motion, this mountain range rises by 2 to 5 cm every year.
When the collision force is greater than the deformation capacity of tectonic plates, faults are likely to form. These faults can range from a few millimetres to several thousand kilometres.
The San Andreas fault in California is over 400 km long.
Formation by compression takes place in the middle of a continental tectonic plate.
Over a long period of time, rocks in a tectonic plate can undergo many horizontal stresses and eventually fold. The resulting folds are at the source of some of the mountains that form in the centre of a tectonic plate.
Intracontinental mountain ranges such as the Pyrenees were formed by compressions occurring within the continental plates.
When a tectonic plate passes over a hot spot, some of the magma can rise to the surface without creating a volcano. This will result in the formation of hills.
Mont St-Hilaire is an example of a hill formed by the passage of a tectonic plate above a hot spot.
Some mountain ranges are ancient volcanoes that have died out. Since tectonic plates are continually moving, a volcano that was initially above a hot spot may move away from where magma could rise to the Earth's surface. This volcano dies as it moves away from the hot spot and a new volcano appears nearby. If this hot spot is located under the ocean, then volcanoes turn into islands.
The Hawaiian archipelago is made up of several volcanoes which, with the movement of tectonic plates, died out and transformed into mountains.
