If you really think about it, a great many things go into a painting. There’s the artist’s vision, sure, but there’s also the pigments and properties of the paint, the mixing of the paints on the palette, the canvas and frame, the types of brushes used, and the physical skill of the painter. Landscapes, likewise, are determined by many factors (even if they never appear in a painting). But for landscapes, a complex system of factors interacts dynamically, continually evolving and producing a masterpiece every step of the way.

The Himalayas are an astoundingly grand landscape; we call them “the roof of the world.” You could simply describe them as the crumpled product of the collision between the Indian and Eurasian tectonic plates, but that would be about as bland as describing the contents of the Louvre as “paint.” Each peak and valley has been slowly sculpted by a collaboration of geologic processes. Researchers have recently uncovered evidence about one of these processes, something with the inartistic name of "tectonic aneurysm."

Floating peaks

It’s reasonable to assume that, in a place like the Himalayas, tectonics pushes a mountain up even as erosion shaves it down. The faster the mountain pushes upward, the harder erosion works to keep it in check. That's because the peaks extend into colder elevations where ice can wedge apart cracks or form rock-grinding glaciers and steepening slopes that drive faster-flowing streams.

All of this is true, but it also misses an amazing part of the story. One of the reasons that mountains are so tall is that they sit on very thick sections of Earth’s less-dense crust, which “floats” on the more-dense mantle. Shave off the top of a mountain range and, like removing a small weight from a floating object, the rock beneath it pops upward a bit. Erosion can cause a mountain to rise—even if there’s no net change in actual elevation.

One of the more dramatic examples of erosion is where the Tsangpo River hooks around the eastern edge of the Himalayas, leaving the Tibetan Plateau to become the mighty Brahmaputra River. There, it passes through the severe Tsangpo Gorge, which we’ve written about before. The difference in elevation between the river and the peaks around it reaches more than double the depth of the Grand Canyon. The Tsangpo River has been busy, eating away at the bedrock as it drops two kilometers over a run of just 100 kilometers.

This also happens to be one of the regions of the Himalayas where the rate of uplift is most extreme. That has gotten many geomorphologists wondering: is the Tsangpo Gorge so deep because the rate of uplift is so great (creating a steep gradient over which the river accelerates), or is the uplift so great because the Tsangpo River has been eroding so much rock and carrying away its weight?

Many have argued that if the latter is true anywhere, it’s probably there. The hypothesized process at work is called a “tectonic aneurysm.” During an aneurysm, blood pushes against a ballooning wall of a blood vessel, further weakening it in a vicious cycle. Within a mountain range, rock at depth is stiffened by the tremendous weight above it. Cut a notch out of the surface (like a river valley), and the rock below becomes a little less stiff than its surroundings. In a region already being squeezed by tectonic forces, this can lead to rock pushing up from beneath, initiating the “ballooning” of an aneurysm.

Under the Tsangpo

The idea is attractive but hard to test. A group of researchers, led by Ping Wang of the China Earthquake Administration’s Institute of Geology, have managed a clever test of it at the Tsangpo Gorge. They noticed that, approaching the gorge, the Tsangpo River on the Tibetan Plateau grows wider and wider before abruptly narrowing. Dammed rivers do the same thing as their reservoirs fill up the valley behind. But what does the terrain look like under the Tsangpo?

To find out, the researchers drilled five holes upstream of the Tsangpo Gorge. They discovered that they had to drill a long way to reach bedrock. Three hundred kilometers upstream, the sediment was about 70 meters thick, but it deepened to over 550 meters nearer the gorge. Just before the gorge, it thinned again until the bedrock emerged at the surface.

The deepest portions occurred where the river was widest. In fact, the depth of the bedrock could be predicted by extrapolating the slope of the valley walls down to where they would meet. Making similar predictions along the entire length, a pretty consistent profile emerges. The bedrock of the valley bottom drops gradually and evenly—despite the fact that the modern river barely drops at all—before rising steeply to the start of the gorge.

This starts to make sense when you look downstream. Through the Tsangpo Gorge, the river drops down incredibly steeply, like a stair step, after which the gradient shallows. If you draw a slope from the newly discovered bedrock valley gradient upstream of the gorge, it lines up beautifully with the slope downstream of the gorge. It's just that there's a sudden interruption, a bedrock peak, right where the gorge starts.

From the bottom of the deepest borehole, the researchers dated sediment samples to see how long they’d been down there. Quartz grains in sand exposed at the surface acquire a sort of “sun burn,” as charged particles from cosmic rays transform atoms inside the mineral into beryllium-10 or aluminum-26—isotopes that you won’t find for any other reason. Since they’re both unstable, decaying over time, researchers can use them to determine how long it’s been since that quartz last acquired a sun burn at the surface.

The answer, in this case, was that the sediment was buried between 2 and 2.5 million years ago. It just so happens that techniques to reveal the uplift history of the rock around the Tsangpo Gorge tell us that uplift accelerated in the last 4 million years.

The researchers conclude that uplift created the Tsangpo Gorge, rather than the erosion of the gorge allowing uplift. As the mountains pushed upward in that region—which is nestled into a sharp corner of the tectonic plate boundary—the gradient upstream flattened while the gradient downstream steepened. The slowing river dumped sediment upstream, staying level with the rising barrier, while the fast flow on the other side incised more and more deeply into the gorge.

In an article published in the same issue of Science, Arizona State researcher Kelin Whipple argues that this doesn’t completely rule out the possibility that the uplift was triggered by even earlier, but less extreme, erosion. But the chances are pretty slim. Unless that can be shown somehow, the “tectonic aneurysm” explanation for this portion of the Himalayas is out.

Even so, Whipple points out the rock there is rising at a rate of five to 10 kilometers every million years—a clip only possible with the burden-removing assistance of serious erosion. A painter can only do so much without her brush.

Science, 2014. DOI: 10.1126/science.1259041, 10.1126/science.aaa0887 (About DOIs).