Scientists may have just toppled a 100-year-old theory about what holds up the highest mountain range on Earth, new research shows.
The Himalayan mountains formed in the collision between the Asian and Indian continents around 50 million years ago, when tectonic forces squeezed Tibet so hard that the region crumpled and its area shrank by almost 620 miles (1,000 kilometers). The Indian tectonic plate eventually slipped under the Eurasian plate, doubling the thickness of Earth’s crust beneath the Himalayas and Tibetan Plateau to the north, and contributing to their uplift.
For a century, the prevailing theory has been that this doubling of the crust alone carries the weight of the Himalayas and the Tibetan Plateau. Research published in 1924 by Swiss geologist Émile Argand shows the Indian and Asian crusts stacked on top of each other, together stretching 45 to 50 miles (70 to 80 km) deep beneath Earth’s surface.
But this theory doesn’t stand up to scrutiny, researchers now say, because the rocks in the crust turn molten around 25 miles (40 km) deep due to extreme temperatures.
“If you’ve got 70 km of crust, then the lowermost part becomes ductile… it becomes like yogurt — and you can’t build a mountain on top of yogurt,” Pietro Sternai, an associate professor of geophysics at the University of Milano-Bicocca in Italy and the lead author of a new study analyzing the geology beneath the Himalayas, told Live Science.
Evidence has long suggested that Arnand’s theory is erroneous, but the idea of two neatly stacked crusts is so appealing that most geologists haven’t questioned it, Sternai said. Historically, “any data that would come along would be interpreted in terms of a single, double-thickness crustal layer,” he said.
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However, the new study reveals there is a piece of mantle sandwiched between the Asian and Indian crusts. This explains why the Himalayas grew so tall, and how they still remain so high today, the authors wrote in the paper, published Aug. 26 in the journal Tectonics.
The mantle is the layer of Earth that sits directly beneath the crust. It is much denser than the crust and, therefore, doesn’t liquefy at the same temperatures. Meanwhile, the crust is so light and buoyant that it behaves similarly to an iceberg, lifting up higher above Earth’s surface the thicker it gets.
Sternai and his colleagues discovered the mantle insert by simulating the collision between the Asian and Indian continents on a computer. The model showed that as the Indian plate slipped beneath the Eurasian plate and started to liquify, blobs of it rose and attached themselves not to the bottom of the Asian crust, but to the base of the lithosphere, which is the rigid outer layer of the planet composed of the crust and upper mantle.
This is fundamental, Sternai said, because it means there is a rigid layer of mantle between the stacked crusts solidifying the whole structure beneath the Himalayas. The two crusts give enough buoyancy to keep the region lifted, while the mantle material provides resistance and mechanical strength. “You’ve got all the ingredients you need to uplift topography and sustain the weight of the Himalayas and Tibetan plateau,” he said.
The researchers then compared their results with seismic data and information gathered directly from rocks. The mantle sandwich in the simulation matched previous evidence that Arnand’s theory couldn’t explain, study co-author Simone Pilia, an assistant professor of geoscience at King Fahd University of Petroleum and Minerals in Saudi Arabia, told Live Science.
“Things actually start to make sense now,” Pilia said. “Observations that seemed to be enigmatic are actually now more easily explained by having a model where you have crust, mantle, crust.”
The study presents strong evidence for this model, but contradicting Arnaud’s 100-year-old theory is controversial because it has been so widely adopted, Pilia said.
“I think the authors are correct that this is controversial,” Adam Smith, a postdoctoral research associate in numerical modeling at the University of Glasgow in Scotland who was not involved in the study, told Live Science in an email. “All prior work generally agreed that all the material beneath the Himalayas came from the crust.”
But the results are still plausible, and they explain a number of geological oddities in the Himalayas, Smith said. “The authors run lots of simulations using different thicknesses for all of the layers, and they seem to always get this bit of mantle sandwiched between the crust of the two plates.”
Douwe van Hinsbergen, a professor of global tectonics and paleogeography at Utrecht University in the Netherlands who wasn’t involved in the study, disagreed that the results are controversial. “It’s a nice new finding and an elegant interpretation,” he told Live Science in an email. “If a continent shoves below another continent, you’d expect a sandwich that consists from top to bottom of crust and mantle lithosphere of the upper (Tibet) plate, and then the crust of the lower (Indian) plate.”
