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We think we know how mountains form. Plate tectonics causes rock to be pushed up at fault boundaries. Except that model is hard to prove, and a new study suggests it might actually be a lot more complicated.

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Funny thing about science. Sooner or later, you have to provide some evidence that whatever you’re going  on about is actually true. Take plate tectonics.

We’ve gotten pretty good at using this model to explain how the movement of Earth’s plates can create things like mountains on the surface. Except, it’s hard to test  whether we’re actually right, because plate tectonics  happens really, really slowly. But a study in the Calabrian mountains of Italy has come up with some of that “evidence” stuff and revealed that things might not always be as simple as we imagine them to be.

We might be totally wrong  about how mountains form. [intro] Plate tectonics is powered largely by forces we cannot see. A few dozen kilometers below the surface, the Earth’s mantle flows in  huge convection currents, a bit like a pan of boiling water. It’s heated from the planet’s deep interior, which makes plumes of warm material rise up towards the surface, before spreading out, cooling, and sinking back to start the cycle again.

And on the surface, cold and rigid tectonic plates making up the crust are pushed and pulled around like floaty toys on a pool. Not that your pool is ever  filled with boiling water, I hope, but… you get the picture. When the tectonic plates meet or diverge, they form dramatic landscapes like rift valleys, volcanoes, and mountain chains.

It’s kind of intuitive that mountains are built when plates collide. Picture the leading edge of a mountain range, forming where one plate is  subducting beneath another one. The top layers of sedimentary  rocks are scraped off and pile up, a bit like the ice scraped off your windshield.

And all that thickened crust acts  like an iceberg made of rock, floating on the fluid mantle underneath. If all that’s true, the faster the plates collide  and thicken the crust, the faster the mountains will rise up. So if we could measure mountain-building speed and compare it to how fast plates collide, we could test that model.

The problem is, most tectonic plates move at about the same speed as your fingernails grow, and mountains take millions of years to pile up. So it’s incredibly hard to  actually test our assumptions. It’s not like we can sit in front of them with a stopwatch to compare their growth with the rate of collision.

However, in a paper published in Nature Geoscience in June 2023, scientists have come up with an innovative new way of  reconstructing past rates of growth, by reading the landscape itself. The study is focused on the  ‘toe’ of southern Italy, where the Calabrian mountains have been formed by the African plate crashing northwards into the Eurasian plate. Researchers from the US have combined several different geological approaches to interpret the shape and  geology of the landscape.

They used ratios of radioactive elements in the rocks to work out their ages, and then interpreted the physical shapes and features of the mountain rocks to figure out where they were when they formed. Together, this helped them piece together the speed of subduction as well as the timing and  speed of mountain building over the last 30 million years. For example, flatter parts of the mountain terrain were formed when rock uplift was slow, whereas steeper sections  suggest faster rates of uplift.

And the traces of rivers cutting  down through existing rocks show when elevations changed quickly as well. Ultimately, these different  lines of evidence showed that the Calabrian mountain building  wasn’t consistent over time, but was more stop-start. And surprisingly, the rates  of uplift were not consistent with the rate of subduction of the African plate.

When subduction was fast, uplift was slow, and when subduction rates slowed, the uplift was fast. This is the opposite of what we’d expect from our simple models of  tectonic crustal thickening. The results suggest that in this case at least, mountain building is more than skin deep, and the researchers have  had to find another theory for how the Calabrian mountains were uplifted.

Remember those convection currents? Their idea is that the part of the African plate that has been subducted underneath Eurasia actively affects how the  mantle underneath convects, and that this in turn affects  the elevation of the crust. It’s known as dynamic topography, and it’s something that’s been  suggested in computer models, but never before seen in nature.

It works like this. As the African plate descends, it drags some of the mantle down with it, and this downward flow is enough to trigger a new convection current  to form in the upper mantle. Mantle material flows down  next to the subducting slab, and is heated and returns to  the surface some distance away.

But this vigorous downwelling of the mantle near to the subduction zone effectively sucks the whole of the crust downwards in this region, and it’s enough to counteract and cancel out any uplift that  happens due to crustal thickening. So subduction is fast, but uplift is slow. This situation doesn’t go on forever, though.

Eventually the slab of crust hits  a transition zone in the mantle, at about 660 kilometers deep. Below this, the lower mantle is much denser, and the swallowed crust just  kind of sits on top of it. Subduction slows, and eventually the slab  disintegrates and tears off, which disrupts that vigorous  convection and downwelling.

Without the suction from the  mantle and the weight of the slab, the crust can bounce back up. Uplift is now fast, even though subduction and actual  crustal thickening have slowed. So while the basic idea isn’t going anywhere, it seems there’s more to this plate  tectonic story than we first thought.

The crustal plates aren’t just pushed around on top of a churning mantle, but have the ability to change  what’s going on underneath as well. And this in turn ends up changing how and when the features on the surface are built. There’s more work to do to find out if this process is unique to these mountains, or happens all around the world.

And the authors hope that their new approach will help reconstruct mountain-building elsewhere. But this study shows us that the Earth is a more complicated interdynamic system than we previously imagined, in which those rocky pool floaties can change the conditions in the pool itself. And better understanding how  and why mountains have grown helps us to more accurately piece  together our planet’s history, and with it the history of… just about everything.

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