The Structure of the Alps
Geologists have studied the Alps more intensively than any other mountains, and the picture they sketch of rock warpings and foldings tells a story of crustal disturbances extending over periods of many millions of years. The origin of these features has been traced to Mesozoic times, although some rock outcroppings date to the earlier Cambrian period. Continental outlines were far different then, and what is now the Mediterranean Sea extended far into central Asia as the Tethys Sea, while marginal areas alternately rose and sank beneath the waves. In the western sector of the Mediterranean, Europe was the northern shore and Africa the southern shore of this sea. Great troughs or geosynclines developed and were filled gradually with sediment washed down from adjacent uplands.
As the weakened crust sagged beneath this added weight, the slopes of the troughs were drawn together in a series of lateral thrustings that caused widespread dislocation. The African shore seems to have edged northward against the relatively stable European foreground, with much crumpling and splintering of rock strata. In some places enormous masses were thrust bodily over submerged layers, in others great folds or wrinkles in the earth crust, called "nappes," were formed. Sometimes these folds, subjected to constant and increasing pressure, virtually collapsed, their strata upheaved at every conceivable angle or even turned bodily upside down.
Certain Alpine areas have been identified as the battlegrounds where these conflicting forces strove. Thus the Jura range seems definitely a part of the European shore, the Dinaric Alps are thought to mark the African shore, and the loftier Pennine range rose from the floor of the oceanic trough, now filled with broken and distorted fragments.
This process of mountain building was no single episode. It was interrupted by prolonged periods of relative calm and by other periods when great areas alternately sank beneath or rose above the sea. Jagged crests were smoothed by long ages of weathering and then were submerged, covered with sedimentary rock, and subjected anew to the forces of erosion. Several such cataclysms marked the course of the conflicting earth movements that built up and tore down the structure of the Alps, so that the present peaks enclose cores of ancient heights long since worn away and buried by later accumulations of sediment.
The rearing of the Alps, which began in the Mesozoic era, was greatly accelerated during the more recent Cenozoic times, which also witnessed the rise of the Himalayas, the Andes, and the giant peaks of the Alaskan coast. And there is reason to believe that the process is still progressing. Certainly erosion is apparent enough in crumbling Alpine precipices and frequent rock slides. There is also some evidence that some of these peaks, which are comparatively youthful mountains, are still slowly rising.
In the formative period, when surface strata were subjected to prolonged strains and stresses, the weight upon the global interior was occasionally relieved so that molten magma welled up to solidify in vast sheets and provide a thickened granitic sublayer or foundation. The remains of older rock formations are sometimes embedded in this foundation; for example, Mont Blanc is an enormous batholith (mass of igneous rock originally stopped below the surface) thrust upward by a surge of viscous matter coated later with schists and gneisses and layers of sedimentary rock.
In the vast squeezing process that produced the Alps, the original area was reduced in width by as much as 150 miles, according to some estimates. It has also been estimated that the mass of the Alps has been reduced by erosion, over tens of millions of years, into little more than a fourth of its original volume.