Prato Sesia

The Prato Sesia geological site features outcrops of the volcanic caldera of the Sesia Supervolcano, which collapsed 282 million years ago during the supervolcano’s catastrophic eruption.

Large blocks of various colours, measuring up to tens of metres in size, are embedded in a yellowish volcanic tuff. This geological chaos, known as a ‘megabreccia’, is the result of the super-eruption: as the caldera collapsed, more than 500^km³ of ash and lapilli (the tuff that binds everything together) were ejected, whilst, at the same time, enormous blocks tumbled from the caldera’s walls into the gigantic chasm.

Among the boulders, one can see all the rocks that predate this most recent event: volcanic rocks, such as the red, glassy rhyolites, the grey andesites, or the tuffs rich in rock fragments, but also fragments of the ancient continental basement, the Laghi Schists, recognisable by their silvery sheen and characteristic schistosity, which resembles a stack of sheets of paper.