Central Mongolia’s Hangay Mountains have long posed a conundrum. Rising 4 kilometers above sea level, the dome-shaped range plays a key role in shaping the region’s climate. But it couldn’t have formed in the same way as most equally tall mountain ranges.

“These mountains in central Mongolia are very far from any plate boundary, about 3,000 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 research finding that geochemical evidence supports a compelling explanation of how these oddball mountains formed. The researchers proposed that at the site of the future mountains, a U-shaped bend in a tectonic plate led to an extra-thick lithosphere. A chunk of that heavy lithosphere eventually broke off and sunk into the mantle. Free of the extra weight, the crust then rebounded upward as the Hangay Mountains.

Bend and Snap

“It’s the first discovery of volcanism for this period.”

Tectonic plates are far from rigid. As they move above, below, and against each other, sections of the plates far from the boundary can develop curves and folds like a scrunched up tablecloth. Curved sections, called oroclines, are common around the world. At about 6,000 kilometers long, the Mongolian orocline is one of the longest, and the Hangay Mountains sit right at the curviest part of the orocline’s U shape.

Li and his colleagues suspected that the Hangays’ location along the orocline is no coincidence. During multiple field expeditions from 2018 through 2026, the researchers collected rock samples from several sites in the Hangay Mountains that showed signs of ancient volcanic activity. Uranium-lead dating of zircons within those samples showed that the area experienced volcanic activity in 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 the Hangay Mountains] during this period.…It’s the first discovery of volcanism for this period.”

The team also analyzed the samples for major and trace elements to determine the depth at which the rocks formed. Their geochemical analysis revealed that the rocks formed in the lithosphere 80 kilometers below the surface. They published these results in Geology in April.

It’s pretty odd that the rocks originated so deep, Li said, because the modern-day lithosphere is only 70 kilometers thick.

The team proposed that when the continental plate folded and created the Mongolian orocline 200 million years ago, the lithosphere bunched up and became thicker in the curve of the U shape. That thicker section of lithosphere, a root at least 80 kilometers thick, would have been unstable in the long term, Li explained.

The lithospheric root would have been too heavy to remain attached to the crust above for long, and a chunk of it would have eventually snapped off. When it sunk, or foundered, into the deep mantle, it would have melted and generated the volcanic activity recorded in the rocks the team studied. Free from the weight of that lithospheric root, the crust above would have rebounded into the dome-shaped mountain range visible today.

Complicated Yet Compelling

“Their story, though complicated, makes a great deal of sense and in a way provides affirmation of a prediction made some time ago regarding oroclines.”

“The story that [the researchers] have put together to explain the massive Hangay topographic ‘dome’ of central west Mongolia is a compelling one that spans more than the past 200 million years of Earth history,” said Stephen Johnston, a tectonics researcher at the University of Alberta in Canada who was not involved with this research. Past research into the Iberian orocline suggested that oroclines might lead to lithospheric thickening, and this explanation of the Hangay Mountains fits that narrative.

“Their story, though complicated, makes a great deal of sense and in a way provides affirmation of a prediction made some time ago regarding oroclines,” Johnston added.

Johnston said that the new explanation of how the Hangay Mountains formed makes him wonder why it took so long—80 million years—between when the orocline formed and when the lithospheric root sank.

“This seems a long time for a gravitationally unstable mantle root to have remained attached to the overlying crust,” he said. He hopes that future work can help determine whether this process has taken place at other oroclines around the world and has simply been overlooked or whether there is something special about the Mongolian orocline.

Li and his team have turned their attention to how the formation of the Hangay Mountains shaped the region’s ancient climate. Today, the towering mountain range prevents moist air from northern Mongolia from reaching the parched Gobi Desert in the south. They hope to connect how a process deep underground, like lithospheric foundering, affected the paleoclimate and, consequently, the region’s habitability.

“It’s very new to try to understand the Earth’s habitability from a deeper sense,” Li said.

—Kimberly M. S. Cartier (@astrokimcartier.bsky.social), Staff Writer

Correction 18 May 2026: The distance between the Hangay Mountains and the Pacific plate margin has been corrected. The location of newly discovered volcanic activity has been corrected.

This news article is included in our ENGAGE resource for educators seeking science news for their classroom lessons. Browse all ENGAGE articles, and share with your fellow educators how you integrated the article into an activity in the comments section below.

Citation: Cartier, K. M. S. (2026), Mongolian mountains rose when the crust bounced back, Eos, 107, https://doi.org/10.1029/2026EO260153. Published on 15 May 2026.

Text © 2026. AGU. CC BY-NC-ND 3.0
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A wide cinematic collection celebrating the evolution of aviation, from fragile early biplanes and daring pioneer pilots to flying boats, wartime fighters, classic airliners, supersonic icons, stealth aircraft, and futuristic aerospace designs. The series combines golden hour light, dramatic skies, ocean crossings, misty runways, military silhouettes, retro travel atmosphere, and science fiction concepts to create a visual timeline of flight as both engineering achievement and human dream.

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The weirdest volcano in the world may be Tanzania’s towering Ol Doinyo Lengai, an active peak that squeezes out a strange, low-temperature lava called carbonatite. Carbonatites are composed of more than 50% carbonate minerals, the same substances that form the ocean’s reefs. At Ol Doinyo Lengai, they are key components of the coldest lava on the planet.

Carbonatites are found on every continent and range in age from today-ish years old (in Tanzania) to about 3 billion years old (in Greenland). What’s more, they’re a major source of critical minerals.

In a new study published in Science Advances, a team of scientists led by Carl Spandler from Adelaide University in Australia identified a compelling correlation between carbonatites and specific sections of Earth’s continents—those proximal to past subduction zones.

Carbonatites and Critical Minerals

In the United States, the federal government defines critical minerals as those essential to the nation’s economic or national security. These minerals must also have supply chains that are vulnerable to distortions such as demand surges and foreign conflict. For example, most of the world’s terbium, used for everything from naval sonar systems to indoor lighting, comes from China. The United States considers terbium a critical mineral because the possibility of political or economic conflict within China or between China and another polity could directly or indirectly threaten the world’s supply of the element.

If you wanted to identify a rock that likely hosts rare earth elements, “carbonatite would be a good place to start.”

Critical minerals are either chemical elements (like terbium) or minerals. Important elements range from the familiar, like the lithium we need for batteries, to the sesquipedalian, like praseodymium, used for high-strength magnets. (Sesquipedalian means “having to do with a very long word.”)

Praseodymium is one of the 17 rare earth elements (terbium is another), all of which are considered critical minerals. Rare earth elements are not actually rare and are often (but not always) found in carbonatites. If you wanted to identify a rock that likely hosts rare earth elements, “carbonatite would be a good place to start,” said Kathryn Goodenough of the British Geological Survey, who was not involved in this study.

Fertilizing the Mantle

Much of Earth’s mantle is rock that remains after magma has been extracted—this mantle has been depleted. But carbonatites must come from mantle that’s quite the opposite—from parts that had to have been fertilized with volatiles containing trace metals, often critical minerals of interest. The question of how the mantle source for carbonatites came to be fertilized has no definitive answer.

Just as a garden can be fertilized in many ways ranging from synthetic sprays to coplanted cover crops, Earth’s mantle can be fertilized via myriad methods. “You must have volatiles or melts rising up from deeper in the mantle that are carrying metals with them,” Goodenough said.

For example, as a slab subducts beneath another tectonic plate, a volcanic arc typically arises above the zone at which the subducting slab reaches about 100 kilometers below Earth’s surface. This is the approximate depth at which the slab releases water, triggering melting in the overlying plate.

But fluids and melts can continue to exit the subducting slab far beyond the trace of the volcanic arc. That far out, the overriding plate almost always comprises a complete section of lithosphere—crustal lithosphere on top and mantle lithosphere on the bottom. The fluids and melts from the underlying slab, rich in halogens, carbon dioxide, phosphorus, and the like, rise into the overriding plate’s mantle lithosphere, changing the rocks via a process called metasomatism, Goodenough explained.

On the other hand, mantle plumes ascending from the core-mantle boundary are thought to be fertilized from a graveyard of subducted slabs that pond in the very deepest part of the mantle.

Spandler and his colleagues focused on testing whether that first method of fertilization, subduction-driven metasomatism, spatially correlates with carbonatites and rare earth element deposits. TL;DR—it does.

Fertilized Mantle Lithosphere

GPlates is a piece of software that allows users to rewind the movements of tectonic plates, exploring how continents have shifted their locations over the past 2 billion years. Using GPlates, Spandler’s coauthors Andrew Merdith and Amber Griffin, also of Adelaide University, mapped 43 polygons that denote regions of subduction lasting 100 million years or longer. These polygons, the authors infer, mark the locations of fertilized mantle lithosphere, which they call FML. These zones are thought to contain the good stuff—the critical minerals of interest.

“If [the correlation were] 100%, I wouldn’t believe it myself.”

Spandler and his colleagues compared the locations of carbonatites and rare earth elements with the polygons. They found that 67% of carbonatites and 72% of rare earth element ore deposits lie within these polygons. This correlation, though not perfect, suggests that mantle lithosphere fertilized by subduction could provide the source for many of these curious and critical deposits.

“If [the correlation were] 100%, I wouldn’t believe it myself because geology doesn’t work that way,” Spandler said.

Two Stepping

Spandler and his colleagues argue that carbonatites form in a two-step process. He emphasized that the new paper focuses on the first step—the process that led to fertilization of the eventual sources for carbonatites and rare earth element deposits.

The second step—the trigger—generates the carbonate-rich magma itself. It’s this event that provides the heat that causes melting of the mantle, said Richard Ernst, a scientist in residence at Carleton University in Canada who was not involved in this study.

“The trigger can be almost anything,” said Spandler, because the lithosphere needs only a nudge to melt. A plume can disrupt the structure of the lithosphere, triggering carbonatite magmatism, but so can continental rifting, he said. Indeed, Ol Doinyo is one of the mountains presiding over the East African Rift (which some scientists think also sits atop a plume).

Previous work by Ernst considered whether plumes could provide at least part of the source for some carbonatites by looking at the age of the deposits and those of nearby large igneous provinces—dramatic, long-lived outpourings of hot basalt thought to result from mantle plumes. In that work, Ernst and his colleague, the late Keith Bell, found the ages of large igneous provinces correlate with the ages of nearby carbonatite deposits; in short, the examples in that paper are potentially linked in both space and time.

Where carbonatite ages match those of nearby flood basalts from large igneous provinces, Spandler said, “I suspect that may just be the trigger mechanism.”

Plume Problems

For some carbonatites, there’s a time difference between when the mantle was fertilized and when the magmas were emplaced, explained Goodenough. “We can track that in several different localities,” she said. This observation would support something like the two-step process outlined above, as opposed to plumes driving the entire sequence.

Another problem with associating carbonatite formation exclusively with plumes, Goodenough said, is that carbonatites require cool conditions that result in relatively minor mantle melting. Plumes, and the large igneous provinces they appear to produce, are hot, and a lot. Plume proponents counter this critique by arguing that carbonatites are often found near the edges of large igneous provinces, away from the hottest zones.

Ernst noted, however, that though Spandler and his colleagues have made the spatial argument for subduction, “they haven’t made the isotopic argument that requires a subduction zone mechanism [for the source].” That sets up a testable hypothesis for future studies that could make use of existing data-rich geochemical studies of deposits within FMLs.

Moreover, even newer research may link the two camps, at least in some cases, with geochemical indicators pointing to both mantle plumes and mantle lithosphere being involved in forming some carbonatites. The latter component, said Ernst, may result from subduction-based fertilization as proposed by Spandler and his colleagues.

The Future of FMLs

“This is just an example of what we could do [with GPlates],” said Spandler. “In the next decade, we’ll see these models getting much more sophisticated and applied to all sorts of things.”

Computing power has improved to allow these models to run in a reasonable time frame. Plus, there’s lots of data. “We have a much better understanding about the history of each little bit of the continental crust around the planet,” he said.

And although people rightly point out that details become fuzzy in plate models that reach into the Proterozoic and beyond, “you’ve just got to pick one model and use it,” said Goodenough. “They’ve…taken the most widely available, repeatable model out there and used that.”

And on the basis of that model, Spandler and colleagues have shown a correlation between subduction—via FMLs—and carbonatites and rare earth element deposits. If someone comes up with another explanation, Spandler said, “that’s fine as well.”

—Alka Tripathy-Lang (@dralkatrip.bsky.social ), Science Writer

Citation: Tripathy-Lang, A. (2026), Ancient subduction may have seeded today’s critical mineral deposits, Eos, 107, https://doi.org/10.1029/2026EO260173. Published on 29 May 2026.

Text © 2026. The authors. CC BY-NC-ND 3.0
Except where otherwise noted, images are subject to copyright. Any reuse without express permission from the copyright owner is prohibited.

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