The Geology of the Dolomites: from Tropical Seas to Breathtaking Mountains

The Geology of the Dolomites is extremely rich and interestng.

The Dolomites are extraordinary mountains; a collection of exceptional geological features unique in the world. Their profound essence lies in the nature of their rocks, and in the dramatic events that have shaped them.

“[…] These mountains, whose peaks rise above the reign of the clouds […] are made up of different species of rocks. The basements, the thickness of which varies, incline differently, bringing them closer or further away from a vertical position, nevertheless directed towards a central point. Their prolongation leads to the formation of sharp points, broken crests and jagged angles that characterise and indicate from afar mountains known as primitive […]”.

This is an excerpt from a letter that Dolomieu – the scientist after whom these mountains are named – wrote in January 1791.

The surprising scenarios offered by these majestic peaks are the result of their particular geological history, and the geology of the Dolomites is extremely complex; the history of their genesis is so important that it cannot be overlooked.

Of course, it is possible to enjoy the dramatic appearance of these magnificent mountains even without knowing a single thing about their geology – and there’s certainly nothing wrong with that.

And yet, so important – and so fascinating – is this aspect that I have devised a page trying to offer – hopefully – an ‘orienteering’ tool for those wishing to learn more: a condensed version of an incredibly long and complex history.

In order to do that I had to simplify a lot; nevertheless, the use of technical terms was, to a certain extent, inevitable. I hope that neither aspect will go to the detriment of comprehension.

A Journey Through Time and Space

The special link between the Dolomites and geology is highlighted in their very name: the term derives from the mineral now known as ‘Dolomite’ (double calcium carbonate), which was first discovered in these mountains by the French scientist Déodat de Dolomieu (1750-1801), who traveled to South Tyrol several times and visited the Dolomites' region in 1789 and 1790.

In 1791 de Dolomieu reported the finding of a rock that he had analysed in 1792 by the Swiss mineralogist Nicolas de Saussure, which led to the discovery of a new mineral. Subsequently, in 1794, to pay homage to his discoverer, Richard Kirwan suggested that the mineral be called 'Dolomite' after de Dolomieu's name.

Ever since the onset of geological research, the Dolomites have represented a point of reference at global level, thanks to their extraordinary accessibility and the clarity with which the geologic phenomena can be directly observed; some of the founding principles of Earth science were deduced here.

Even today, scientists and researchers from all over the world come to the Dolomites to confront themselves by comparing notes and deepening here the history of our planet more closely “in the field”, thus confirming the continued interest in these magnificent mountains.

Another fundamental aspect determining the overall value of the Dolomites from a geological point of view is the fact that they represent and illustrate a significant part of the Earth's history in a consistent and very detailed way. In particular, the interval between the Upper Permian and the Triassic (between 270 and 200 million years ago) is well documented, and can be appreciated in spectacular detail.

Just to reiterate the aspect of global reference that these mountains hold from the viewpoint of the history of geology is the fact that, from an historical perspective too, many significant time intervals of the Triassic were defined in the Dolomites, and take their name after parts of the region, thus underlining the importance of these mountains as a reference point; for example, the term ‘Ladinian’ comes from Ladin, the language spoken in the region’s heartland, while ‘Fassanian’ comes from the Val di Fassa and ‘Cordevolian’ from the Cordevole valley.

However, perhaps the most characteristic geological aspect is linked to the presence of ancient fossil atolls; indeed, the Dolomites display one of the best conserved examples of fossil reefs and of tropical environments of the Mesozoic worldwide, including fossil evidence of the organisms that formed them.

The succession of rocks illustrates the resurgence and evolution of life over time, after the most dramatic extinction known in geological history, which is known to have taken place at the threshold between the Permian and the Triassic (251 million years ago), leading to the disappearance of more than 90% of all living species at the time.

Furthermore, here is also clearly and dramatically visible the interaction between the atolls and the coral reefs of the Triassic, and the massive volcanic eruptions that have characterized that distant period of time.

The lack of tectonic deformation, the impressive outcrops and the thick layers of sediment accumulated – along with the considerable lateral continuity – highlight another exceptional characteristic of the geology of the Dolomites: the possibility of interpreting and telling their geological history over time (‘vertically’) as well as in space (‘horizontally’).

In particular, the ‘vertical’ interpretation would seem to allow to browse these mountains as the pages of a gigantic stone book, thus revealing the history of planet Earth – whilst the ‘horizontal’ reading gives one the chance to physically experience the Prehistoric geography of these ancient seas and islands.

Here it is also still possible to walk over the ancient lagoons and visit the sea margins, viewing where corals and sponges used to thrive on the banks where waves crashed; making one's way down, one can even descend along the old escarpments to finally reach the bottom of the sea, a thousand metres below.

The Dolomites are thus truly extraordinary mountains, and the nine stations making up the Dolomites UNESCO 'Serial Heritage Site' offer a unique and exceptional collection of geological features of global significance.

A Book Written in Stone

The fundamental essence of the Dolomites lies in the rocks and the extraordinary events shaping them. Their scenery is therefore the specific result of a geological history that can be traced back at least 280 million years, when Europe and Africa were still merged in a supercontinent known as Pangaea: here, at that time, there was a tropical sea – a great oceanic gulf called the Tethys.

On the edges of this gulf, during the Permian (290 to 251 million years ago), an ancient mountain range – at that stage nearly flattened – began to sink, making the deposition of large amounts of sediments possible.

Subsidence – particularly in the area of the western Dolomites – was accompanied by intense volcanic activity that led to the deposition of porphyry, which locally still forms the substrate onto which the other sedimentary deposits are laid.

This slow sinking process gradually allowed the sea to invade the entire region, which then became a warm and shallow expanse of water. From the beginning of the Triassic (about 251 million years ago) and for more than 8 million years, the depth of that sea changed cyclically, sometimes leading to the temporary emergence of lands, followed by new phases of erosion and sinking or periods when the water was much deeper.

However, starting from around 240 million years ago, a large number of organisms – which required light to live – began to build reefs in order to keep pace with the progressive lowering of the seabed and survive the rapid subsidence of the latter.

Thus an archipelago of islands was formed and began to emerge, where those organisms were able to thrive, creating atolls and lagoons separated from one another by sea inlets and stretches of water sometimes more than 1,000 metres deep.

The life of these islands, resulting from the incessant work of these small organisms capable of fixing mineral salts in their skeletons – or stabilising the sediment – is recounted by the phenomenal mountains still around us today, which by now represent a fossil archipelago unique in the world.

Another important event characterized this period and contributed to the specificity of the Dolomites: volcanism.

In the Upper Ladinian time (240 to 230 million years ago) large parts of the region were affected by major volcanic eruptions – submarine at first, then also sub-aerial. Lava, tufa and other volcanic products rapidly went to fill in – and sometimes bury – the reefs, often fossilizing them and at times also significantly changing the contours of the landscape and the geography of the Dolomites as a whole.

Towards the end of the Ladinian time (around 236 million years ago), the volcanoes ceased their activity, broke up, and were later eroded; as a result, large quantities of pebbles and dark sands were produced, and these came to settle in the surrounding seas.

After this phase of upheaval and the subsequent rest, the organisms responsible for reef-building could resume their work, and a new generation of coral reefs was formed. For a further 7 million years these coral reefs progressed and continued to grow, affected in their development by variations in the sea level and by the evolution of organisms.

The sea became increasingly more shallow, as large amounts of sediments coming from the surrounding cliffs and the land above sea level which lay to the south (below the current Po valley and the Venetian plains) led to the filling of the stretches of sea between the islands; thus, a vast coastal marine plain started to form.

During the Early Norian (around 228 million years ago) a new phase of widespread subsidence began in the Dolomites' region, and allowed the sea to invade the coastal plains once again: this is when a powerful series of deep layers of carbonate deposits (sometimes even more than 1,000 metres thick) started to gradually build up.

This muddy plain, governed by the action of tides, was inhabited by the first dinosaurs, as shown by the footprints found impressed in the Dolomite rocks in several locations.

Between the end of the Triassic and the Lower Jurassic (approx. 206-190 million years ago) there was a new phase of subsidence, connected with the opening up of a new ocean towards the west and the breaking-up of the Pangaea, leading first to the depositing of shallow-sea limestone sediments and then to the complete flooding of all the sea plains.

During the Upper Jurassic and throughout the whole length of the Cretaceous periods (between approximately 170-65 million years ago), the whole area experienced a high level of pelagic sedimentation, whose evidence is in the significant succession of fine limestone and marl still to be found today.

The processes leading to these sediments emerging and becoming a mountain range were active instead from the end of the Cretaceous and up to a few million years ago during the Tertiary. These processes were activated by the clash between Africa and Europe, and the resulting deformation of the ancient Tethys prompted these sediments to emerge; this, finally, led the Alps to become the current range we admire today.

In this scenario, the Dolomites hold once again a special place: tectonic deformation, which was very intense elsewhere, was quite mild here; thus the original relationships between the sedimentary units have been more easily preserved.

The Dolomites: an Astonishing Saga of Rocks

So, to recap, it is clear by this point that the Dolomites have not always been mountains. During the Permian this area was a plain furrowed by rivers, whereas during the Triassic it became a wide tropical sea, dotted with small atolls and the occasional volcano. Over time, these islands increased in size, becoming similar to the actual coral reefs of the Caribbean islands.

Then, when the Atlantic ocean was formed the whole area sank. At that point, Africa detached itself from the Pangaea and tilted towards Europe; in this way, the compression formed the Alpine chain: this is when the Dolomites emerged from the sea, rising up for thousands of meters.

At the limit between the Permian and Triassic, almost 90% of all marine species living at the time became extinct. After that, in the millions of years immediately following, a large number of new species appeared, destined to dominate the planet for the whole duration of the Mesozoic (which includes the Triassic, Jurassic and Cretaceous periods); among these, the most famous were undoubtedly the dinosaurs.

The second phase of extinction came on the border between the Cretaceous and the Paleogene, during which the dinosaurs disappeared – along with other species such as the ammonites.

All in all, three processes have generated the Dolomites: lithogenesis (namely, the transformation of terrestrial and marine sediments into rock); orogenesis, linked to the rising of the Alpine chain; and morphogenesis – the process responsible for carving out the valleys and rock faces as a result of the action of atmospheric agents such as ice, water and gravity.

The faults are fractures generated by the tectonic activity. These broke up the Dolomite region into many sectors, raising or lowering them, thus allowing us today to appreciate all the different types of rock involved.

Spectacular Rocks Sculpted in Time

The Dolomites represent a collective unit made up of various mountain ranges that show a remarkable level of geo-morphological uniformity. They contain an extensive and exemplary range of case studies of phenomena and shapes that result above all from their complex geological structure as well as from past and present climatic conditions.

These peculiar shapes include peaks, towers, needles, pinnacles; dolomite and limestone escarpments; ridges and spurs of volcanic rock; gentle slopes of clayey terrain; strata (rock layers), scree slopes and talus cones; detritus from landslides; plateaus and terraces; lakes and deep gorges carved out by streams.

An original way of interpreting these forms in the landscape is linked to their morphological geo-diversity, intended both in relation to the differences with other mountain ranges and on the basis of their inherent genetic variability.

First of all, in global terms the Dolomites are characterized by monumental, original and spectacular qualities that set them quite clearly apart from all other mountains in the world.

Furthermore, even within the context of the Alpine chain, they offer a particularly wide – varied, complex and emblematic – range of morphologic features: these are structural forms linked above all to distant or more recent movements of the terrestrial crust (as visible for example in escarpments and fault lines, ridges cut by fractures, stream captures) or related to the different types of rocks (as seen, for instance, in the majestic peaks overlooking less steep slopes, plateaus, ledges).

All these morphologies and phenomena overlap one another, and due to the variety and complexity they create, their forms – for all their heterogeneity and complexity – offer an educational, almost complete scientific range of case studies within the Dolomites.

The morphology is linked either to current climatic conditions or to processes that took place during more recent geological eras; amongst them, we find some traces that can be ascribed to pre-glacial times, or there is evidence related to some other intermediate temperate phases between the ice-ages.

But dominant above all are the forms linked to glacial erosion and accumulation: "roches moutonnées" – as they are called – polished and layered by the action of glaciers; hanging valleys, cirques and moraine deposits; traces of ancient frozen terrain and evidence of the pressure exerted by the glacial masses.

The morphology that can be related to more recent and current climatic conditions is linked above all to the cycles of freezing/thawing and to the force of gravity: strata, scree cones and ridges at the base of the slopes; heaps of stones with icy nuclei, also self-propelled; avalanche tracks and cones.

A recurrent aspect of this same variety and geo-morphologic complexity is represented by landslides of all possible types, including some considerable, clearly visible ones – really spectacular "manual-like" examples which have, by now, become part of the international scientific literature.

Meanwhile, at local level, another set of examples is also offered by the vast range of karstic phenomena and formations, both at superficial level – like corrosion forms, ridged fields (Karrenfelds), sink-holes (doline), springs and wells – and, at subterranean level, underground caves and swallow holes.

One can affirm that the Dolomites represent an open-air field laboratory at high-altitude, with a geo-morphological heritage of exceptional value at global level. Extraordinarily, these mountains are also one of the most easily accessible sites in the world; therefore, they are ideal for research, educational purposes and other didactic activities  as well as for developing new Earth Science theories.

Ultimately, the Dolomites are an open book that can help us comprehend – and marvel at – the wonders of creation, even through the eyes of a layperson.

And the Evolution Continues...

The geo-morphological evolution that we can observe today is linked to various causes: characteristics of the rocks and their structural discontinuity; current climatic conditions; more or less intense meteorological events and – last but not least – human activities.

One can also note that relic morphological features from the past still condition the aspect of the mountains and the dynamic of the relief: waterfalls keep precipitating with a strong erosional power, plunging down from hanging valleys of glacial origin; moraine debris are, time and again, repeatedly subject to the processes of degradation and dislocation (through collapses too); the thawing of ancient permafrost can lead to landslides, resulting from new freezing/thawing processes (in compact rocks) and water absorption phenomena (in clayey soil).

Broken rocks also undergo further fragmentation as a result of the volumetric expansion of frozen water, and are affected by debris falls as well, which, in turn, are involved in mass debris flows – sometimes including gravitational collapses.

Plateaus and terraces are the privileged locations for geo-morphologic static situations, and tend to undergo a standstill in terms of geo-morphological evolution, with a particular inclination to evolve into wood cover or they can become suitable as agricultural terrain.

In certain locations, small kettle lakes and marshes of ancient glacial origin show in places emblematic and representative stratigraphic sequences, with organic findings which can be dated in order to assist with the reconstruction of ancient, fossil landscapes.

Lastly, the pressures exerted by the glacial masses – especially where they flowed into the valleys – may have led to the creation of potential landslides – surfaces liable to detach themselves along micro-fault lines: these have usually contributed to form some of the most evident, spectacular and dramatic landslide phenomena that have ever taken place in the region.

It is also worth mentioning that recently some major collapses of massive amounts of rocks have occurred; for example, in the Gardena/Grödnertal, Badia/Gadertal and Fiscalina/Fischleintal valleys.

In the last few years, there have also been several rock falls on peaks at altitudes exceeding 2,000 metres in the Odle/Geisler, Tofane, Cinque Torri and Pomagnanon groups: this is mainly the consequence of the thawing of ancient permafrost that was trapped in a fossil state in crevices and in other gaps between the rocks.

The rising temperatures of recent summers have led to the thawing of portions of this fossil ice; cracks in the rocks were thus filled with thawed water (in addition to that deriving from precipitation). During the following winter, new ice formed inside the same fractures, with an increase of around one tenth in the volume of water and the subsequent widening of the gaps. In the following summer, these same cracks were then filled with an even larger amount of water, which later froze again, and this process further widened, deformed – and ultimately broke up – the rocks.

Progressive cycles of freezing/thawing have thus created a greater tendency for portions of rock to detach and eventually break off, thus leading to their subsequent collapse.

In clayey sections, similar quantities of thawed water have caused instead a greater tendency to the fluidization of rocks, making them more viscous and thus producing new – or reactivating old – slides or mudflows, as those that are taking place (and which can be seen) on the slopes overlooking the upper reaches of certain valleys of the Dolomites such as the Badia/Gadertal, Boite and Cordevole.

So, to conclude, the geological history of the Dolomites is all but finished; rather, it is a living process which still continues – albeit very slowly – under our very eyes.

So, when we walk on these most majestic of mountains, we should take a moment to pause, from time to time, to just ponder on the fact that we are treading on a living landscape which breathes, moves and changes all the time; a landscape that guards within itself the key to better comprehend the planet we are living in.

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