Mount Everest gets a little taller every year

The peak is already the highest in the world, at roughly 29,029 feet above sea level. But over millennia, it has risen 50 to 165 feet—and its elevation continues to creep up.

Mountain heights adjust constantly from erosion and tectonic shifts. But new research suggests that in Everest’s region of the Himalayas, the Earth’s crust is also rebounding, a phenomenon that occurs when a huge weight that has been pressing down on the surface is removed.

The relaxing of the crust in this area started some 89,000 years ago, when a rogue river system began washing away substantial amounts of rocks and sediment, according to the new study, published in the journal Nature Geoscience in September. With a lightened load, the crust bobbed upward, as a boat might after unloading cargo. The rebound adds about 0.2 to 0.5 millimeter to Everest’s height above sea level each year.

The clues to what happened come from tracing the ancient path of the river Arun, which used to flow north of Everest but changed course and joined with the river Kosi, running farther south. Once combined, the swollen rivers carved a deep gorge 40 miles east of Everest, dragging away rock and mud.

The authors of the study used models that considered the region’s topography as well as elevation along the river’s path to calculate the time frame in which the river changed course. By calculating the mass of the material carried away, they could estimate the rebound.

“The story they put forward is compelling," said Lizzie Dingle, a geomorphologist at Durham University in the U.K., who wasn’t involved with the study. Geologists have observed similar uplift of the crust elsewhere, Dingle said. “We see the same thing when large ice sheets and glaciers melt."

Areas of Greenland and Antarctica have been rising slowly, geologists have found, as the crust relaxes there after losing ice to melt.

Everest and the Himalayas rose up from the seabed about 45 million years ago, when the Indian continental plate collided with the Eurasian plate, crumpling its crust. The crunched rock rose higher over millennia as the Indian plate continued to push north. Today, tectonic activity continues to lift the range by about 2 millimeters a year.

At the same time, glaciers and landslides are removing material from the mountain range, slowly whittling away its height and offsetting, to some degree, increases to its elevation, said Adam Smith, a geologist at the University of Glasgow and a co-author of the study.

“It all seems so insignificant," Smith said, “but then you pile it up over millions of years, and you get amazing things happening."