Geology of Puglia: last piece of the vanished continent Adria

Puglia is a geological exception in Italy. It is one of the last pieces of the almost vanished continent of Adria, recently named Greater Adria because it was larger than thought, perhaps as big as Greenland. Puglia, the heel of Italy, has only recently merged with the boot itself, through the tectonic forces that also caused the rise of Italy's Apennines and volcanoes. The limestone caves, white cliffs, astonishing turquoise seawater and even the unique trullo houses say something about the geology of this mythical corner of the Mediterranean.

Text: Kathelijne Bonne

Puglia or Apulia stretches from the mountainous Gargano Peninsula (the spur of the boot) to the extreme southeastern tip of Italy. I visited the region in full summer, under the smoldering heat wave now known as Cerberus, a name I also used on an article of the environmental problems of Naples. Despite my overheated brain, the geology of the region of the trullo houses never fails to fascinate, and I always wondered how my beloved Puglia fits into the larger framework of Italian and European natural history. That framework has come into focus just recently, thanks to geologists of the University of Utrecht, who reconstructed the evolution of the Mediterranean in meticulous detail and in which Adria occupies a very prominent place.

Italy, and in fact all of the Mediterranean, is the active front of colliding tectonic plates. The irregular southern frontiers of Europe, with all its protrusions and islands, do not at all resemble the larger, more tightly delineated landmasses such as Africa, Australia and the Americas. Europe is more like Southeast Asia or the Caribbean in terms of geographic fragmentation and tectonic complexity.

Europe: a tectonic puzzle

Europe consists of a number of older tectonic plates that have split off from the great southern continent of Gondwana. They drifted northward, on the caterpillar tracks of the planet, to collide with Baltica, the ancient core of Europe. Adria, one of these plates, tore away from Gondwana some 240 million years ago (Triassic Period) to plunge beneath Europe, a process that is still ongoing, as we shall see.

The outer limits of Adria were conjectural until in 2019 the Utrecht-based researchers finally "tamed" Adria and mapped its outlines. Adria turned out to be much larger than current remaining pieces (Puglia, Malta, Adriatic Sea) suggested. Greater Adria, so christened by Prof. Dr. Douwe van Hinsbergen, was about the size of Greenland. The publication of the original results in the journal Gondwana Research led to an avalanche of papers on the "vanished continent," a title that always captures the imagination. I was given permission to reuse the map below (from the press release "Mountain formation and plate tectonics in Mediterranean for the first time integrally investigated"), on which you can see Greater Adria, adrift between Europe and Africa, during Early Cretaceous times and on which Puglia occupies only a tiny space.

Southern Europe is generally mountainous, yet Puglia is fairly flat and the rocks lay horizontal; they are not deformed. That makes Puglia unique throughout the Mediterranean, where most of the land is somehow lifted, broken or intensely folded by mountain building forces. However, it is only a matter of time before Puglia too will be in the grinding teeth of plate tectonics.

Adria: coral reefs in a clear sea

During the Cretaceous Period (145 to 65 million years ago), much of Adria was submerged beneath the clear waters of the tropical Tethys Sea, a vibrant, shallow world of coral reefs and shellfish, not unlike the Bahamas today. The sedimentary deposits and fossils of the Tethys can now be found across North America, the Sahara, and in high Himalayan peaks. The famous Carrara marble of northern Italy has Tethyan origins too.

Cretaceous sea levels were at an all-time high due to the greenhouse climate, so Europe was a tropical archipelago of islands. The Tethys stretched so far south into Africa that a marine connection formed with the South Atlantic across Nigeria; a sea teeming first with giant marine reptiles and then with legged primordial whales.

The Mediterranean is the last remnant of the once great Tethys, and is getting smaller as the African and European plates move toward each other, sometimes even isolating Mediterranean waters from global circulation, which once made this ancient sea evaporate.

In short, southern Europe is an active front where one plate plunges under the other, in the process called subduction.

The heel of the boot

In subduction, two plates collide and one plate, the heaviest, sinks into the Earth's mantle. Meanwhile, the Earth's crust is folded and raised into a mountain range along which planetary tensions are released through the rupturing of faults and the erupting of volcanoes.

In Italy, we see the following (see image below): The Adria plate (orange) plunges westward and downward (blue line) under the Apennines (yellow). The red dots are volcanoes. This succession of geo-regions is easy to see if you drive along the motorway east to west across the peninsula, from Bari to Naples: The hilly limestone plateau of Apulia (Adria) gives way to a flatter region (the subduction zone: Bradanic Trough, see further) with crop fields and windmills, which rises into the high mountains (Irpinia in the Apennines), to finally descend to Naples where the volcanoes are.

As Adria subducted, its coral reefs moved ever westward, ever closer to the Apennine Mountain range, until the reefs were uplifted above sea-level, creating an archipelago of islets (now Puglia). This archipelago was still separated from the Apennines by a deep-sea trough, the Bradanic Trough: the place where Adria plunges into the depths. This deep-sea trough gradually filled in with sediment and became increasingly shallow. Five hundred thousand years ago, during the Pleistocene, it finally silted up and became part of the land. The heel of Italy had merged with the boot.

Further west, the subducting plate melts the rock above it, creating magma reservoirs that feed Italy's stately string of volcanoes, from the inactive volcanoes of Tuscany and Lazio, across Campi Flegrei, Vesuvius, Etna and the Aeolian Islands.

Earthquakes and millionaires

Seismic risk is high in the Apennines. Unlike in southern Turkey, where catastrophic earthquakes are caused by transform faults, in Italy the rupture of the earth's crust takes place along supposedly "normal" faults. But for the victims understanding the earthquake mechanism is utterly futile. Tens of thousands of people already lost their lives in the tragic earthquakes of Messina (1908), Irpinia (1930, 1980), L'Aquila (2009) and many others.

An even greater number of people were injured or left homeless, and many feel abandoned. Funds for reconstruction are all too often drained by the mafia. The decadent list of millionaires gets a little longer after each disaster. In the last major L'Aquila earthquake, some scientists were sentenced for downplaying the earthquake hazard, but then the sentences themselves came under fire. And so, the heated debate flared up again about the (un)predictability of natural disasters.

But as difficult as it is to estimate how, where and when nature will strike, nothing prevents authorities from engaging in mitigation and making preparations in seismic risk areas, ... if that is what they want to spend public money on.

Trullo houses

In Puglia, the earthquake risk is not high, the guide assured us in the famous Castellana caves, where razor-sharp stalactites dangle high above our heads. The risk zone is just a little too far to cause devastation here. The geological processes of Puglia are rather slow, like the soil forming processes that lead to amazing red soils (Terra rossa, or Chromic Luvisols in this case, which will come up in another article), and infiltrating rainwater that dissolves the underlying limestone, forming caves. Along the shorelines of both the Adriatic and Ionian seas, white limestone has other appreciated effects: white beaches and cliffs, and clear turquoise sea water, not muddled by sediment.

Above ground, limestone is used as building stone, making for beautifully white towns. Fine-layered limestone slabs, found only in one part of Puglia, were used to make the roof tiles of the trullo houses. The cone-shaped trulli are scattered throughout rural Puglia and were built by farmers as temporary granaries. Over time, they became cottages or farms, and today many trulli has been repurposed into hotels, restaurants and shops.

The oldest existing trulli date from the 17th century, but the original form possibly dates back to Antiquity, and perhaps even Prehistory. Greeks, Phoenicians, Messapians and Romans all exerted an influence on form and style. The highest concentration of trulli is still determined by geology and the fine-layeredness of the limestone: especially in Valle d'Itria, the best thickness for regular roof tiles is found. For the pinnacle of trulli pleasure, tourists should head to the trulli capital Alberobello, a UNESCO World Heritage Site.

To this day, the history of the trulli is hotly and eagerly debated, and its cultural heritage is as unique as Puglia's geology.

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The story of Puglia is akin to that of the Carrara marble, from which Michelangelo chose the noblest pieces to create his David. You can read more about Italian volcanoes in "Volcanoes of Europe," or in a previous article about the threats of Campi Flegrei volcano near Naples. Vesuvius comes up in "Volcanoes, plastic and mafia," and in Plastic eruption in the Gulf of Naples. As part of the great Mediterranean basin, you may enjoy reading more on that time when the Med dried up almost completely and what happened with fauna. "Tethys Ocean: the vanished ocean of the world is now in the mountains," is still often read, and is a good complement to the article on Puglia you've just read. Some more basic knowledge about plate tectonics and its discoverers (after more than 100 years of opposition) will also help to better understand Italy's natural history.

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Sources: 

Douwe J.J. Van Hinsbergen, Trond H. Torsvik, Stefan M. Schmid, Liviu C. Maţenco, Marco Maffione, Reinoud L.M. Vissers, Derya Gürer, Wim Spakman, Orogenic architecture of the Mediterranean region and kinematic reconstruction of its tectonic evolution since the Triassic, Gondwana Research, Volume 81, 2020, Pages 79-229, ISSN 1342-937X, https://doi.org/10.1016/j.gr.2019.07.009.

Van Meulenbrouck, 2019, Universiteit Utrecht Nieuws, Internationale pers duikt massaal op onderzoek naar Groot-Adrië, https://www.uu.nl/nieuws/internationale-pers-duikt-massaal-op-onderzoek-naar-groot-adrie.

Van Meulebrouck, 2019, Universiteit Utrecht Nieuws, Gebergtevorming en platentektoniek in Middellandse Zeegebied voor het eerst integraal onderzocht. https://www.uu.nl/nieuws/verdwenen-continent-middellandse-zeegebied.

Tropeano et al., 2023. Geological Uniqueness and Potential Geotouristic Appeal of Murge and Premurge, the First Territory in Puglia (Southern Italy) Aspiring to Become a UNESCO Global Geopark. Geosciences 2023, 13, 131. https://doi.org/10.3390/geosciences13050131.

Tropeano, Marcello & Sabato, Luisa & Pieri, Piero, 2002. Filling and cannibalization of a foredeep: the Bradanic Trough, Southern Italy. Geological Society, London, Special Publications. 191. 55-79. 10.1144/GSL.SP.2002.191.01.05.

The Guardian, 2016, A timeline of major earthquakes to hit Italy. https://www.theguardian.com/world/2016/aug/24/timeline-major-earthquakes-italy

The Guardian, 2016, Italy must block mafia from earthquake rebuild, says prosecutor. https://www.theguardian.com/world/2016/aug/28/italy-earthquake-mafia-construction-contracts

Wikipedia: 2009 L'Aquila earthquake. https://en.wikipedia.org/wiki/2009_L%27Aquila_earthquake.

BBC, 22 Oct 2012, L'Aquila quake: Italy scientists guilty of manslaughter. https://www.bbc.com/news/world-europe-20025626.

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Kathelijne: As a nature lover and earth scientist I am intrigued by how life, air, rocks, soil, ocean and societies interact on geological and human timescales.

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