The Earth's mantle is a mysterious place, and the Azores Plateau in the North Atlantic has long been a puzzle for geologists. The region's unusually thick oceanic crust and water-rich lavas have traditionally been attributed to a mantle plume, a column of hot rock rising from deep within the Earth. However, a new study challenges this conventional wisdom, suggesting that the answer may lie not in heat, but in a cold relic left behind by an ancient ocean.
The Azores Plateau, a broad rise in the North Atlantic dotted with nine volcanic islands, has always been a bit of an anomaly. Normal ocean crust is about four miles thick, but here it swells to as much as 19 miles. For thick crust like this, the textbook answer has always been a mantle plume. However, Dr. Jianfeng Yang at the Chinese Academy of Sciences (CAS) in Beijing led a team that was puzzled by the misfit.
Their simulations of mantle flow and melting suggested that a mantle plume alone could not account for the observations. The trail led to the transition zone, a layer of mantle rock roughly 250 to 400 miles below the surface, where minerals hold a lot of water. A diamond carried up from there proved it, holding a water-rich grain. One study showed that deep Earth is anything but dry.
Water reaches such depths through subduction, the slow process where one plate slides beneath another and sinks into the mantle. The descending slab hauls water-soaked rock down. Much gets wrung out near the surface, but some rides into the transition zone and stays. Those sinking slabs are old ocean floors, plates that finished their life and slid into the depths. Over hundreds of millions of years, their cargo of water collects in pockets of the transition zone.
In the simulations, the team set a mid-ocean ridge drifting across the seafloor. Beneath it sat mantle rock containing a few tenths of a percent water. Then they let the physics run. Water changes how rock behaves. Even a trace lets mantle rock melt at lower temperatures than dry rock could. As the ridge wandered over the damp patch, that wet material welled upward and began to melt. It produced far more magma than comparable dry rock.
The drifting was the engine. These are merely model results and only a proposed mechanism, not a snapshot of the seafloor. In the models, that long bout of melting built oceanic crust between six and 12 miles thick, several times the ocean norm and squarely in the range measured under the Azores. The mechanism reproduced features that plume models had struggled to explain. The melting tracked a moving ridge instead of a fixed point.
One thing that immediately stands out is that the source rock in the model was damp, the lavas carried the wet fingerprint real Azores rocks show. Researchers already knew the deep Earth holds water, and that plumes can raise plateaus. What no one had shown was a migrating ridge drawing that buried water up to build a thick crust on its own. In this scenario, water played the dominant role rather than heat.
This flips the usual story for volcanoes erupting far away from any plate boundary. Instead of unusually hot mantle rising from depth, the engine can be ancient water freed from the transition zone and tapped by a passing ridge. The plume is no longer the only suspect. It also recasts those long-dead slabs as active players. Plates that sank into the mantle hundreds of millions of years ago may still influence volcanism at the surface, via the water they left behind.
This raises a deeper question: what other secrets might the deep Earth hold? Broad stretches of deep rock carry odd chemical signatures that long puzzled geochemists, faint traces of surface material locked far below. Recycled water rising from ancient slabs offers one possible explanation for those signatures, a possibility that one recent paper keeps probing.
Before this work, the thick crust and wet lavas of the Azores sat awkwardly inside a plume-only picture. Now there is an alternative. The model suggests that water recycled from ancient plates, stored hundreds of miles below and drawn up by a wandering ridge, could build a plateau on its own. That gives geologists a fresh lens for the many ocean plateaus and lone volcanoes that never sat neatly over a plume. Some may be wet-mantle features, not hot ones.
Mapping where ancient slabs stored water could help identify where similar volcanism may occur. It also tightens the bond between Earth's surface and its depths. Water that once filled an ocean can vanish for ages, then return to help raise islands. The planet keeps a long memory, and traces of ancient oceans may still shape the surface today. This study is published in Nature Communications.
Personally, I think this new study is a fascinating development in our understanding of the Earth's mantle. It challenges our assumptions and opens up new avenues for exploration. What makes this particularly fascinating is the idea that ancient water, stored deep within the Earth, can still influence the surface in such profound ways. It raises a deeper question: how much more do we have to learn about our planet's inner workings?
