Fifty million years ago, India slammed into Eurasia（欧亚大陆） , 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（聚合） today, though at an ever-slowing pace. University of Michigan geomorphologist（地貌学家） 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," said Clark, an assistant professor in the Department of Earth and Environmental Sciences in the College of Literature, Science, and the Arts.

"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 said. "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（地幔，斗篷） , not the height of the mountains, is the critical factor that will determine when the Himalayan-Tibetan mountain-building episode ends. 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 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, according to 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.