How the Hangay Mountains in Mongolia Formed: A Tectonic Plate Mystery Solved

The Hangay Mountains in central Mongolia have long puzzled geologists. Rising 4 kilometers above sea level, this dome-shaped range plays a crucial role in shaping the region’s climate. Yet its formation defies conventional explanations for mountain ranges of similar height.

“These mountains in central Mongolia are very far from any plate boundary, about 5,000 kilometers away from the Pacific margin,” said Pengfei Li, a geologist at the Chinese Academy of Sciences’ Guangzhou Institute of Geochemistry. “It’s very hard to understand why we have such a mountain range so far from the plate boundary.”

Li recently led a study that provides a compelling explanation for the mountains’ origin. The research, published in Geology in April, suggests that a U-shaped bend in a tectonic plate created an unusually thick section of the lithosphere. Over time, this heavy lithospheric root broke off and sank into the mantle. The loss of weight allowed the crust to rebound upward, forming the Hangay Mountains.

Bend and Snap: The Role of the Mongolian Orocline

Tectonic plates are not rigid; they bend, fold, and shift as they move. Sections of plates far from boundaries can develop curves called oroclines. The Mongolian orocline, stretching about 6,000 kilometers, is one of the longest in the world. The Hangay Mountains sit at the curviest part of its U shape.

Li and his team suspected this location was no coincidence. Between 2018 and 2026, they conducted multiple field expeditions, collecting rock samples from the Hangay Mountains. These samples showed signs of ancient volcanic activity. Uranium-lead dating of zircons revealed that the area experienced volcanism during the early Cretaceous period, 124–114 million years ago.

“When I saw the age, I was surprised,” Li said. “120 million years—no one had ever reported volcanoes [in Mongolia] during this period.…It’s the first discovery of volcanism for this period.”

The team also analyzed the rocks for major and trace elements to determine their formation depth. Their geochemical analysis showed the rocks originated 80 kilometers below the surface in the lithosphere. This was surprising because the modern lithosphere is only 70 kilometers thick.

How the Lithospheric Root Triggered Mountain Formation

The researchers proposed that when the continental plate folded to create the Mongolian orocline 200 million years ago, the lithosphere thickened at the curve of the U shape. This thickened section, or lithospheric root, became unstable over time. “The lithospheric root would have been too heavy to remain attached to the crust above for long,” Li explained. Eventually, a chunk of the root snapped off and sank into the deep mantle, a process called lithospheric foundering.

As the heavy root sank, it melted, triggering volcanic activity and causing the crust to rebound upward. This rebound formed the Hangay Mountains, resolving a long-standing geological mystery.