Scientists May Have Finally Solved a Decades-Old Mystery Beneath the Pacific Ocean

A thermochemical plume may explain how Earth’s largest oceanic plateau formed beneath the sea.

Buried beneath the western Pacific is a volcanic structure so vast that it rivals the size of a continent. The Ontong Java Plateau is the largest oceanic plateau on Earth, built from immense volumes of lava and underlain by crust far thicker than that of the surrounding seafloor. Yet one crucial question has remained unresolved: What kind of process could have created something so enormous without lifting it above sea level?

That contradiction has challenged geologists for decades. The leading mantle-plume model predicts that a surge of unusually hot material from deep inside Earth should have produced major surface uplift before the eruptions began. But geological evidence indicates that most of the plateau formed underwater.

Researchers from the South China Sea Institute of Oceanology of the Chinese Academy of Sciences now propose a different explanation. By using thermodynamic modeling to test the leading formation scenarios, they found that the plateau may have been produced by a thermochemical mantle plume—a rising body of mantle that was not only hotter than its surroundings, but also chemically distinct.

Oceanic plateaus are enormous submarine volcanic provinces marked by elevated seafloor and exceptionally thick crust. Many formed during brief but intense episodes of volcanism that released extraordinary quantities of magma. Because most remain submerged, they are largely shielded from the weathering and erosion that erase or distort similar volcanic structures on land.

Scientists have traditionally attributed their formation to one of two processes: deep mantle plumes or unusually rapid seafloor spreading. Neither explanation, however, has fully accounted for the unusual size, composition, and mostly submarine formation of the Ontong Java Plateau.

Two models compete

In the mantle plume model, hot material rises from deep inside Earth over tens of millions of years until it reaches the bottom of an oceanic plate. There, the hot mantle spreads outward beneath the plate. As pressure drops, some of the mantle melts, producing large amounts of magma that erupt as lava and build a broad volcanic plateau.

The second idea points to unusually rapid seafloor spreading. In that scenario, mantle rock beneath mid-ocean ridges melts as pressure decreases, generating enough magma to create a large igneous province.

New model solves contradictions

The new study, published in Nature Geoscience, proposes a different explanation for the Ontong Java Plateau: a thermochemical plume formation process.

The Ontong Java Plateau formed mostly during the Early Cretaceous period on the Pacific Plate and is the largest existing oceanic plateau on Earth. For years, many researchers favored the mantle plume explanation. But a purely thermal plume creates a problem. It would be expected to lift the plateau above sea level, which does not match evidence that most of the plateau formed underwater.

The seafloor spreading model also leaves key questions unresolved. Radiometric ages from drilled basaltic rocks on the Ontong Java Plateau do not closely match the ages of nearby magnetic lineations. That mismatch suggests the plateau probably formed within a plate, rather than near mid-ocean ridges.

Simulations favor mixed plumes

To compare the competing ideas, the researchers used thermodynamic modeling to reconstruct the mantle’s thermal and chemical conditions when the Ontong Java Plateau formed. They tested both the mantle plume scenario and the seafloor spreading scenario.

The simulations showed that the seafloor spreading model would require either an unrealistically high mantle potential temperature or a large amount of dense, easily melted pyroxenite. By contrast, a thermochemical plume could account for several major features of the plateau. A plume with a temperature anomaly of 135 to 200 ℃ and up to 13% dense, easily melted pyroxenite could explain the plateau’s crustal thickness patterns, lava composition, and mostly underwater formation.

Using the observed differences in crustal thickness and lava composition across the plateau, the researchers developed a spatial evolution model for a thermochemical mantle plume head.

Other plateaus may fit

The findings suggest that thermochemical plumes may be more important in building oceanic plateaus than scientists previously recognized.

“In addition to the Ontong Java Plateau, many other oceanic plateaus also occur with heterogeneous mantle sources, suggesting the common involvement of thermochemical plumes in oceanic plateau formation,” said Prof. Jinchang Zhang, first author of the study. “This mechanism differs substantially from the purely thermal plume model that has long been assumed.”

Reference: “Ontong Java Plateau formed by a thermochemical mantle plume” by Jinchang Zhang, Xubo Zhang, Shuangshuang Chen, Eric L. Brown, Jiangyang Zhang, Jian Lin, Rui Gao, William W. Sager and Jun Korenaga, 11 June 2026, Nature Geoscience.
DOI: 10.1038/s41561-026-02019-9

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