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• Two opposite types of faults, normal and reverse, help explain our undersea mountain ridges.

• Scientists found that larger normal faults gave way to narrower ones caused by “unfaulting.”

• This unfaulting explains why there are surprising “reversal earthquakes” under the oceans.

In newly published research, scientists from Paris, New York, and China try to explain a tectonic behavior deep under the ocean. Instead of things going with the flow of the simple convection patterns we’ve come to know and love, directional ridges filled with magma are triggering undersea earthquakes with reverse faults—appearing to go backward against convection.

There’s a lot to unpack here, but the research could open the door to a new understanding of why Earth’s crust looks the way it does, especially beneath our oceans.

The Earth’s surface is the outermost portion of its crust, and landmasses are like especially stacked crust. The Himalayan plateau is so thick and heavy that it’s actually sinking over time as part of the natural cycles that keep Earth’s shape. By having such a high concentration of extremely tall mountains and landmass, the Himalayan region is practically begging to be cut down to size.

In contrast, Earth’s crust is thinnest beneath the oceans, which explains why the oceans are the site of ridges that form as tectonic plates regularly pull apart and allow magma from beneath to peek through. There is, technically, new Earth-surface formed every day in these undersea hotspots—that’s called tectonic extension. The magma pushes through, and the tectonic plates continue to slide toward the land masses like a conveyor belt. When plate mass reaches the edges of the continents, it is forced under and remelted to begin the process again. This conveyor belt circulation is called convection, and it’s a major part of plate tectonics.

But sometimes, something unexpected happens. There are small areas of tectonic contraction mixed in with the extension process, creating a kind of two-steps-forward, one-step-back situation. This is most noticeable in the thickest land masses, and it’s how mountain ranges are typically formed.

Extension and contraction each have an associated type of fault. Normal faults describe where two plates are pulling apart, allowing the side resting above to slide down on an angle. Reverse faults are the opposite: plates push together, and the resting side is shoved up and over. There are other types and subtypes of faults represented around the world, with complicating additional forces and directions added depending on the landmass.

The seafloor generally keeps things simple with fault types, but the researchers explain that, in the 1970s, scientists noticed something interesting. Normal faults that are quite spread out—with the thinnest portion in the center—are found toward the middle of mid-oceanic ridges where new magma emerges. But faults get narrower and narrower as you move away from the ridge. They’re still normal faults, but they’ve been partly reversed or “unfaulted” back toward a smooth surface.

So, what’s going on there?

In this paper, the team reports that magma is at least partly responsible for smushing these normal faults together. When magma punctuates the seafloor in certain places, it creates compressive forces, pushing the normal faults closer together and leading to small reversal incidents.

Those small reversal incidents are earthquakes showing reverse faults—something that has long puzzled seismologists and those who study the ocean floor. Overall, the scientists estimate that the closure of these outer normal faults reduces the height of new abyssal hills by as much as 50 percent. Intuitively, this could help explain why the seafloor remains overall very flat despite the stark rises of the mid-oceanic ridges.

The ocean floor remains largely uncharted because of the challenges of studying it. By figuring out the cause of these puzzling reversal earthquakes, scientists can truly break new ground within the field.

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