A geologic collision in motion
The Apennine mountains, the rocky spine of the Italian peninsula, are the direct result of a slow-motion collision between continents. The mountain-building event, or orogeny, began around 20 million years ago in the Miocene epoch and continues today. This process involves the Adriatic microplate, a chunk of crust connected to the African plate—subducting, or diving beneath the Eurasian plate. As the Adriatic seafloor descends, layers of sedimentary rock deposited over millions of years in the ancient Tethys Sea are scraped off, folded, and stacked on top of each other.
This immense compression created a fold-and-thrust belt. Geologists use a technique called balanced cross-sections to estimate the scale of this deformation. By conceptually unfolding the crumpled rock layers, they calculate that at least 100 kilometers of the original seafloor has been horizontally compressed to create the mountain range. The highest peak in the Apennines, Corno Grande in the Gran Sasso massif of Abruzzo, reaches an elevation of 2,912 meters (9,554 feet). Its limestone and dolomite composition shows its origin as ancient marine sediments. The main thrusting phase that built the Gran Sasso ended in the Late Pliocene to Early Pleistocene.
A paradox of pull and push
The geology of the Apennines is unusual because the eastern and western sides of the peninsula are behaving in opposite ways. On the eastern, Adriatic side, the crust is being compressed, creating the fold-and-thrust belt. Simultaneously, on the western, Tyrrhenian side, the crust is extending and thinning. This stretching process began around 9-10 million years ago and created the Tyrrhenian Sea as a back-arc basin. The extensional forces on the western side have produced a series of normal faults, which cause much of the region's seismic activity.
This tectonic process is still active. The convergence between the plates continues at a rate of about 3 millimeters per year. This ongoing strain is released through earthquakes. The central Apennines are one of Italy's most seismically active zones, having experienced numerous destructive events, including the 2009 L'Aquila and the 2016-2017 Central Italy sequences. These earthquakes primarily occur on the extensional normal faults that run along the mountain chain's axis, a result of the peninsula being pulled apart as it is simultaneously pushed from the east.