How Rain Might Make Mountains Grow

 we’ve talked about all kinds of big, complicated questions — like what happened at the start of the universe. But sometimes, the questions that are hardest to answer are actually some of the simplest ones. Like, take mountains. Mountains have a profound impact on the rainfallpatterns that shape our weather and climate. But how does rain affect mountain ranges? It seems like an easy question, but geologists have been debating the answer for more than a century. And some of their ideas aren’t what you’dexpect. Like, what if raindrops actually make mountainsgrow? If it’s true, the implications could shapehow we view the formation of the biggest features on our planet’s surface. And after decades of searching, the finalpieces of the puzzle are starting to emerge — thanks to some radiation from space. Oddly enough, the key to this idea is erosion:the process of wind, water, or ice wearing down a mountain by carryingaway small pieces of soil and rock. And intuitively, knocking off bits of a mountainought to make it smaller. And if erosion were the whole story, that would pretty much be the end of it. But that misses the complex effects of platetectonics. Plate tectonics describes how Earth’s crustis broken into moving chunks. But what matters here is what happens whenthose pieces come together.

 When two plates collide, their edges can getpushed up, creating a mountain range. And erosion can affect that process, bothwhile it’s happening and long after. It all comes down to erosion’s ability to remove material from the mountains— and thus, weight. See, when mountains get lighter, they act differently. For instance, during the slow collision between plates, lighter plates will experience less frictionas slabs of rock slide past each other on their journey upwards. And less friction means the tectonic forceshave an easier time pushing the mountain upwards — making a taller mountain. Then, there’s another factor called isostaticequilibrium. That’s the idea that the tectonic platesare essentially floating on the mantle, and their height is the balance point betweengravity pulling down and buoyancy pushing up. Just like how an empty boat sits higher inthe water than a fully-loaded one, as mountains are eroded away and become lighter,they can float higher. So erosion can give an extra lift from below by accelerating the tectonic forces already in motion.

 Now, it’s worth acknowledging what mightseem like an obvious problem with this: Even if erosion makes a mountain lighter, it seems like wearing away the rock should also make the mountain shorter. And here, the key is that geologists think a lot of this erosion happens not on a mountain’s peak, but in river valleys lower down. These are areas of intense erosion and, by cutting into the side of a mountain, they can remove a lot of weight without removing a lot of height. So, overall, it seems straightforward! Rain creates rivers, rivers erode mountains,the mountains become lighter and ultimately, taller. Except, here’s the problem: All of this rests on that first idea, that rain drives erosion through rivers. But geologists have had an incredibly toughtime proving that. Like, there are a lot of things that provide water for rivers, including rain, underground aquifers, and the seasonal melting of glaciers. So, to claim that rain is the primary driver of the erosion that might grow mountains, someone needs to show that changes in rainfallactually correspond to changes in erosion. And isolating those details has proven so difficult to do that it took until 2020 to do this in a tectonically-active mountainrange. 

To do it, a team looked at how rain affectsrivers cutting into the Earth’s tallest mountains, the Himalayas. And to measure the rate of erosion, they reliedon something surprising: cosmic rays from space. Cosmic rays are highly energetic particlesthat come from the Sun and beyond. And when they strike rock, they can sometimes interact with the mineralsinside to produce a form of the element berylliumcalled beryllium-10. But this only happens for rocks near the surface,like during or after erosion. So when you look at a sample of eroded rock,the more beryllium-10 atoms you see in it, the longer it’s been since it was eroded out of a rock. In the study, researchers used that information to figure out how fast erosion was happening in Himalayan river valleys in Bhutan and Nepal. 

Next, they built computer simulations of the rivers themselves, including their watersheds. Then, they entered rainfall data into theirriver models to see if they could reproduce erosion rates that match what they observedusing beryllium-10. And it worked! Erosion from their modeled rivers does infact depend on how much rain falls in the area! ...and somehow, this is only one piece ofthe puzzle. The next step is to link these findings witha model of the tectonic forces, to quantify how strongly this erosion affectsthe height of a mountain. But if scientists couldn’t say with confidencethat rain contributes strongly to the erosion effects of rivers, the rest would be kind of a moot point. So score one for finding the evidence needed to back up our hypotheses! It’s not always the most glamorous part of science, but it’s one of the most important for understanding our world. And if you like considering questions like this, you might also really enjoy a new album by Patrick Olson called Music for Scientists. It’s a tribute to those who’ve dedicatedtheir lives to science-driven work, and was inspired by the space between humancreativity and what Richard Feynman calls “the inconceivablenature of nature.”