Италийн Апеннины нурууны дор дэлхийн царцдас “цахилгаан мэт” салж байна

Deep beneath our feet, the slow machinery of Earth is always turning.

Continents collide. Tectonic plate edges slip, one beneath the other, into the mantle. Mountains rise, and chasms yawn, and beneath it all, the ever-churning process of convection – at a pace so gradual that, to human eyes, the world appears unchanging.

For us, working backward from surface geology to the chthonic processes still sculpting it is a bit like trying to reconstruct a movie from a single frame.

And one particularly puzzling frame can be found in the Apennine mountain range that forms the backbone of the Italian peninsula.

Here, the crust is pulling apart along the mountain range, even as it is being squeezed together along its outer edge. Parts of the region are rising; others are sinking. And earthquakes on either side of the range tell similarly contradictory stories.

Now, geologists led by Stefano Tavani of the University of Florence think they have figured out the answer: Earth’s crust is ‘unzipping’ under Italy, producing the strange geology on the land above.

It’s a process known as delamination, in which the dense lower crust and attached lithosphere peel away from the crust above and sink into the mantle.

Beneath the Apennines, that process is not occurring everywhere at the same time, according to Tavani and his colleagues; rather, just like a zipper, it has a single front, known as a hinge, where the peeling is happening, and that front is slowly migrating under Italy toward the Adriatic foreland.

But to understand why this is having the effect it does on Earth’s surface, we need to rewind the movie.

The Apennine Mountains run for some 1,200 kilometers (745 miles) along the length of the Italian peninsula and have long posed a geological puzzle.

Like many mountain ranges, the Apennines were built by tectonic plates pushing together, crumpling and thickening Earth’s crust over millions of years, producing soaring peaks.

But the tectonic system beneath Italy didn’t simply keep pushing in the same way. As the slab of rock sinking into the mantle gradually retreated, the crust behind the growing mountain range was pulled apart, opening the Tyrrhenian Sea.

This left the Apennines in an unusual situation. Even as the outer edge of the mountain range continued to be squeezed together, the crust farther back was being stretched apart.

“The paradox of how horizontal contraction and extension can occur simultaneously in convergent mountain belts remains a fundamental and largely unresolved problem in continental dynamics,” reads a description of the contradiction in a 2006 Annals of Geophysics report.

For millions of years, these opposing movements were parts of the same tectonic system. Between around 10 and 2 million years ago, roughly 100 kilometers of shortening in the central Apennines was matched by a similar amount of extension in the Tyrrhenian region behind them.

This simultaneous contraction and extension has previously been attributed to slab rollback – the retreat of the sinking slab, stretching the crust behind the mountain front even as contraction continued farther east.

But starting around 2 million years ago, something changed. The major phase of extension that opened the Tyrrhenian Sea came to an end, and shortening along the Apennine front subsequently slowed dramatically.

The paradoxical deformation of the mountain range, however, continued. Something else seemed to be going on.

To investigate, Tavani and his colleagues brought together several different views of the mountain range, from decades of earthquake and GPS measurements to satellite radar observations and maps of the boundary between Earth’s crust and mantle – a region affectionately referred to as the Moho, short for the Mohorovičić discontinuity.

A compelling pattern emerged. The different kinds of deformation appeared to be centered around the same structure deep beneath the Apennines.

For more than 500 kilometers along the mountain range, the researchers found a zone where the Moho beneath the Tyrrhenian side overlaps the Moho beneath the Adriatic side.

They interpret this doubled crust as the region where the lower crust is peeling away – the moving front of the unzipping process – like the point at which a piece of tape lifts away from a surface as you peel it.

The earthquakes cluster around it, too. Behind and above the front, earthquake mechanisms mostly indicate that the crust is being pulled apart; ahead of it, they mostly indicate compression.

GPS measurements tell a similar story. Across the mountain belt, the researchers measured around 4 millimeters per year of extension, while toward its outer edge, some of that movement is balanced by roughly 2 millimeters per year of contraction.

The researchers describe this as “accordion-like” deformation: the mountain belt stretches internally while simultaneously shortening at its front.

Slab rollback could explain such a pattern during the earlier evolution of the Apennines. But that explanation isn’t enough for what we see now. Tavani and his colleagues argue that ongoing delamination beneath the mountains provides the missing internal engine.

Ahead of the migrating hinge, the lower crust and lithospheric mantle remain attached to the sinking slab, which pulls the crust downward. As the hinge passes and the lower layers peel away, however, that downward load is released.

Related: Earth’s Crust Is ‘Dripping’ Under The Andes, Scientists Say

The remaining crust can then unbend and rebound upwards, as denser material beneath it is replaced by more buoyant mantle. This process produces extension behind the hinge even as the still-attached crust ahead of it experiences compression and subsidence.

It’s not a complete picture; the model is deliberately simplified, and questions remain about the precise structure of the slab beneath the Apennines. More sophisticated models will be needed to understand the full complexity of the mantle and crust as they deform over time.

But the result suggests that the Apennines could be offering geologists a rare gift – “an empirical, geodetically constrained documentation of a laterally migrating delamination hinge that is tracking mantle-lithospheric peel-back in real-time,” the researchers write.

Not too shabby for a movie assembled from a single frame.

The findings have been published in Communications Earth & Environment.