This year’s Tour de France, contrary to its name, starts in Italy and San Marino. The riders will initiate their three-week adventure in Florence – the “Cradle of the Renaissance“. It is also the birthplace of the legendary two-time winner Gino “il Pio” Bartali. The opening stage is 200 kilometers long. It is a hilly stage stretching across the territories of Tuscany and Emilia-Romagna with a finish in the notorious seaside party town of Rimini. But that’s not the shaky start we mean.
For this stage, “hilly” is probably an understatement. In reality the riders will climb for over 3600 meters as they will (almost!) cross the Northern Apennines. These are part of an approximately 1200 kilometers long mountain chain extending across most of Italy. Apart from the numerous culinary delicacies and heritage sites, this region is infamous for its frequent earthquake activity.
Did you know that a large part of the Northern Apennines is today hidden under the sediments of the Po plain and the Adriatic Sea? In this blog post we will explore the buried northern Apennines and why they host so many earthquakes. Additionally, we find out the one thing the stage start and finish cities have in common and how today’s race could span across three countries – if only it took place some 20,000 years ago. Let’s start with stage 1: a shaky start.
Let’s start with tectonics
In order to try to understand why there are earthquakes in the Northern Apennines today, we first have to try to understand how they formed. Generally speaking, the Northern Apennines owe their existence to the convergence of the Eurasian and African plates.
Of course, the underlying story is, like the Tour de France, very complex, dramatic and even involves a few plot twists. Or should we say “plate twists”? We start in the Jurassic period, about 150 million years ago, when the Piemont-Liguria Ocean (a smaller part of the Alpine Tethys Ocean) opened between the European plate and the Adria plate (a smaller part of the African plate). In other words, they moved away from each other.
The first plot twist happened in the Late Cretaceous period, about 80 million years ago. The Adria plate started to move towards the European plate. This movement initiated the closure of the ocean and the overriding of the Adria plate over the European part of the ocean. As the convergence continued the European and Adria plates collided while simultaneously bulldozing and squishing together sediments and older rocks resulting in the formation of the Apennines. In this Giro d’Italia blog by Douwe van Hinsbergen you can find more information.
The second plot twist was in the form of a subduction polarity reversal in the Late Eocene, some time around 34 million years ago. The plates basically swapped their roles and the Adria plate started to sink beneath the European plate. All these events played an important role in shaping the Apennines that we can observe today.
Slow plates
Presently, the plates are still closing in on each other. The Northern Apennines are moving towards the northeast with a staggering speed of 3 mm per year. They will certainly not escape beneath the riders’ wheels.
The Northern Apnennines are overtaking the Adria plate. All this cumulated energy has to be released somehow. One way to achieve this is by earthquakes which occur on faults. In fact, the figure below shows that the Northern Apennines have been shaken by many moderate and strong earthquakes. This unfortunately resulted in considerable damage and loss of life.
Hidden faults
A careful reader might notice that most of the earthquakes are located in the middle of the Adriatic Sea or in the Po Plain. They are both flat and show no evidence of faults on the surface. So where are the faults that host the earthquakes then?
Time for the final plot twist. Seismic studies (using sound to investigate the Earth’s structure) reveal that a significant part of the Northern Apennines actually extends towards the northeast. It is today buried by sediments younger than 23 million years. The buried front of the Northern Apennines is represented by a (aptly named) fold-and-thrust belt which frequently hosts relatively strong shallow earthquakes. One of the more recent events was the 2012 Emilia-Romagna earthquake sequence.
As the sedimentation rates have been higher than the tectonic uplift, the Northern Apennines thrust front is steadily being buried by sediment. In other words: the tectonic uplift couldn’t keep up with the sedimentation. The now buried Appenines are out of sight but not out of mind for many earthquake scientists and local residents. So, in order for the riders to completely cross the (partly hidden) Northern Apennines on the first stage, they would have to add a couple of tens of kilometers towards the northeast.
Pietraforte
Earthquakes are notorious for damaging old buildings. It would be horrible if a big one happened near Florence which has been a UNESCO World Heritage site for more than forty years. The character of the city lies in the light brown (ochre) colour of the “pietraforte sandstone”. It’s a building stone which covers many of Florence’s medieval buildings. In fact, they already used pietraforte (or “strong rock” in Italian) in Florence during the Roman period. Construction really boomed after the twelfth century with the city’s expansion. That is no wonder as the sandstone outcrops were suitably located very close to the city. Very sustainable, locally sourced building materials.
The pietraforte was formed during the Late Cretaceous, between 100 and 65 million years ago, by turbidity currents. These are a mixture of water and sediment that travels down a slope and deposits sediment when it loses momentum because it reaches a flat area. This is most common in the deep parts of the ocean. Therefore, the pietraforte can be considered a marine deposit. So technically speaking, the riders are within touching distance to the sea in Rimini as well as in Florence. Shake it up!
A geological fun fact
Today’s race takes place across two countries – Italy and San Marino. If stage one took place approximately 20,000 years ago and would be some 100 kilometers longer it would also reach the area of present-day Croatia. The post-race refreshment would then be called a “bevanda” instead of a “spritz”. But how would the riders cross the Adriatic Sea? Fortunately, they would not have to.
During the Last Glacial Maximum, which as the name suggests, marks the largest glacier extent of the Last Glacial, so much water was stored in the glaciers that the sea level across the globe was approximately 125 meters lower compared to today. That means that you could walk the shortest route from the Apennines to the Dinarides without getting your feet wet. That is if you don’t count crossing the River Po in the middle. Too bad they did not have bikes back then. It would make the journey a lot faster.
We all hope that stage one will be exciting and full of surprises. Fortunately, it already is from a geological point of view. Today you hopefully learned something new about the formation of the Northern Apennines, their earthquakes and that both the start and finish have something marine in common. However, we will have to wait for another glaciation to see if the organizers will extend the stage to another country. For this year, we can only hope the only shaking will be in the form of pre-race jitters.
NB: Blogs in other languages than English are all auto-translated. Our writers are not responsible for any language and spelling mistakes.
