Geology of the Alps

From Academic Kids


The Alps arose as a result of the pressure exerted on sediments of the Tethys Ocean basin as its Mesozoic and early Cenozoic strata were pushed against the stable Eurasian landmass by the northward-moving African landmass. Most of this occurred during the Oligocene and Miocene epochs. The pressure formed great recumbent folds, or nappes, that rose out of what had become the Tethys Sea and pushed northward, often breaking and sliding one over the other to form gigantic thrust faults. Crystalline rocks, which are exposed in the higher central regions, are the rocks forming Mont Blanc, the Matterhorn, and high peaks in the Pennine Alps and Hohe Tauern.

Geological History

Geologists have documented that in late Paleozoic times all continents were clustered into one supercontinent called Pangea. Pangea began to break apart in the later Paleozoic (Permian) times. Two main continents, Gondwana (southern) and Laurasia (northern), were slowly formed as a new ocean, the Tethys Ocean, opened in an East-West direction. After this, the Atlantic began to open (in a North-South direction), forming the Tethys Sea. These four continents would become present day North and South America, Eurasia and Africa.

The break-up history of Pangea exemplifies dispersal of supercontinents under the process of Plate Tectonics. At the present time, an oceanic basin similar to what the early Tethys Ocean may have been like is opening below the Red Sea, continuing down through Africa, forming the Rift Valley. Eventually, a new ocean will cut through east Africa, dividing a large section of land from the main continent.

As the Tethys Ocean basin continued to widen and deepen, the elements of weathering were always at work on the surrounding landmasses. The effects of wind and water were able to chemically and mechanically erode and destroy the mountain ranges which we know were present on the earlier European continent. During the approximately 100 million years of open ocean, the rivers on the surrounding landmasses transported layers of mud, sand and gravel into the depths of the Tethys Ocean and formed compacted sediment layers several thousand meters in thickness. These same processes are continuing in our present oceans.

About 100 million years ago the process of spreading was halted in the Tethys Ocean and the African continent broke away from the South American plate, beginning a northward movement. As a result of this process, the soft layers of ocean sediment were compressed and folded as they were slowly thrust upwards. Caught in the middle of the merging continents, the area of the Tethys Sea between Africa and Eurasia began to shrink.

The tremendous forces at work in the lower continental foundation caused the European base to bend downward into the molten core and soften. The southern (African) landmass then continued its northward movement over some one thousand kilometers. The slow folding and pleating of the sediments as they rose up from the depths is believed to have initially formed a series of long east-west island chains.

In the final stage of the Tethys Sea's disappearance (its remainder would become the Atlantic Ocean), the large mass of material that was originally far to the south was pressed onto and over the deep ocean layers. This is why in some areas of the Alps you find younger sediments overlaid by much older material. Basically, as Africa moved north, the European continent was subducted underneath it. The Alps are made up of piles of sliced off sheets (termed nappes by Alpine geologists) from the European margin, with sheets of the southern continental margin on top. Also, as this was happening the elements were at work weathering these soft materials at a high rate and filling the newly-made valley (what was formerly the Thethys Sea basin) with sediment layers of mixed composition.

The present day view of the Alps is also complicated by the fact that in regions where there was early extensive weathering it was possible for the deep molten granite to well up to the surface and mix with the sediments. In fact, in some valleys which have been deeply cut by rivers it is possible to see crystalline rocks that were the base of the original proto-continent that itself had a lifetime of some 400 million years before splitting into its present pieces.

Thus, the European Alps consist mainly of material that was transported over a thousand kilometers northward by the movement of the African plate. The formation of the Mediterranean Sea is a more recent development and does not mark the northern shore of the African landmass. Satellite photos clearly show the Alps as a crescent-shaped series of folds stretching from southern France to eastern Austria that mark Europe's collision with Africa. The same mechanism shaped the Himalayan range, where the Indian continent is moving northward and pressing into and under the Asian landmass.

In a later period, after the mountains had been formed, there was a final tremendous push that deformed the earlier northern European shore region. Here a series of low limestone mountains (Jura range, 1200-1600 m) were created. These stretch from near Paris to southern Germany and stretch along the northern border of Switzerland, forming the characteristic landscape of the Neuchatel region. The process of mountain building continues to this day. Measurements in the road and railway tunnels show that the Alps continue to rise somewhere between a millimeter and a centimeter each year. This is held in an overall balance by weathering effects. Also, there are many active seismic areas under the mountains that show that stresses continue to be released along deep fault lines.

The actual landscape we see today is a recent development -- only some two million years old. This was the period where some five known ice ages have done much to remodel the region. The tremendous glaciers that flowed out of the mountain valleys repeatedly covered all of the Swiss plain and shoved the topsoil into the low rolling hills seen today. They scooped out the lakes and rounded off the limestone hills along the northern border.

The last glacier advance ended only "yesterday" in geologic terms (some 10,000 years ago in this area) and left the large lake now known as Lake Neuchatel. The ice in this region reached some 1000 meters in depth and flowed out of the region behind Lake Geneva some 100 km to the south. Today large granite boulders are found scattered in the forests in the region. These were carried and pushed by the glaciers that filled this part of the western plain for some 80,000 years during the last ice age. From their composition it has been possible to determine the precise area from which they began their journey. As the last ice age ended, it is believed that the climate changed so rapidly that the glaciers retreated back into the mountains in only some 200 to 300 years time.

Besides leaving an arctic-like wasteland of barren rock and gravel, the huge moraine of material that was dropped at the front of the glaciers blocked huge masses of melt water that poured onto the central plain during this period. A huge lake resulted, flooding the region to a depth of several hundred meters for many years. The old shoreline can be seen in some places along the low hills at the foot of the mountains -- the hills actually being glacial side-moraines. As the Aare River, which now drains western Switzerland into the Rhine River, eventually opened the natural dam, the water levels in the plain fell to near the present levels.

In the last 150 years man has changed the flow and levels of all the rivers and most of the extensive wetlands and small lakes have disappeared under the effects of farming and other development.

The Alps were the first mountain system to be extensively studied by geologists, and many of the geologic terms associated with mountains and glaciers originated there. The term "Alps" has been applied to mountain systems around the world that exhibit similar traits.

Historical Views of Alpine Geology

The geology reflected in the remainder of this article comes from the 1911 Encyclopedia Britannica and reflects views from the first decade of the 20th Century. It is a good example of the descriptive geology of that time, but worthless when it comes to why these ranges exist and some of the structural commentary. It is a pre-Plate Tectonics view and is no longer valid.

The Alps form but a small portion of a great zone of crumpling mountain ranges that stretch in a series of curves from the Atlas Mountains to the Himalayas. Within this zone the crust of the earth has been ridged up into a complex system of creases or folds out of which the great mountain chains of southern Europe and Asia have been carved by atmospheric agencies. Superficially, the continuity of the zone is broken at intervals by gaps of greater or lesser extent, but these are due, in part at least, to the subsidence of portions of the folded belt and their subsequent burial by more recent accumulations. Such a gap is that between the Alps and the Carpathians, but a glance at a geological map of the region will show that the folding was probably at one time continuous.

Leaving, however, the larger question of the connection between the great mountain ranges of Europe and Asia, we find that the Alps are formed of a series of wrinkles or folds, one behind another, frequently arranged en echelon. The folds generally run in the direction of the chain, and together they form an arc around the plain of Lombardy and Piedmont. Outside this arc lies a depression along which the waters of the upper Danube and the lower Rhone find their way towards the sea; and beyond rise the ancient crystalline masses of Bohemia, the Black Forest and the central plateau of France, together with the intervening Mesozoic beds of southern Germany and the Jura. The depression is filled by Miocene and later beds, which for the most part lie flat and undisturbed as they were laid down. Beyond the depression also, excepting in the Jura Mountains, there is no sign of the folding which has raised the Alpine chain. Some of the older beds indeed are crumpled, but the folding is altogether different in age and in direction from that of the Alps.

Within the crust of the earth, whether by the contraction of the interior or in any other way, tangential pressures were set up. Since the crust is not of uniform strength throughout, only the weaker portions yielded to the pressure; and these were crumpled up against the more resisting portions and sometimes were pushed over them. In the case of the Alps it seems natural enough that the crystalline masses of Bohemia, the Black Forest and the central plateau of France should be firmer than the more modern sedimentary deposits; but it is not so easy to understand why the Mesozoic rocks of southern Germany resisted the folding, while those of the Jura yielded. It should, however, be borne in mind that the resisting mass is not necessarily at the surface. Such is in outline the process by which the Alps were elevated; but when the chain is examined in detail, it is found that its history has not been uniform throughout; and it will be convenient, for purposes of description, to divide it into three portions, which may be called the Eastern Alps, the Swiss Alps, and the Western Alps.

The Eastern Alps consist of a central mass of crystalline and schistose rocks flanked on each side by a zone of Mesozoic beds and on the north by an outer band of Tertiary deposits. On the Italian side there is usually no zone of folded Tertiaries and the Mesozoic band forms the southern border of the chain. Each of these zones is folded within itself, and the folding is more intense on the Bavarian side than on the Italian, the folds often leaning over towards the north. The Tertiary zone of the northern border is of especial significance and is remarkable for its extent and uniformity. It is divided longitudinally into an outer zone of Molasse and an inner zone of Flysch. The line of separation is very clearly defined; nowhere does the Molasse pass beyond it to the south and nowhere does the Flysch extend beyond it to the north. The Molasse, in the neighbourhood of the mountains, consists chiefly of conglomerates and sandstones, and the Flysch consists of sandstones and shales; but the Molasse is of Miocene and Oligocene age, while the Flysch is mainly Eocene. The relations of the two series are never normal. Along the line of contact, which is often a fault, the oldest beds of the Molasse crop out, and they are invariably overturned and plunge beneath the Flysch. A few miles farther north these same beds rise again to the surface at the summit of an anticlinal which runs parallel to the chain. Beyond this point all signs of folding gradually cease and the beds he flat and undisturbed.

The Flysch is an extraordinarily thick and uniform mass of sandstones and shales with scarcely any fossils excepting fucoids. It is intensely folded and is constantly separated from the Mesozoic zone by a fault. Throughout the whole extent of the Eastern Alps it is strictly limited to the belt between this fault and the marginal zone of Molasse. Eocene beds, indeed, penetrate farther within the chain, but these are limestones with nummulites or lignite-bearing shales and have nothing in common with the Flysch. But although the Flysch is so uniform in character, and although it forms so well defined a zone, it is not everywhere of the same age. In the west it seems to be entirely Eocene, but towards the east intercalated beds with Inoceramus, etc., indicate that it is partly of Cretaceous age. It is, in fact, a facies and nothing more. The most probable explanation is that the Flysch consists of the detritus washed down from the hills upon the flanks of which it was formed. It bears, indeed, very much the same relation to the Alps that the Siwalik beds of India bear to the Himalayas.

The Mesozoic belt of the Bavarian and Austrian Alps consists mainly of the Triassic, Jurassic and Cretaceous (Mesozoic) beds playing a comparatively subordinate part. But between the Trias of the Eastern Alps and the Triassic of the region beyond the Alpine folds there is a striking contrast. North of the Danube, in Germany as in England, red sandstones, shales and conglomerates predominate, together with beds of gypsum and salt. It was a continental formation, such as is now being formed within the desert belt of the globe. Only the Muschelkalk, which does not reach so far as England, and the uppermost beds, the Rhaetic, contain fossils in any abundance. The Trias of the Eastern Alps, on the other hand, consists chiefly of great masses of limestone with an abundant fauna, and is clearly of marine origin. The Jurassic and Cretaceous beds also differ, though in a less degree, from those of northern Europe. They consist largely of limestone; but marls and sandstones are by no means rare, and there are considerable gaps in the succession indicating that the region was not continuously beneath the sea. Tithonian fossils, characteristic of southern Europe, occur in the upper Jurassic, while the Gosau beds, belonging to the upper Cretaceous, contain many of the forms of the Hippuritic Sea. Nevertheless, the difference between the deposits on the two sides of the chain shows that the central ridge was dry land during at least a part of the period.

The central zone of crystalline rock consists chiefly of gneisses and schists, but folded within it is a band of Palaeozoic rocks which divides it longitudinally into two parts. Palaeozoic beds also occur along the northern and southern margins of the crystalline zone. The age of a great part of the Palaeozoic belts is somewhat uncertain, but Permian, Cretaceous, Carboniferous, Devonian and Silurian fossils have been found in various parts of the chain, and it is not unlikely that even the Cambrian may be represented.

The Mesozoic belt of the southern border of the chain extends from Lago Maggiore eastwards. Jurassic and Cretaceous beds play a larger part than on the northern border, but the Triassic still predominates. On the west the belt is narrow, but towards the east it gradually widens, and north of Lago di Garda its northern boundary is suddenly deflected to the north and the zone spreads out so as to include the whole of the Dolomite mountains of Tirol. The sudden widening is due to the great Judicaria fault, which runs from Lago d'Idro to the neighbourhood of Meran, where it bends round to the east. The throw of this fault may be as much as 2000 metres, and the drop is on its south-east side, i.e. towards the Adriatic. It is probable, indeed, that the fault took a large share in the formation of the Adriatic depression. On the whole, the Mesozoic beds of the southern border of the Alps point to a deeper and less troubled sea than those of the north. Clastic sediments are less abundant and there are fewer breaks in the succession. The folding, moreover, is less intense; but in the Dolomites of Tirol there are great outbursts of igneous rock, and faulting has occurred on an extensive scale.

West of a line which runs from Lake Constance to Lago Maggiore the zones already described do not continue with the same simplicity. The zone of the Molasse is little changed, but the Flysch is partly folded in the Mesozoic belt and no longer forms an absolutely independent band. The Triassic has almost disappeared, and what remains is not of the marine type characteristic of the Eastern Alps but belongs rather to the continental facies which occurs in Germany and France. Jurassic and Cretaceous beds form the greater part of the Mesozoic band. On the southern side of the chain the Mesozoic zone disappears entirely a little west of Lago Maggiore and the crystalline rocks rise directly from the plain.

Perhaps the strangest problem in the whole of Switzerland is that presented by the so-called Klippen. Within the Alps, when normally developed, we may trace the individual folds for long distances and observe how they arise, increase and die out, to be replaced by others of similar direction. But at times, within or on the border of the northern Eocene trough, the continuity of the folds is suddenly broken by mountain masses of quite different constitution. These are the Klippen, and they are especially important in the Chablais region and between the Lakes of Geneva and Thun. Not only is the folding of the Klippen wholly independent of that of the zone in which they lie, but the rocks which form them are of foreign facies. They consist chiefly of Jurassic and Triassic beds, but it is the Trias and the Jura of the Eastern Alps and not of Switzerland. Moreover, although they interrupt the folding of the zone in which they occur, they do not disturb it: they do not, in fact, rise through the zone, but lie upon it like unconformable masses -- in other words, they rest upon a thrust-plane. Whence they have come into their present position is by no means clear; but the character of the beds which form them indicates a distant origin. It is interesting to note, in this connection, that the pebbles of the Swiss Molasse are not generally such as would be derived from the neighbouring mountains, but resemble the rocks of the Eastern Alps. The Klippen are, no doubt, the remains of a much larger mass brought into the region upon a thrust-plane, and much of the Molasse has been derived from its destruction. Although the explanation here given of the origin of the Swiss Klippen is that which now is usually accepted, it should be mentioned that other theories have been proposed to account for their peculiarities.

In the Western Alps the outer border of Molasse persists; but it no longer forms so well-defined a zone, and strips are infolded amongst the older rocks. The Eocene has altogether lost its independence as a band and occurs only in patches within the Mesozoic zone. The latter, on the other hand, assumes a greater importance and forms nearly the whole of the subalpine ranges. It consists almost entirely of Jurassic and Cretaceous beds, the Trias in these outer ranges being of very limited extent. The main chain is formed chiefly of crystalline and schistose rocks, which on the Italian side rise directly from the plain without any intervening zone of Mesozoic beds. But it is divided longitudinally by a well-marked belt of stratified deposits, known as the zone of the Brianconnais, composed chiefly of Carboniferous, Triassic and Jurassic beds. The origin of the schistose rocks has long been under discussion, and controversy has centred more particularly around the schistes lustres, which are held by some to be of Triassic age and by others to be pre-Carboniferous and even, perhaps, Archaean. Partly in consequence of the uncertainty as to the age of these and other rocks, there is considerable difference of opinion as to the structure of the Western Alps. According to the view most widely accepted in France the main chain as a whole forms a fan, the folds on the eastern side leaning towards Italy and those on the western side towards France. The zone of the Brianconnais lies in the middle of the fan.

From the above account it will at once appear that between the convex and the concave margins of the Alpine chain there is a striking difference. Upon the outer side of the arc the central zone of crystalline rocks is flanked by Mesozoic and Tertiary belts; towards the west, indeed, the individuality of these belts is lost, to a large extent, but the rocks remain. Upon the inner side the Tertiary band is found only in the eastern part of the chain, while towards the west, first the Tertiary and then the Mesozoic band disappears against the modern deposits of the low land. The appearance is strongly suggestive of faulting; and probably the southern margin of the chain lies buried beneath the plain of northern Italy.

The chain of the Alps was not raised by a single movement nor in a single geological period. Its growth was gradual and has not been uniform throughout. In the Eastern Alps the central ridge seems to have been in existence at least as early as Triassic times, but it has since been subject to several oscillations. The most conspicuous folding, that of the Mesozoic and Tertiary belts, must have occurred in Tertiary times, and it was not completed till the Miocene period. The structure of the zones in the Bavarian Alps seems to suggest that the chain grew outwards in successive stages, each stage being marked by the formation of a boundary fault. A precisely similar structure is seen in the Himalayas.


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