Chilly winds, rugged slopes, flurries of snow – mountains, in general, are such majestic features of the planet, aren’t they? While they could form through one or more of the mechanisms of faulting, folding and volcanism, some of the most awe-inspiring mountains occur in Fold-Thrust Belts (FTB) found at tectonic plate boundaries. They could be a product of subduction of an oceanic plate under a continental one (or rather the continental part of a plate because no tectonic plate is composed entirely of continental material) and the resulting volcanism, or of terrane accretion (wherein oceanic ridges and islands get added to the mainland, thus causing uplift), or of intercontinental collision (with the exhumation of sedimentary rocks and the wedging of one plate under the other causing the uplift). The three of these processes generally occur successively.
We shall look at the Himalayas and the Alps, two mountain ranges resulting from continent-continent collision, to see if the similarity between them extends beyond this. The Himalayas occur in a roughly 2500 km long stretch through Northeastern Pakistan, Northern India, Southern Tibet, Nepal, Sikkim and Bhutan. Rather than a single broad belt, they are grouped into four distinct parallel longitudinal belts from the south to the north as: (a) Sub-Himalayas or Siwalik Range; (b) the Lesser or Lower Himalayas ; (c) the Great Himalaya Range; (d) Tethys or Tibetan Himalayas . The Alps, on the other hand, are also long, stretching over 1,200 km from Austria and Slovenia in the east, through Italy, Switzerland, Liechtenstein and Germany to France in the west. They are divided into Western, Central, and Eastern segments, each of which consists of several distinct ranges.
If we look at a physical map of the world, we can see that the Alps are not exactly connected to the Himalayas. Yet, both the ranges could be said to have formed from the same orogenic event – the closing of the Tethys ocean that spanned the interval between the two supercontinents Laurasia and Gondwanaland around 152 Ma (Mega annum or million years ago). With many further rifting events, at about 50 to 40 Ma, the Himalayas started forming when the Indian plate collided with the Eurasian plate, and the Alps started building about 35 to 25 Ma, with the African plate colliding with the Eurasian plate. All these events starting from about 65 Ma fall under the umbrella term of ‘Alpide orogeny’ , resulting not only in the formation of the Alps and the Himalayas, but also of the Hindukush, the Zagros and the other ranges in between. Accordingly, this chain of mountains is known as the alpide belt. It is a narrow zone with local compression and is the most seismically active region after the Pacific Ring of Fire. It extends for thousands of Km along the southern margin of the Eurasian plate.
If both the Alps and the Himalayas have basically formed by the closing of the Tethys ocean, why are the Himalayas much taller than the alps? Why are the Himalayas still growing, while the Alps aren’t? Why are there more numerous and damaging earthquakes in the Himalayas, than in the Alps? Well, firstly the closing of the Tethys is an oversimplified statement, and with regards to the India-Eurasia collision, there is thought to have been an additional intra-oceanic subduction, and hence two downgoing slabs under the Eurasian plate. The double subduction phenomenon as well as the Reunion plume activity is thought to explain the extremely high convergence rate of about 18 cm/yr sustained for some 20 Ma in the Cretaceous period (Pusok and Stegman, 2020)! (To put it in perspective, tectonic plates are generally known to move at rates less than 10 cm/yr.) Yet, the answers to all these questions essentially have to do with the rates of convergence of the tectonic plates involved. Very simplistically, the faster the convergence, the taller the mountains.
The Alps, it is said, are no longer growing due to tectonic activity. It has been suggested that the Alps owe their seemingly non-plate tectonic structure to extensive post-collisional deformation (Dietrich Roeder 1977). It is no longer growing in height, still it can be said to remain in a somewhat dynamic steady-state. To explain the previous statement, the multiple rivers associated with the Alpine system erode a great deal of sediments from the mountains. Due to the lost mass, the Alps rise in order to reach an isostatic equilibrium (It is somewhat similar to what happens if the weight of an object floating in water is decreased – the object bobs up even more. This analogy is not exact mainly because the earth’s mantle is not liquid, but rather a solid that seemingly flows over a geologic time-scale of millions of years). In essence, this rise (of about 1 mm/yr) and the loss of mass and hence height by erosion balance each other out. Thus, the Alps are no longer “growing” in the sense of the word.
In other orogenic systems (including the Himalayas), continued compression and plate convergence after continent-continent collisions are being recognized as a major structure-forming and energy-consuming geotectonic element (Bird et al., 1975). The India- Eurasia collision even today has a convergence rate of approximately 6 cm/yr resulting in the Himalayas growing vertically at a rate greater than 1 cm/yr (after accounting for continued subduction of the Indian plate under the Eurasian one, the huge amount of erosion and isostatic uplift). Of course, apart from the eventual slowing down of the convergence rate of the plates, the growth of the mountains will also stagnate, with rise being compensated for by lateral extension, a process suspected by some, to have started already.
Detailed studies of earthquakes in the Alps and the Himalayas shows that the frequency and magnitude of large earthquakes in the densely populated regions close to mountain chains depend on the collision rate of the tectonic plates (Zilio et al. 2017). This is because the faster they collide, the cooler the temperatures and the larger the areas that generate earthquakes. This increases the relative number of large earthquakes. The team compared earthquakes recorded in the Alps and Himalayas. Their results imply that the plate collisions in the Alps are more ductile than those in the Himalayas, reducing the hazard of earthquakes.
Alright, so the danger of earthquakes is lesser in the Alps than in the Himalayas. Are the Alps then safer than the Himalayas for trekkers and tourists in other ways too? The answer is yes. The snow/ice conditions of the Alps tend to be more stable, more predictable and consequently safer to climb and ski. Contrastingly, there is a miniscule amount of skiing opportunity in the Himalayas, restricted to a few months in a place called Auli. More strikingly, the Alps are way more accessible than the Himalayas, in that to get close to one of the taller Himalayan peaks, one must trek on rugged paths for at least a few days, and be well-equipped for the trek. In the Alps though, one can find oneself sipping coffee in a quaint little cafe virtually at the bottom of Mt Blanc, watching the progress of climbers on its slopes, without much exertion. Understandably, easy accessibility translates to increased commercialisation and loss of the element of untouched wilderness. It may be a trade-off between physical activity and the etherealness of the experience. Nevertheless, any mountain experience is exceedingly scenic – whether you choose to visit the Alps or the Himalayas or any other range for that matter. The mountain air will still do you a lot of good, like it did for Heidi!
A special thanks to my dear friend Sumedha D for the collaboration.
Originally published in The Qrius Rhino.
References:
Adina E. Pusok and Dave R. Stegman, 2020. The convergence history of India-Eurasia records multiple subduction dynamics processes. Science Advances 06 May2020: Vol. 6, no. 19, eaaz8681, DOI: 10.1126/sciadv.aaz8681
Bird, P., ToksSz, M.N. and Sleep, N.H., 1975. Thermal and mechanical models of conti- nent-continent convergence zones. J. Geophys. Res., 80(32): 4405-4416.
Dietrich Roeder, Continental convergence in the Alps, Tectonophysics, Volume 40, Issues 3–4, 1977, Pages 339-350, https://doi.org/10.1016/0040-1951(77)90072-5
Luca Dal Zilio, Ylona van Dinther, Taras V. Gerya, Casper C. Pranger, Seismic behaviour of mountain belts controlled by plate convergence rate, Earth and Planetary Science Letters, Volume 482, 2018, Pages 81-92, https://doi.org/10.1016/j.epsl.2017.10.053 .
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