Italian Dolomites Geology: Adeene and the Kilometer of Dolostone / by Adeene Denton

On Research

As the only geologist around during my travels, my geologic observations are both the product of the reading I’ve done on the Alps (via O. Adrian Pfiffner’s excellent book Geology of the Alps) and my geologic background.  I’ve covered the topics as best I can, but I recommend doing your own reading! As you can see from the images in this post, the Dolomites are covered in roads and hiking trails, making it very easy for the exploration-minded tourist to get around.  It was hands-down the best country for getting from place to place, and I cannot give the trail-maintenance groups and hut system enough credit.

 Welcome to the Dolomites! This is the view into Austria from the top of the Sassongher.

Welcome to the Dolomites! This is the view into Austria from the top of the Sassongher.

An Introduction to the Alps

To say I was excited to visit the Alps as a geologist would be an understatement. The Alps are, without a doubt, the site of the oldest geologic investigations in the world thanks to their fortuitous location in Western Europe. Modern geologic theory about how mountain-building works was formulated in the Alps, thanks to the large amount of scientists in the area and the mountains’ relative accessibility thanks to centuries of occupation in almost every valley.  I visited the Alps before I became a geologist, and was captivated by the terrain I saw even if I couldn't articulate why.  As I returned a trained geologist (with a lot to learn), I got so much more out of my time in the Alps.

Alpine geology is classic, and the first 19th-century studies defined many stratigraphic sections found in the Alps as type sections for geologic ages and epochs.  While on this trip I found out via my reference texts that the Jura Mountains, which lie on the northwest side of the Alps, are the namesake of the Jurassic Period on the geologic timescale and my mind was blown (a better geologist would've already known this, but I’m here to learn).  The Alps may not be particularly old or particularly large, but their history and structural features remain compelling for many geologists.

This post is different from my previous posts about geologic history in that my focus isn’t Italy as a whole, but the region of the Dolomites and their context within the Alps.  The Dolomites are located in northern Italy, which places them in the Southern Alps and largely disconnects them from the geology of the Po River valley and the rest of Italy to the south.  The Alps are incredibly complex in their own right, and have been split into sections both laterally (the Western, Central, and Eastern Alps) and longitudinally (the Northern Alps, the central metamorphic core complex, and the Southern Calcareous Alps).

 A simplified geologic maps of the Alps - note the complex fault structures.  The Dolomites are the yellow in the south (SA unit).

A simplified geologic maps of the Alps - note the complex fault structures.  The Dolomites are the yellow in the south (SA unit).

What makes the Dolomites important? 

The Dolomites may not be interesting from a structural point of view (there’s plenty of other places in the Alps for that), but they do contain million of years’ worth of history about the Adriatic shoreline during the Triassic (248-206 million years ago).  They are also the reason that the southern portion of the Alps is known as the Southern Limestone Alps, and form the type section for the entire region.  For those who are unfamiliar, the Dolomites are made of – shockingly – mostly dolomite and related carbonaceous rocks that originate from a succession of carbonate platforms that formed on the shoreline, built by living organism with carbonaceous shells.  Also called "the Pale Mountains" by inhabitants, they are easily recognizable by their pale white and grey coloring (the grey comes from weathering).

The Dolomites are impressive because of their sheer size – in some locations, the Dolomia Principale member can be up to 1 km thick, and the formations are unusually widespread.  Carbonate platforms can develop in many places in the world if conditions are right, including Belize, the Maldives, and Papua New Guinea today, but few can persist for as long as this one did.  Carbonates require shallow, clear, and warm water and typically can only exist in a relatively small range of water depths along the coast (typically 5-15 m, but this can vary).  Because of this, they are very vulnerable to outside forces such as subsidence, climate change, and input from nearby rivers.  If the coastline sinks below a viable range or if nearby rivers add too much siliciclastic mud and silt, the carbonates will drown or become buried in a relatively short time, geologically speaking.

 Not quite a kilometer of dolomite here, but quite a thick section.

Not quite a kilometer of dolomite here, but quite a thick section.

The carbonates on the Adriatic coast benefitted from an unusually even shoreline and climatic conditions that allowed them to grow fast enough to keep up with subsidence in the Triassic.  The coastline was on the east and south of Pangaea at the time, before the supercontinent began its breakup.  It was during the Triassic that the massive Dolostone layers were deposited, but more carbonaceous layers followed in successive eras?.  Tectonic forcing became important in the Jurassic when the Piemont ocean opened along the Adriatic coast, which broke up the large platform.  The platform’s breakup and the slow spreading rates created neighboring warm, shallow basins, perfect for the deposition of nodular limestone and other forms of dolostone. 

Subsidence overtook deposition by the end of the Jurassic, and the old shoreline was subsequently buried in places (though uncovered in others, leaving unconformities) until the Alps began building at the end of the Cretaceous, 65 million years ago, unearthing the carbonate platform, which had been cemented together to become the formations of the Dolomites.

 Pictured: two different sections within the Dolomite - the upper, massive dolostone unit and the lower, less continuous unit that is carbonaceous, but more susceptible to erosion.

Pictured: two different sections within the Dolomite - the upper, massive dolostone unit and the lower, less continuous unit that is carbonaceous, but more susceptible to erosion.

The Dolomite vs. Limestone Controversy

Over the course of my journey in the Dolomites, I found myself explaining to many people that dolomite and limestone are not the same thing.  I do not mean to sound pedantic by explaining this, but I want to provide an understandable explanation for any of the laypeople reading this blog.  Names and chemical formulas can be confusing and hard to visualize, and mineralogy was not my strong suit in my geologic studies either.  For those people struggling with the difference, this is for you!

The difference between dolomite and limestone comes down to chemistry – carbonate mud and/or limestone is comprised predominately of calcite (CaCO3).  Dolomite is believed to form when the calcite in limestone is modified by groundwater that is magnesium-rich or through evaporation, converting the mineral calcite into dolomite (CaMg(CO3)2) in a reaction known as dolomitization.  Dolomitization replaces some of the calcium ions with magnesium ions, and the process can change limestone in whole or in part – if the reaction is only partial, we might call it “dolomitic limestone” instead.  While dolomites are prominent in the rock record, there are very few modern dolomites to study, and those that do exist – in anaerobic conditions off the coast of Rio de Janeiro – may not be representative of the typical dolomitization process.

 Pictured: definitely Dolomite, not limestone. Probably.

Pictured: definitely Dolomite, not limestone. Probably.

How might you tell the difference between dolomite and limestone in the field? Well, it’s not easy.  One of the classic carbonate identification tests is to pour hydrochloric acid (HCL) on the suspected rock.  If it fizzes (“effervesce” would be the technical term) nearly immediately, it’s limestone.  Dolomite will produce a very weak effervescence in response, but this can be hard to spot.  Dolomite is also slightly harder than limestone, but good luck successfully proving this in the field, especially because dolomite and limestone exist more on a continuum than as two completely separate rock types.  Taking it back to the lab is really the best way to firmly know.

Fortunately for me, I had handy cross-sections of the primary formations of the Dolomites with me, and was armed with the knowledge that almost everything I saw was dolomite.

The Location of the Dolomites

We may know how the dolomites were deposited and what exactly they are, but how did they get to their present location?

As I mentioned earlier, uplift of the Alps began around 65 million years ago, long after the proto-dolomites were submerged beneath the waves, when the African and Eurasian plates collided.  The Tethyan Ocean, which had previously existed between them, was mostly subducted but sections of the ocean crust remained incorporated into the metamorphic core complex at the center of the Alps.  The core of the Alps has been folded and fractured much more than the outsider regions – hence the sharp verticality of the Swiss Alps (think Mont Blanc, the Matterhorn, etc.) that lends them such fame. 

 I cannot in good conscience write about the Alps and not include a Matterhorn picture, so here it is. It looks just as amazing in real life, I promise.

I cannot in good conscience write about the Alps and not include a Matterhorn picture, so here it is. It looks just as amazing in real life, I promise.

The Dolomites underwent very little deformation during their uplift, which is reflected in their relatively flat tops and tendency towards plateaus rather than jagged peaks.  During the collision, multiple large thrust faults formed as Africa plunged beneath Eurasia, and huge sheets of rock (also known as nappes) were shoved out and across the exposed surface of Europe.  While this explanation is quite simplified, the basics are correct; different sections of the Dolomites contain localized folding and faulting, but overall the region remained relatively undisturbed compared to other sections of the Alps.  The Dolomites are also relatively low compared to the Swiss Alps to the north, another consequence of being on the edge rather than at the center of deformation.

In Conclusion

The Dolomites are very physically imposing and fascinating to look at, but are actually rather simple to understand geologically, particularly in comparison to the rest of the Alps.  In the midst of folding, faulting and large-scale metamorphism, the Dolomites rose up mostly intact and unaltered.  Their presence defines life in northern Italy - erosion of the Dolomites provides fertile soil coating the valley floors, as well as the looming threat of landslides.  Landslides are fairly common in the dolomites; while dolomite erodes less quickly than limestone, it is not impenetrable, and areas of high water supply (especially from snowmelt) are often under threat.

I struggled writing this post only because the Dolomites are so amazingly consistent.  I hiked in and around the Dolomiti, from Bolzano to Cortina, and saw the different sections of carbonaceous rock that I expected to see, with the slightest traces of glacial activity on one of the higher plateaus.  Glaciers, which dominate other areas of the Alps, have stayed away from the lower-elevation Dolomites, and in turn the Dolomites remain subject to erosion as the only force shaping them in the past hundreds of years.  

 A happy geologist in the Dolomites - a quick hike easily gets you above the tree line!

A happy geologist in the Dolomites - a quick hike easily gets you above the tree line!

I would definitely recommend visiting the Dolomites if you can - the mountains are the most accessible I've ever hiked in, and the scenery is breathtaking.  If you would like read more about the geology, check out Geology of the Alps! But I will warn you - it gets pretty technical.

Stayed tuned for my last travel blog post next week on the history and culture of the Dolomites! I hope you've enjoyed these posts :)