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The Sesia Val Grande UNESCO Geopark, in Piedmont, combines spectacular geology of international significance with biodiversity and human culture.
The Supervolcano
A case that is unique in the world
In the heart of the Western Alps lies the fossilised remains of a supervolcano, revealing its deepest parts. Around 300 million years ago, when there was a single continent on Earth called Pangaea, a volcano erupted, spewing out an immense quantity of material and releasing energy equivalent to 250 atomic bombs.
Between 60 and 30 million years ago, the same processes that formed the Alps lifted and rotated the section of the Earth’s crust where the erupted volcano was located, revealing its magma chamber down to a depth of around 30 km: this is a phenomenon unique in the world!
Evidence of its presence can be seen in an area stretching from Valsesia and Valsessera right up to the shores of Lake Maggiore.
An important discovery
A unique fossilised supervolcano has been discovered between Valsesia and Valsessera, in the Western Alps. The research was carried out by Silvano Sinigoi, Professor of Petrography at the University of Trieste, and James Quick, Vice-President of Southern Methodist University in Dallas. Professor Silvano Sinigoi, a former student of the geologist Giorgio Rivalenti, Professor of Petrology and Petrography at the University of Modena (a pioneer in understanding the geology of Valsesia), has been studying the geology of the Valsesia region since as far back as 1979 (continuing Prof. Rivalenti’s work), and his collaboration with Prof. James Quick dates back to the 1980s.
In 2009, following the publication of the first comprehensive study on the subject in the international scientific journal *Geology*, the news immediately spread around the world, featuring in the national press, online and on television. On 2 October 2009, Prof. Sinigoi presented the research findings at a lecture held at the Pro Loco theatre in Borgosesia.
It had already been known for over a century that volcanic rocks outcrop in the lower Valsesia, just as it had long been known that very deep layers of the Earth (mantle peridotites) emerged in the Balmuccia area and that, taken together, the rocks outcrop along the Valsesia valley between Balmuccia and Gattinara form a cross-section through the Earth’s crust.
The significant new finding was to demonstrate, thanks to modern geochronological techniques, that the magmatic rocks intruding into this crustal section and the volcanic rocks outcrop between Borgosesia and the Po Valley belonged to a single magmatic system active between 290 and 280 million years ago and now extinct.
Magma mixing in the Aniceti Basic Complex
Sesia Gorges between Scopa and Balmuccia
Kinzigite outcrop near Crevola
Supervolcano Megabreccia at Prato Sesia
Supervolcano caldera at Romagnano Sesia
Geosite of the Sesia Supervolcano in Agnona
An open-air geological laboratory
Around 60 million years ago, following the opening of the Atlantic Ocean and the subsequent drift of the African continent, the collision between Africa and Europe led to the formation of the Alps and, in the Valsesia area, caused the crust to fold at a 90° angle, exposing the deepest parts of the volcano’s magma chamber. Thanks to this overturning of the Earth’s crust, it is now possible to observe directly what was originally situated 25 kilometres below the surface. This is a geological structure that has now become a fossilised feature, exposing some of the most hidden and deepest parts of the magmatic system beneath the volcano, which are generally inaccessible. This will enable scholars from around the world – geologists and volcanologists alike – to understand what really happens beneath an active volcano.
Thanks to the evidence provided by the structures that have emerged along the Valsesia valley, researchers will have a comprehensive model for interpreting both the geophysical profiles and the magmatic processes at work beneath active calderas, understand the fundamental processes that influence eruptions, where the enormous quantities of lava are stored, and explain even more clearly the links between tectonic plate movements and volcanic eruptions.
A complex story
Around 290 million years ago, there was a widespread thermal anomaly affecting much of Europe, causing the partial melting of the mantle at depths of several tens of kilometres. The basaltic magma produced by the melting of the mantle rose and intruded into the deepest part of the crust. Subsequent intrusions of magma from the mantle gradually led to the growth of a large magmatic complex of basic composition, intruding into the deepest part of the Valsesia crustal section, known as the Basic Complex. The basic magma interacted with the crust, successively incorporating and assimilating ever-higher levels of the crust. At the same time, part of the crustal rocks melted due to the heat transmitted by the basic magma, which was cooling and beginning to crystallise. The partial melting of the crust gave rise to granitic melts, which migrated upwards and, for around 10 million years, fuelled the growth of plutons in the shallower part of the crust (known as the Graniti dei Laghi) or were erupted, giving rise tovolcanic activity.
The mixing of magmas of mantle origin with those derived from the melting of the crust led to the formation of ‘hybrid’ magmas, from which the rocks constituting the majority of the Basic Complex crystallised. In turn, the acidic melts derived from the crust that fed the granitic bodies were contaminated by material from the mantle.
Between 290 and 280 million years ago, in the deepest part of the crust, the basic magma cooled to form a kind of magmatic ‘granite’ (crystalline mush), consisting partly of crystals and partly of interstitial melt. Above the Basic Complex, the crustal material was still partially melting, whilst higher up the acidic plutons were also cooling, giving rise to a crystalline mush. At the same time,effusive activity was taking place on the surface.
Shortly afterwards, around 280 million years ago, a violent eruption took place, accompanied by the collapse of the system, resulting in the formation of a caldera at least 13 km in diameter. Within a very short time – just a few days – the roof of the magma chamber collapsed and hundreds of cubic kilometres of pyroclastic material were ejected: one of the most violent geological events on record.
The intrusion of mantle material that led to the formation of the Basic Complex was therefore the ‘thermal driver’ that triggered the large-scale formation of acidic magmas through partial melting of the crust. It was these acidic magmas that gave rise to plutons and significant volcanic activity, culminating in the final super-eruption.
