Prayer flags on a high-elevation pass on the Tibetan Plateau. The plateau and the Himalaya Mountains formed as a result of India’s collision with Asia over the past 50 million years. Photo by Marin Clark.
Fifty million years ago, India slammed into Eurasia, a collision that gave rise to the tallest landforms on the planet, the Himalaya Mountains and the Tibetan Plateau.
India and Eurasia continue to converge today, though at an ever-slowing pace. U-M geomorphologist and geophysicist Marin Clark wanted to know when this motion will end and why. She conducted a study that led to surprising findings that could add a new wrinkle to the well-established theory of plate tectonics — the dominant, unifying theory of geology.
“The exciting thing here is that it’s not easy to make progress in a field (plate tectonics) that’s 50 years old and is the major tenet that we operate under,” says Clark, an assistant professor in the Department of Earth and Environmental Sciences in LSA.
“The Himalaya and Tibet are the highest mountains today on Earth, and we think they’re probably the highest mountains in the last 500 million years,” she says. “And my paper is about how this is going to end and what’s slowing down the Indian plate.”
Clark’s paper is scheduled for online publication Feb. 29 in the journal Nature.
In it, she suggests that the strength of the underlying mantle, not the height of the mountains, is the critical factor that will determine when the Himalayan-Tibetan mountain-building episode ends. The Earth’s mantle is the thick shell of rock that separates the crust above from the core below.
According to the theory of plate tectonics, the outer part of the Earth is broken into several large plates, like pieces of cracked shell on a boiled egg. The continents ride on the plates, which move relative to one another and occasionally collide. The tectonic plates move about as fast as your fingernails grow, and intense geological activity — volcanoes, earthquakes and mountain-building, for example — occurs at the plate boundaries.
The rate at which the Indian sub-continent creeps toward Eurasia is slowing exponentially, says Clark, who reviewed published positions of northern India over the last 67 million years to evaluate convergence rates. The convergence will halt — putting an end to one of the longest periods of mountain-building in recent geological history — in about 20 million years, she estimates.
Clark. Photo courtesy Marin Clark.
And what will cause it to stop?
Until now, conventional wisdom among geologists has been that the slowing of convergence at mountainous plate boundaries was related to changes in the height of the mountains. As the mountains grew taller, they exerted an increasing amount of force on the plate boundary, which slowed the convergence.
But in her Nature paper, Clark posits that a different model, one based on the strength of the uppermost mantle directly beneath the mountains, best explains the observed post-collisional motions of the Indian plate.
By “strength” Clark means the uppermost mantle’s ability to withstand deformation, a property called viscous resistance. Clark suggests that the relatively strong mantle directly beneath Tibet and the Himalayas acts as a brake that slows — and will eventually halt — the convergence of the two continents.
“My paper is arguing that it’s not the height of the mountains, it’s the strength of the mantle that’s controlling this slowing,” Clark says. “This is something that hasn’t been considered before and basically grew out of field observations in northern Tibet.”
