That deeper rock is far enough down that it can be warmed by heat from the Earth's interior.
"It's like you put that into your microwave and turned it up just a bit," Hodges said. "It doesn't melt, but it begins to flow."
The softer rock gradually oozes from deep below the Earth, through the crust, and onto the surface, slowly adding new layers, and therefore new height, to the Himalaya. But there it cools, forming rigid rock that blocks any further flow.
That's where the strengthening monsoons may have come into play.
All that rain rapidly eroded the newly hardened rock, allowing the underlying layers to continue to flow, the study suggests.
As the intensifying monsoons brought more rainstorms, the deep rock could push to the surface more frequently, resulting in faster growing mountains—that's the theory, anyway.
The Evidence
The researchers found evidence supporting the theory in core samples drilled from deep seabeds off China, India, and Pakistan. The cores included millions of years of sediments washed down from various parts of Asia.
Careful analysis of the minerals in these samples indicated how quickly the rock from which they were formed weathered before being washed out to sea—an indication of how fast the uplands were eroding and, therefore, a clue to the strength of bygone monsoons.
Other chemical analyses reveal how long ago the eroded Himalayan rock cooled from its warm-Brie state, revealing the eras in which the Himalaya were growing.
The researchers found a strong correlation between the growth of the mountains and the strength of the monsoons, all the way back to 23 million years ago, according to the study, published this week in the journal Nature Geoscience.
Adrian Hartley, a geologist at the University of Aberdeen, U.K., has studied the link between climate and mountain building in the Andes. He was not directly involved in the current study, although he has colleagues who were.
"This is a very elegant model, which ties a number of different observations and datasets," Hartley said by email.
Nevertheless, he notes, questions remain.
"What happened before 23 million years ago? Was Himalayan elevation insufficient to enhance the monsoon? Or is that we just don't have the [geological] records to prove it?"
Geologist Sean Willett, who was not involved with the study, is cautious about the strength of the findings.
"There are quite a few steps between 'monsoon intensity' and clay mineralogy in the deep sea cores," said Willet, a professor at Geological Institute of the Eidgenössische Technische Hochschule in Switzerland, by email.
Still, he said, it's interesting that the dates at which many of the Himalayan rocks cooled turned out to coincide so strongly with the apparent change in climate.
"The idea that this was driven by a particularly erosive climate is intriguing."
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