The Himalayas “breathe”, and mountains grow and shrink in cycles

If we could quickly advance our planetary clock, the Earth’s surface would wrinkle with activity. Continents will drift across the globe, oceans will open and close, and new mountains will rise toward the sky.

However, even as mountains rise, they also regress periodically when stress from tectonic collisions causes earthquakes. These events occur in a cycle, like a giant rocky box that paints uneven breaths, explains Luca Dal Zelio, a geophysicist at the California Institute of Technology.

The forces driving this cycle are incredibly complex, and nowhere is nowhere more evident than at the 1,400 miles of jagged peaks that make up the Himalayas. Identifying the fundamentals behind this scale is vital to understanding the local risks of earthquakes, which threaten the hundreds of millions of people living within them.

In a new research paper published in Nature Reviews, Dal Zilio and colleagues pooled the results of more than 200 previous studies of Himalayan geology to lay out the complex mechanisms behind these geological respiration processes – plus the many remaining challenges.

Since similar geological “breaths” have been documented around the world, the new work is key to understanding the processes that carve the many mountain ranges on Earth – and to discover the dangers that these ranges may pose as well. The length of the Himalayas and their geological complexity make it a wonderful natural laboratory, says study co-author Judith Hubbard, a structural geologist at Nanyang Technological University in Singapore.

“It’s as if the Earth is doing the experiment for us,” she says.

The birth of a giant

The planet’s tectonic plates are in constant motion, reshaping the surface when it disintegrates and collides. The Himalayas are the dramatic result of one such tectonic buildup about 50 million years ago, when the Indo-continental plate collided with the Eurasian Plate. Both land masses are dense and thriving, so as the continents compress and India begin to roll under Eurasia, the landscape collapses and the crust thickens, raising the majestic peaks. (Learn more about the mysteries behind this continental collision.)

To this day, India continues to march north at a rate of roughly two inches every year. But the Earth is not sliding smoothly under Eurasia, and as India compresses, the Eurasian Plate is clumping and swelling. This process pushes the mountains slightly higher into the sky in a long inhalation. Ultimately, the pressure reaches its breaking point, and land masses move in a ground-shaking earthquake – the geological version of exhaling or coughing.

The cycle was deadly in 2015, when a 7.8-magnitude earthquake sank an area of ​​the Himalayan mountain range by nearly two feet.

Different areas in a mountain range may produce different types or intensity of exhalation. While some people cough violently, Hubbard says, others may produce a series of hiccups. And just because a portion of the mountains is blowing in one direction at a time doesn’t mean it will do so again.

“Even the same patch can have different behaviors at different times,” says Rebecca Pendik, a geophysicist at the University of Montana, who was not involved in the new research. “And pretty much nobody has a clue why.”

To understand these complexities, scientists need to relate mountain building processes that take place at completely different time scales – from the slow, slow velocity of tectonic plates to the near-instantaneous shifts of an earthquake. This isn’t easy: different measurements are needed to understand each phenomenon, which often draws researchers from different geological disciplines. (The latter comes with its own unique challenges, as Hubbard says, “Sometimes the same word means two different things to two different people.”)

The new paper attempts to make some of these time-stretching connections in the Himalayas, and study how each affects the way future earthquakes unfold.

Transit time

One of the main ways to bridge the gaps between time scales is to look at the shape of the fracture between the two tectonic plates. In the Himalayas, this rift stretches 1,400 miles, and has many gaps and bends – remnants of the ancient shattering that raised the mountains in the first place. These features have slowly evolved in the thousands of years since then, and they can influence earthquake development today.

In one of the research papers included in the new review, Hubbard and colleagues found that subsurface curves surround the part of the fault that shifted during the 2015 earthquake. This indicates that the structures imposed restrictions on the extent of the interruption, and thus the size of the earthquake.

Dal Zilio says that other structures that formed through the ages may have existed across the length of the range, and can likewise determine how far an earthquake has spread near the surface. The review paper highlights how gathering the range of available information can help scientists develop a more complete understanding – and more robust computer models – not only of how the range grows, but also of its lethal potential.

“The ultimate goal is to know the types of earthquakes we can expect and the types of damage they will cause,” says Hubbard, adding that doing so requires a lot of detective work. “If we’re trying to recognize this exhalation or coughing process but the ground isn’t breathing out or coughing, it’s really hard to know.”

To help bridge the gaps, some researchers are studying the scars left by historical earthquakes, she notes. Other researchers are working to create a more complete map of fault bends, turns, and thickness of sediments near the surface – both of which could affect the location and intensity of seismic vibration in the future. But that’s a big challenge, says Shashank Narayan, a graduate student at the Indian Institute of Technology, Kanpur, who is not the author of the new paper.

He learned this from his personal experience mapping the structure of the Himalayan Fracture along a cross-section roughly a mile in the middle of the Himalayas using a sonar-like process. Geologists often rely on seismic waves to decode what lies deep in the Earth, where their speed and dispersion can reveal different types of rocks or underground structures. Narayan and his team created their own waves by hitting the ground with a heavy weight and then “listening” from a distance with instruments known as geophones.

Narayan says the terrain in many other areas is a major obstacle to such surveys. To the west of their location, for example, the mountains undergo drastic altitude changes over short distances. “You can’t put a single sensor in this area,” he says.

Rotate things forward

As the mountain’s inhalation and exhalation cycles continue, the system itself will change as well, further complicating the image. Some of the accumulated stress from each inhale permanently deforms the rocks, which remains even after the next geological exhalation. Hubbard notes that if all the stress was eliminated with every cough, there wouldn’t be any mountains left.

Over time, as India continues to move north under Eurasia, other landscape features will shift. First, it will jump into the position of the active fault rift, gradually turning south as it finds new paths to the surface.

Pendik also says, “At some point, Nepal will cease to exist.” The advancement of the Indian plate over tens of thousands of years is constantly creeping its southern border towards the north, slowly putting pressure on Nepal.

“In this very long time scale, nothing is too constant,” she says. The phrase “arranged in stone” is not the correct phrase.

Despite all the lingering doubts, Bendick says she was impressed with the review paper because of the wide range of data the team was able to put together, linking every available measurement to the mountain building process.

“I was not blown away by one piece, but how important it is for all of these things to work together to shape the world out of my window, essentially, and the dangers to which human societies are exposed,” she says.

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