As two well-known LIFE writers, Lorus Milne and Margery Milne, once wrote:
“There are many surprising things about mountains, but perhaps the most astonishing of all is the very fact that they exist.”
We spend more and more time indoors, we are getting used to living in ever narrower and more claustrophobic spaces. Our gaze no longer wanders towards the horizon, and when we happen to be able to admire a view we really have to agree with Lorus and Margery Milne's statement. It is surprising to see a mountain rise at the far end of the horizon, there where the plain stops and the hard, jagged slopes begin, the escarpments, the peaks, the towers and the steep rock faces.
It is precisely at moments like these that the greatness of the world, the immensity of the mountains, leaves us spellbound with an uncontrollable sense of wonder.
Science too leaves us amazed with its extraordinary revelations. Geology in particular has reached such a knowledge of mountains that, first of all, one thing is certain: what we consider the great reliefs of the planet are the product of immeasurable forces that act over extremely long timescales (so-called geological time). These are thrusts, movements and collisions between rock masses that take place with imperceptible slowness. This is probably why, from antiquity right up to recent times, mountains were regarded as perennial and unchanging entities. In fact it is only some forty years since scientists managed to formulate a theoretical model able to explain volcanoes, earthquakes and the formation of mountains in a unified way.
The first doubts about the immutability of mountains emerged with the discovery of fossils trapped between the rock strata of the high peaks. These doubts remained just that until as late as the middle of the twentieth century, when the origin and the birth of mountains began to be clarified scientifically (orogenesis: oros = mountain; génesis = birth).
Orogenetic theories today include a great number of hypotheses, some of them widely verified, others still fiercely contested or invalidated. The debate on the factors that contribute to the formation of mountains is still open and the studies are certainly not complete.
Paradoxically, what helped us understand the slow movements of the Earth's crust were the ocean floors, whose origin is very different from that of the land masses. They are made of much younger rocks (200 million years, against the 4 billion years of the oldest continental rocks). The rocks that make up the ocean floors are mainly basaltic, and so they are denser and heavier than continental rocks, which instead have a granitic composition. Moreover the lithosphere (what we commonly think of as the Earth's crust) is only about 65 km thick near the ocean floors, while its thickness can reach as much as 400 km near the continental plates. The explanation for this difference would lie in the fact that continental rocks, lighter than the basaltic rocks of the ocean floors, tend to float on the mantle of fluid materials that would make up the so-called asthenosphere.
Thanks to research carried out on the ocean floor the theory of the tectonics of crustal blocks, also called plates, was born. A plate is a portion of lithosphere hundreds or even thousands of kilometres wide that, like a raft, floats on the Earth's fluid mantle, carrying the lighter continental mass on its back. The largest plates are the North American plate, made up of North America and part of the floor of the northern Atlantic Ocean; the Antarctic plate, the African plate and the Eurasian plate.
Mid-ocean ridges are the dividing margin between two neighbouring plates. Along these ridges new rock material would form through continuous, slow production. The insertion of new rock material would make the continental plates appear to move apart, pushed by the forces generated by the formation of new ocean floor. Since there are no gaps between one plate and the next, the movement spreads to all the plates, which move apart, collide or bump into each other, with decisive consequences for the shape of the Earth's surface.
We can sum up three possible situations:
- The first situation would be that two plates can separate, pushed in two
opposite directions, in the same way as happens along the mid-ocean ridges. Precisely because of the supply of new rock material, the mid-Atlantic plate and the North American plate are steadily moving apart by about five centimetres a year:
- The second case is when two plates slide past each other, causing, as an effect of their mutual friction, earthquakes of considerable intensity (as in the case of the San Andreas fault);
- The third and last case is the one that, as far as the formation of mountains is concerned, interests us most. If the colliding margins belong to two continental plates, these first lock together, then one of the two plates slides under the other. The surface part of the two fronts is crushed, folded and lifted, forming mountain ranges, while the part that sinks works its way into the Earth's mantle. This last case is that of the birth of the Alps or of the formation of the Himalayan range.
To see how these forces shaped our peninsula, read about the formation of the Italian mountains.