Forging Mountains
In 1953 Edmund Hillary and his Sherpa partner Tenzing Norgay became the first confirmed climbers to summit Mount Everest nearly 100 years after the mountain was confirmed to be the highest in the world by a British surveying team led by George Everest (the namesake of the mountain). Mountains have always signified the height of human endurance with many having attempted the same feat as Hilary prior and post his success with varying levels of triumph. Mountaineer George Mallory, who unfortunately died on the mountain, famously stated when asked why he wanted to climb Everest “because it’s there”; but why is it there?
Everest and the Himilayas in general are, geologically speaking, a young mountain range beginning to form during the Eocene (55–38 million years ago) when the Indian tectonic plate collided with the Eurasian plate representing the closure of the Tethys ocean. However, orogenesis (mountain building) isn’t always as simple as two blocks of crust squishing together over a long period of time. There are three main methods of orogenesis; collisional (Himalayas), accretionary (Andes) and intracratonic (Tian Shan), with there also being sub-types however I shall only focus on the three generally.
Before explaining these orogenies, I will first explain what a subduction zone is as this is the primary geodynamic process for collisional and accretionary orogens. Subduction zones are found on the borders of tectonic plates and are, in the simplest terms, areas of the planet where a piece of crust sinks underneath another before being destroyed in the fiery depths of the Earth’s mantle. The most well-known subduction systems are that of destructive margins most famous of which is the ring of fire which encompasses the entire border of the pacific ocean.
Oceanic crust sinks below the continental crust along the edge of the pacific ocean and is destroyed; however, this process produces mountains and volcanoes such as Mount Fiji, Mount St Helens and the Andes range along the coast of western South America — this is accretionary orogenesis. As the oceanic crust subducts beneath the continent, a wedge of the mantle exists between the two-crust pieces. As the oceanic crust is subducted, it gets compressed by the immense pressures and is dehydrated as water is leached from the rock. The addition of water to the mantle wedge above lowers the mantle's melting point, causing it to liquefy and rise as magma, which penetrates the continent above producing a volcanic arc. A back-arc basin can form as well and is an area of low-land where the crust is extended and stretched behind the volcanic arc. This back-arc extension process is responsible for creating the Sea of Japan, which separates the island from mainland Asia.
This process of subduction and accretionary orogenesis terminates when continent meets continent. As a continent approaches another, the ocean between them closes and as such subduction ends. When the two pieces of continent connect one is forced beneath the other however they do not subduct properly as continental crust is more buoyant than oceanic crust. Instead, they are forced together, resulting in the thickening of the crust and formation of mountains such as I described above for the Himilayas. As collision of continents occurs along the same subductive margins as accretionary orogens, collisional orogens often override them and pieces of old ocean crust can be preserved in the mountain belt known as ophiolites and suture zones. This is why paradoxically you can often find marine fossils in some mountain ranges. Some of the rock involved in this process is subject to immensely high pressures until they are recrystallised. If the continent is subducted to 80km plus depth, micro-diamonds can begin to form in the fabric of the mountain.
The final type of orogenesis is that of intracratonic means; the word intracratonic simply refers to the fact that these orogenies occur in the centre of a tectonic plate, not at a tectonic border, unlike their accretionary and collisional counterparts. Intracratonic mountains are the rarest and least understood and not a great deal is known about their geodynamic process. However, is it believed that tectonic stress at the borders of plates propagates horizontally into the centre of the plate reactivating old weaknesses in the rock and causing uplift. Some also believe that stress can come vertically from the mantle uplifting the crust. An example of this is the Tian Shan mountain range of central Asia.
So the next time you’re admiring a mountain or planning your next expedition not only then can you appreciate it’s majesty but also the violence, patience and planet shaping processes behind the aesthetic centrepieces of Earth.
