Geologists found 'gigantic fortresses' beneath the Earth's crust in the mantle

Geologists have stumbled on what they call gigantic fortresses lying almost 1,800 miles below our feet in Earth’s mantle.

New seismic evidence shows how these subterranean bulwarks are older and tougher than the cooler debris of dead ocean plates that surround them, turning a long‑running mystery into a concrete data story.

Each fortress spans an area comparable to a continent beneath Africa and the Pacific, and registers hotter than its frigid surroundings.

Utrecht University‘s seismologist Arwen Deuss, working with colleagues across Europe, Australia, and the United States, led the crew that pieced together the picture.

Fortresses in Earth’s mantle

The scientific shorthand for the fortresses is large low seismic velocity provinces or LLSVPs, continent‑scale masses perched at the core‑mantle boundary about 1,800 to 1,900 miles down.

They dampen shear waves and slow them, marking them out as zones of hot, sluggish rock that differs sharply from the stiff material around them.

Seismic tomography first lit them up in global maps during the 1990s. Two hulking blobs emerged, one under Africa and one under the central Pacific, each more than 3,000 miles across.

Later surveys confirmed that the African LLSVP towers roughly 600 miles high while its Pacific twin rises even farther.

Yet whether they were temporary stains or ancient fixtures remained unsettled, in part because most models assumed vigorous, homogenizing flow in the lower mantle.

Taken together, the two provinces occupy about eight percent of the mantle’s volume, a reminder that Earth’s interior is far from uniform.

Listening to the planet’s bell

Seismologists treat the Earth like a giant bell that rings when a great quake strikes. The catastrophic 1994 Bolivia event provided tones that hummed for hours, carrying clues from the deepest layers and offering clean, low‑noise data.

“Large earthquakes make the whole Earth ring like a bell with different tones,” said Deuss after reviewing decades of records.

Her team tuned in not only to the pitch shifts but also to how loudly each mode persisted, a seldom‑used metric called quality factor.

That loudness, known as damping, revealed something odd. Vibrations grew quiet in the cold slab graveyard but stayed loud inside the hot fortresses, implying low internal friction and unexpectedly efficient energy transmission.

Against expectation, the LLSVPs sapped little energy from the waves. The finding overturned the simple view that heat alone controls attenuation, forcing scientists to search for another variable.

Texture tells a deeper story

The key lies in grain boundaries, the microscopic seams between crystals in mantle minerals. Fewer seams mean fewer places for energy to bleed away, a principle familiar to anyone who has tried to push water through a coarse versus fine filter.

Subducted plates recrystallize into tiny grains as they plunge, multiplying boundaries and soaking up seismic energy. They show up as the quietest patches in the global map, matching the high‑damping ring around the Pacific.

Inside the fortresses, grains have had eons to grow fat. Large crystals line up like chunky bricks, stiffening the rock and letting waves glide through with little loss, a behavior confirmed in high‑pressure laboratory rigs.

Laboratory tests on olivine confirm the trend, showing that doubling grain size can nearly halve seismic attenuation at mantle temperatures.

That experimental curve fits the numbers seen in the new global model, strengthening the grain‑size explanation.

Restless mantle and giant fortresses

Modeling suggests the grain growth needed to reach that size takes at least 500 million years, perhaps far longer.

The fortresses, therefore, predate many supercontinents that have since broken apart, including Pangea and its earlier cousins.

Such rigidity helps them ignore the slow churn of mantle convection. They stand their ground while cooler slabs crawl and sink around them, like boulders lodged in a riverbed of flowing rock.

“There is less flow in Earth’s mantle than is commonly thought,” Deuss explained, noting that the presence of these immovable masses forces scientists to rethink convection cycles. The textbook picture of a well‑stirred mantle no longer fits.

A layered mantle, part conveyor and part castle, now seems more likely. That hybrid view changes every surface process driven from below, from the drift of continents to the cycling of carbon.

Why the finding matters at the surface

The hot edges of the fortresses appear to be launchpads for mantle plumes that punch up to volcano chains such as Hawaii. Their buoyant rise fuels not only island arcs but also the massive basalt floods seen in Earth’s deep past.

Large igneous provinces linked to mass extinctions tend to cluster above these plume zones. Knowing that the plumes come from fortress margins ties surface cataclysms to deep‑mantle architecture.

Mountain belts form where plates collide, but the deep heat flow that drives those collisions may trace back to these hot roots. In that sense, the fortresses could set the tempo of orogeny and rifting alike.

Over geologic time, their stability may even shape the drift path of continents, steering plates around the mantle’s slow‑moving keystones. That possibility will feed new computer models that link deep convection to plate motions.

What scientists will chase next

Researchers plan denser global arrays and machine‑learning tools to catch fainter normal modes. Each new quake becomes another flashlight aimed into the depths, refining the 3‑D map with every event.

Geochemists will compare volcanic rocks to lab‑made analogs, hunting chemical fingerprints of the fortresses.

A match would close the loop between seismic images and real material, settling the debate over whether the fortresses differ in composition as well as temperature.

Mineral physicists also want to recreate fortress conditions in diamond‑anvil presses, squeezing and heating samples to map how grain growth unfolds through deep time. The work could reveal why some minerals coarsen while others stay fine and brittle.

Meanwhile, geoneutrino observatories may soon detect radioactive decay signatures that differ inside and outside the fortresses.

Any contrast would add a new dimension to the planet‑sized puzzle, linking deep geology to particle physics.

The study is published in Nature.

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